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Sales, marketing and customer service
sales-engagement
outbound sequences and dialer
Outreach, Apollo
System
Techniques · 30
Primitives · 102
sales-engagement
Deterministic simulation testing
distributed · medium: Run the whole system in one thread under a seeded scheduler with simulated network, disk and clock, so every failure replays from its seed.
›
Durable workflow execution
distributed · medium: Ordinary code made crash-proof by logging every side-effect result and replaying the code from that log; activities run at least once, workflow logic must be deterministic.
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Incremental sync cursors
distributed · medium: The client keeps one opaque token; each pull returns only the changes after it plus the next token, so sync cost scales with what changed, not with the dataset.
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Primary-backup replication
distributed · medium: One node orders all writes and ships them to standbys; the acknowledgement rule (none, one, all) sets what a crash loses, and an external arbiter decides who takes over.
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RNN transducer (RNN-T)
machine-learning · medium: Streaming sequence model that sums all monotone alignments in a frame-by-label lattice and conditions each label on earlier ones.
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Text classification
machine-learning · medium: Assign one or more labels to a text; climb a ladder from regexes to linear models on n-grams, embeddings, fine-tuned encoders and LLM prompts, only as far as the data and budget demand.
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Webrtc
media · medium: WebRTC stack: ICE, DTLS-SRTP, RTP, congestion control and data channels for real-time media
Hierarchical timing wheel
systems · low: Timers bucketed by expiry into wheels of slots, coarser wheels for far deadlines: O(1) insert and cancel for millions of timeouts.
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Mime parsing
storage · low: RFC 5322/2045 headers, multipart bodies and transfer encodings
SPF, DKIM and DMARC sender authenti…
networking · low: SPF authorizes sending IPs, DKIM signs the message, DMARC requires either to match the visible From domain and says what to do on failure.
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Token bucket
systems · low: A bucket of up to b tokens refilled at rate r; requests spend tokens, enforcing an average rate while allowing bursts of b.
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E164 number parsing
data-structures · low: Parse, validate and normalize phone numbers to E.164
Hdr histogram
data-structures · low: Log-linear bucketed histograms with bounded relative error for latency percentiles
Process supervision
hardware-os · low: A parent process starts services, reaps their exits and restarts them under a delay and burst-limit policy, then stops them with SIGTERM before SIGKILL.
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Sip signaling
media · low: SIP call setup, dialogs, SDP offer/answer and registrar/proxy routing
Open and click tracking
networking · low: Tracking pixels and signed redirect links for engagement events
Mutual TLS
security · low: TLS where the client also proves a certificate key, so every connection carries a verified, rotatable machine identity.
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OAuth 2 and OpenID Connect
security · low: Delegated authorization (OAuth 2) and login on top of it (OIDC); use authorization code + PKCE for every user-facing client.
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Email threading
storage · low: Threading messages by Message-ID/References (JWZ algorithm) and subject fallback
Business day calendars
systems · low: Holiday calendars, business-day adjustment rules (following, modified following) and settlement cycles
Calendar date arithmetic
systems · low: Civil dates, time zones (tzdb), DST gaps, month-end anchoring and duration math
Graceful draining
systems · low: On a termination signal, stop taking new work, finish in-flight work until a deadline, tell clients to reconnect elsewhere, then exit; clients resume from a token.
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Cooperative scheduling
concurrency · low: Green threads/coroutines yielding at await points on a user-space scheduler
+ 23 more techniques
+ 86 more primitives
System
Techniques · 30
Primitives · 102
sales-engagement
Deterministic simulation testing
distributed · medium: Run the whole system in one thread under a seeded scheduler with simulated network, disk and clock, so every failure replays from its seed.
›
Durable workflow execution
distributed · medium: Ordinary code made crash-proof by logging every side-effect result and replaying the code from that log; activities run at least once, workflow logic must be deterministic.
›
Incremental sync cursors
distributed · medium: The client keeps one opaque token; each pull returns only the changes after it plus the next token, so sync cost scales with what changed, not with the dataset.
›
Primary-backup replication
distributed · medium: One node orders all writes and ships them to standbys; the acknowledgement rule (none, one, all) sets what a crash loses, and an external arbiter decides who takes over.
