https://yanevskiv.com/ 2026-07-21T22:25:50+00:00 https://yanevskiv.com/ https://yanevskiv.com/_media/logo.png text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/115200?rev=1781320400&do=diff 115200 115200 is the default baud rate for Arduino USB-UART communication. picocom You specify the baudrate using -b option. $ picocom /dev/ttyACM0 -b 115200 To exit picocom, use Ctrl + A, X. This means holding control and pressing 'A' and 'X'. Don't release control while switching from 'A' to 'X'. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ab-testing?rev=1781320400&do=diff A/B testing A/B testing is a method of comparing two variants of a feature by exposing each to a different subset of users simultaneously and measuring the effect on a target metric. Variant A is typically the control (current behaviour); variant B is the treatment (the change being evaluated). text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/aba-problem?rev=1781320400&do=diff ABA problem The ABA problem is a correctness hazard in lock-free code that uses compare-and-swap (CAS). CAS reads a value, does some work, then atomically updates the location only if it still holds the value you read — otherwise retrying. The assumption baked in is that a matching value means nothing changed. This is wrong: another thread can change the location from A to B and back to A between your read and your CAS, and CAS cannot tell the difference. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/agile?rev=1781320400&do=diff Agile Agile is a broad family of software development approaches that emphasise iterative delivery, collaboration with customers, and responding to change over following a fixed plan. The term was formalised in 2001 when seventeen software developers published the Agile Manifesto, which states four values: text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/amdahls-law?rev=1781320400&do=diff Amdahl's law Amdahl's law is a formula that predicts the maximum speedup you can get from parallelizing a program, given that some fraction of it is inherently sequential. It was stated by Gene Amdahl in 1967. The key insight is that the sequential portion of a program acts as a hard ceiling on how much parallelism can help, no matter how many processors you throw at it.$T$$p$$1 - p$$n$$\frac{p}{n}$$1 - p$$n$$$T(n) = T \left( (1 - p) + \frac{p}{n} \right)$$$S(n)$$n$$$S(n) = \frac{T}{T(n)} = \fr… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/applying-gates-to-zero-state?rev=1781320400&do=diff Applying gates to $\lvert 0\rangle$ (Zero state) Gate Matrix form Derivation $I$ gate (Identity gate) $I = \begin{pmatrix}1&0\\0&1\end{pmatrix}$ $I\lvert 0\rangle = \begin{pmatrix}1&0\\0&1\end{pmatrix}\begin{pmatrix}1\\0\end{pmatrix} = \begin{pmatrix}1\\0\end{pmatrix} = \lvert 0\rangle$ $X$ gate (NOT gate) $X = \begin{pmatrix}0&1\\1&0\end{pmatrix}$ $X\lvert 0\rangle = \begin{pmatrix}0&1\\1&0\end{pmatrix}\begin{pmatrix}1\\0\end{pmatrix} = \begin{pmatrix}0\\1\end{pmatrix} = \lvert 1\rangl… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/arduino-uno-r4-wifi?rev=1781320400&do=diff Arduino Uno R4 WiFi Documentation: * <https://docs.arduino.cc/hardware/uno-r4-wifi/> text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/array?rev=1781320400&do=diff Array An array is a fixed-size sequence of elements of the same type stored in contiguous memory. The index of an element determines its address: base + index * sizeof(element). This means any element is reachable in O(1) time without traversal, and the memory layout is cache-friendly — iterating over an array touches sequential cache lines rather than chasing pointers. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/artifact?rev=1781320400&do=diff Artifact An artifact in software engineering is a file or package produced by a build process that is intended for use or distribution. Source code is the input; artifacts are the outputs. Examples: a compiled binary, a .jar file, a Docker image, a firmware text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/assert.h?rev=1781320400&do=diff assert.h If you have ever had a function produce a wrong result somewhere deep in a call stack with no indication of where things went wrong, assert.h is what you reach for. It provides the assert macro, which checks a condition at runtime and immediately kills the program with a diagnostic message that tells you exactly which file, which line, and which condition failed. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/atomic-openmp?rev=1781320400&do=diff Atomic (OpenMP) count++ compiles to three instructions: load, increment, store. When two threads execute this simultaneously, both can load the same value, both increment it, and both store back — the net result is one increment instead of two. This is the classic read-modify-write race condition. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/b-plus-tree?rev=1781320400&do=diff B+ tree A B+ tree is a variant of the B-tree where all values are stored in the leaf nodes and internal nodes hold only keys used for routing. Leaf nodes are linked together in a sorted doubly linked list. This distinction has a significant practical effect: range queries traverse only the leaf layer rather than the entire tree, and all records are equidistant from the root, giving uniform access time. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/b-tree?rev=1781320400&do=diff B-tree A B-tree is a self-balancing search tree generalised from a binary search tree to allow more than two children per node. Each node holds between $t-1$ and $2t-1$ keys (where $t \geq 2$ is the minimum degree), sorted in order. A node with $k$ keys has $k+1$ child pointers. All leaves are at the same depth, so the tree is perfectly height-balanced, and the height is O(log n). text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/barrier-openmp?rev=1781320400&do=diff Barrier (OpenMP) Barrier is a synchronisation point where all threads in a team wait until every member has arrived. OpenMP inserts an implicit barrier at the end of every parallel, for, and sections region. All threads wait there until every member of the team has arrived. An explicit text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/bash-internal?rev=1781320400&do=diff List of internal Bash shell commands text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/basis-state?rev=1781320400&do=diff Basis state Basis state is a state vector that is part of a basis. A basis is set of two or more quantum states out of which all other quantum states can be built through linear combination. A qubit is a two-level quantum system so has two basis states. The standard basis $\lvert 0\rangle, \lvert 1\rangle}$$\lvert\text{cat}\rangle$$\lvert\text{dog}\rangle$$d = 2$$\lvert\psi\rangle$$a,b\in\mathbb C$$\mathbb C^2$$\mathbb C^2$$$ \lvert\psi\rangle = \begin{pmatrix}a\\b\end{pmatrix},\quad\lvert\psi… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/bell-state-qiskit?rev=1781320400&do=diff Bell state (Qiskit) Bell states implementation using Qiskit. All four Bell states are prepared from $\lvert 00\rangle$ using a Hadamard gate and a CX gate, with optional single-qubit corrections. from qiskit import QuantumCircuit from qiskit.quantum_info import Statevector def bell(z_flip=False, x_flip=False): qc = QuantumCircuit(2) qc.h(0) qc.cx(0, 1) if z_flip: qc.z(0) # relative phase flip if x_flip: qc.x(1) # bit flip on second qubit return qc p… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/bell-state?rev=1781320400&do=diff This page has moved to Bell states (plural). text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/bell-states?rev=1781320400&do=diff Bell states Bell states are the four maximally entangled two-qubit states, named after physicist John Bell. They form an orthonormal basis for the two-qubit Hilbert space $\mathbb{C}^4$ and are the canonical examples of quantum entanglement. No product state can approximate them without a dramatic increase in entropy.$$\lvert\Phi^+\rangle = \frac{1}{\sqrt{2}}(\lvert 00\rangle + \lvert 11\rangle) \qquad \lvert\Phi^-\rangle = \frac{1}{\sqrt{2}}(\lvert 00\rangle - \lvert 11\rangle)$$$$\lvert\Psi^+… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/binary-tree?rev=1781320400&do=diff Binary tree A binary tree is a hierarchical structure where each node holds a value and has at most two children, called left and right. The node with no parent is the root. Nodes with no children are leaves. The height of a tree is the number of edges on the longest path from root to leaf; a tree with n nodes has height between O(log n) (balanced) and O(n) (degenerate — essentially a linked list). text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/bloch-sphere?rev=1781320400&do=diff Bloch sphere Bloch sphere is a geometrical representation of a single qubit state as a point on the surface of a unit sphere in three-dimensional space. It provides an intuitive way to visualize qubit states and the effect of quantum gates as rotations. The Bloch sphere is named after physicist Felix Bloch.$\lvert\psi\rangle = a\lvert 0\rangle + b\lvert 1\rangle$$a, b \in \mathbb{C}$$|a|^2 + |b|^2 = 1$$\theta \in [0, \pi]$$\varphi \in [0, 2\pi)$$$\lvert\psi\rangle = \cos\frac{\theta}{2}\lvert 0… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/blocking-mpi?rev=1781320400&do=diff Blocking and non-blocking (MPI) A read() call on a file descriptor blocks: it doesn't return until the kernel has data to hand back. That is convenient but ties up the thread for the duration of the wait. POSIX offers aio_read() as an alternative: issue the request, do other work, and check completion later with text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/blue-green-deployment?rev=1781320400&do=diff Blue-green deployment Blue-green deployment is a release strategy where two identical production environments (called blue and green) are maintained simultaneously. At any moment, one environment is live and the other is idle. To deploy a new version, you deploy it to the idle environment, run smoke tests, then switch traffic from the live environment to it. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/born-rule?rev=1781320400&do=diff Born rule Born rule is a postulate in quantum mechanics that gives an interpretation of probability amplitudes. Namely, the probability amplitude $c\in\mathbb{C}$ associated with some state $\lvert i\rangle$ encodes the probability of finding the quantum system in the state $\lvert i\rangle$. The value of this probability — according to the Born rule — equals the modulus squared of the probability amplitude $c$$$P_i = |c|^2$$$\lvert\psi\rangle$$a\in\mathbb{C}$$\lvert 0\rangle$$b\in\mathbb{C}$$\… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/brooks-law?rev=1781320400&do=diff Brooks's law Brooks's law states that adding manpower to a late software project makes it later. It was articulated by Fred Brooks in his 1975 book The Mythical Man-Month, based on his experience managing the development of OS/360 at IBM. The reason is that new team members are not immediately productive. They need time to understand the codebase, the domain, and the team's conventions. During that ramp-up period they consume the time of the experienced members who have to train and answer que… text/html 2026-06-15T09:18:41+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/c-headers?rev=1781515121&do=diff C headers C headers offer an interface to the C standard library. List of headers * assert.h * complex.h * ctype.h * errno.h * fenv.h * float.h * inttypes.h * iso646.h * limits.h * locale.h * math.h * setjmp.h * signal.h * stdalign.h * stdarg.h * stdatomic.h * stdbit.h * stdbool.h * stdckdint.h * stddef.h * stdint.h * stdio.h * stdlib.h * stdmchar.h * stdnoreturn.h * string.h * tgmath.h * threads.h * time.h * uchar.h * wchar.h * wctype.h Links * <https://en.… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/c-standard-library?rev=1781320400&do=diff C standard library C standard library is one library that's always linked by default when you compile C programs. To use the library, a set of headers is offered which you can include by default. For example, <stdlib.h> and <stdio.h> in this code are headers from the C standard library (but there are also headers which are text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/canary-release?rev=1781320400&do=diff Canary release A canary release is a deployment strategy where a new version of software is rolled out to a small subset of users before being promoted to the full user base. The name references the practice of sending canaries into coal mines to detect toxic gas before sending miners in. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/canonical-states?rev=1781320400&do=diff Canonical states Canonical states are the six single-qubit states corresponding to the six cardinal points of the Bloch sphere — the $\pm z$, $\pm x$, and $\pm y$ poles. They are the eigenstates of the three Pauli operators $Z$, $X$, $Y$ and are the most frequently encountered states in single-qubit quantum computing.$\pm z$$\lvert 0\rangle$$\lvert 1\rangle$$$\lvert 0\rangle = \begin{pmatrix}1\\0\end{pmatrix} \qquad \lvert 1\rangle = \begin{pmatrix}0\\1\end{pmatrix}$$$\pm x$$\lvert +\rangle$$\l… text/html 2026-06-13T03:14:39+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ccx-gate?rev=1781320479&do=diff CCX (Toffoli) gate CCX gate (doubly-controlled X, also called Toffoli gate or CCNOT) is a three-qubit gate that flips the target qubit if and only if both control qubits are $\lvert 1\rangle$. It was introduced by Tommaso Toffoli in 1980 and is the key building block for reversible classical computation inside a quantum circuit.$\lvert a, b, c\rangle \mapsto \lvert a, b, c \oplus (a \wedge b)\rangle$$\oplus$$\wedge$$8 \times 8$$\lvert 000\rangle$$\lvert 111\rangle$$$\text{CCX} = \begin{pmatrix}… text/html 2026-06-13T03:19:41+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ccz-gate?rev=1781320781&do=diff CCZ gate CCZ gate (doubly-controlled Z) is a three-qubit gate that applies a Pauli Z to the target qubit if and only if both control qubits are $\lvert 1\rangle$, and does nothing otherwise. Because Z acts as a phase flip, the CCZ gate is fully symmetric: any permutation of the three qubits produces the same unitary.$\lvert a, b, c\rangle \mapsto (-1)^{a \wedge b \wedge c}\lvert a, b, c\rangle$$\lvert 111\rangle$$8 \times 8$$\lvert 000\rangle$$\lvert 111\rangle$$$\text{CCZ} = \begin{pmatrix} 1&… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ci-cd?rev=1781320400&do=diff CI/CD CI/CD (continuous integration / continuous delivery) is the practice of automatically building, testing, and deploying software on every change. The goal is to keep the gap between “code written” and “code running in production” as small as possible, and to catch problems as close to the source as possible rather than discovering them weeks later during a manual release. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/circular-buffer?rev=1781320400&do=diff Circular buffer A circular buffer (also called a ring buffer) is a fixed-capacity Queue backed by a flat Array with two indices: a read head and a write head. When either index reaches the end of the array it wraps back to zero, making the array appear circular. This gives O(1) enqueue and dequeue with no allocation and no data movement, at the cost of a fixed maximum size. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/cnot-gate-cudaq?rev=1781320400&do=diff CX gate (CUDA-Q) CX gate implementation using CUDA-Q. The example prepares the Bell state $\lvert\Phi^+\rangle = \frac{1}{\sqrt{2}}(\lvert 00\rangle + \lvert 11\rangle)$. // Compile: nvq++ main.cpp -o main // Run: ./main #include <cudaq.h> struct kernel { __qpu__ void operator()() { cudaq::qvector<2> q; h(q[0]); // |00> -> |+0> cx(q[0], q[1]); // CX -> Bell state |Phi+> mz(q); } }; int main() { auto counts = cudaq::sample(kernel{}); … text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/cnot-gate-custatevec?rev=1781320400&do=diff CX gate (cuStateVec) CX gate implementation using cuStateVec. The example prepares the Bell state $\lvert\Phi^+\rangle = \frac{1}{\sqrt{2}}(\lvert 00\rangle + \lvert 11\rangle)$. // Compile: nvcc main.cu -o main -lcustatevec // Run: ./main #include <stdio.h> #include <math.h> #include <cuda_runtime.h> #include <custatevec.h> int main() { const int nQubits = 2; const int dim = 1 << nQubits; cuDoubleComplex h_sv[4] = {{1,0},{0,0},{0,0},{0,0}}; // |00> cuDoubleComplex *d_… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/cnot-gate-qiskit?rev=1781320400&do=diff CX gate (Qiskit) CX gate implementation using Qiskit. The example prepares the Bell state $\lvert\Phi^+\rangle = \frac{1}{\sqrt{2}}(\lvert 00\rangle + \lvert 11\rangle)$. from qiskit import QuantumCircuit from qiskit.quantum_info import Statevector qc = QuantumCircuit(2) qc.h(0) # |00> -> |+0> qc.cx(0, 1) # CX: control=q0, target=q1 -> Bell state print(Statevector(qc)) # Statevector([0.70710678+0.j, 0.+0.j, 0.+0.j, 0.70710678+0.j], dims=(2, 2)) text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/collapse-openmp?rev=1781320400&do=diff Collapse (OpenMP) Collapse is an OpenMP clause that merges multiple perfectly-nested loops into a single flat iteration space before distributing work across threads. When a loop nest has too few outer iterations to keep all threads busy, distributing only the outer loop leaves most threads idle. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/collectives-mpi?rev=1781320400&do=diff Collectives (MPI) Many parallel patterns require all processes to participate in a coordinated operation: distributing input, combining results, or synchronising before the next phase. These can always be assembled from point-to-point sends and receives, but writing them manually is verbose and misses optimisation opportunities. A broadcast, for instance, could be written as rank 0 sending the value to each other process in a loop: text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/communicator-duplication-mpi?rev=1781320400&do=diff Communicator duplication (MPI) When a C program calls a library, the library inherits all open file descriptors. If the library reads from stdin without knowing the application also reads from it, input gets silently consumed by the wrong side. The standard fix is to text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/communicators-mpi?rev=1781320400&do=diff Communicators (MPI) When two unrelated programs both use TCP on the same machine, their traffic doesn't collide: each connection is scoped by port numbers that distinguish one stream from another. MPI uses a communicator for the same purpose. A communicator is a group of processes combined with a context that scopes all message traffic: a message sent on one communicator cannot be received on a different communicator, even between the same pair of ranks with the same tag. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/complex.h?rev=1781320400&do=diff complex.h complex.h is what you include when you need the maths to work directly with complex numbers, rather than manually tracking real and imaginary parts in separate double variables. Added in C99, it gives you _Complex types, the imaginary unit $e^{i\pi} \approx -1$ text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/coreutils?rev=1781320400&do=diff List of coreutils commands coreutils (or GNU core utilities) are a set of commands inherited from Unix and implemented by the GNU project. These commands are pretty much available on all GNU/Linux systems, no matter the distro. The following is a table of coreutil commands. You can use text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/cp-1?rev=1781320400&do=diff $\mathbb{CP}^1$ Complex projective line (or $\mathbb{CP}^1$) is the space in which pure qubit states live. It is defined as the quotient of $\mathbb{C}^2 \setminus \{0\}$ by $\mathbb{C}^\times$. $$\mathbb{CP}^1 \stackrel{\text{def}}{=} (\mathbb{C}^2 \setminus\{0\})/\mathbb{C}^\times\qquad \mathbb{C}^\times\stackrel{\text{def}}{=}\mathbb{C} \setminus \{0\}$$ Motivation To do quantum computing, we usually use the Hilbert space $\mathbb{C}^2$. That's the Hilbert space in which all qubit state v… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/critical-sections-openmp?rev=1781320400&do=diff Critical sections (OpenMP) In POSIX threads, mutual exclusion is explicit: you call pthread_mutex_lock before a shared update and pthread_mutex_unlock after. Critical sections in OpenMP do the same thing with a pragma. The enclosed block is protected by an implicit mutex: only one thread at a time executes it, and threads that arrive while another is inside are blocked until it exits. Unlike text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ctype.h?rev=1781320400&do=diff ctype.h ctype.h gives you the character classification and conversion functions you reach for when parsing input character by character. If you have ever written if (c >= 'a' && c <= 'z') to check for a lowercase letter, these functions are the right way to do it — they handle the full range of text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/curses.h?rev=1781320400&do=diff curses.h Writing raw escape sequences to position the cursor and handle keyboard input is painful and terminal-specific. curses.h (typically ncurses) abstracts all of that away: you tell it to put text at row 5, column 10, and it figures out the right escape sequence for whatever terminal the user has. It also tracks what is on screen so it only sends the changes on each refresh, rather than redrawing everything. text/html 2026-06-19T10:07:31+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/cx-gate?rev=1781863651&do=diff CX gate (CNOT) CX gate (controlled-X, also called CNOT or controlled-NOT) is a two-qubit gate that flips the target qubit if and only if the control qubit is $\lvert 1\rangle$, and does nothing otherwise. It is the standard entangling gate in quantum computing. $$\text{CX} = \begin{pmatrix}1 & 0 & 0 & 0\\ 0 & 1 & 0 & 0\\ 0 & 0 & 0 & 1\\ 0 & 0 & 1 & 0\end{pmatrix}$$ The rows and columns are ordered $\lvert 00\rangle, \lvert 01\rangle, \lvert 10\rangle, \lvert 11\rangle$$$\text{CX}\lvert 00\ran… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/cy-gate?rev=1781320400&do=diff CY gate CY gate (controlled-Y) is a two-qubit gate that applies a Pauli Y to the target qubit if and only if the control qubit is $\lvert 1\rangle$, and does nothing otherwise. $$\text{CY} = \begin{pmatrix}1 & 0 & 0 & 0\\ 0 & 1 & 0 & 0\\ 0 & 0 & 0 & -i\\ 0 & 0 & i & 0\end{pmatrix}$$ The rows and columns are ordered $\lvert 00\rangle, \lvert 01\rangle, \lvert 10\rangle, \lvert 11\rangle$, with the first qubit as control and the second as target: $$\text{CY}\lvert 00\rangle = \lvert 00\rangle \… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/cz-gate?rev=1781320400&do=diff CZ gate CZ gate (controlled-Z) is a two-qubit gate that applies a Pauli Z to the target qubit if and only if the control qubit is $\lvert 1\rangle$, and does nothing otherwise. Because Z only adds a phase of $-1$ to $\lvert 1\rangle$, the gate is symmetric: either qubit can be treated as the control.$$\text{CZ} = \begin{pmatrix}1 & 0 & 0 & 0\\ 0 & 1 & 0 & 0\\ 0 & 0 & 1 & 0\\ 0 & 0 & 0 & -1\end{pmatrix}$$$\lvert 00\rangle, \lvert 01\rangle, \lvert 10\rangle, \lvert 11\rangle$$\lvert 11\rangle$$$… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/data-sharing-openmp?rev=1781320400&do=diff Data sharing (OpenMP) A local variable in a C function lives on the stack and is private to that call. When multiple threads run the same code, each gets its own stack, so locals are naturally separate. Variables declared outside a parallel region are different: all threads share the same memory location. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/deadlock-mpi?rev=1781320400&do=diff Deadlock (MPI) The classic multithreaded deadlock: thread A holds mutex 1 and waits for mutex 2, while thread B holds mutex 2 and waits for mutex 1. Neither can proceed because each is blocked on a resource the other holds. MPI deadlocks follow the same pattern with blocking sends taking the place of mutex acquires. If process 0 calls text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/density-matrix?rev=1781320400&do=diff Density matrix Density matrix (written as $\rho$) is a matrix representation of a quantum state. It's a more general way to represent a quantum state compared to the state vector $\lvert\psi\rangle$. While a state vector can only represent pure states, a density matrix can represent mixed states as well, making it the correct tool for open quantum systems, noisy circuits, and statistical ensembles of quantum states.$\lvert\psi\rangle$$$\rho = \lvert\psi\rangle\langle\psi\rvert$$$\lvert 0\rangl… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/derived-datatypes-mpi?rev=1781320400&do=diff Derived datatypes (MPI) MPI's built-in types (MPI_INT, MPI_DOUBLE, etc.) describe contiguous arrays of a single type. When the data you want to send is scattered in memory — a column of a row-major matrix, alternating elements, fields from a struct — you have two options. The first is to copy the elements manually into a flat temporary buffer: text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/deutsch-jozsa?rev=1781320400&do=diff Deutsch-Jozsa algorithm Deutsch-Jozsa algorithm is a quantum algorithm that determines whether a function $f:\{0,1\}^n \to \{0,1\}$ is constant or balanced using exactly one quantum query. It was proposed by David Deutsch and Richard Jozsa in 1992 as a generalization of Deutsch's algorithm to $n$$f$$2^n$$2^{n-1} + 1$$n + 1$$n$$\lvert 0\rangle^{\otimes n}$$\lvert 1\rangle$$n+1$$n$$n$$$\lvert 0\rangle^{\otimes n}\lvert 1\rangle \xrightarrow{H^{\otimes n+1}} \frac{1}{\sqrt{2^n}}\sum_{x=0}^{2^n-1}\… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/deutsch?rev=1781320400&do=diff Deutsch's algorithm Deutsch's algorithm is the simplest quantum algorithm that demonstrates a quantum speedup over any classical algorithm. It determines whether a binary function $f:\{0,1\} \to \{0,1\}$ is constant (same output for both inputs) or balanced (different outputs for the two inputs) using only a single quantum query, whereas any classical algorithm requires two.$f$$U_f$$\lvert x\rangle\lvert y\rangle \mapsto \lvert x\rangle\lvert y \oplus f(x)\rangle$$f$$f(0)$$f(1)$$U_f$$\lvert 0\r… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/devops?rev=1781320400&do=diff DevOps DevOps is a set of practices and a cultural philosophy that aims to shorten the feedback loop between writing code and running it in production by breaking down the traditional separation between development and operations teams. The name is a portmanteau of development and operations. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/dirac-notation?rev=1781320400&do=diff Dirac notation Dirac notation (or bra-ket notation) is a convenient way to name vectors in a Hilbert space $\mathcal{H}$ (via “kets”) and its dual space $\mathcal{H}^*$ (via “bras”). Kets are written as $\lvert\text{ket}\rangle$ and bras are written as $\langle\text{bra}\rvert$. The text inside a bra or ket has no intrinsic meaning — it is simply a label, much like naming a variable in programming.$\lvert\psi\rangle$$\mathbb{C}^2$$a, b \in \mathbb{C}$$$\lvert\psi\rangle = \begin{pmatrix}a\\b\en… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/do-while-0?rev=1781320400&do=diff do { ... } while (0) In C programming language,do { ... } while (0) is a loop which does a single iteration. A compiler will optimize it away completely, so the syntax produces no actual machine code. Most commonly, it's used to make function-macros. Example: /* * - clearing psr.i is implicitly serialized (visible by next insn) * - setting psr.i requires data serialization * - we need a stop-bit before reading PSR because we sometimes * write a floating-point register right before rea… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/docker?rev=1781320400&do=diff docker What is docker? Docker is a tool for managing containers. Containers are somewhat like virtual machine instances except a lot more lightweight. Think of an application e.g. a web app. You want to run the application, but you don't want to pollute your host OS with all the baggage that comes with it (libraries, dependencies, config files, services, text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/dynamic-array?rev=1781320400&do=diff Dynamic array A dynamic array is a resizable sequence backed by a heap-allocated Array. It provides O(1) indexed access like a fixed array, but grows automatically when capacity is exceeded. When the backing array is full and a new element is appended, the implementation allocates a larger array (typically 2×), copies all elements over, and frees the old allocation. The amortised cost of appending is O(1) because the copying work is spread across all the inserts that preceded the resize. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/eat-your-own-dog-food?rev=1781320400&do=diff Eating your own dog food Eating your own dog food (or dogfooding) is the practice of using the product you are developing internally, before or alongside releasing it to external users. The phrase comes from a 1988 Microsoft email where a manager challenged his team to use the company's own Lanman networking software internally. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/eigenstate?rev=1781320400&do=diff Eigenstate Eigenstate is refers to a quantum state associated with a quantum gate that remains unchanged after applying the gate. text/html 2026-06-15T09:18:41+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/embedded-engineering?rev=1781515121&do=diff Embedded engineering Embedded engineering is the discipline of writing software for microcontrollers (MCUs) and other resource-constrained hardware that is dedicated to a single task or a fixed set of tasks. Unlike general-purpose computing, an embedded system usually has no operating system, no display, and no user interacting with it directly — it just runs, often for years without being rebooted, inside some larger device. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/embedded-glossary?rev=1781320400&do=diff Embedded engineering glossary * QNX (Quantum Network eXecutive) - A proprietary real-time operating system (RTOS) * Lock-free queue - An efficient MT-safe queue implemented only using atomic instructions with no spinlocks, semaphores or mutexes. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/eof?rev=1781320400&do=diff EOF EOF is an acronym that stands for “End of file” Shell In shell, EOF is commonly used as a terminator in what is called a heredoc (“here document”). It's when you want to supply a multiline text to a command to its standard output, using <<. $ cat > hello.txt << EOF Here, you can write anything you like. It's not over until you actually type E-O-F. You can cancel with Ctrl + C. EOF text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/eqn-electromagnetism?rev=1781320400&do=diff (WIP) Equations of Electromagnetism Electrostatics Coulomb's law Attractive force $Q_2$ feels thanks to $Q_1$'s electric field: $$\mathbf F_ = \frac{1}{4\pi\varepsilon_0}\frac{Q_1Q_2}{r^2}\mathbf r_{012}$$ Electric field Electric field created by a single point charge $Q$: $$\mathbf E = \frac{1}{4\pi\varepsilon_0}\frac{Q}{r^2}\mathbf r_0$$ Electric field created by multiple point charges $Q_1, Q_2, ..., Q_N$: $$\mathbf E = \frac{1}{4\pi\varepsilon_0}\sum_{i=1}^{N}\frac{Q_i}{r^2_i}\mathbf r… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/errno.h?rev=1781320400&do=diff errno.h When a standard library function or system call fails, it tells you two things: the return value (NULL, -1, or similar) tells you that it failed, and errno.h tells you why. Without errno, you would know something went wrong but not whether the file did not exist, you lacked permission, or the disk was full. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/exit-status?rev=1781320400&do=diff Exit status Exit status lets the user of your program know if the program ended in success or failure. Commonly, 0 is success, any other value is failure (most commonly, failure is 1 -- a bit counter intuitive at first!) Your users can inspect it with text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/false-sharing-openmp?rev=1781320400&do=diff False sharing (OpenMP) False sharing is a performance problem that occurs when two threads write to different variables that happen to occupy the same cache line. Cache lines are typically 64 bytes wide. The hardware cache-coherence protocol must keep that line consistent across cores, so every write by one thread invalidates the cached copy in the other and forces a round-trip to main memory. The threads are accessing distinct variables and no actual race condition exists, but performance coll… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/fdsa?rev=1781320400&do=diff fdsafdsafdsafdas fdsafdsafds text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/feature-branch?rev=1781320400&do=diff Feature branch A feature branch is a git branch created for the development of a single feature or change, kept separate from the main branch until the work is complete and reviewed. The developer works on the branch in isolation, then opens a pull request to merge it back. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/feature-flag?rev=1781320400&do=diff Feature flag A feature flag (also called a feature toggle or feature switch) is a conditional in the code that enables or disables a feature at runtime without deploying new code. The feature is deployed but dark; it is activated by changing a configuration value, not by a release. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/fenv.h?rev=1781320400&do=diff fenv.h fenv.h gives you access to the floating-point environment: the status flags that record when something unusual happened during a computation (overflow, division by zero, invalid result like NaN), and the rounding mode that controls how inexact results are rounded. Before C99 formalised this header, there was no standard way to check whether a computation had silently produced a NaN or infinity. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/fft?rev=1781320400&do=diff FFT FFT (or Fast Fourier Transform) is an algorithm that computes DFT (or “Discrete Fourier Transform”). There are several FFT algorithms but the most popular one is called Cooley–Tukey FFT. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/float.h?rev=1781320400&do=diff float.h float.h tells you the hard limits of floating-point arithmetic on the platform you are compiling for: how many decimal digits double can represent reliably, the largest and smallest values, and machine epsilon. You reach for it when writing numerical code that needs to be correct across platforms rather than just on your development machine. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/flush-openmp?rev=1781320400&do=diff Flush (OpenMP) C's volatile keyword tells the compiler that a variable can change outside normal program flow and must not be cached in a register: every read goes to memory and every write is committed immediately. OpenMP threads face a similar problem at a larger scale: the compiler and CPU are free to keep shared variables in registers or store-buffers, so one thread's writes may not be visible to another without explicit synchronisation. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/forward-list?rev=1781320400&do=diff Forward list A forward list is a singly linked list with a single pointer to the head and no tail pointer. Each node holds a value and a next pointer; there is no prev. Traversal is one-directional: from head toward NULL. It is the minimal linked sequence — less memory per node than a text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/function-macro?rev=1781320400&do=diff Function macro Function macro is a C preprocessor macro that looks like a function call when you use it. This is a basic example with a single expression. sqr(x) looks and feels like a function even though it's a macro. A good habit to get into is to always enclose arguments into parenthesis when you define function macros. For example, text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/general-writing-guide?rev=1781320400&do=diff General writing guide General writing guide (this article) is a guide to writing articles in this wiki. Audience The intended audience is someone technically inclined — comfortable reading code and following a logical argument, but not necessarily with a formal background in every subject the wiki covers. Two things to keep in mind while writing:$\alpha$$e^{i\theta}$$H^\dagger = H$$$X = \begin{pmatrix} 0 & 1 \\ 1 & 0 \end{pmatrix}$$ text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/getopt.h?rev=1781320400&do=diff getopt.h Parsing argv by hand gets old fast once you have more than two flags. getopt.h is a POSIX/GNU header that provides getopt for short options (-v, -o file) and getopt_long for long options (--verbose, --output=file). It handles the argument loop, the option string, and the text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ghz-state-qiskit?rev=1781320400&do=diff GHZ state (Qiskit) GHZ state implementation using Qiskit. The three-qubit GHZ state $(\lvert 000\rangle + \lvert 111\rangle)/\sqrt{2}$ is prepared by applying a Hadamard gate to the first qubit and then chaining CX gates to spread the superposition to the remaining qubits. from qiskit import QuantumCircuit from qiskit.quantum_info import Statevector qc = QuantumCircuit(3) qc.h(0) # |000> -> (|000> + |100>) / sqrt(2) qc.cx(0, 1) # -> (|000> + |110>) / sqrt(2) qc.cx(0, 2) # -> (|000> + … text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/gist-docker?rev=1781320400&do=diff Docker (Reference) This page includes a myriad docker commands for reference. Basic Containers Run ubuntu:latest image (will do docker pull automatically if you don't have it) docker run -it --rm ubuntu:latest Images Pull ubuntu:latest image text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/gist-git?rev=1781320400&do=diff Git (Examples) Basics Creating a repository with an initial commit: git init Adding a file to the staging area: git add file.txt Un-adding a file from the staging area: git restore --staged file.txt Showing what's in the staging area: text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/gist-semaphores?rev=1781320400&do=diff (WIP) Semaphores (Examples) This page includes some examples of semaphores in multi-threading environments. The syntax is borrowed from EasyMT. Basic Synchronization Two threads a() and b() alternate their execution regardless of the scheduler. Output: $B$$B$ text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/git?rev=1781320400&do=diff git Git is a tool for managing repositories. Repositories are like folders, except it's easier to track the changes that happen inside them. Think of a project you've been working on e.g. a game in written in C++. You've been working on your game for weeks and now you have quite a bit of code. You're starting to be a little afraid of making changes text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/gitignore?rev=1781320400&do=diff .gitignore .gitignore is a file You can ignore everything in a given di F.A.Q. How do I ignore a directory without making a commit? Just put * in .gitignore of the directory you want to ignore (e.g. ./local directory) # ./local/.gitignore # Ignore everything in directory `./local` * text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/global-phase?rev=1781320400&do=diff Global phase Global phase is a property of a quantum state that emerges from the mathematical description but is physically completely unobservable. Multiplying an entire quantum state by a complex number of unit magnitude $e^{i\phi}$ produces a state that is physically identical to the original in every possible measurement.$a = Ae^{i\alpha}$$b = Be^{i\beta}$$A, B \in \mathbb{R}$$\alpha, \beta \in (-\pi, \pi]$$$\lvert\psi\rangle = Ae^{i\alpha}\lvert 0\rangle + Be^{i\beta}\lvert 1\rangle$$$\lve… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/gnu-linux?rev=1781320400&do=diff GNU/Linux GNU/Linux is a free operating system that replaced the older operating system called Unix. It's ubiquitous on servers and supercomputers. It's an increasingly popular choice for desktops, but not yet common. It's also the operating system behind SteamOS. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/goodharts-law?rev=1781320400&do=diff Goodhart's law Goodhart's law states that when a measure becomes a target, it ceases to be a good measure. It was formulated by British economist Charles Goodhart in 1975 in the context of monetary policy, but it applies broadly to any system where behaviour is optimised toward a metric. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/graph?rev=1781320400&do=diff Graph A graph is a structure consisting of a set of vertices (nodes) and a set of edges that connect pairs of vertices. Edges can be directed (one-way, like a one-way street) or undirected (bidirectional). Edges can carry a weight representing cost, distance, or capacity. Graphs model any relationship between entities: road networks, dependency graphs, social networks, circuit netlists, and state machines. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/greenspuns-tenth-rule?rev=1781320400&do=diff Greenspun's tenth rule Greenspun's tenth rule (despite there being no rules one through nine) is an aphorism by Philip Greenspun: “Any sufficiently complicated C or Fortran program contains an ad hoc, informally-specified, bug-ridden, slow implementation of half of Common Lisp. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/grover?rev=1781320400&do=diff Grover's algorithm Grover's algorithm is a quantum search algorithm that finds a marked element in an unstructured database of $N$ items in $O(\sqrt{N})$ queries. It was invented by Lov Grover in 1996 and provides a quadratic speedup over classical brute-force search, which requires $O(N)$$U_f$$\lvert x^*\rangle$$U_f\lvert x\rangle = -\lvert x\rangle$$x = x^*$$U_f\lvert x\rangle = \lvert x\rangle$$\lvert s\rangle = H^{\otimes n}\lvert 0\rangle^{\otimes n}$$\lvert x^*\rangle$$G$$U_f$$D = 2\lvert… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/gustafsons-law?rev=1781320400&do=diff Gustafson's law Gustafson's law is a formula that predicts the speedup of a parallel program when the problem size is allowed to grow with the number of processors. It was proposed by John Gustafson in 1988 as a rebuttal to the pessimism of Amdahl's law. The key shift is moving from $T$$n$$\alpha$$1 - \alpha$$n$$n$$(1-\alpha)T$$n(1-\alpha)T$$\alpha T$$$T_1 = \alpha T + n(1 - \alpha)T = T\bigl(\alpha + n(1 - \alpha)\bigr)$$$$S(n) = \frac{T_1}{T} = \alpha + n(1 - \alpha) = n - \alpha(n - 1)$$$\fr… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/h-gate-cudaq?rev=1781320400&do=diff Hadamard gate (CUDA-Q) Hadamard gate implementation using CUDA-Q. // Compile: nvq++ main.cpp -o main // Run: ./main #include <cudaq.h> struct kernel { __qpu__ void operator()() { cudaq::qubit q; h(q); // |0> -> |+> mz(q); } }; int main() { auto counts = cudaq::sample(kernel{}); counts.dump(); // ~50% |0>, ~50% |1> } text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/h-gate-custatevec?rev=1781320400&do=diff Hadamard gate (cuStateVec) Hadamard gate implementation using cuStateVec. // Compile: nvcc main.cu -o main -lcustatevec // Run: ./main #include <stdio.h> #include <math.h> #include <cuda_runtime.h> #include <custatevec.h> int main() { const int nQubits = 1; const int dim = 1 << nQubits; cuDoubleComplex h_sv[2] = {{1,0},{0,0}}; // |0> cuDoubleComplex *d_sv; cudaMalloc(&d_sv, dim * sizeof(cuDoubleComplex)); cudaMemcpy(d_sv, h_sv, dim * sizeof(cuDoubleComplex), cu… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/h-gate-qiskit?rev=1781320400&do=diff Hadamard gate (Qiskit) Hadamard gate implementation using Qiskit. from qiskit import QuantumCircuit from qiskit.quantum_info import Statevector qc = QuantumCircuit(1) qc.h(0) # |0> -> |+> = (|0> + |1>) / sqrt(2) print(Statevector(qc)) # Statevector([0.70710678+0.j, 0.70710678+0.j], dims=(2,)) text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/h-gate?rev=1781320400&do=diff Hadamard gate Hadamard gate (or H gate) is a single-qubit gate that maps the computational basis states $\lvert 0\rangle$ and $\lvert 1\rangle$ to equal superpositions. It is the most commonly used gate for creating superposition and appears at the start of most quantum algorithms. $$H = \frac{1}{\sqrt{2}}\begin{pmatrix}1 & 1\\ 1 & -1\end{pmatrix}$$$$H\lvert 0\rangle = \frac{\lvert 0\rangle + \lvert 1\rangle}{\sqrt{2}} = \lvert +\rangle \qquad H\lvert 1\rangle = \frac{\lvert 0\rangle - \lvert 1… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/harvard-architecture?rev=1781320400&do=diff Harvard architecture Harvard archictecture is a type of computer archictecture. It's a type of architecture where code and data are kept in separate memories. The Harvard architecture can be contrasted against von-Neumann archictecture. In the latter, code and data are both stored in the same working memory (RAM). The CPU has to fetch both the instructions and data by accessing the same memory bus, leading to what is known as von-Neumann bottleneck. This doesn't really matter, of course text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/hash-map?rev=1781320400&do=diff Hash map A hash map (also called a hash table) is a data structure that maps arbitrary keys to values in O(1) average time. A hash function converts a key to an integer index into a backing Array of buckets. The ideal hash function distributes keys uniformly so that each bucket holds roughly one entry; when two keys hash to the same bucket, that is a text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/header-guard?rev=1781320400&do=diff Header guard Header guards are a pattern used in C and C++ programming languages when writing header files. Header files (.h files) are files that are included in source files (.c and .cpp files) near the top (hence the name) using the #include <...