›
RNN transducer (RNN-T)
machine-learning · medium: Streaming sequence model that sums all monotone alignments in a frame-by-label lattice and conditions each label on earlier ones.
›
Text classification
machine-learning · medium: Assign one or more labels to a text; climb a ladder from regexes to linear models on n-grams, embeddings, fine-tuned encoders and LLM prompts, only as far as the data and budget demand.
›
Webrtc
media · medium: WebRTC stack: ICE, DTLS-SRTP, RTP, congestion control and data channels for real-time media
Bounce processing
networking · medium: Parse DSNs and feedback loops, classify hard/soft bounces, maintain suppression lists
Email delivery smtp
networking · medium: Outbound SMTP: queues, MX lookup, retries, bounces, feedback loops and IP warm-up
Coverage-guided fuzzing
security · medium: Mutate inputs, run an instrumented target, and keep any input that reaches a new code edge; feedback turns blind guessing into incremental search.
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A/B testing (online controlled expe…
statistics · medium: Randomly split units into arms by a salted hash, log exposure, compare metrics with a confidence interval, and guard the result with sample-ratio, guardrail and peeking checks.
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Change data capture
storage · medium: Decode the database's own commit log into row-change events in commit order, with a resume position, so any consumer can mirror the data exactly.
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Point-in-time recovery
storage · medium: Periodic base backups plus a continuously archived change log let you rebuild the state as of any instant, then branch from there.
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Distributed rate limiting
systems · medium: Enforce one rate limit across N servers: a central atomic counter, a limit split per node, or batched token leases, trading exactness against coordination cost.
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Delta debugging
compilers · medium: Ddmin minimization of failing inputs or change sets
Durable job queue
distributed · medium: Persisted tasks that workers lease for a visibility timeout, ack on success and retry with backoff; at-least-once execution that survives worker and queue crashes.
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Fault injection
distributed · medium: Inject latency, errors, kills and partitions via proxies, netem or hooks
Jitter buffer
media · medium: A receiver-side buffer holds packets just long enough, a delay tracked from recent arrivals, to reorder them and hand out media at a steady rate.
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NAT traversal (STUN, ICE, TURN)
media · medium: Learn your public address with STUN, swap candidate addresses over signalling, test pairs with ICE checks, and fall back to a TURN relay if no direct path works.
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Beam search decoding
ml-training · medium: Keep the k best partial sequences by summed log-probability at every step, set finished ones aside, and return the best (n-best) by a length-normalised score.
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Delay based congestion control
networking · medium: One-way delay gradient estimation for real-time media (GCC, transport-cc)
Linear-time regular expression matc…
search-sort · medium: Match a regular expression in O(pattern x text) worst case by tracking all live parse positions at once, then cache the state sets as a lazily built DFA for speed.
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TLS 1.3
security · medium: The standard encrypted, authenticated transport; one round trip, only forward-secret AEAD suites, and little left to misconfigure.
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Hypothesis testing
statistics · medium: Pick a test statistic, compare it to its distribution under "no effect", read off a p-value, and control error rates across one test or many.
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Sequential hypothesis testing
statistics · medium: Group-sequential and alpha-spending designs for interim analyses without inflating error
ARIES recovery
storage · medium: Three-pass crash recovery (analysis, redo, undo) for steal/no-force buffer pools; repeats history, then rolls back losers with compensation log records.
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Packet loss concealment
media · medium: Synthesize missing audio frames from prior signal (pitch repetition, neural PLC)
Syscall interception
security · medium: Intercept syscalls via ptrace, seccomp user notify or a user-space kernel
Sequential probability ratio test
statistics · medium: Add up the log-likelihood ratio per observation and stop when it crosses a fixed bound; the least data of any test with the same error rates (for two point hypotheses).
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Buffer pool
storage · medium: The database's own page cache (frames, page table, pin counts, dirty bits, replacement) so it, not the OS, controls eviction, write order and I/O.
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Hierarchical timing wheel
systems · low: Timers bucketed by expiry into wheels of slots, coarser wheels for far deadlines: O(1) insert and cancel for millions of timeouts.