>_ text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/heap?rev=1781320400&do=diff Heap A heap is a Binary tree stored in an Array that maintains the heap property: in a max-heap, every parent is greater than or equal to its children, so the maximum element is always at the root. In a min-heap the relationship is reversed and the minimum is at the root. The tree is text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/hhl?rev=1781320400&do=diff HHL algorithm HHL algorithm (Harrow-Hassidim-Lloyd algorithm) is a quantum algorithm for solving systems of linear equations $Ax = b$ where $A$ is an $N\times N$ sparse Hermitian matrix. Proposed in 2009, it achieves an exponential speedup over classical linear solvers under certain conditions.$O(Ns\kappa\log(1/\varepsilon))$$s$$\kappa$$\varepsilon$$O(\log(N)s^2\kappa^2/\varepsilon)$$N$$\lvert b\rangle$$A$$\lambda_j$$A$$\lvert\lambda_j\rangle \mapsto (C/\lambda_j)\lvert\lambda_j\rangle$$A^{-1}$… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/hilbert-space?rev=1781320400&do=diff Hilbert space Hilbert space is a complete inner product space. It is vector space that is equipped with an inner product such that the metric induced by the inner product is complete within the space. Completeness means that all Cauchy sequences converge within the same space. Hilbert space is a special case of the more general Banach space, both of which are concepts from functional analysis.$\mathbb{R}$$\mathbb{C}$$\lvert\psi\rangle$ text/html 2026-06-15T09:23:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/home?rev=1781515400&do=diff Home Hi, I'm Ivan! This website is my encyclopedia about various technical topics. The topics I cover on this wiki are mostly related to technology, but sometimes I also write about mathematics and physics. Three major topics on this website are: 1. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/hopf-fibration?rev=1781320400&do=diff Hopf fibration Hopf fibration is a map connecting the state vector representation of a qubit $\lvert\psi\rangle \in \mathbb{C}^2$ to a point on the Bloch sphere $(x, y, z) \in \mathbb{R}^3$. It explains why a qubit — an object living in the complex space $\mathbb{C}^2$ — can be visualized as a point on a real three-dimensional sphere.$a, b \in \mathbb{C}$$|a|^2 + |b|^2 = 1$$\mathbb{C}^2$$S^3$$S^3/U(1) \cong S^2$$\pi: S^3 \to S^2$$S^2$$S^3$$U(1)$$\pi$$\sigma_x, \sigma_y, \sigma_z$$$\pi:\mathbb{C… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/hotfix?rev=1781320400&do=diff Hotfix A hotfix is an urgent patch applied directly to a production release to fix a critical bug, without going through the normal development cycle. The name reflects the intent: it is applied while the system is “hot” (running in production) and cannot wait for the next scheduled release. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/hsp?rev=1781320400&do=diff Hidden subgroup problem Hidden subgroup problem (HSP) is a class of problems that provides a unified framework for many quantum algorithms. The problem is: given a group $G$, a set $X$, and a function $f: G \to X$ that is constant on cosets of some subgroup $H \leq G$ and takes distinct values on different cosets, find the hidden subgroup $H$$\mathbb{Z}_2$$\mathbb{Z}_2^n$$\mathbb{Z}$$\mathbb{Z}_N$$G$$G$$\frac{1}{\sqrt{|H|}}\sum_{h\in H}\lvert x_0 h\rangle$$x_0$$G$$G$$H$$O(\log|G|)$$H$$G$$S_n$$D… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/hybrid-openmp-mpi?rev=1781320400&do=diff Hybrid MPI+OpenMP (MPI) A naive parallel program launches one MPI rank per core. That works, but it means every rank keeps its own copy of the data (there is no shared address space between MPI processes), and the number of messages scales with total core count. A better model on a multi-core node is one MPI rank per node (or per socket), with OpenMP threads filling the cores within it. Inter-node communication goes through MPI; intra-node parallelism uses shared memory through OpenMP. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/i-gate-cudaq?rev=1781320400&do=diff I gate (CUDA-Q) I gate implementation using CUDA-Q. // Compile: nvq++ main.cpp -o main // Run: ./main #include <cudaq.h> struct kernel { __qpu__ void operator()() { cudaq::qubit q; h(q); // prepare |+> // identity is a no-op; the state is unchanged mz(q); } }; int main() { auto counts = cudaq::sample(kernel{}); counts.dump(); // ~50% |0>, ~50% |1> } text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/i-gate-custatevec?rev=1781320400&do=diff I gate (cuStateVec) I gate implementation using cuStateVec. // Compile: nvcc main.cu -o main -lcustatevec // Run: ./main #include <stdio.h> #include <cuda_runtime.h> #include <custatevec.h> int main() { const int nQubits = 1; const int dim = 1 << nQubits; cuDoubleComplex h_sv[2] = {{1,0},{0,0}}; // |0> cuDoubleComplex *d_sv; cudaMalloc(&d_sv, dim * sizeof(cuDoubleComplex)); cudaMemcpy(d_sv, h_sv, dim * sizeof(cuDoubleComplex), cudaMemcpyHostToDevice); cust… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/i-gate-qiskit?rev=1781320400&do=diff I gate (Qiskit) I gate implementation using Qiskit. from qiskit import QuantumCircuit from qiskit.quantum_info import Statevector qc = QuantumCircuit(1) qc.h(0) # prepare |+> qc.id(0) # identity — state unchanged print(Statevector(qc)) # Statevector([0.70710678+0.j, 0.70710678+0.j], dims=(2,)) text/html 2026-06-15T15:22:49+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/i-gate?rev=1781536969&do=diff $I$ gate (Identity gate) I gate (or identity gate) is a single-qubit quantum gate that leaves the state of a qubit unchanged. It is represented by the $2 \times 2$ identity matrix and drawn as a plain wire in quantum circuit diagrams. $$I = \begin{pmatrix}1 & 0\\ 0 & 1\end{pmatrix}$$ The identity gate is trivial on its own but appears in multi-qubit notation to indicate that a qubit is carried through a time step without any operation being applied. For example, applying $H$$H \otimes I$ text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/i-state-qiskit?rev=1781320400&do=diff $\lvert i\rangle$ (Qiskit) Plus-i state $\lvert i\rangle$ implementation using Qiskit. The $\lvert i\rangle$ state is prepared by applying a Hadamard gate followed by an S gate to $\lvert 0\rangle$. from qiskit import QuantumCircuit from qiskit.quantum_info import Statevector qc = QuantumCircuit(1) qc.h(0) # |0> -> |+> qc.s(0) # |+> -> |i> = (|0> + i|1>) / sqrt(2) print(Statevector(qc)) # Statevector([0.70710678+0.j, 0.+0.70710678j], dims=(2,)) text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ifupdown?rev=1781320400&do=diff ifupdown ifupdown is the traditional network manager on GNU/Linux systems. It has since been replaced by systemd-networkd. It was especially commmon in Debian systems before Debian 12 (Bookworm). The “if” part is short for “interface”. “up” and “down” refer to text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/incremental-build-model?rev=1781320400&do=diff Incremental build model The incremental build model is a software development lifecycle model in which the system is built and delivered in a series of increments, each adding functional capability to the previous one. The first increment delivers a working core; subsequent increments add features until the full system is complete. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/intro?rev=1781320400&do=diff Intro Hi, I'm Ivan! This website is my encyclopedia about various technical topics. The topics I cover are mostly related to technology, but sometimes I also write about mathematics and physics. I'm an embedded software engineer, currently pivoting to high-performance engineering (HPC). I like working directly with hardware or low-level software. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/introduction-to-quantum-computing?rev=1781320400&do=diff Introduction to quantum computing Quantum computing is a theory of programming a quantum computer. If you're reading this article, you're likely curious about quantum computing and would like to learn more. How does a quantum computer work? What makes it different from a classical computer? How exactly do you program it? If you don't have a background in physics quantum computing may initially seem math-heavy and rather difficult to understand. An assumption I'm going to make is that the reade… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/inttypes.h?rev=1781320400&do=diff inttypes.h inttypes.h solves a portability trap that bites you the first time you try to printf a uint64_t. On one platform uint64_t is unsigned long, so %lu works. On another it is unsigned long long, so you need %llu. Get it wrong and you get a compiler warning on one platform and silently wrong output on another. The format macros in this header expand to whichever specifier is correct for the current platform. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/iso646.h?rev=1781320400&do=diff iso646.h iso646.h defines macros that let you spell out operators in plain English: and for &&, or for ||, not for !, and so on. It exists because some older national character sets used the code points for &, |, ~, and ! for other symbols, so alternative spellings were standardised in C95. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/iswap-gate-cudaq?rev=1781320400&do=diff iSWAP gate (CUDA-Q) iSWAP gate implementation using CUDA-Q. The example prepares $\lvert 10\rangle$ and applies iSWAP to produce $i\lvert 01\rangle$. // Compile: nvq++ main.cpp -o main // Run: ./main #include <cudaq.h> struct kernel { __qpu__ void operator()() { cudaq::qvector<2> q; x(q[0]); // q[0]=|1>, q[1]=|0> => |10> iswap(q[0], q[1]); // |10> -> i|01> mz(q); } }; int main() { auto counts = cudaq::sample(kernel{}); counts.du… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/iswap-gate-custatevec?rev=1781320400&do=diff iSWAP gate (cuStateVec) iSWAP gate implementation using cuStateVec. The example prepares $\lvert 10\rangle$ and applies iSWAP to produce $i\lvert 01\rangle$. // Compile: nvcc main.cu -o main -lcustatevec // Run: ./main #include <stdio.h> #include <cuda_runtime.h> #include <custatevec.h> int main() { const int nQubits = 2; const int dim = 1 << nQubits; // Prepare |10>: q0=1, q1=0 -> index 2 (binary 10) cuDoubleComplex h_sv[4] = {0}; h_sv[2] = make_cuDoubleComplex(1.0… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/iswap-gate-qiskit?rev=1781320400&do=diff iSWAP gate (Qiskit) iSWAP gate implementation using Qiskit. The example prepares $\lvert 10\rangle$ and applies iSWAP to produce $i\lvert 01\rangle$. from qiskit import QuantumCircuit from qiskit.quantum_info import Statevector qc = QuantumCircuit(2) qc.x(1) # q0=|0>, q1=|1> => |10> in math ordering qc.iswap(0, 1) # |10> -> i|01> print(Statevector(qc)) # Statevector([0.+0.j, 0.+1.j, 0.+0.j, 0.+0.j], dims=(2, 2)) text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/iswap-gate?rev=1781320400&do=diff iSWAP gate iSWAP gate is a two-qubit gate that swaps the $\lvert 01\rangle$ and $\lvert 10\rangle$ amplitudes while multiplying each by $i$, and leaves $\lvert 00\rangle$ and $\lvert 11\rangle$ unchanged. $$\text{iSWAP} = \begin{pmatrix}1&0&0&0\\ 0&0&i&0\\ 0&i&0&0\\ 0&0&0&1\end{pmatrix}$$ $$\text{iSWAP}\lvert 01\rangle = i\lvert 10\rangle \qquad \text{iSWAP}\lvert 10\rangle = i\lvert 01\rangle$$ The factor of $i$ distinguishes iSWAP from the plain SWAP gate. SWAP permutes amplitudes without … text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/kanban?rev=1781320400&do=diff Kanban Kanban is a workflow management method that visualises work in progress and limits the amount of work allowed at each stage. The name comes from the Japanese word for “signboard”; the method originated in Toyota's manufacturing system in the 1940s and was adapted for software by David Anderson around 2007. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/kdp20260110-1?rev=1781320400&do=diff KDP Midterm (2026-01-10) Question 1 Write and explain the Fine grain ticket algorithm implemented with $\text{FA}$ opeartion. If we replaced $\text{FA}$ with $\text{TS}$ (test-and-set) operation implemented in the following way, would it be possible to implement a fine-grained solution? If yes, implement it. Is the critical section entry fair? Why / why not?$\text{FA}$$\text{TS}$ text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/kdp20260110-2?rev=1781320400&do=diff KDP Midterm (2026-01-10) Question 2 Consider a dining table that can seat at most $N$ people ($N > 2$). A person may take some food, sit at the table (sitAtTable()), eat, leave the table (leaveTable()) and walk away. No person should eat alone i.e. sit at the table alone. Also, every person that takes food must necessarily sit down in order to eat. Create a solution for this probalom using a text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/kdp20260218-1?rev=1781320400&do=diff KDP Exam (2026-02-18) Question 1 Solve the dining philsophers problem by using signal and continue monitor discipline and by using covering condition technique. Ensure philosphers start eating in the order of their arrival. Solution #define N 5 /* number of philosophers */ monitor DiningPhilosophers { enum { THINKING, HUNGRY, EATING }; int phil[N]; cond delay[N]; queue<int> q; bool canEat(int id) { return (phil[(id - 1 + N) % N] != EATING) && (phil[(id + 1)… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/kdp20260218-2?rev=1781320400&do=diff KDP Exam (2026-02-18) Question 2 The Roller Coaster Problem. Assume there are $N$ passengers and $M$ cars in one roller coaster. Passengers alternate between walking around the amusement park and riding the roller coaster. One car can take at most $K$ passengers where $K < N$$K$ text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/kdp20260218-3?rev=1781320400&do=diff (WIP) KDP Exam (2026-02-18) Question 3 Using synchronous message passing (synch_send, receive) write a distributed solution for The Dining Philosophers problem. In the distributed solution philosopher processes may only communicate with corresponding fork processes and fork processes may only communicate with corresponding philosopher processes. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ket-0?rev=1781320400&do=diff $\lvert 0 \rangle$ (Zero state) The zero state $\lvert 0\rangle$ is one of the two computational basis states of a qubit. It is the quantum analogue of a classical 0 bit, and it is the standard initial state used in most quantum circuits. The other computational basis state is $$\lvert 0\rangle = \begin{pmatrix}1\\0\end{pmatrix}$$$\lvert 0\rangle$$(0, 0, 1)$$+1$$Z\lvert 0\rangle = \lvert 0\rangle$$\lvert 0\rangle$$\lvert +\rangle = (\lvert 0\rangle + \lvert 1\rangle)/\sqrt{2}$$I\lvert 0\rangle … text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ket-00?rev=1781320400&do=diff $\lvert 00\rangle$ The state is the two-qubit computational basis state where both qubits are $\lvert 0\rangle$. It is the tensor product $\lvert 0\rangle\otimes\lvert 0\rangle$ — a product state with no entanglement between the qubits. It is the default initial state of every qubit register in a quantum circuit.$$\lvert 00\rangle = \lvert 0\rangle\otimes\lvert 0\rangle = \begin{pmatrix}1\\0\\0\\0\end{pmatrix}$$$\lvert 00\rangle$$0$$H\otimes I$$\lvert 00\rangle$$\lvert 00\rangle$$I_0\lvert 00\… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ket-01?rev=1781320400&do=diff $\lvert 01\rangle$ The state is a two-qubit computational basis state with qubit 0 in $\lvert 0\rangle$ and qubit 1 in $\lvert 1\rangle$. It is the tensor product $\lvert 0\rangle\otimes\lvert 1\rangle$ — an unentangled product state. It is reached from $\lvert 00\rangle$ by a single $X$ gate on qubit 1. $$\lvert 01\rangle = \lvert 0\rangle\otimes\lvert 1\rangle = \begin{pmatrix}0\\1\\0\\0\end{pmatrix}$$ Measuring qubit 0 yields $0$ with certainty; measuring qubit 1 yields $1$$H\otimes I$$\l… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ket-1?rev=1781320400&do=diff $\lvert 1\rangle$ (One state) The one state $\lvert 1\rangle$ is one of the two computational basis states of a qubit. It is the quantum analogue of a classical 1 bit. The other computational basis state is $\lvert 0\rangle$. $$\lvert 1\rangle = \begin{pmatrix}0\\1\end{pmatrix}$$ On the Bloch sphere, $\lvert 1\rangle$ corresponds to the south pole at coordinates $(0, 0, -1)$$-1$$Z\lvert 1\rangle = -\lvert 1\rangle$$\lvert 1\rangle$$\lvert -\rangle = (\lvert 0\rangle - \lvert 1\rangle)/\sqrt{2… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ket-10?rev=1781320400&do=diff $\lvert 10\rangle$ The state is a two-qubit computational basis state with qubit 0 in $\lvert 1\rangle$ and qubit 1 in $\lvert 0\rangle$. It is the tensor product $\lvert 1\rangle\otimes\lvert 0\rangle$ — an unentangled product state. It is the SWAP of $\lvert 01\rangle$, reached from $\lvert 00\rangle$ by a single $X$ gate on qubit 0. $$\lvert 10\rangle = \lvert 1\rangle\otimes\lvert 0\rangle = \begin{pmatrix}0\\0\\1\\0\end{pmatrix}$$ Measuring qubit 0 yields $1$$0$$H\otimes I$$\lvert 10\ra… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ket-11?rev=1781320400&do=diff $\lvert 11\rangle$ The state is the two-qubit computational basis state where both qubits are in $\lvert 1\rangle$. It is the tensor product $\lvert 1\rangle\otimes\lvert 1\rangle$ — an unentangled product state. It is reached from $\lvert 00\rangle$ by $X$ on both qubits, and from $\lvert 10\rangle$ or $\lvert 01\rangle$ by $X$ on the remaining qubit. $$\lvert 11\rangle = \lvert 1\rangle\otimes\lvert 1\rangle = \begin{pmatrix}0\\0\\0\\1\end{pmatrix}$$ Measuring either qubit yields $1$$H\oti… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ket-ghz?rev=1781320400&do=diff GHZ state GHZ state (Greenberger-Horne-Zeilinger state) is a maximally entangled quantum state of three or more qubits. The three-qubit GHZ state is an equal superposition of all-zeros and all-ones. $$\lvert\text{GHZ}\rangle = \frac{1}{\sqrt{2}}(\lvert 000\rangle + \lvert 111\rangle)$$ The GHZ state is named after Daniel Greenberger, Michael Horne, and Anton Zeilinger, who introduced it in 1989. It is a natural generalization of the two-qubit Bell state $\lvert\Phi^+\rangle$$0$$1$$n$$n \geq 2… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ket-minus-i?rev=1781320400&do=diff $\lvert -i\rangle$ (Minus-i state) The minus-i state $\lvert -i\rangle$ (also written $\lvert -y\rangle$) is an equal superposition of $\lvert 0\rangle$ and $\lvert 1\rangle$ with a relative phase of $-i$. It is one of the six cardinal states on the Bloch sphere, sitting at the negative $y$-axis at coordinates $(0, -1, 0)$. $$\lvert -i\rangle = \frac{1}{\sqrt{2}}(\lvert 0\rangle - i\lvert 1\rangle) = \frac{1}{\sqrt{2}}\begin{pmatrix}1\\-i\end{pmatrix}$$ On the Bloch sphere, $\lvert -i\rangle$… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ket-minus?rev=1781320400&do=diff $\lvert -\rangle$ (Minus state) The minus state $\lvert -\rangle$ is an equal superposition of $\lvert 0\rangle$ and $\lvert 1\rangle$ with a relative minus sign. It is one of the six cardinal states on the Bloch sphere, sitting at the negative $x$-axis at coordinates $(-1, 0, 0)$. $$\lvert -\rangle = \frac{1}{\sqrt{2}}(\lvert 0\rangle - \lvert 1\rangle) = \frac{1}{\sqrt{2}}\begin{pmatrix}1\\-1\end{pmatrix}$$ On the Bloch sphere, $\lvert -\rangle$ is an eigenstate of the Pauli-X gate with eig… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ket-phi-minus?rev=1781320400&do=diff $\lvert\Phi^-\rangle$ (Phi-minus state) The phi-minus state $\lvert\Phi^-\rangle$ is one of the four Bell states — the maximally entangled two-qubit states. It is an equal superposition of $\lvert 00\rangle$ and $\lvert 11\rangle$ with a relative minus sign. Like $\lvert\Phi^+\rangle$, both qubits always give the same outcome in the computational basis; the minus sign is only visible in phase-sensitive measurements.$$\lvert\Phi^-\rangle = \frac{1}{\sqrt{2}}(\lvert 00\rangle - \lvert 11\rangle) … text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ket-phi-plus?rev=1781320400&do=diff $\lvert\Phi^+\rangle$ (Phi-plus state) The phi-plus state $\lvert\Phi^+\rangle$ is one of the four Bell states — the maximally entangled two-qubit states. It is an equal superposition of $\lvert 00\rangle$ and $\lvert 11\rangle$ with the same sign, meaning both qubits are always found in the same state when measured in the computational basis. The other $\Phi$$$\lvert\Phi^+\rangle = \frac{1}{\sqrt{2}}(\lvert 00\rangle + \lvert 11\rangle) = \frac{1}{\sqrt{2}}\begin{pmatrix}1\\0\\0\\1\end{pmatrix… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ket-plus-i?rev=1781320400&do=diff $\lvert +i\rangle$ (Plus-i state) The plus-i state $\lvert +i\rangle$ (also written $\lvert i\rangle$ or $\lvert y\rangle$) is an equal superposition of $\lvert 0\rangle$ and $\lvert 1\rangle$ with a relative phase of $+i$. It is one of the six cardinal states on the Bloch sphere, sitting at the positive $y$-axis at coordinates $(0, 1, 0)$. $$\lvert +i\rangle = \frac{1}{\sqrt{2}}(\lvert 0\rangle + i\lvert 1\rangle) = \frac{1}{\sqrt{2}}\begin{pmatrix}1\\i\end{pmatrix}$$ On the Bloch sphere, $\… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ket-plus?rev=1781320400&do=diff $\lvert +\rangle$ (Plus state) The plus state $\lvert +\rangle$ is an equal superposition of $\lvert 0\rangle$ and $\lvert 1\rangle$ with equal positive amplitudes. It is one of the six cardinal states on the Bloch sphere, sitting at the positive $x$-axis at coordinates $(1, 0, 0)$. $$\lvert +\rangle = \frac{1}{\sqrt{2}}(\lvert 0\rangle + \lvert 1\rangle) = \frac{1}{\sqrt{2}}\begin{pmatrix}1\\1\end{pmatrix}$$ On the Bloch sphere, $\lvert +\rangle$ is an eigenstate of the Pauli-X gate with eig… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ket-psi-minus?rev=1781320400&do=diff $\lvert\Psi^-\rangle$ (Psi-minus state) The psi-minus state $\lvert\Psi^-\rangle$ is one of the four Bell states — the maximally entangled two-qubit states. It is the only antisymmetric Bell state: swapping the two qubits picks up a minus sign, $\text{SWAP}\lvert\Psi^-\rangle = -\lvert\Psi^-\rangle$. It is also called the singlet state$\Psi$$$\lvert\Psi^-\rangle = \frac{1}{\sqrt{2}}(\lvert 01\rangle - \lvert 10\rangle) = \frac{1}{\sqrt{2}}\begin{pmatrix}0\\1\\-1\\0\end{pmatrix}$$$\lvert\Psi^+\r… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ket-psi-plus?rev=1781320400&do=diff $\lvert\Psi^+\rangle$ (Psi-plus state) The psi-plus state $\lvert\Psi^+\rangle$ is one of the four Bell states — the maximally entangled two-qubit states. It is an equal superposition of $\lvert 01\rangle$ and $\lvert 10\rangle$ with the same sign, meaning the two qubits always give opposite outcomes when measured in the computational basis. The other $\Psi$$$\lvert\Psi^+\rangle = \frac{1}{\sqrt{2}}(\lvert 01\rangle + \lvert 10\rangle) = \frac{1}{\sqrt{2}}\begin{pmatrix}0\\1\\1\\0\end{pmatrix}$… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ket-w?rev=1781320400&do=diff W state W state is an entangled three-qubit state that is an equal superposition over all configurations with exactly one qubit in state $\lvert 1\rangle$ and the others in $\lvert 0\rangle$. $$\lvert W\rangle = \frac{1}{\sqrt{3}}(\lvert 001\rangle + \lvert 010\rangle + \lvert 100\rangle)$$ The W state was identified by Wolfgang Dür, Guifré Vidal, and J. Ignacio Cirac in 2000 as part of their classification of three-qubit entanglement. Unlike the $n$$n$$\lvert 1\rangle$$$\lvert W_n\rangle = \… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/kpsewhich?rev=1781320400&do=diff kpsewhich kpsewhich is a LaTeX-related command. It's a diagnostic command that's used to look up the full path of a file you know the LaTeX compiler would use implicitly. In some way, it is similar to the which command, which shows the full path of a command if you would run it in the shell (e.g. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/kraus-operator?rev=1781320400&do=diff Kraus operator Kraus operators $\{K_k\}$ are a set of matrices used to represent a quantum channel — a completely positive trace-preserving (CPTP) map $\mathcal{E}$ that sends density matrices to density matrices. Any physically valid quantum channel can be expressed in Kraus form.$$\mathcal{E}(\rho) = \sum_k K_k\rho K_k^\dagger$$$\sum_k K_k^\dagger K_k = I$$\text{tr}(\mathcal{E}(\rho)) = \text{tr}(\rho) = 1$$K_k$$\mathcal{E}(\rho)$$1-p$$p$$X$$\lvert 0\rangle \leftrightarrow \lvert 1\rangle$$$K… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/landauers-principle?rev=1781320400&do=diff Landauer's principle Landauer's principle establishes the thermodynamic lower bound on energy required to erase information from a physical system (e.g. a computer register). At temperature $T$, the minimum energy needed to erase information is: $$E = (\text{bits erased}) \times k_B T \ln 2$$ Where $k_B$ is the Boltzman's constant.$S_0 = 64\times k_B\ln 2$$S_1 = 0$$\Delta S = S_0 - S_1$$Q = T \Delta S$$T = 300K$$$ E_\text{bit} \approx 2.9\times 10^{-21} J$$$$ E_\text{bit} \approx 0.018 \text… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/latex-syntax?rev=1781320400&do=diff LaTeX syntax For addition, you use the + symbol e.g. a + b = c: $$ a + b = c$$ For subtraction, you use the - symbol e.g. a - b = c $$ a - b = c$$ For multiplication, you just write ab or put a dot in between with \cdot (meaning: “centered dot”) like a \cdot b. Using a * b$a * b$$$a\cdot b = c$$$a / b$$${a \over b} = c$$$$ a = b \pmod{N}$$$$e^{i\phi} = \cos(\phi) + i\sin(\phi)$$$$\ln(1 + x) = 1 + x + $$$$\int \iint \iiint \oint \oiint \oiiint$$ text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/latex?rev=1781320400&do=diff LaTeX Latex (stylized as $\LaTeX$) is a markup language used to create scientific papers. It's the language used to produce PDF documents, like the ones you'll commonly see on arXiv. Latex has a simple declarative syntax that looks like \command[opt1][opt2]...[optN]{arg1}{arg2}...{argN} text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/limits.h?rev=1781320400&do=diff limits.h limits.h tells you the integer type limits for the platform you are compiling on. You reach for it when you need to guard against overflow before it happens, or when you want to write code that works correctly regardless of whether long is 32 or 64 bits. Writing text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/lindbald-master-equation?rev=1781320400&do=diff Lindbald master equation Lindbald master equation is the equation that generalizes the Schrodinger equation. The following is the equation written in terms of density matrix $\rho$. $$\frac{d\rho}{dt} = -\frac{i}{\hbar}[H, \rho] + \sum_k\left(L_k^\dagger L_k - \frac{1}{2}\{L_k^\dagger L_k, \rho\}\right)$$ text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/lindblad-equation-qutip?rev=1781320400&do=diff Lindblad equation (QuTiP) Lindblad equation implementation using QuTiP. QuTiP's mesolve() integrates the full Lindblad master equation when the c_ops list is non-empty: $$\frac{\mathrm{d}\rho}{\mathrm{d}t} = -\frac{i}{\hbar}[H,\,\rho] + \sum_k\!\left(L_k\rho L_k^\dagger - \frac{1}{2}\{L_k^\dagger L_k,\,\rho\}\right)$$ Each element of c_ops is a collapse operator $C_k$; QuTiP constructs the full dissipator term internally as $C_k\rho C_k^\dagger - \frac{1}{2}\{C_k^\dagger C_k, \rho\}$$k$$C_k … text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/lindblad-equation?rev=1781320400&do=diff Lindblad equation Lindblad equation (also called the GKSL equation, after Gorini, Kossakowski, Sudarshan, and Lindblad) is the most general Markovian master equation for the density matrix $\rho$ of an open quantum system. It extends the von Neumann equation by adding a dissipator term that models incoherent processes — energy loss, dephasing, and other forms of coupling to an environment.$$\frac{\mathrm{d}\rho}{\mathrm{d}t} = -\frac{i}{\hbar}[H,\,\rho] + \sum_k\!\left(L_k\rho L_k^\dagger - \fr… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/linked-list?rev=1781320400&do=diff Linked list A linked list is a sequence of nodes where each node holds a value and a pointer to the next node. The nodes need not be contiguous in memory; the pointer is what links them. The head pointer is the entry point; the last node's next pointer is NULL. Insertion and deletion at any position where you already hold a pointer costs O(1) — no elements shift — but reaching a position by index requires traversing from the head, costing O(n). text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/liskov-substitution-principle?rev=1781320400&do=diff Liskov substitution principle The Liskov substitution principle (LSP) states that if S is a subtype of T, then objects of type T may be replaced with objects of type S without altering the correctness of the program. It was formulated by Barbara Liskov in a 1987 keynote. It is the L in text/html 2026-06-15T08:54:11+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-algorithms?rev=1781513651&do=diff List of algorithms * sorting-algorithms * dijkstra-algorithm text/html 2026-06-15T09:23:16+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-articles?rev=1781515396&do=diff List of articles * List of tools * List of paths * List of commands * List of hardware * List of C headers * List of C concepts * List of C++ headers * List of C++ concepts * List of embedded engineering concepts * List of parallel computing concepts * List of quantum computing concepts * List of software engineering concepts * List of data structures * List of algorithms * List of miscellaneous topics * List of writing guides text/html 2026-06-15T08:54:11+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-c-concepts?rev=1781513651&do=diff List of C concepts * C standard library * #pragma once * do { ... } while (0) * Function macro * main() return * Exit status * Header guard * Embedded engineering glossary text/html 2026-06-15T08:54:11+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-c-headers?rev=1781513651&do=diff List of C headers * assert.h * complex.h * ctype.h * errno.h * fenv.h * float.h * inttypes.h * iso646.h * limits.h * locale.h * math.h * setjmp.h * signal.h * stdalign.h * stdarg.h * stdatomic.h * stdbit.h * stdbool.h * stdckdint.h * stddef.h * stdint.h * stdio.h * stdlib.h * stdmchar.h * stdnoreturn.h * string.h * tgmath.h * threads.h * time.h * uchar.h * wchar.h * wctype.h * curses.h * getopt.h text/html 2026-06-15T08:54:11+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-commands?rev=1781513651&do=diff List of commands * List of util-linux commands * List of LaTeX commands * List of system commands * List of HPC commands text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-concepts-in-quantum-control?rev=1781320400&do=diff List of concepts in quantum control * List of optimal quantum control algorithms * list-of-hamiltonians * pulse-shaping * drag-pulse * rabi-cycle * spin-echo * optimal-pulse-control * rotating-wave-approximation text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-concepts-in-quantum-error-correction?rev=1781320400&do=diff List of concepts in quantum error correction * List of quantum error correction codes * list-of-lindbald-operators * list-of-kraus-operators * Density matrix * Lindbald master equation * Kraus operator * lindbald-operator * list-of-lindbald-operators * quantum-channel * cptp-map * superoperator * fault-tolerance text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-concepts?rev=1781320400&do=diff List of concepts * C * header-guards * #pragma once * C++ * Name mangling * Rule of 5 * Miscellaneous * Numbers every programmer should know text/html 2026-06-15T08:54:11+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-cpp-concepts?rev=1781513651&do=diff List of C++ concepts * Name mangling * Rule of 5 * CRTP * constexpr text/html 2026-06-15T08:54:11+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-cpp-headers?rev=1781513651&do=diff List of C++ headers C++98 * <algorithm> * <bitset> * <complex> * <deque> * <exception> * <fstream> * <functional> * <iomanip> * <ios> * <iosfwd> * <iostream> * <istream> * <iterator> * <limits> * <list> * <locale> * <map> * <memory> * <new> * <numeric> * <ostream> * <queue> * <set> * <sstream> * <stack> * <stdexcept> * <streambuf> * <string> * <typeinfo> * <utility> * <valarray> * <vector> C++11 * <array> * <atomic> * <chrono> * <codecvt> * <condition_var… text/html 2026-06-15T08:54:11+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-data-structures?rev=1781513651&do=diff List of data structures * Array * Forward list * Linked list * Dynamic array * Stack * Queue * Circular buffer * Matrix * Binary tree * Heap * Red-black tree * B-tree * B+ tree * Hash map * Graph text/html 2026-06-15T08:54:11+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-embedded-engineering-concepts?rev=1781513651&do=diff List of embedded engineering concepts * List of logic gates * rtos * xv6 * riscv * x86-64 * mcu * spi * uart * i2c * gpio * wcet * Harvard architecture text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-essential-quantum-computing-concepts?rev=1781320400&do=diff List of essential quantum computing concepts * List of quantum gates * List of quantum states * List of quantum algorithms * Qubit * Two qubits * Quantum register * Quantum gate * Born rule * Quantum state * State vector * Pauli gates * Global phase * Basis state * Eigenstate * relative-phase * clifford-gate * Hopf fibration * $\mathbb{CP}^1$ * unitary-gate * quantum-measurement * Bloch sphere * Dirac notation * dagger-operation * matrix-transpose * complex-conjugate *… text/html 2026-06-15T08:54:11+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-hardware?rev=1781513651&do=diff List of hardware * ThinkPad * Arduino Uno R4 WiFi * weller-we-1010 text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-headers-cpp?rev=1781320400&do=diff List of headers in C++ <https://en.cppreference.com/w/cpp/headers.html> text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-hpc-commands?rev=1781320400&do=diff List of HPC commands * nvcc * cuobjdump text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-kdp-exercises?rev=1781320400&do=diff List of KDP exercises * KDP Midterm (2026-01-10) Question 1 * KDP Midterm (2026-01-10) Question 2 * KDP Exam (2026-02-18) Question 1 * KDP Exam (2026-02-18) Question 2 * (WIP) KDP Exam (2026-02-18) Question 3 * kdp20260218-4 text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-latex-commands?rev=1781320400&do=diff List of LaTeX commands * pdflatex * kpsewhich text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-logic-gates?rev=1781320400&do=diff List of logic gates * and-gate * or-gate * not-gate * nand-gate * nor-gate text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-miscellaneous-quantum-computing-concepts?rev=1781320400&do=diff List of miscellaneous quantum computing concepts * schrodinger-picture * heisenberg-picture * interaction-picture * measurement-problem * intepretation-of-quantum-mechanics * copenhagen-interpretation * many-world-interpretation text/html 2026-06-15T08:54:11+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-miscellaneous-topics?rev=1781513651&do=diff List of miscellaneous topics * bytebeat * code-golf * List of KDP exercises text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-optimal-quantum-control-algorithms?rev=1781320400&do=diff List of optimal quantum control algorithms * grape * crab * goat text/html 2026-06-15T15:07:18+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-parallel-computing-concepts?rev=1781536038&do=diff List of parallel computing concepts * introduction-to-parallel-computing * synchronization-primitve * SAXPY * Semaphore * mutex * Numbers every programmer should know * Amdahl's law * Gustafson's law * cache-coherence * cuda * OpenMP * MPI * queuing-theory * Lock-free queue * numa * false-sharing * ABA problem * Trace monoid * hazard-pointer * cache-coherence * Roofline model * numa * embarrassingly-parallel * fork-join-model * rcu * lock * lock-convoy * lock-content… text/html 2026-06-15T08:54:11+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-paths?rev=1781513651&do=diff List of paths * /bin * /dev * /boot * /etc * /home * /lib * /mnt * /opt * /proc * /root * /sbin * /srv * /tmp * /usr * /var text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-quantum-algorithms?rev=1781320400&do=diff List of quantum algorithms * Quantum Fourier transform * Quantum phase estimation * Shor's algorithm * Grover's algorithm * Deutsch-Jozsa algorithm * QAOA * VQE text/html 2026-06-15T08:54:11+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-quantum-computing-concepts?rev=1781513651&do=diff List of quantum computing concepts * Qubit * Dirac notation * Born rule * Global phase * Bloch sphere * Quantum state * Quantum register * Quantum gate * Quantum circuit * Quantum algorithm * Quantum error correction * Schrödinger equation text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-quantum-computing-libraries?rev=1781320400&do=diff List of quantum computing libraries * qiskit * q-sharp text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-quantum-error-correction-codes?rev=1781320400&do=diff List of quantum error correction codes * repetition-code * shor-code * surface-code text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-quantum-gates?rev=1781320400&do=diff List of quantum gates * $I$ gate (Identity gate) * $X$ gate (NOT gate) * Y gate * Z gate * Rotation gates * Rotation-X gate * Rotation-Y gate * Rotation-Z gate * S gate * T gate * Hadamard gate * Phase gate * Unitary gate * ccnot-gate * CX gate (CNOT) * SWAP gate * iSWAP gate text/html 2026-06-15T08:54:11+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-software-engineering-concepts?rev=1781513651&do=diff List of software engineering concepts * Brooks's law * Parkinson's law * Kanban * Extreme programming * Agile * Test-driven development * CI/CD * DevOps * Blue-green deployment * Canary release * Unit test * Regression * Test automation * Kanban * Feature flag * Production * A/B testing * Hotfix * Feature branch * Eating your own dog food * Spike solution * SOLID * Liskov substitution principle * Smart-and-gets-things-done * DevOps * Incremental build model * Prototyp… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-software-engineering-tools?rev=1781320400&do=diff List of software engineering tools * jira * confluence * ms-teams * harvestapp * notion text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-syntaxes?rev=1781320400&do=diff List of syntaxes * c-syntax * cpp-syntax * LaTeX syntax text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-system-commands?rev=1781320400&do=diff List of system commands * ifupdown text/html 2026-06-15T08:54:11+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-tools?rev=1781513651&do=diff List of tools * git * docker * Shell * tmux * ssh * cmake * nvim * gdb * clangd * Terminal * LaTeX * VPS * qemu * gcc * perf * nsight * valgrind * numactl text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-util-linux-commands?rev=1781320400&do=diff List of util-linux commands util-linux is a collection of commands provided by the Linux Kernel Organization. A similar set of commands is GNU core utilities. The following are commands provided by until-linux. You can do man <command> or use the --help option to see what each command does. text/html 2026-06-15T08:54:11+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/list-of-writing-guides?rev=1781513651&do=diff List of writing guides * General writing guide * Writing guide on quantum computing * Writing guide on parallel computing text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/locale.h?rev=1781320400&do=diff locale.h locale.h controls how your program formats and interprets culture-specific data: whether the decimal separator is . or ,, what the currency symbol is, which characters count as letters. By default a C program runs in the "C" locale — ASCII, text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/lock-free-queue?rev=1781320400&do=diff Lock-free queue A mutex-protected queue is the natural starting point for producer-consumer communication between threads. Under light load it works fine, but under high contention the lock becomes a bottleneck: a thread preempted while holding it stalls every other producer and consumer until it is scheduled again. A text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/main-return?rev=1781320400&do=diff main() return The return value of int main(); function is called the program's exit status. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/master-openmp?rev=1781320400&do=diff Master (OpenMP) Master is an OpenMP directive that restricts execution of a block to thread 0. Unlike `single`, there is no implicit barrier. Other threads skip the block entirely and continue immediately, making it suitable for non-critical side effects like printing progress or updating a log counter where stalling the whole team would be wasteful. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/math-books?rev=1781320400&do=diff Math books * <https://zb260.user.srcf.net/notes/III/modrep.pdf> text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/math.h?rev=1781320400&do=diff math.h math.h is the standard mathematical function library for floating-point arithmetic. It covers trigonometry, exponentiation, logarithms, rounding, and a handful of utility functions. Every function operates on double by default; append f for float text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/mathematical-maturity?rev=1781320400&do=diff Mathematical Maturity Trivial example One of the earliest awakenings of my mathematical maturity was when I was a kid, thinking one of the simplest equations there is: $$ a\cdot x = b$$ I knew how to solve this 99% of the time: $$x = b / a$$ Is this correct? Let's take the equation $2\cdot x = 6$$b = 0$$x = 0$$a = 0$$a = 0$$b = 3$$x$$0 = 3$$a = 0$$b = 0$$x$$0 = 0$$a\cdot x = b$$a\cdot x = b$ text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/matmul?rev=1781320400&do=diff Matmul Matmul is an algorithm in high-performance computing (HPC). It is a BLAS level 2 (“Basic Linear Algebra Subprograms”) algorithm. Definition Matrices Let $\mathbf A\in\mathbb{R}^{A_\text{rows}\times A_\text{cols}}$ and $\mathbf B\in \mathbb R^{B_\text{rows}\times B_\text{cols}}$ be square matrices over real numbers and let $A_\text{rows} = B_\text{cols}$. Matmul defined as a matrix multiplication $\mathbf A\cdot \mathbf B$$\mathbf C\in\mathbb{R}^{A_\text{rows}\times B_\text{cols}}$$$ … text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/matrix?rev=1781320400&do=diff Matrix A matrix is a two-dimensional rectangular array of values organised in rows and columns. In C, a matrix is stored as a flat Array in row-major order: all elements of row 0 come first, then row 1, and so on. The element at row i, column j of an M × N$y = Ax$$O(mn)$$C = AB$$O(mnk)$ text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/message-ordering-mpi?rev=1781320400&do=diff Message ordering (MPI) TCP guarantees that bytes sent on a stream arrive in order. If you send() “hello” and then send() “world” on the same socket, the receiver gets “hello” first. MPI provides the same guarantee for point-to-point messages: messages sent from process A to process B on the same communicator with the same tag arrive in the order they were sent. If A calls text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/minus-i-state-qiskit?rev=1781320400&do=diff $\lvert -i\rangle$ (Qiskit) Minus-i state $\lvert -i\rangle$ implementation using Qiskit. The $\lvert -i\rangle$ state is prepared by applying a Hadamard gate followed by an $S^\dagger$ gate to $\lvert 0\rangle$. from qiskit import QuantumCircuit from qiskit.quantum_info import Statevector qc = QuantumCircuit(1) qc.h(0) # |0> -> |+> qc.sdg(0) # |+> -> |-i> = (|0> - i|1>) / sqrt(2) print(Statevector(qc)) # Statevector([0.70710678+0.j, 0.