›
Mime parsing
storage · low: RFC 5322/2045 headers, multipart bodies and transfer encodings
SPF, DKIM and DMARC sender authenti…
networking · low: SPF authorizes sending IPs, DKIM signs the message, DMARC requires either to match the visible From domain and says what to do on failure.
›
Token bucket
systems · low: A bucket of up to b tokens refilled at rate r; requests spend tokens, enforcing an average rate while allowing bursts of b.
›
E164 number parsing
data-structures · low: Parse, validate and normalize phone numbers to E.164
Hdr histogram
data-structures · low: Log-linear bucketed histograms with bounded relative error for latency percentiles
Process supervision
hardware-os · low: A parent process starts services, reaps their exits and restarts them under a delay and burst-limit policy, then stops them with SIGTERM before SIGKILL.
›
Sip signaling
media · low: SIP call setup, dialogs, SDP offer/answer and registrar/proxy routing
Open and click tracking
networking · low: Tracking pixels and signed redirect links for engagement events
Mutual TLS
security · low: TLS where the client also proves a certificate key, so every connection carries a verified, rotatable machine identity.
›
OAuth 2 and OpenID Connect
security · low: Delegated authorization (OAuth 2) and login on top of it (OIDC); use authorization code + PKCE for every user-facing client.
›
Email threading
storage · low: Threading messages by Message-ID/References (JWZ algorithm) and subject fallback
Business day calendars
systems · low: Holiday calendars, business-day adjustment rules (following, modified following) and settlement cycles
Calendar date arithmetic
systems · low: Civil dates, time zones (tzdb), DST gaps, month-end anchoring and duration math
Graceful draining
systems · low: On a termination signal, stop taking new work, finish in-flight work until a deadline, tell clients to reconnect elsewhere, then exit; clients resume from a token.
›
Cooperative scheduling
concurrency · low: Green threads/coroutines yielding at await points on a user-space scheduler
Record replay
distributed · low: Record every nondeterministic input (time, randomness, I/O, model and tool outputs, per-tick player input) and re-execute deterministically for debugging, replays and state rebuild
PRNGs (PCG, xoshiro)
numerics · low: Small, fast, statistically strong non-cryptographic generators: PCG32 and xoshiro256**, with seeding, parallel streams and unbiased ranges.
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Event sourcing
finance · low: Persist state changes as an append-only stream of immutable events per entity; current state is a fold over them, guarded by expected-version appends.
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Message sequence numbers
distributed · low: A per-stream counter on every message lets the receiver detect loss, duplicates and reordering, ack cheaply and resume after reconnect with "send from N".
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Cursor pagination
networking · low: Keyset/opaque-cursor pagination for stable incremental reads from APIs and tables
Write-ahead log
storage · low: Append each change to a checksummed sequential log and fsync it before touching data pages; the basis of crash durability in nearly every database.
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Leases
distributed · low: A lock that expires on its own: bounded-time ownership that survives crashes and partitions, given bounded clock drift and fencing tokens on writes.
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Health checking
systems · low: Decide per backend whether to route to it using probes and real-traffic errors, with consecutive-count hysteresis, capped ejection and a panic fallback.
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Recurrent networks
machine-learning · low: GRU and LSTM cells for streaming sequence models
Short-time Fourier transform and sp…
signal-processing · low: Slide a window over a signal and FFT each frame to get a time-frequency picture; the front end of audio ML, vibration and radar analysis.
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Byte-pair encoding (BPE) tokenizati…
machine-learning · low: Learn a subword vocabulary by repeatedly merging the most frequent adjacent pair; byte-level BPE encodes any input losslessly with 32k-200k tokens.
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Calibration
machine-learning · low: Make predicted probabilities match observed frequencies: measure with ECE, fix post hoc with temperature, Platt or isotonic scaling on held-out data.
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Linear and logistic regression
machine-learning · low: Fit a weighted sum of features to a number (least squares) or a probability (logistic); the baseline every model must beat.
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Transformer block
ml-training · low: The decoder-only pre-norm block (attention plus MLP on a residual stream) behind every modern LLM, with exact shapes, parameter, FLOP and KV-cache counts.
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RTP and RTCP
media · low: RTP stamps each media packet with sequence number, media-clock timestamp and source ID; RTCP reports loss, jitter, round trip and clock mapping within a bandwidth cap.