-0.70710678j], dims=(2,)) text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/minus-state-qiskit?rev=1781320400&do=diff $\lvert -\rangle$ (Qiskit) Minus state $\lvert -\rangle$ implementation using Qiskit. The $\lvert -\rangle$ state is prepared by applying an X gate followed by a Hadamard gate to $\lvert 0\rangle$. from qiskit import QuantumCircuit from qiskit.quantum_info import Statevector qc = QuantumCircuit(1) qc.x(0) # |0> -> |1> qc.h(0) # |1> -> |-> = (|0> - |1>) / sqrt(2) print(Statevector(qc)) # Statevector([ 0.70710678+0.j, -0.70710678+0.j], dims=(2,)) text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/mixed-state?rev=1781320400&do=diff Mixed state Mixed state is a quantum state that cannot be described by a single state vector $\lvert\psi\rangle$. It arises when a quantum system is entangled with its environment or when there is classical uncertainty about how the system was prepared. Mixed states are represented by density matrices $\rho$$\rho = \lvert\psi\rangle\langle\psi\rvert$$\text{tr}(\rho^2) = 1$$\lvert 0\rangle$$p$$\lvert 1\rangle$$1-p$$$\rho = p\lvert 0\rangle\langle 0\rvert + (1-p)\lvert 1\rangle\langle 1\rvert = \… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/mpi?rev=1781320400&do=diff MPI MPI (Message Passing Interface) is a standard for distributed-memory parallel programming in C, C++, and Fortran. Unlike OpenMP where you add pragmas and the threads share your program's memory, with MPI you launch N independent copies of your program, each with its own private address space, and they coordinate by explicitly sending and receiving messages. Because the communication model makes no assumption that processes share any hardware, MPI programs scale from a laptop to a cluster wi… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/multiqubit-gates?rev=1781320400&do=diff Multiqubit gates Multiqubit gates are quantum gates that act on two or more qubits simultaneously. They are the mechanism by which entanglement is created and destroyed in a quantum circuit — a single-qubit gate acting on one qubit at a time can never entangle two qubits that started in a product state.$2 \times 2$$4 \times 4$$8 \times 8$$n$$2^n \times 2^n$$U$$U$$\lvert 1\rangle$$X$$X$$U$$CU$$4 \times 4$$\lvert 0\rangle$$U$$\lvert 1\rangle$$n$$\{H, T, \text{CX}\}$$\epsilon$$O(\log^c(1/\epsilon)… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/my-bookmarks?rev=1781320400&do=diff My bookmarks C++ * <https://en.cppreference.com/w/> * <https://json.nlohmann.me/api/basic_json/> LaTeX Online tools: * <https://openai.com/index/introducing-prism/> * <https://www.mathcha.io/> * <https://www.overleaf.com/> Documentation PDFs: * <https://pgfplots.sourceforge.net/pgfplots.pdf> * <https://www.bu.edu/math/files/2013/08/tikzpgfmanual.pdf> * <https://feog.github.io/chap1dm.pdf> Wikipedia Related to mathematics: * <https://en.wikipedia.org/wiki/Siamese_neural_network… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/name-mangling?rev=1781320400&do=diff Name mangling Name mangling is how C++ compilers encode object types then and reconstruct the type back. Let's say we got a following piece of C++ code. namespace Hello { struct World { template <typename T> T add(T x, T y) { return x + y; } }; } text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/net-config?rev=1781320400&do=diff Network Configuration This article is about network configuration in GNU/Linux systems ifupdown Dynamic configurations Dynamic configuration with ifupdown (IPv4) # /etc/network/interfaces iface ens3 inet dhcp Dynamic configuration with ifupdwon (IPv6) text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/nih-syndrome?rev=1781320400&do=diff NIH syndrome NIH syndrome stands for “Not invented here” syndrome. It's when you deliberately avoid using a premade solution in favor of your own. For example, you might prefer to implement your own dynamic array even though std::vector<int> already exist. The commonly given rationale is that you'll probably find the implementation you made easier to understand (you wrote it!) and you'll be able easily make it fit your needs. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/nisq?rev=1781320400&do=diff NISQ NISQ (Noisy Intermediate-Scale Quantum) refers to the current era of quantum computing, characterized by devices with tens to a few thousand qubits that are too noisy for full fault-tolerant quantum error correction. The term was coined by John Preskill in 2018.$10^{-3}$$10^{-2}$ text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/nonblocking-collectives-mpi?rev=1781320400&do=diff Non-blocking collectives (MPI) Non-blocking point-to-point communication lets a process post a send or receive and continue computing while the transfer happens in the background (see blocking and non-blocking). Non-blocking collectives extend the same idea to collective operations. MPI-3 introduced variants that return immediately with a request handle rather than blocking until the operation completes. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/noon-state?rev=1781320400&do=diff NOON state NOON state is an entangled quantum state of two modes in which $N$ photons are in one mode and zero in the other, in an equal superposition with the reverse. It is written as: $$\lvert N\!:\!0\rangle\!\rangle = \frac{1}{\sqrt{2}}\left(\lvert N\rangle_a\lvert 0\rangle_b + \lvert 0\rangle_a\lvert N\rangle_b\right)$$ NOON states arise naturally in quantum metrology and quantum lithography. Their key property is phase sensitivity that scales as $1/N$$1/\sqrt{N}$$\sqrt{N}$$\phi$$a$$N$$N… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/nowait-openmp?rev=1781320400&do=diff Nowait (OpenMP) Nowait is an OpenMP clause that removes the implicit barrier at the end of a work-sharing construct. Threads that finish their portion of the work early proceed to the next statement immediately instead of waiting for stragglers. This is only safe when the subsequent code does not depend on the results of the current construct. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/numbers-every-programmer-should-know?rev=1781320400&do=diff Numbers every programmer should know Numbers every programmer should know is a short document listing latencies of various computer operations. The list is given by a computer scientist Jeff Dean. It illustrates how various high-level operations exponentially take longer time to perform compared to various low-level operations. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/nvcc?rev=1781320400&do=diff nvcc nvcc (or Nvidia C/C++ compiler) is a C++ compiler created by Nvidia that adds CUDA kernel syntax. $ nvcc kernel.cu text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/one-sided-mpi?rev=1781320400&do=diff One-sided communication (MPI) mmap with MAP_SHARED lets two processes on the same machine map the same physical memory. One process writes through its pointer and the other immediately sees the update — no explicit send or receive, no coordination required from the reader. One-sided MPI communication (also called text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/openmp?rev=1781320400&do=diff OpenMP OpenMP is a shared-memory parallelism API for C, C++, and Fortran. If you have a loop that takes too long and you want it to use all the cores on your machine instead of just one, OpenMP is usually the shortest path there. It works via compiler directives ($s$$1/s$ text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/p-gate-cudaq?rev=1781320400&do=diff Phase gate (CUDA-Q) Phase gate implementation using CUDA-Q. The example applies $P(\pi/2) = S$ to $\lvert +\rangle$, rotating it to $\lvert i\rangle = \frac{1}{\sqrt{2}}(\lvert 0\rangle + i\lvert 1\rangle)$. // Compile: nvq++ main.cpp -o main // Run: ./main #include <cudaq.h> #include <cmath> struct kernel { __qpu__ void operator()() { cudaq::qubit q; h(q); // |0> -> |+> r1(M_PI / 2.0, q); // P(pi/2) = S gate mz(q); } }; … text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/p-gate-custatevec?rev=1781320400&do=diff Phase gate (cuStateVec) Phase gate implementation using cuStateVec. The example applies $P(\pi/2) = S$ to $\lvert +\rangle$, rotating it to $\lvert i\rangle = \frac{1}{\sqrt{2}}(\lvert 0\rangle + i\lvert 1\rangle)$. // Compile: nvcc main.cu -o main -lcustatevec // Run: ./main #include <stdio.h> #include <math.h> #include <cuda_runtime.h> #include <custatevec.h> int main() { const int nQubits = 1; const int dim = 1 << nQubits; cuDoubleComplex h_sv[2] = {{1,0},{0,0}}; // |0>… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/p-gate-qiskit?rev=1781320400&do=diff Phase gate (Qiskit) Phase gate implementation using Qiskit. The example applies $P(\pi/2) = S$ to $\lvert +\rangle$, rotating it to $\lvert i\rangle = \frac{1}{\sqrt{2}}(\lvert 0\rangle + i\lvert 1\rangle)$. from qiskit import QuantumCircuit from qiskit.quantum_info import Statevector import numpy as np qc = QuantumCircuit(1) qc.h(0) # |0> -> |+> qc.p(np.pi / 2, 0) # P(pi/2) = S: |+> -> |i> print(Statevector(qc)) # Statevector([0.70710678+0.j, 0.+0.70710678j], dims=(2,)) text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/p-gate?rev=1781320400&do=diff Phase gate Phase gate (or P gate) is a single-qubit gate that applies a phase shift $\phi$ to the $\lvert 1\rvert$ component of a qubit while leaving the $\lvert 0\rangle$ component unchanged. It is a rotation about the $z$-axis of the Bloch sphere. $$P(\phi) = \begin{pmatrix}1 & 0\\ 0 & e^{i\phi}\end{pmatrix}$$ Applied to a general qubit $\lvert\psi\rangle = a\lvert 0\rangle + b\lvert 1\rangle$, the phase gate multiplies the $\lvert 1\rangle$$e^{i\phi}$$$P(\phi)\lvert\psi\rangle = a\lvert 0\… text/html 2026-06-15T09:34:54+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/parallel-computing-overview?rev=1781516094&do=diff Overview Paradigms Paradigm Memory model API Typical scale Shared-memory Common address space OpenMP, pthreads Single node Distributed-memory Private per process MPI Multi-node cluster GPU Host + device CUDA, HIP, OpenCL Single GPU Hybrid Mixed $S = \dfrac{1}{s + (1-s)/p}$$s$$p$$S = p - s(p-1)$$p$$\min(\pi,\; I \cdot \beta)$ text/html 2026-06-15T15:47:42+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/parallel-computing?rev=1781538462&do=diff Parallel computing Parallel computing is a style of programming where a computation is broken into parts that run simultaneously across multiple processors, cores, or machines. The motivation is straightforward: a single core has a clock speed ceiling, and modern CPUs gain performance by adding more cores rather than running each core faster. To take advantage of that, programs have to be written with parallelism in mind. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/parallel-io-mpi?rev=1781320400&do=diff Parallel I/O (MPI) POSIX pwrite(fd, buf, n, offset) lets multiple processes write to different offsets in the same file without interfering: the offset is passed explicitly rather than shared, so there is no race on the file position. The straightforward approach in a parallel program is for each process to compute its own offset and call text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/parallel-loops-openmp?rev=1781320400&do=diff Parallel loops (OpenMP) The most common parallel pattern in C is a loop over independent array elements: each iteration writes to a different output location and doesn't read results that other iterations produce. for (int i = 0; i < N; i++) a[i] = i * 0.5; text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/parkinsons-law?rev=1781320400&do=diff Parkinson's law Parkinson's law states that work expands to fill the time available for its completion. It was first articulated by C. Northcote Parkinson in a 1955 essay in The Economist, satirising bureaucratic growth in the British Civil Service. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/past-disclaimer?rev=1781320400&do=diff Disclaimer This website is not an authoritative source. It is written by one person -- me. It's not meant to serve as documentation or a definitive guide to the technology I talk about. I write about topics that interest me and I explain them in the way I understand them text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/path-bin?rev=1781320400&do=diff /bin /bin is the directory for essential user-facing command binaries that must be available before any other filesystem is mounted. It contains the tools needed to boot the system, enter single-user mode, and recover from a broken install: sh, ls, text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/path-boot?rev=1781320400&do=diff /boot /boot holds the files the bootloader needs to start the kernel: the kernel image itself, the initial RAM disk, and the bootloader configuration. Everything in /boot is read before the root filesystem is fully operational. Once the kernel is running and the system is up, nothing in text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/path-dev?rev=1781320400&do=diff /dev /dev is the directory that exposes hardware devices and kernel abstractions as files. Reading and writing a file in /dev communicates directly with a device driver. This is the “everything is a file” principle made concrete: a disk, a terminal, a random number generator, and a null sink all appear as regular file paths that ordinary text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/path-etc?rev=1781320400&do=diff /etc /etc holds system-wide configuration files. Every file in /etc is a plain text file or a directory of text files; no binaries live here. Editing a file in /etc changes the behaviour of the corresponding service or subsystem for all users on the machine. User-level configuration lives in dotfiles under text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/path-home?rev=1781320400&do=diff /home /home contains the personal directories of non-root users. Each user gets a subdirectory named after their login: /home/alice, /home/bob. The shell sets $HOME to this path at login, and programs use it as the default location for user-specific data and configuration. Dotfiles — files and directories whose names begin with text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/path-lib?rev=1781320400&do=diff /lib /lib holds the shared libraries required by the binaries in /bin and /sbin. These are the .so files the dynamic linker loads at runtime. The C standard library (libc.so.6), the dynamic linker itself (ld-linux.so.2), and kernel modules all live here. The constraint is the same as for text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/path-mnt?rev=1781320400&do=diff /mnt /mnt is the conventional location for temporarily mounting filesystems by hand. There is no daemon managing it and no automatic population — a sysadmin mounts something here, uses it, and unmounts it. It is a scratch space for one-off mounts: a USB drive being inspected, a disk image being repaired, another OS partition being accessed for recovery. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/path-opt?rev=1781320400&do=diff /opt /opt holds self-contained optional software packages that install outside the normal distribution package manager. Each package gets its own subdirectory, and inside it the layout mirrors the root hierarchy: bin/, lib/, share/, etc/. Nothing in text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/path-proc?rev=1781320400&do=diff /proc /proc is a virtual filesystem (procfs) with no backing storage on disk. Every file and directory in it is generated on the fly by the kernel when read. It is the primary interface for reading kernel state and per-process information from userspace, using ordinary text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/path-root?rev=1781320400&do=diff /root /root is the home directory of the root user (superuser). It is deliberately separate from /home — placing it directly on the root filesystem means it is available even if the /home partition fails to mount or does not exist. A sysadmin logging in for emergency recovery or single-user mode can always reach text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/path-sbin?rev=1781320400&do=diff /sbin /sbin holds system administration binaries: tools used to manage the machine that are typically run as root. The distinction from /bin is intent: /bin contains tools any user might need (ls, cat, cp), while /sbin contains tools for system configuration and maintenance ( text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/path-srv?rev=1781320400&do=diff /srv /srv holds data that is served to the outside world by services running on this machine. The idea is to give web servers, FTP servers, and similar daemons a dedicated, clearly named location for their content, separate from system configuration in text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/path-tmp?rev=1781320400&do=diff /tmp /tmp is the system-wide directory for temporary files. Any process can create files here. The kernel cleans it on reboot; many distributions also run a periodic cleanup of files older than a few days. On most modern systems /tmp is a tmpfs mount, meaning it lives entirely in RAM (and swap) rather than on disk — writes to text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/path-usr?rev=1781320400&do=diff /usr /usr is the secondary hierarchy and holds the bulk of the system's installed software. On a fresh install the majority of disk space under / is consumed by /usr. It mirrors the top-level structure: /usr/bin for user binaries, /usr/lib for libraries, text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/path-var?rev=1781320400&do=diff /var /var holds variable data: files whose content grows and changes during normal system operation. Log files, mail spools, package manager caches, lock files, and runtime state all live here. Separating this from /usr (which is largely static after install) makes it practical to mount text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/pauli-gate-cudaq?rev=1781320400&do=diff Pauli gates (CUDA-Q) Pauli gates implementation using CUDA-Q. // Compile: nvq++ main.cpp -o main // Run: ./main #include <cudaq.h> struct apply_i { __qpu__ void operator()() { cudaq::qubit q; mz(q); } }; struct apply_x { __qpu__ void operator()() { cudaq::qubit q; x(q); mz(q); } }; struct apply_y { __qpu__ void operator()() { cudaq::qubit q; y(q); mz(q); } }; struct apply_z { __qpu__ void operator()() { cudaq::qubit q; h(q); z(q); h(q); mz(q); } }; int main() { cu… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/pauli-gate-custatevec?rev=1781320400&do=diff Pauli gates (cuStateVec) Pauli gates implementation using cuStateVec. // Compile: nvcc main.cu -o main -lcustatevec // Run: ./main #include <stdio.h> #include <math.h> #include <cuda_runtime.h> #include <custatevec.h> static void apply_and_print(custatevecHandle_t handle, cuDoubleComplex *d_sv, cuDoubleComplex gate[4], const char *label) { cuDoubleComplex h_sv[2] = {{1,0},{0,0}}; // reset to |0> cudaMemcpy(d_sv, h_sv, 2 * sizeof(cuDoubleComplex), cu… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/pauli-gate-qiskit?rev=1781320400&do=diff Pauli gates (Qiskit) Pauli gates implementation using Qiskit. from qiskit import QuantumCircuit from qiskit.quantum_info import Statevector qc = QuantumCircuit(4) qc.id(0) # |0> -> |0> qc.x(1) # |0> -> |1> qc.y(2) # |0> -> i|1> qc.h(3) qc.z(3) # |+> -> |-> for i, label in enumerate(['I', 'X', 'Y', 'Z']): sub = QuantumCircuit(1) sub.append(qc.data[i].operation, [0]) if label == 'Z': sub.h(0) sub.z(0) print(f"{label}: {Statevector(sub)}") text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/pauli-gates?rev=1781320400&do=diff Pauli gates Pauli gates are the four single-qubit gates $I$, $X$, $Y$, $Z$ corresponding to the identity and the three Pauli matrices. They form the basis of single-qubit quantum operations and appear throughout quantum error correction, where $X$, $Y$, and $Z$ represent the three fundamental error types a qubit can suffer.$$I = \begin{pmatrix}1 & 0\\0 & 1\end{pmatrix}\qquad X = \begin{pmatrix}0 & 1\\ 1 & 0\end{pmatrix}\qquad Y = \begin{pmatrix}0 & -i \\ i & 0\end{pmatrix}\qquad Z = \begin{pmat… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/pdflatex?rev=1781320400&do=diff pdflatex pdflatex is the most common LaTeX compiler. You compile latex source code by running pdflatex main.tex. This creates a PDF file main.pdf. It also creates a log file main.log and an auxiliary file main.aux used to build references. You can ignore these latter two and text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/perf?rev=1781320400&do=diff perf What is perf? perf is a profiling and performance analysis tool built into the Linux kernel. It talks directly to the CPU's hardware performance monitoring unit (PMU) — a set of on-chip counters that can track things like retired instructions, cache misses, branch mispredictions, and stalled cycles — and lets you measure any of them against a running program. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/performance-model-mpi?rev=1781320400&do=diff Performance model (MPI) Every write() syscall has a fixed overhead regardless of how many bytes you write: there is a kernel crossing, buffer management, and bookkeeping before the first byte moves. For a local file the overhead is small relative to I/O cost, but over a network the per-message overhead can be much larger than the cost of moving the data itself. The $\alpha + \beta n$$\alpha$$\beta$$\alpha$ text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/persistent-communication-mpi?rev=1781320400&do=diff Persistent communication (MPI) HTTP/1.0 opened a new TCP connection for every request, paying a three-way handshake each time. HTTP/1.1 introduced keep-alive: one connection, many requests, the setup cost paid once. MPI persistent communication applies the same idea to message passing. In an iterative algorithm where the same process pair exchanges data every iteration with the same buffer and count — a halo exchange in a stencil code, for example — text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/playground?rev=1781320400&do=diff \documentclass{standalone} \begin{document} \end{document} text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/plus-state-qiskit?rev=1781320400&do=diff $\lvert +\rangle$ (Qiskit) Plus state $\lvert +\rangle$ implementation using Qiskit. The $\lvert +\rangle$ state is prepared by applying a Hadamard gate to $\lvert 0\rangle$. from qiskit import QuantumCircuit from qiskit.quantum_info import Statevector qc = QuantumCircuit(1) qc.h(0) # |0> -> |+> = (|0> + |1>) / sqrt(2) print(Statevector(qc)) # Statevector([0.70710678+0.j, 0.70710678+0.j], dims=(2,)) text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/point-to-point-mpi?rev=1781320400&do=diff Point-to-point communication (MPI) In a single-process C program, functions communicate through shared memory: one writes to a variable and another reads it. Across processes with separate address spaces — especially on different machines — that doesn't work. The low-level solution is sockets: text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/posix-headers?rev=1781320400&do=diff POSIX headers POSIX headers are C programming language headers as defined by POSIX. Links * <https://pubs.opengroup.org/onlinepubs/9799919799/idx/head.html> text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/pragma-once?rev=1781320400&do=diff #pragma once In C and C++ programming languages #pragma once is a non-standard preprocessor directive that is equivalent to header guards. You just put #pragma once at the top, rather than wrapping the entire header within header guards. Header guards: // image.h #ifndef __IMAGE_H__ #define __IMAGE_H__ struct Image { ... }; #endif /* __IMAGE_H__ */ text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/prefix-reductions-mpi?rev=1781320400&do=diff Prefix reductions (MPI) A prefix reduction (scan) turns an array of N values into N partial results, where each output element is the reduction of all input elements up to that index. The sequential exclusive prefix sum: int offsets[N]; offsets[0] = 0; for (int i = 1; i < N; i++) offsets[i] = offsets[i-1] + counts[i-1]; text/html 2026-06-13T03:49:41+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/probability-amplitude?rev=1781322581&do=diff Probability amplitude Probability amplitude is a complex number associated with a possible outcome of a quantum measurement. The probability of that outcome is the squared modulus of the amplitude, a rule known as the Born rule. Classical probability uses real numbers in $[0, 1]$$\lvert\psi\rangle = a\lvert 0\rangle + b\lvert 1\rangle$$a$$b$$$P_0 = |a|^2 \qquad P_1 = |b|^2$$$$|a|^2 + |b|^2 = 1$$$n$$2^n$$\sum_x |c_x|^2 = 1$$\mathbb{C}^{2^n}$$c_1$$c_2$$|c_1 + c_2|^2$$|c_1|^2 + |c_2|^2$$\lvert 0\… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/probing-mpi?rev=1781320400&do=diff Probing for messages (MPI) A standard recv() on a socket requires a pre-allocated buffer. If the incoming message is larger than the buffer, data gets truncated or the call fails. The POSIX escape hatch is MSG_PEEK: pass it as a flag and recv() fills your buffer but leaves the data in the socket queue, letting you inspect the size before committing to a real receive. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/process-groups-mpi?rev=1781320400&do=diff Process groups (MPI) MPI_Comm_split partitions all processes by a color value, which works well when membership can be computed from a formula — even/odd rank, row and column in a grid. When the subset is defined by an explicit list of ranks with no clean formula, text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/production?rev=1781320400&do=diff Production Production is the live environment where software runs and serves real users. It is distinguished from development (a developer's local machine), staging (a production-like environment used for testing before release), and CI (the automated build environment). text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/prototype?rev=1781320400&do=diff Prototype A prototype in software engineering is a preliminary version of a system built to explore a design, validate an assumption, or demonstrate feasibility. It is intentionally incomplete and typically not intended for production use. The goal of a prototype is to reduce uncertainty before committing to a full implementation. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/pure-state?rev=1781320400&do=diff Pure state Pure state is a quantum state that is completely described by a single state vector $\lvert\psi\rangle$. It carries the maximum possible information about a quantum system. When information about the system leaks into the environment the state becomes mixed — this process is called decoherence, and it is one of the primary challenges in quantum computing.$0\,\text{V}$$5\,\text{V}$$\lvert\psi\rangle = a\lvert 0\rangle + b\lvert 1\rangle$$a$$b$$\lvert\psi\rangle$$\rho = \lvert\psi\rang… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/qaoa?rev=1781320400&do=diff QAOA QAOA (Quantum Approximate Optimization Algorithm) is a hybrid quantum-classical algorithm for combinatorial optimization problems. It was introduced by Edward Farhi, Jeffrey Goldstone, and Sam Gutmann in 2014 and is one of the leading candidates for near-term quantum advantage on $p$$(\boldsymbol\gamma, \boldsymbol\beta)$$\lvert s\rangle = H^{\otimes n}\lvert 0\rangle^{\otimes n}$$H_C$$H_B = \sum_i X_i$$p$$$\lvert\boldsymbol{\gamma},\boldsymbol{\beta}\rangle = e^{-i\beta_p H_B}e^{-i\gamma_… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/qft?rev=1781320400&do=diff Quantum Fourier transform Quantum Fourier transform (QFT) is the quantum analogue of the discrete Fourier transform (DFT). It maps a computational basis state $\lvert j\rangle$ to a superposition whose amplitudes encode the DFT of the input amplitudes, for $N = 2^n$ states on $n$$$\text{QFT}\lvert j\rangle = \frac{1}{\sqrt{N}}\sum_{k=0}^{N-1} e^{2\pi ijk/N}\lvert k\rangle$$$O(n^2)$$n$$O(N \log N) = O(n \cdot 2^n)$$n$$n(n+1)/2$$$R_k = P\!\left(\frac{2\pi}{2^k}\right) = \begin{pmatrix}1 & 0 \\ 0 … text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/qpe?rev=1781320400&do=diff Quantum phase estimation Quantum phase estimation (QPE) is a quantum algorithm that estimates the eigenvalue phase $\phi$ of a unitary operator $U$ given access to an eigenstate $\lvert u\rangle$ of $U$. It is one of the most important subroutines in quantum computing, used as a building block in $U\lvert u\rangle = e^{2\pi i\phi}\lvert u\rangle$$\phi \in [0,1)$$\phi$$n$$n$$O(2^n)$$U$$n$$\lvert 0\rangle^{\otimes n}$$\lvert u\rangle$$n$$U^{2^k}$$k = 0, 1, \ldots, n-1$$k$$U^{2^k}$$$\lvert 0\rangl… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/quantum-algorithm?rev=1781320400&do=diff Quantum algorithm Quantum algorithm is an algorithm designed to run on a quantum computer, exploiting quantum phenomena such as superposition, entanglement, and interference to solve certain problems faster than any known classical algorithm. Quantum algorithms are expressed as sequences of quantum gates applied to a quantum register, followed by measurements. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/quantum-circuit?rev=1781320400&do=diff Quantum circuit Quantum circuit is a model of quantum computation in which a sequence of quantum gates is applied to a quantum register. Quantum circuits are the quantum analogue of classical digital circuits, but instead of Boolean logic gates acting on bits they use unitary matrices acting on qubits.$\lvert 0\rangle^{\otimes n}$$d$$n$$nd$$\{H, T, \text{CX}\}$$\varepsilon$$O(\text{polylog}(1/\varepsilon))$$\lvert 0\rangle$$\lvert 1\rangle$ text/html 2026-06-15T09:18:41+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/quantum-computing?rev=1781515121&do=diff Quantum computing Quantum computing is a model of computation that uses quantum-mechanical systems to store and process information. The fundamental unit is the Qubit, the quantum analog of a classical bit. A classical bit is either 0 or 1 at any given moment. A qubit can be in a superposition of both — and multiple qubits can be entangled, meaning their states are correlated in ways that have no classical analog. Those two properties, superposition and entanglement, are the source of quantum c… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/quantum-dummy-text?rev=1781320400&do=diff Quantum concepts A Qubit lives in a two-dimensional complex Hilbert space $\mathbb{C}^2$. You write its state in Dirac notation as $\lvert\psi\rangle = a\lvert 0\rangle + b\lvert 1\rangle$, where $a$ and $b$ are complex numbers called probability amplitudes, constrained by $|a|^2 + |b|^2 = 1$. The two basis states $\lvert 0\rangle$ and $\lvert 1\rangle$ are the quantum analogs of classical 0 and 1, but before any measurement the qubit genuinely occupies both at once — the amplitudes are not a d… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/quantum-error-correction?rev=1781320400&do=diff Quantum error correction Quantum error correction (QEC) is the set of techniques used to protect quantum information from errors caused by decoherence and imperfect gates. Unlike classical bits, which can be copied and checked against redundant copies, quantum states cannot be cloned — any measurement collapses the state. QEC works around this by encoding a single logical qubit into an entangled state of several physical qubits, allowing errors to be detected and corrected without learning anyt… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/quantum-gate?rev=1781320400&do=diff Quantum gate A quantum gate is a unitary operation applied to one or more qubits. It is the quantum computing analog of a classical logic gate, but with two key differences: quantum gates are represented by matrices rather than truth tables, and they are always reversible — every quantum gate has an inverse.$n$$2^n \times 2^n$$U$$U^\dagger U = I$$\lvert\psi\rangle$$U\lvert\psi\rangle$$U$$U^\dagger$$2 \times 2$$U(\theta, \phi, \lambda)$$4 \times 4$$n$ text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/quantum-register?rev=1781320400&do=diff Quantum register Quantum register is a register made out of qubits. It is the quantum equivalent of a classical CPU register. Classical CPU regsiters are made out of bits and are used to store number values. For example, a classical 16-bit register, like Intel 8086's $c = \frac{1}{\sqrt 2}(1 + i)$$|c_{63}|^2 = 0.75$$63$$\lvert\text{Qreg}\rangle$$\lvert\cdot\rangle$$$\begin{aligned} \lvert\text{Qreg}\rangle = \sum_{k = 0}^{k = 2^{16} - 1} c_k\lvert k\rangle = (x_0 + i y_0)\lvert\text{0}\rangle … text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/quantum-state?rev=1781320400&do=diff Quantum state A quantum state is a complete mathematical description of a quantum system. For a single qubit, a quantum state is a unit vector in a two-dimensional complex Hilbert space; for $n$ qubits, the state lives in a $2^n$-dimensional space formed by the tensor product of the individual qubit spaces.$\lvert\psi\rangle = \alpha\lvert 0\rangle + \beta\lvert 1\rangle$$\alpha, \beta \in \CC$$|\alpha|^2 + |\beta|^2 = 1$$|\alpha|^2$$|\beta|^2$$\lvert 0\rangle$$\lvert 1\rangle$$\rho$$\lvert 0\r… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/qubit-gates-in-c?rev=1781320400&do=diff Qubit gate Simulation In C99, a header <complex.h> is provided that implements complex numbers. State As we've seen, a qubit is a two-state configuration with complex probability amplitudes distributed between the two states. This gives us the following implementation of a single qubit: $$\lvert\psi\rangle = \alpha\lvert 0\rangle + \beta\lvert 1\rangle,\qquad \alpha,\beta\in\mathbb C$$$$\lVert\psi\lVert^2 = |\alpha|^2 + |\beta|^2 = \alpha\bar{\alpha} + \beta\bar{\beta}$$$$\lvert\psi\rangle\r… text/html 2026-06-13T03:46:46+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/qubit?rev=1781322406&do=diff Qubit Qubit (or quantum bit) is the basic unit of quantum information. The Hilbert space of a single qubit is $\mathbb{C}^2$, meaning two complex amplitudes to describe it, not one. A classical bit can only be 0 or 1. A qubit can be in a superposition: a combination of $\lvert 0\rangle$$\lvert 1\rangle$$$\lvert\psi\rangle = \begin{pmatrix}a\\b\end{pmatrix} = a\underbrace{\begin{pmatrix}1\\ 0\end{pmatrix}}_{\lvert 0\rangle} + b\underbrace{\begin{pmatrix}0\\1\end{pmatrix}}_{\lvert 1\rangle} = a\… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/queue?rev=1781320400&do=diff Queue A queue is a first-in, first-out (FIFO) sequence. Elements are added at the back (enqueue) and removed from the front (dequeue). A queue can be implemented over a Linked list with a head and tail pointer, over a Circular buffer for a bounded version, or as a lock-free queue for concurrent use. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/r-gate-cudaq?rev=1781320400&do=diff Rotation gates (CUDA-Q) Rotation gates implementation using CUDA-Q. The example applies $R_x(\pi/2)$ to $\lvert 0\rangle$, producing $\frac{1}{\sqrt{2}}(\lvert 0\rangle - i\lvert 1\rangle)$. // Compile: nvq++ main.cpp -o main // Run: ./main #include <cudaq.h> #include <cmath> struct kernel { __qpu__ void operator()() { cudaq::qubit q; rx(M_PI / 2.0, q); // R_x(pi/2) on |0> mz(q); } }; int main() { auto counts = cudaq::sample(kernel{}); counts.du… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/r-gate-custatevec?rev=1781320400&do=diff Rotation gates (cuStateVec) Rotation gates implementation using cuStateVec. The example applies $R_x(\pi/2)$ to $\lvert 0\rangle$, producing $\frac{1}{\sqrt{2}}(\lvert 0\rangle - i\lvert 1\rangle)$. // Compile: nvcc main.cu -o main -lcustatevec // Run: ./main #include <stdio.h> #include <math.h> #include <cuda_runtime.h> #include <custatevec.h> int main() { const int nQubits = 1; const int dim = 1 << nQubits; cuDoubleComplex h_sv[2] = {{1,0},{0,0}}; // |0> cuDoubleComp… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/r-gate-qiskit?rev=1781320400&do=diff Rotation gates (Qiskit) Rotation gates implementation using Qiskit. The example applies $R_x(\pi/2)$ to $\lvert 0\rangle$, producing $\frac{1}{\sqrt{2}}(\lvert 0\rangle - i\lvert 1\rangle)$. from qiskit import QuantumCircuit from qiskit.quantum_info import Statevector import numpy as np qc = QuantumCircuit(1) qc.rx(np.pi / 2, 0) # R_x(pi/2) on |0> print(Statevector(qc)) # Statevector([0.70710678+0.j, 0. -0.70710678j], dims=(2,)) text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/rabi-cycles?rev=1781320400&do=diff Rabi oscillations Rabi oscillations (or Rabi cycles) are periodic oscillations in the state of a driven two-level quantum system. When a qubit is driven by a resonant electromagnetic field, its population continuously oscillates between $\lvert 0\rangle$ and $\lvert 1\rangle$ at the Rabi frequency $\Omega$$\omega = \omega_0$$\omega_0$$\Omega$$\lvert 1\rangle$$\lvert 0\rangle$$$P_1(t) = \sin^2\!\left(\frac{\Omega t}{2}\right), \qquad P_0(t) = \cos^2\!\left(\frac{\Omega t}{2}\right)$$$\lvert 0\ra… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ramsey-interference?rev=1781320400&do=diff Ramsey interferometry Ramsey interferometry is a technique for measuring the transition frequency of a quantum system with very high precision using two separated pulses. It was developed by Norman Ramsey in 1950 as an improvement on Rabi's continuous-drive spectroscopy, and it forms the basis of modern atomic clocks and qubit frequency calibration.$\pi/2$$T$$\pi/2$$\lvert +\rangle$$\lvert 0\rangle$$T$$\delta T$$\delta = \omega - \omega_0$$\omega$$\omega_0$$\pi/2$$$P_1(T) = \frac{1}{2}(1 - \cos… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/red-black-tree?rev=1781320400&do=diff Red-black tree A red-black tree is a self-balancing binary search tree that maintains O(log n) height through a set of colour invariants enforced on every insert and delete. Each node is coloured red or black, and the invariants constrain how colours can be arranged: the root is black, red nodes can only have black children, and every path from any node to a descendant NULL leaf passes through the same number of black nodes. These rules together bound the height to at most $2 \log_2(n+1)$ text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/reduction-mpi?rev=1781320400&do=diff Reduction (MPI) A reduction combines all elements of a collection into a single value using an associative operator. In sequential C, that is a loop: double sum = 0.0; for (int i = 0; i < N; i++) sum += arr[i]; In a parallel program, each process holds a different slice of the data and computes a partial result. To get the global value, those partial results need to be combined. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/reduction-openmp?rev=1781320400&do=diff Reduction (OpenMP) Summing an array in serial C is a straightforward accumulation loop: double sum = 0.0; for (int i = 0; i < N; i++) sum += a[i]; Adding #pragma omp parallel for to this loop creates a race condition: multiple threads read sum text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/regression?rev=1781320400&do=diff Regression A regression is a bug that was not present in a previous version of the software and was introduced by a subsequent change. The term comes from the Latin regressus (going back): something that previously worked has gone back to being broken. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/rho-zero?rev=1781320400&do=diff $\rho_0$ (Zero state density matrix) is the density matrix of the zero state. As a pure state its density matrix carries no more information than the ket, but the density matrix representation is the right starting point for studying what happens to $\lvert 0\rangle$ when it couples to an environment — when the ket description breaks down.$$\rho_0 = \lvert 0\rangle\langle 0\rvert = \begin{pmatrix}1\\0\end{pmatrix}\begin{pmatrix}1 & 0\end{pmatrix} = \begin{pmatrix}1 & 0\\0 & 0\end{pmatrix}$$$\l… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/roofline-model?rev=1781320400&do=diff Roofline model The roofline model is a visual performance model that tells you whether a kernel is limited by compute throughput or memory bandwidth. It was introduced by Williams, Waterman, and Patterson in 2009. The model gives you a single number to characterize a kernel — its $$\text{AI} = \frac{\text{FLOP}}{\text{bytes transferred}}$$$\Pi$$\beta$$$P \leq \min\!\left(\Pi,\ \beta \cdot \text{AI}\right)$$$x$$y$$\beta$$\Pi$$$\text{AI}_{\text{ridge}} = \frac{\Pi}{\beta}$$$\mathbf{y} \leftarrow … text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/rotation-gates?rev=1781320400&do=diff Rotation gates Rotation gates $R_x(\theta)$, $R_y(\theta)$, $R_z(\theta)$ are single-qubit gates that rotate the Bloch vector by angle $\theta$ about the $x$-, $y$-, and $z$-axes respectively. They are defined via the matrix exponential of the corresponding Pauli matrices: $$R_x(\theta) = e^{-i\theta X/2} \qquad R_y(\theta) = e^{-i\theta Y/2} \qquad R_z(\theta) = e^{-i\theta Z/2}$$ Matrix form Using $e^{-i\theta P/2} = \cos(\theta/2)I - i\sin(\theta/2)P$ (which follows from $P^2 = I$ for any… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ruin-theory?rev=1781320400&do=diff Ruin theory (personal finance) This article is supposed to illustrate using Cramer-Lundberg model from actuarial science and applying to personal finance. This model was invented in 1930s for insurance companies. Insurance companies never seem to go bankrupt yet ordinary people are often get financial struggles. While it's easy to blame insurance companies for cheating at the game and ripping us off (and there is certainly a lot of truth to that), perhaps we can also learn the game they're play… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/rule-of-5?rev=1781320400&do=diff Rule of 5 In C++, rule of 5 * Destructor * Copy constructor * Move constructor * Copy assignment operator * Move assignment operator #pragma once struct Image { public: Image(); /* Todo */ }; Links * <https://en.cppreference.com/w/cpp/language/rule_of_three.html> text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/rwa?rev=1781320400&do=diff Rotating wave approximation Rotating wave approximation (RWA) is an approximation in quantum optics and quantum control that simplifies the Hamiltonian of a driven quantum system by discarding rapidly oscillating terms. It reduces the full time-dependent driven Hamiltonian to a simpler time-independent one in the rotating frame, making $\omega_0$$\omega$$\delta = \omega - \omega_0$$\omega + \omega_0$$\omega$$\rho \to e^{i\omega t Z/2}\rho e^{-i\omega t Z/2}$$$H_{\text{RWA}} = \frac{\hbar}{2}\be… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/rx-gate?rev=1781320400&do=diff Rotation-X gate gate rotates a qubit by angle $\theta$ about the $x$-axis of the Bloch sphere. It is one of the three rotation gates and is defined as the matrix exponential of the Pauli-X operator: $$R_x(\theta) = e^{-i\theta X/2} = \begin{pmatrix}\cos\dfrac{\theta}{2} & -i\sin\dfrac{\theta}{2}\\[6pt] -i\sin\dfrac{\theta}{2} & \cos\dfrac{\theta}{2}\end{pmatrix}$$ Applied to the computational basis states: $$R_x(\theta)\lvert 0\rangle = \cos\tfrac{\theta}{2}\lvert 0\rangle - i\sin\tfrac{\th… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ry-gate?rev=1781320400&do=diff Rotation-Y gate gate rotates a qubit by angle $\theta$ about the $y$-axis of the Bloch sphere. It is one of the three rotation gates and is defined as the matrix exponential of the Pauli-Y operator: $$R_y(\theta) = e^{-i\theta Y/2} = \begin{pmatrix}\cos\dfrac{\theta}{2} & -\sin\dfrac{\theta}{2}\\[6pt] \sin\dfrac{\theta}{2} & \cos\dfrac{\theta}{2}\end{pmatrix}$$ Applied to the computational basis states: $$R_y(\theta)\lvert 0\rangle = \cos\tfrac{\theta}{2}\lvert 0\rangle + \sin\tfrac{\theta}… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/rz-gate?rev=1781320400&do=diff Rotation-Z gate gate rotates a qubit by angle $\theta$ about the $z$-axis of the Bloch sphere. It is one of the three rotation gates and is defined as the matrix exponential of the Pauli-Z operator: $$R_z(\theta) = e^{-i\theta Z/2} = \begin{pmatrix}e^{-i\theta/2} & 0\\[6pt] 0 & e^{i\theta/2}\end{pmatrix}$$ Applied to the computational basis states: $$R_z(\theta)\lvert 0\rangle = e^{-i\theta/2}\lvert 0\rangle \qquad R_z(\theta)\lvert 1\rangle = e^{i\theta/2}\lvert 1\rangle$$ The gate is dia… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/s-gate?rev=1781320400&do=diff S gate S gate (or gate) is a single-qubit gate that adds a phase of $\pi/2$ to the $\lvert 1\rangle$ state while leaving $\lvert 0\rangle$ unchanged. It is a special case of the phase gate with $\phi = \pi/2$, and satisfies $S^2 = Z$. $$S = \begin{pmatrix}1 & 0\\ 0 & i\end{pmatrix}$$ On the computational basis, $S\lvert 0\rangle = \lvert 0\rangle$ and $S\lvert 1\rangle = i\lvert 1\rangle$. On the Bloch sphere, $S$ is a $\pi/2$ rotation about the $z$-axis; it maps $\lvert +\rangle$$\lvert +i\… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/saxpy?rev=1781320400&do=diff SAXPY SAXPY is an algorithm in high-performance computing (HPC). Math Vectors Let $\mathbf{y}^{(t)}, \mathbf{x}^{(t)}\in\mathbb{R}^n$ be vectors over real numbers at some discrete time $t\in\mathbb{N}$ and let $a\in\mathbb{R}$ be a real parameter. SAXPY is defined as a vector update of $\mathbf{y}$ at time $t+1$ according to the formula: $$\mathbf{y}^{(t+1)} = a\mathbf{x}^{(t)} + \mathbf{y}^{(t)}$$ Elements $\mathbf{y}^{(t)} = (y_0^{(t)}, y_1^{(t)}, \dots, y_n^{(t)})$$\mathbf{x}^{(t)} = (… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/scatter-and-gather-mpi?rev=1781320400&do=diff Scatter and gather (MPI) The basic pattern for distributing work is: rank 0 holds the full dataset, divides it into N equal chunks, and sends one to each process. Without collectives, that is a loop of sends: if (rank == 0) for (int i = 0; i < size; i++) MPI_Send(&data[i * chunk], chunk, MPI_INT, i, 0, MPI_COMM_WORLD); else MPI_Recv(local, chunk, MPI_INT, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE); text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/scheduling-openmp?rev=1781320400&do=diff Scheduling (OpenMP) Scheduling in OpenMP controls how loop iterations are divided among threads. By default, parallel for splits iterations into roughly equal static chunks assigned to threads upfront. This works well when each iteration takes the same time. When iteration cost varies, static assignment leaves some threads idle while others are still running. A loop where some values of text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/schrodinger-equation-qutip?rev=1781320400&do=diff Schrödinger equation (QuTiP) Schrödinger equation implementation using QuTiP. QuTiP's sesolve() (schrödinger equation solver) integrates the equation directly in state-vector form. It takes the Hamiltonian H, an initial ket psi0, and a time array tlist, then returns a list of ket vectors $\lvert\psi(t)\rangle$$$i\hbar\frac{\mathrm{d}}{\mathrm{d}t}\lvert\psi\rangle = H\lvert\psi\rangle$$$\langle O\rangle = \langle\psi(t)\lvert O\lvert\psi(t)\rangle$ text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/schrodinger-equation?rev=1781320400&do=diff Schrödinger equation The Schrödinger equation is the equation of motion for a quantum state. In the qubit picture the state is a complex vector $\lvert\psi\rangle$ in a Hilbert space, and the equation is a first-order linear ODE on that vector. The generator of the evolution is a Hermitian operator $H$$$i\hbar\frac{\mathrm{d}}{\mathrm{d}t}\lvert\psi\rangle = H\lvert\psi\rangle$$$\lvert\psi\rangle$$t + dt$$t$$$\lvert\psi(t+dt)\rangle = U(t+dt,\,t)\,\lvert\psi(t)\rangle$$$dt$$U(t,t) = I$$U(t+dt, … text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/sections-openmp?rev=1781320400&do=diff Sections (OpenMP) Suppose you have a few independent operations — compress a buffer, encrypt a header, write a log — that share no data and could run simultaneously. A loop doesn't fit because the operations are distinct, not iterations of the same work. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/semaphore?rev=1781320400&do=diff Semaphore Semaphore is a synchronization primitive. It's used to control thread execution in multithreaded environments. A semaphore acts like an integer. An integer supports two well-known opeartions: increment i++ and decrement i--. A semaphore works in a similar way, except a semaphore cannot be decremented below zero. This is because semaphore takes action to protect its value against bankrupcy by taking hostage of threads that attempt to make it negative. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/send-modes-mpi?rev=1781320400&do=diff Send modes (MPI) MPI_Send has a behaviour that surprises many first-time users: whether it blocks or returns immediately depends on message size and the implementation's internal buffer. For small messages, most implementations copy the data into an internal buffer and return right away. For messages larger than that buffer, text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/setjmp.h?rev=1781320400&do=diff setjmp.h setjmp.h provides setjmp and longjmp: a mechanism for jumping directly from a function back to an earlier point in the call stack, bypassing all the normal return chain in between. It is the C equivalent of throwing an exception that is caught several frames up — but without any of C++'s object cleanup machinery, which makes it both simpler and more dangerous. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/shared-memory-windows-mpi?rev=1781320400&do=diff Shared memory windows (MPI) On a multi-core node, MPI processes communicate through the messaging layer by default, even when they share the same physical memory. A send between two processes on the same node still involves serialisation overhead and memory copies that shared memory would avoid. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/shell?rev=1781320400&do=diff Shell What is a shell? Shell is a program that lets you to run other programs in the terminal. Shell allows you to pass command line arguments, set environment variables and redirect the program's stardard input / output / error. It allows you to chain programs, such that output from one program is redirected as input to another program. It also allows you to run the programs in the background. Basically, a shell allows you to easily orchestrate programs in the terminal. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/shor?rev=1781320400&do=diff Shor's algorithm Shor's algorithm is a quantum algorithm that factors large integers in polynomial time. It was invented by Peter Shor in 1994 and is one of the most consequential results in quantum computing, because it would break the RSA public-key cryptosystem if run on a sufficiently large fault-tolerant quantum computer.$O(e^{O(n^{1/3}\log^{2/3} n)})$$n$$O(n^3)$$O(n)$$N$$a$$\gcd(a, N) = 1$$r$$a^r \equiv 1 \pmod{N}$$r$$a^{r/2} \not\equiv -1 \pmod{N}$$\gcd(a^{r/2} \pm 1, N)$$N$$1/2$$x \in \… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/signal.h?rev=1781320400&do=diff signal.h signal.h is how your program hears from the OS when something happens outside its normal execution: the user pressed Ctrl-C, a timer expired, you divided by zero, or another process sent you a notification. Signals are asynchronous — they can arrive between any two instructions — which makes them tricky to handle correctly. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/simd-openmp?rev=1781320400&do=diff SIMD (OpenMP) A modern CPU can add four floats in a single instruction rather than one, loading them into a wide register and operating on all four lanes simultaneously. This is SIMD (single instruction, multiple data), exposed on x86 as SSE/AVX and on ARM as NEON. The compiler attempts to use these instructions automatically (auto-vectorisation), but it can be blocked by pointer aliasing, non-unit strides, or conditionals it cannot prove safe. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/single-openmp?rev=1781320400&do=diff Single (OpenMP) Single is an OpenMP directive that runs the enclosed block on exactly one thread, whichever arrives first. All other threads skip the block and wait at an implicit barrier at the end, so the whole team is synchronised before continuing. It is commonly used for initialisation, I/O, and task generation. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/single-qubit-gates?rev=1781320400&do=diff Qubit gates Qubit gates (single-qubit gates) are $2 \times 2$ unitary matrices that act on a single Qubit. Every such gate corresponds to a rotation on the Bloch sphere. The most general single-qubit gate (up to global phase) is the U gate $U(\theta, \phi, \lambda)$, which spans all of $SU(2)$; every named single-qubit gate is a special case of it.$I$$X$$Y$$Z$$\pi$$x$$y$$z$$X$$\lvert 0\rangle \leftrightarrow \lvert 1\rangle$$Z$$\lvert 0\rangle$$\lvert 1\rangle$$Y = iXZ$$X^2 = Y^2 = Z^2 = I$$Z$$… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/smart-and-vapid?rev=1781320400&do=diff Smart-and-gets-things-done Smart-and-gets-things-done is a hiring heuristic from Joel Spolsky's 2000 essay “The Guerrilla Guide to Interviewing”. Spolsky argued that the only two things that matter when hiring a software developer are whether they are smart and whether they actually get things done. All other interview criteria are either proxies for these two properties or irrelevant. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/solid?rev=1781320400&do=diff SOLID SOLID is a set of five object-oriented design principles intended to make code more understandable, maintainable, and extensible. The acronym was coined by Robert C. Martin (Uncle Bob). S — Single responsibility principle A class should have only one reason to change. In practice: a class that handles both data persistence and business logic is harder to change than one that does only persistence. When the database schema changes and the business rules change, they change independently,… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/spike-solution?rev=1781320400&do=diff Spike solution A spike solution is a short, throwaway experiment written to explore a technical question or reduce uncertainty. The name comes from Extreme programming: Kent Beck used “spike” to mean driving something sharply through a problem to understand it, the way you drive a spike through wood to test the material. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/ssh?rev=1781320400&do=diff ssh What is ssh? SSH (Secure shell) is a tool that allows you to remotely control another computer from your own. Login without password On the client, you should run the following command: ssh-keyring This command is going to generate two files in your text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/stack?rev=1781320400&do=diff Stack A stack is a last-in, first-out (LIFO) sequence. The only accessible element is the top; you push to add and pop to remove, always at the same end. A stack can be implemented over an Array (with a top index) or a Linked list (with insertions at the head). The array-backed version is more cache-friendly; the linked version avoids a fixed capacity. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/state-vector?rev=1781320400&do=diff State vector State vector (written as ket $\lvert\psi\rangle$) is the vector representation of a quantum state. Quantum state is often written as a linear combination of basis states where the coefficients are probability amplitudes. For example, a qubit is often written in the following way using Dirac notation.$$\lvert\psi\rangle = a\lvert 0\rangle + b\lvert 1\rangle$$$\mathbb{C}^2$$\mathbb{C}^2$$\lvert\psi\rangle$$\mathbb{C}^2$$$\lvert\psi\rangle = \begin{pmatrix}a\\b\end{pmatrix}\qquad a,… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/stdalign.h?rev=1781320400&do=diff stdalign.h stdalign.h (C11) gives you alignas and alignof — the convenient macro wrappers around the _Alignas and _Alignof keywords. Alignment controls where in memory an object is placed: a 16-byte-aligned object must start at an address divisible by 16. You need this most often for SIMD, where vector load instructions require specific alignment, and occasionally for hardware registers. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/stdarg.h?rev=1781320400&do=diff stdarg.h stdarg.h provides the machinery for writing variadic functions — functions that accept a variable number of arguments, like printf. Without it you would have no portable way to iterate over the unnamed arguments. It defines va_list, and the text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/stdatomic.h?rev=1781320400&do=diff stdatomic.h stdatomic.h (C11) gives you atomic types and operations for lock-free concurrent programming. An atomic operation completes without interruption: no other thread can observe the object in a half-updated state. This is the C standard's portable answer to compiler-specific builtins like text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/stdbit.h?rev=1781320400&do=diff stdbit.h Before C23, if you wanted to count the leading zeros in an integer or compute its population count, you either reached for GCC's __builtin_clz and __builtin_popcount, wrote a loop, or pulled in a platform-specific intrinsic. None of these were portable. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/stdbool.h?rev=1781320400&do=diff stdbool.h Before C99, every codebase had its own boolean convention. Some used int with 0 for false and non-zero for true, some #defined TRUE and FALSE, some used char. stdbool.h (C99) ends that by giving portable names to the convention the language already used: text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/stdckdint.h?rev=1781320400&do=diff stdckdint.h Signed integer overflow in C is undefined behaviour. Unsigned overflow wraps silently. Both have caused real security bugs — length calculations that overflow and produce a smaller allocation than expected, counters that wrap around to zero. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/stddef.h?rev=1781320400&do=diff stddef.h size_t, NULL, offsetof — you have seen these in practically every C codebase. They all come from stddef.h, which defines the handful of fundamental types and macros that the rest of the standard library depends on. Most headers pull it in transitively, so you often get it without including it directly. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/stdint.h?rev=1781320400&do=diff stdint.h Write int assuming it is 32 bits, run the code on a 16-bit microcontroller, and things break silently. stdint.h (C99) solves this by providing integer types with guaranteed widths, so you can write code that means exactly what it says regardless of platform. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/stdio.h?rev=1781320400&do=diff stdio.h printf("Hello, world\n") — nearly every C tutorial starts there, which means stdio.h is usually the first header anyone touches. It provides buffered file and console I/O: FILE, stdin/stdout/stderr, printf, scanf, fopen/fclose, and a whole family of read/write functions. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/stdlib.h?rev=1781320400&do=diff stdlib.h stdlib.h is the junk drawer of the C standard library. It covers dynamic memory allocation, process control, type conversions, random numbers, sorting, and searching — everything that did not fit cleanly into the other headers. #include <stdlib.h> // dynamic memory void *p = malloc(1024); // allocate 1024 bytes, uninitialized void *z = calloc(16, sizeof(int)); // allocate and zero 16 ints p = realloc(p, 2048); // resize allocation free(p); … text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/stdmchar.h?rev=1781320400&do=diff stdmchar.h The older Unicode headers in C (<wchar.h>, <uchar.h>) have a common problem: they do not tell you how much output buffer space a conversion will need, they do not handle malformed input gracefully, and they do not support transcoding between arbitrary encodings. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/stdnoreturn.h?rev=1781320400&do=diff stdnoreturn.h If you have a die() or fatal() function that calls exit, the compiler does not automatically know it never returns. It will warn about missing return values in callers and may generate unnecessary dead-code paths. stdnoreturn.h (C11) defines the text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/string.h?rev=1781320400&do=diff string.h String manipulation in C means working directly with pointers into null-terminated byte arrays. string.h provides the standard set of functions for this: measuring lengths, copying, concatenating, comparing, searching, and operating on raw memory blocks. The string functions work on text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/strunk-and-white?rev=1781320400&do=diff Strunk & White Strunk & White is a guide on writing text in the English language. The guide is contained in a book titled “The Elements of Style”. It was written written by William Strunk Jr. in the 1910s and later expanded by E. B. White in the 1950s. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/swap-gate-cudaq?rev=1781320400&do=diff SWAP gate (CUDA-Q) SWAP gate implementation using CUDA-Q. The example prepares $\lvert 10\rangle$ and applies SWAP to produce $\lvert 01\rangle$. // Compile: nvq++ main.cpp -o main // Run: ./main #include <cudaq.h> struct kernel { __qpu__ void operator()() { cudaq::qvector<2> q; x(q[0]); // q[0]=|1>, q[1]=|0> => |10> swap(q[0], q[1]); // |10> -> |01> mz(q); } }; int main() { auto counts = cudaq::sample(kernel{}); counts.dump(); … text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/swap-gate-custatevec?rev=1781320400&do=diff SWAP gate (cuStateVec) SWAP gate implementation using cuStateVec. The example prepares $\lvert 10\rangle$ and applies SWAP to produce $\lvert 01\rangle$. // Compile: nvcc main.cu -o main -lcustatevec // Run: ./main #include <stdio.h> #include <cuda_runtime.h> #include <custatevec.h> int main() { const int nQubits = 2; const int dim = 1 << nQubits; // Prepare |10>: q0=1, q1=0 -> index 2 (binary 10) cuDoubleComplex h_sv[4] = {0}; h_sv[2] = make_cuDoubleComplex(1.0, 0.… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/swap-gate-qiskit?rev=1781320400&do=diff SWAP gate (Qiskit) SWAP gate implementation using Qiskit. The example prepares $\lvert 10\rangle$ and applies SWAP to produce $\lvert 01\rangle$. from qiskit import QuantumCircuit from qiskit.quantum_info import Statevector qc = QuantumCircuit(2) qc.x(1) # q0=|0>, q1=|1> => |10> in math ordering qc.swap(0, 1) # |10> -> |01> print(Statevector(qc)) # Statevector([0.+0.j, 1.+0.j, 0.+0.j, 0.+0.j], dims=(2, 2)) text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/swap-gate?rev=1781320400&do=diff SWAP gate SWAP gate is a two-qubit gate that exchanges the states of its two qubits. It is symmetric — neither qubit plays the role of control or target — and it is its own inverse: $\text{SWAP}^2 = I$. $$\text{SWAP} = \begin{pmatrix}1&0&0&0\\ 0&0&1&0\\ 0&1&0&0\\ 0&0&0&1\end{pmatrix}$$ $$\text{SWAP}\lvert 00\rangle = \lvert 00\rangle \qquad \text{SWAP}\lvert 01\rangle = \lvert 10\rangle \qquad \text{SWAP}\lvert 10\rangle = \lvert 01\rangle \qquad \text{SWAP}\lvert 11\rangle = \lvert 11\rangle… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/syntax?rev=1781320400&do=diff [GitHub][gh] fdsafdsa text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/t-gate?rev=1781320400&do=diff T gate T gate (or gate, or gate) is a single-qubit gate that adds a phase of $\pi/4$ to the $\lvert 1\rangle$ state while leaving $\lvert 0\rangle$ unchanged. It is a special case of the phase gate with $\phi = \pi/4$, and is the square root of the S gate: $T^2 = S$. $$T = \begin{pmatrix}1 & 0\\ 0 & e^{i\pi/4}\end{pmatrix}$$ On the computational basis, $T\lvert 0\rangle = \lvert 0\rangle$ and $T\lvert 1\rangle = e^{i\pi/4}\lvert 1\rangle$. On the Bloch sphere, $T$ is a $\pi/4$ rotation abou… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/tasks-openmp?rev=1781320400&do=diff Tasks (OpenMP) Tasks are the OpenMP mechanism for irregular parallelism. parallel for only handles loops with a known iteration count. For work like recursive algorithms, tree traversals, and producer-consumer patterns, #pragma omp task packages a unit of work that any idle thread in the current team can pick up and execute. Tasks are created inside a text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/terminal?rev=1781320400&do=diff Terminal Terminal is conceptually anything that looks like a keyboard + a screen attached to a teletype device (TTY). It gives humans the ability to use the teletype device. When you type something in a terminal, the terminal sends your characters to the teletype device. When a teletype device responds, the terminal prints the result back to you. A terminal can also reinterpret the characters coming from the teletype device and change its behavior in some way (e.g. change cursor location or tu… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/test-automation?rev=1781320400&do=diff Test automation Test automation is the practice of running tests programmatically rather than manually. Instead of a person clicking through an application to verify that features work, a program does it. The tests run on every change, give a pass/fail result within minutes, and never forget to check something. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/test-driven-development?rev=1781320400&do=diff Test-driven development Test-driven development (TDD) is a practice where you write the test before you write the code. The test fails first — because the code does not exist yet — then you write the minimum code to make it pass, then you clean up. The cycle repeats for every new piece of behaviour. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/tgmath.h?rev=1781320400&do=diff tgmath.h Remembering to write sqrtf for float, sqrt for double, and sqrtl for long double is tedious and error-prone — forget the suffix and you silently get the wrong precision. tgmath.h (C99) provides type-generic wrappers that dispatch to the right variant based on the argument type, so you can just write text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/thinkpad?rev=1781320400&do=diff ThinkPad X-series x220ai text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/thread-affinity-openmp?rev=1781320400&do=diff Thread affinity (OpenMP) On a laptop or single-socket desktop, every core reaches the same RAM at the same speed. On a multi-socket server, each socket has its own bank of RAM (NUMA, non-uniform memory access): accessing local memory is fast, but crossing the inter-socket interconnect to reach the other socket's RAM costs roughly 2–3× more. By default, the OS is free to migrate threads between cores and sockets, silently moving a thread away from the data it allocated. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/thread.h?rev=1781320400&do=diff threads.h POSIX pthreads work great on Linux but are not available everywhere. threads.h (C11) provides a portable threading API — thread creation, mutexes, condition variables, and thread-local storage — with a standardised interface that any conforming C11 implementation must provide. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/three-qubit-gates?rev=1781320400&do=diff Three-qubit gates Three-qubit gates are $8 \times 8$ unitary matrices acting on a three-qubit system. They appear in fault-tolerant quantum computation and quantum arithmetic circuits. Any three-qubit gate decomposes into two-qubit and single-qubit gates, though the decompositions can be deep.$\lvert 1\rangle$$$\text{CCX}\lvert c_1 c_2 t\rangle = \lvert c_1 c_2\rangle \otimes \lvert t \oplus (c_1 \wedge c_2)\rangle$$$\lvert 1\rangle$$T$$T$$T^\dagger$$H$$S$$T$$T$$T$ text/html 2026-06-13T03:37:51+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/three-qubits?rev=1781321871&do=diff Three qubits Three-qubit system is a quantum system of three qubits. The Hilbert space is $\mathbb{C}^8$, meaning eight complex amplitudes to describe it, not three. A single qubit needs two amplitudes; two qubits need four; three qubits need eight. The state space doubles with every qubit you add. That exponential growth is what makes quantum computers interesting.$$\lvert\psi\rangle = \begin{pmatrix}c_{000}\\c_{001}\\c_{010}\\c_{011}\\c_{100}\\c_{101}\\c_{110}\\c_{111}\end{pmatrix}, \qquad \… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/time-measurement-mpi?rev=1781320400&do=diff Time measurement (MPI) In a single-process program, clock_gettime(CLOCK_MONOTONIC, &ts) gives the elapsed time from one CPU's perspective. In a parallel program, each process runs on a potentially different core with a clock that is not perfectly synchronised to the others. Timing on just one process gives its view of the wall time, which can be shorter than the actual parallel section if that process happens to finish early. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/time-measurement-openmp?rev=1781320400&do=diff Time measurement (OpenMP) Time measurement in OpenMP is done with omp_get_wtime(), which returns elapsed wall-clock time in seconds as a double. Calling it before and after a parallel region gives the actual time the user waited, which is what matters for speedup measurement. CPU time is not useful here: it sums across all threads and grows with thread count rather than shrinking. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/time.h?rev=1781320400&do=diff time.h Getting the current time is a one-liner. Timing how long something takes, converting between timezones, or formatting a timestamp all have more moving parts than you would expect. time.h provides the types and functions for all of it: time_t (seconds since the Unix epoch), text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/tmux?rev=1781320400&do=diff tmux What is tmux? Tmux is a terminal multiplexer. It allows you to run multiple terminals in a single terminal. In this sense, it is an alternative to screen command. Tmux runs a background session that you can attach to. It gives you the ability to create multiple windows within a session. Then it also gives you the ability to create multiple panes within a window, and tile them. Each pane contains a terminal. Thus, you can think of tmux as a text/html 2026-06-15T08:55:49+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/todo?rev=1781513749&do=diff To do This page contains articles which I'm in the process of writing or plan to write about. Website The following are some features I need to implement on the site: * Editor: * Add: Live preview while editing (requires backend API support) * text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/toffoli-gate-cudaq?rev=1781320400&do=diff Toffoli gate (CUDA-Q) Toffoli gate implementation using CUDA-Q. The example prepares $\lvert 110\rangle$ (both controls $\lvert 1\rangle$, target $\lvert 0\rangle$) and applies the Toffoli gate, flipping the target to produce $\lvert 111\rangle$. // Compile: nvq++ main.cpp -o main // Run: ./main #include <cudaq.h> struct kernel { __qpu__ void operator()() { cudaq::qvector<3> q; x(q[0]); // q[0] = |1> x(q[1]); … text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/toffoli-gate-custatevec?rev=1781320400&do=diff Toffoli gate (cuStateVec) Toffoli gate implementation using cuStateVec. The example prepares $\lvert 110\rangle$ (both controls $\lvert 1\rangle$, target $\lvert 0\rangle$) and applies the Toffoli gate, flipping the target to produce $\lvert 111\rangle$. // Compile: nvcc main.cu -o main -lcustatevec // Run: ./main #include <stdio.h> #include <cuda_runtime.h> #include <custatevec.h> int main() { const int nQubits = 3; const int dim = 1 << nQubits; // Prepare |110>: q0=1, q1=… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/toffoli-gate-qiskit?rev=1781320400&do=diff Toffoli gate (Qiskit) Toffoli gate implementation using Qiskit. The example prepares $\lvert 110\rangle$ (both controls $\lvert 1\rangle$, target $\lvert 0\rangle$) and applies the Toffoli gate, flipping the target to produce $\lvert 111\rangle$. from qiskit import QuantumCircuit from qiskit.quantum_info import Statevector qc = QuantumCircuit(3) qc.x(0) # q0 = |1> qc.x(1) # q1 = |1> => state is |110> qc.ccx(0, 1, 2) # Toffoli: both controls |1>, flip target -> |111> print(St… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/topics?rev=1781320400&do=diff Topics I plan to write about Wikipedia * Trace theory * Pi calculus * History monoid Concepts * Hazard pointer * ABA problem * Cache coherence (MSI MESI MOESI DragonFly) text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/trace-monoid?rev=1781320400&do=diff Trace monoid A trace monoid (also called a free partially commutative monoid, or a Mazurkiewicz trace monoid after Antoni Mazurkiewicz who introduced them in 1977) is an algebraic structure that captures the notion of concurrent computation. The idea is simple: if two actions in a program do not share any data, the order in which you execute them should not matter. A trace monoid makes this precise by taking the set of all instruction sequences and declaring two sequences equivalent if one can … text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/two-qubit-gates?rev=1781320400&do=diff Two-qubit gates Two-qubit gates are $4 \times 4$ unitary matrices acting on a pair of qubits. Unlike single-qubit gates, they can create entanglement between qubits that were previously in a product state. Any two-qubit unitary can be decomposed into at most three $n$$\lvert +\rangle\lvert 0\rangle$$(c_x, c_y, c_z)$ text/html 2026-06-13T03:23:59+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/two-qubits?rev=1781321039&do=diff Two qubits Two-qubit system is a quantum system of two qubits. The Hilbert space is $\mathbb{C}^4$, meaning four complex amplitudes to describe it, not two. A single qubit needs two amplitudes; two qubits need four; three qubits need eight. The state space doubles with every qubit you add. That exponential growth is what makes quantum computers interesting.$$\lvert\psi\rangle = \begin{pmatrix}c_{00}\\c_{01}\\c_{10}\\c_{11}\end{pmatrix}, \qquad \lvert\psi\rangle = c_{00}\underbrace{\begin{pmatr… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/u-gate-cudaq?rev=1781320400&do=diff U gate (CUDA-Q) U gate implementation using CUDA-Q. The example applies $U(\pi/2, 0, \pi) = H$ to $\lvert 0\rangle$ via its Euler decomposition, producing $\lvert +\rangle$. // Compile: nvq++ main.cpp -o main // Run: ./main #include <cudaq.h> #include <cmath> struct kernel { __qpu__ void operator()() { cudaq::qubit q; // U(theta, phi, lambda) via Euler decomposition: Rz(phi) Ry(theta) Rz(lambda) rz(0.0, q); // phi = 0 ry(M_PI / 2.0, q); … text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/u-gate-custatevec?rev=1781320400&do=diff U gate (cuStateVec) U gate implementation using cuStateVec. The example applies $U(\pi/2, 0, \pi) = H$ to $\lvert 0\rangle$, producing $\lvert +\rangle$. // Compile: nvcc main.cu -o main -lcustatevec // Run: ./main #include <stdio.h> #include <math.h> #include <cuda_runtime.h> #include <custatevec.h> int main() { const int nQubits = 1; const int dim = 1 << nQubits; cuDoubleComplex h_sv[2] = {{1,0},{0,0}}; // |0> cuDoubleComplex *d_sv; cudaMalloc(&d_sv, dim * sizeof… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/u-gate-qiskit?rev=1781320400&do=diff U gate (Qiskit) U gate implementation using Qiskit. The example applies $U(\pi/2, 0, \pi) = H$ to $\lvert 0\rangle$, producing $\lvert +\rangle$. from qiskit import QuantumCircuit from qiskit.quantum_info import Statevector import numpy as np qc = QuantumCircuit(1) qc.u(np.pi / 2, 0, np.pi, 0) # U(pi/2, 0, pi) = H print(Statevector(qc)) # Statevector([0.70710678+0.j, 0.70710678+0.j], dims=(2,)) text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/u-gate?rev=1781320400&do=diff Unitary gate Unitary gate is the most general single-qubit unitary gate, parameterized by three real angles $\theta$, $\phi$, $\lambda$. Every single-qubit unitary (up to global phase) can be expressed as $U(\theta, \phi, \lambda)$ for some choice of these angles, making it the canonical representation of a single-qubit gate.$$U(\theta, \phi, \lambda) = \begin{pmatrix}\cos\dfrac{\theta}{2} & -e^{i\lambda}\sin\dfrac{\theta}{2}\\[8pt] e^{i\phi}\sin\dfrac{\theta}{2} & e^{i(\phi+\lambda)}\cos\dfrac… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/uchar.h?rev=1781320400&do=diff uchar.h wchar_t seemed like the solution to Unicode, but its encoding is implementation-defined — 16-bit UTF-16 on Windows, 32-bit UTF-32 on Linux — making it useless for writing portable Unicode code. uchar.h (C11) provides char16_t and char32_t text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/unit-test?rev=1781320400&do=diff Unit test A unit test is a test that verifies a single, isolated unit of code — typically a function or a class method — in isolation from its dependencies. The goal is to make the test fast, deterministic, and precise about what it is testing. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/virtual-topologies-mpi?rev=1781320400&do=diff Virtual topologies (MPI) In a 2D stencil on a P×Q process grid, each process needs the ranks of its north, south, east, and west neighbors. Computing those ranks manually requires knowing the grid dimensions and handling wraparound explicitly: int row = rank / Q, col = rank % Q; int north = ((row - 1 + P) % P) * Q + col; int south = ((row + 1) % P) * Q + col; text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/von-neumann-equation-qutip?rev=1781320400&do=diff von Neumann equation (QuTiP) von Neumann equation implementation using QuTiP. QuTiP's mesolve() (master equation solver) integrates the Lindblad master equation. Passing an empty collapse-operator list c_ops=[] disables the dissipator, reducing it exactly to the von Neumann equation:$$\frac{\mathrm{d}\rho}{\mathrm{d}t} = -\frac{i}{\hbar}[H,\,\rho]$$$O$$\langle O\rangle = \mathrm{tr}(\rho\, O)$ text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/von-neumann-equation?rev=1781320400&do=diff von-Neumann equation von-Neumann equation is basically the Schrodinger equation where the quantum state is represented by a density matrix $\rho$, rather than a state vector $\lvert\psi\rangle$. It takes the following form. $$\frac{\mathrm d\rho}{\mathrm dt} = -\frac{i}{\hbar}[H, \rho]$$ Where $H$ is the Hamiltonian and $\rho$ is the density matrix and they both vary in time. Meanwhile $i$$\hbar$$$\dot\rho = -\frac{i}{\hbar}[H, \rho]$$$[H, \rho]$$[H, \rho] = H\rho - \rho H$$$\dot\rho = -\frac… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/vps?rev=1781320400&do=diff VPS What is a VPS? VPS (Virtual private server) is a virtual computer that you can buy from a cloud provider. It can do almost anything a real computer can. A VPS slice is often a virtual machine (QEMU + KVM) given to you by a cloud privder, with GNU/Linux pre-installed. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/vqe?rev=1781320400&do=diff VQE VQE (Variational Quantum Eigensolver) is a hybrid quantum-classical algorithm for estimating the ground state energy of a quantum system. It was proposed in 2014 by Peruzzo et al. and is one of the most important algorithms for near-term NISQ devices, with applications in quantum chemistry and materials science.$\lvert\psi(\boldsymbol\theta)\rangle$$H$$E_0$$$E_0 \leq \langle\psi(\boldsymbol\theta)\rvert H\lvert\psi(\boldsymbol\theta)\rangle$$$\lvert\psi(\boldsymbol\theta)\rangle$$\boldsymbo… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/w-state-qiskit?rev=1781320400&do=diff W state (Qiskit) W state implementation using Qiskit. The three-qubit W state $\lvert W\rangle = (\lvert 001\rangle + \lvert 010\rangle + \lvert 100\rangle)/\sqrt{3}$ is prepared using the recursive decomposition $\lvert W_3\rangle = \frac{1}{\sqrt{3}}\lvert 1\rangle\lvert 00\rangle + \sqrt{\frac{2}{3}}\lvert 0\rangle\lvert W_2\rangle$. import numpy as np from qiskit import QuantumCircuit from qiskit.quantum_info import Statevector qc = QuantumCircuit(3) # Step 1: Ry on q[0] to set amplitud… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/wchar.h?rev=1781320400&do=diff wchar.h If you need to work with non-ASCII text in C — printing accented characters to a terminal, comparing strings that contain Unicode letters, formatting locale-aware dates — you will eventually run into wchar_t. wchar.h (C95) provides wide character I/O and string functions for it: the wide-character mirror of text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/wctype.h?rev=1781320400&do=diff wctype.h isalpha('é') returns 0. The narrow character classification functions in <ctype.h> only handle ASCII (values 0-127); everything else is either wrong or undefined behaviour. wctype.h (C95) is the wide-character counterpart that works on the full Unicode character set via text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/wip-example?rev=1781320400&do=diff (WIP) Work in Progress text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/writing-guide-on-header-files?rev=1781320400&do=diff Writing guide on header files Writing guide on header files is a supplement to the writing guide specifically for articles about C and C++ header files. The goal of a header article is not to replicate documentation. cppreference and the POSIX manual pages already do that exhaustively. An article here should give a programmer a genuine understanding of the header: why it exists, how it feels to use it, what it gets wrong, and how the key parts are implemented under the hood. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/writing-guide-on-parallel-computing?rev=1781320400&do=diff Writing guide on parallel computing Writing guide on parallel computing (this article) covers conventions specific to parallel computing articles in this wiki. Read the general writing guide first; this document supplements it. The central challenge Parallel computing articles fail in one consistent way: they explain what a primitive does before establishing why a C programmer would care. The reader lands on text/html 2026-06-15T08:54:11+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/writing-guide-on-quantum-computing?rev=1781513651&do=diff Writing guide on quantum computing Writing guide on quantum computing (this article) covers conventions specific to quantum computing articles in this wiki. Read General writing guide first for structure, paragraph, and code block conventions that apply across all articles.$H^\dagger = H$$(AB)^\dagger = B^\dagger A^\dagger$$\mathrm{tr}(ABC) = \mathrm{tr}(CAB)$$H$$X$$Y$$Z$$\rho$$[A, B] = AB - BA$$\{A, B\} = AB + BA$$\hbar$$\hbar = 1$$X$$\pi$$x$$L_k\rho L_k^\dagger$$L_k^\dagger\rho L_k$$[H,\rho] … text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/x-gate-cudaq?rev=1781320400&do=diff X gate (CUDA-Q) X gate implementation using CUDA-Q. // Compile: nvq++ main.cpp -o main // Run: ./main #include <cudaq.h> struct kernel { __qpu__ void operator()() { cudaq::qubit q; x(q); // |0> -> |1> mz(q); } }; int main() { auto counts = cudaq::sample(kernel{}); counts.dump(); // 100% |1> } text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/x-gate-custatevec?rev=1781320400&do=diff X gate (cuStateVec) X gate implementation using cuStateVec. // Compile: nvcc main.cu -o main -lcustatevec // Run: ./main #include <stdio.h> #include <cuda_runtime.h> #include <custatevec.h> int main() { const int nQubits = 1; const int dim = 1 << nQubits; cuDoubleComplex h_sv[2] = {{1,0},{0,0}}; // |0> cuDoubleComplex *d_sv; cudaMalloc(&d_sv, dim * sizeof(cuDoubleComplex)); cudaMemcpy(d_sv, h_sv, dim * sizeof(cuDoubleComplex), cudaMemcpyHostToDevice); cust… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/x-gate-qiskit?rev=1781320400&do=diff X gate (Qiskit) X gate implementation using Qiskit. from qiskit import QuantumCircuit from qiskit.quantum_info import Statevector qc = QuantumCircuit(1) qc.x(0) # |0> -> |1> print(Statevector(qc)) # Statevector([0.+0.j, 1.+0.j], dims=(2,)) text/html 2026-06-15T15:23:22+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/x-gate?rev=1781537002&do=diff $X$ gate (NOT gate) X gate (or Pauli-X gate, or quantum NOT gate) is a single-qubit gate that flips $\lvert 0\rangle$ to $\lvert 1\rangle$ and $\lvert 1\rangle$ to $\lvert 0\rangle$, making it the quantum analog of a classical NOT gate. It is one of the three Pauli gates. $$X = \begin{pmatrix}0 & 1\\ 1 & 0\end{pmatrix}$$ The gate flips the computational basis states and leaves the Hadamard basis states ($\lvert +\rangle$$\lvert -\rangle$$$X\lvert 0\rangle = \lvert 1\rangle \qquad X\lvert 1\ra… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/x220-gpu-rendering-with-hyprland?rev=1781320400&do=diff GPU Rendering Artifacts on ThinkPad X220 with Hyprland — Diagnosis & Fix Hardware & Software Context ------ Machine Lenovo ThinkPad X220 (4291SWP) GPU Intel HD Graphics 3000 — Sandy Bridge, Gen 6 Kernel driver i915 Mesa driver crocus (Gallium3D for Intel Gen 4–8) text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/xp-programming?rev=1781320400&do=diff Extreme programming Extreme programming (XP) is a software development methodology introduced by Kent Beck in 1996. The core idea is to take practices that are known to work — testing, code review, frequent releases, collaboration — and push them to their logical extreme. If testing is good, test everything. If code review is good, review code continuously by having two people write it together. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/y-gate-cudaq?rev=1781320400&do=diff Y gate (CUDA-Q) Y gate implementation using CUDA-Q. // Compile: nvq++ main.cpp -o main // Run: ./main #include <cudaq.h> struct kernel { __qpu__ void operator()() { cudaq::qubit q; y(q); // |0> -> i|1>; global phase not observable at measurement mz(q); } }; int main() { auto counts = cudaq::sample(kernel{}); counts.dump(); // 100% |1> } text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/y-gate-custatevec?rev=1781320400&do=diff Y gate (cuStateVec) Y gate implementation using cuStateVec. // Compile: nvcc main.cu -o main -lcustatevec // Run: ./main #include <stdio.h> #include <cuda_runtime.h> #include <custatevec.h> int main() { const int nQubits = 1; const int dim = 1 << nQubits; cuDoubleComplex h_sv[2] = {{1,0},{0,0}}; // |0> cuDoubleComplex *d_sv; cudaMalloc(&d_sv, dim * sizeof(cuDoubleComplex)); cudaMemcpy(d_sv, h_sv, dim * sizeof(cuDoubleComplex), cudaMemcpyHostToDevice); cust… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/y-gate-qiskit?rev=1781320400&do=diff Y gate (Qiskit) Y gate implementation using Qiskit. from qiskit import QuantumCircuit from qiskit.quantum_info import Statevector qc = QuantumCircuit(1) qc.y(0) # |0> -> i|1> print(Statevector(qc)) # Statevector([0.+0.j, 0.+1.j], dims=(2,)) text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/y-gate?rev=1781320400&do=diff Y gate Y gate (or Pauli-Y gate) is a single-qubit gate represented by the second Pauli matrix. It combines the actions of the X gate (bit flip) and the Z gate (phase flip), and additionally multiplies the amplitude by $i$. It is one of the three Pauli gates. $$Y = \begin{pmatrix}0 & -i \\ i & 0\end{pmatrix}$$ The gate maps the computational basis states with an accompanying imaginary factor, and its eigenstates are the $Y$$$Y\lvert 0\rangle = i\lvert 1\rangle \qquad Y\lvert 1\rangle = -i\lver… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/yanevskiv-wiki?rev=1781320400&do=diff Wiki This is a meta article about this website. This website is my creation Challenges Writing this website presents some unique challenges. How do I write down my notes but also make the information useful to the reader? If this website was only about putting my notes, I wouldn't have to use words at all. I could make articles where I just slap some links and call it a day. That would be useful to me, but not to the wider audience. text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/z-gate-cudaq?rev=1781320400&do=diff Z gate (CUDA-Q) Z gate implementation using CUDA-Q. The Z gate applied to $\lvert 0\rangle$ is invisible at measurement (it leaves $\lvert 0\rangle$ unchanged), so this example prepares $\lvert +\rangle$ first, applies Z, then rotates back to the computational basis to confirm the phase flip. // Compile: nvq++ main.cpp -o main // Run: ./main #include <cudaq.h> struct kernel { __qpu__ void operator()() { cudaq::qubit q; h(q); // |0> -> |+> z(q); // |+> -> |-… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/z-gate-custatevec?rev=1781320400&do=diff Z gate (cuStateVec) Z gate implementation using cuStateVec. The Z gate applied to $\lvert 0\rangle$ is invisible at measurement (it leaves $\lvert 0\rangle$ unchanged), so this example prepares $\lvert +\rangle$ first to make the phase flip observable via interference. // Compile: nvcc main.cu -o main -lcustatevec // Run: ./main #include <stdio.h> #include <math.h> #include <cuda_runtime.h> #include <custatevec.h> int main() { const int nQubits = 1; const int dim = 1 << nQubits;… text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/z-gate-qiskit?rev=1781320400&do=diff Z gate (Qiskit) Z gate implementation using Qiskit. The Z gate applied to $\lvert 0\rangle$ is invisible at measurement (it leaves $\lvert 0\rangle$ unchanged), so this example prepares $\lvert +\rangle$ first to make the phase flip observable via interference. from qiskit import QuantumCircuit from qiskit.quantum_info import Statevector qc = QuantumCircuit(1) qc.h(0) # |0> -> |+> qc.z(0) # |+> -> |-> print(Statevector(qc)) # Statevector([ 0.70710678+0.j, -0.70710678+0.j], dims=(2,)) text/html 2026-06-13T03:13:20+00:00 Anonymous (anonymous@undisclosed.example.com) https://yanevskiv.com/z-gate?rev=1781320400&do=diff Z gate Z gate (or Pauli-Z gate, or phase flip gate) is a single-qubit gate that applies a phase flip to the $\lvert 1\rangle$ state while leaving $\lvert 0\rangle$ unchanged. It is the third Pauli gate and a special case of the phase gate with $\phi = \pi$. $$Z = \begin{pmatrix}1 & 0\\ 0 & -1\end{pmatrix}$$ The gate leaves the computational basis states $\lvert 0\rangle$ and $\lvert 1\rangle$ as eigenstates, and swaps the Hadamard basis states:$$Z\lvert 0\rangle = \lvert 0\rangle \qquad Z\lve…