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Sdp offer answer
media · low: SDP session descriptions and offer/answer negotiation
SRTP
media · low: Per-packet encryption, authentication and replay protection for RTP and RTCP that keeps the headers readable, with the packet index derived from the 16-bit sequence number.
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Sctp protocol
networking · low: SCTP multi-streaming, multi-homing and message framing for telecom signalling
Smtp protocol
networking · low: SMTP/ESMTP sessions, pipelining, STARTTLS and reply codes
Sliding-window counters
systems · low: Count events in the last W seconds with a ring of sub-bucket counters; the two-bucket weighted estimate gives rate limits in 16 bytes per key.
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DNS message format and client looku…
networking · low: Binary query/response format of DNS (RFC 1035) and how a stub client uses it: record types, name compression, UDP/TCP/EDNS sizes, TTL caching.
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Exponential backoff with jitter
systems · low: Retry after delays that grow geometrically and are randomized, so failed clients spread out instead of hammering a recovering service in synchronized waves.
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Code coverage instrumentation
compilers · low: Insert counters into compiled code to record which blocks, edges or lines ran; a spanning tree of the control-flow graph cuts the counters needed.
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Fast hash functions
data-structures · low: Non-cryptographic hashes built from 64×64→128-bit multiplies and xor-shifts; pick by key size, trust level and whether the hash is persisted.
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Sanitizers
security · low: ASan/UBSan/TSan shadow memory and instrumentation to catch memory bugs
Bootstrap confidence intervals
statistics · low: Resampling-based confidence intervals for arbitrary statistics
CUPED (variance reduction with pre-…
statistics · low: Subtract the part of each unit's outcome that a pre-experiment covariate predicts; the effect estimate stays unbiased and its variance shrinks by 1 - rho^2.
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Experiment layering (overlapping ex…
statistics · low: Partition the tunable parameters into layers, hash each unit with a per-layer salt so layers are independent, and give experiments inside a layer disjoint bucket ranges.
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Deterministic bucketing
systems · low: Hash a stable unit id with a per-purpose salt into a bucket; "in" means bucket below the percentage, so assignment is stateless, repeatable and only grows on ramp-up.
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Consumer offsets
distributed · low: Each consumer group stores one integer per partition, the next position to read, so a restarted consumer resumes where it left off with at-least-once delivery.
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Object storage
cloud · low: S3-compatible blob storage at cents per GB-month with 11 nines durability; pick by egress price, request price and latency, not by storage price alone.
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Copy on write snapshots
storage · low: Point-in-time snapshots by sharing pages/extents and copying only on write
Consistent hashing
distributed · low: Hash nodes and keys onto a ring; each key belongs to the next node clockwise, so membership changes move only about 1/n of keys.
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GCRA
systems · low: Generic Cell Rate Algorithm: a token bucket stored as one theoretical arrival time per key, updated with one compare-and-swap; exact limits in 8 bytes.
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Failure bisection
search-sort · low: Binary search over commits, batches or inputs to isolate a failure
Dead-letter queue
distributed · low: A side queue for messages that keep failing: after N delivery attempts or a permanent error, park them with the reason so the main queue keeps moving.
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Idempotency keys
finance · low: Client-chosen unique key per logical operation, stored with a request fingerprint and the final response, so retries replay the first result instead of acting twice.
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Transparent interception
networking · low: Redirect traffic to a local proxy (iptables TPROXY/REDIRECT, eBPF) without app changes
A/V sync and media clocks
media · low: How audio and video stay lip-synced: per-stream timebases, PTS/DTS, PCR and RTCP clock mapping, and a master clock that slave streams chase by dropping, repeating or resampling.
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Exponential smoothing (ETS, Holt-Wi…
statistics · low: Forecast by keeping a few running states (level, trend, seasonal) updated by a fraction of each forecast error; O(1) memory per series and a strong baseline.
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Network address translation
networking · low: NAT mapping tables, endpoint-independent mapping/filtering, hairpinning and port allocation
Message authentication codes (HMAC,…
security · low: A keyed short tag proving a message is unmodified and came from a key holder; pick HMAC, CMAC, KMAC or a one-time polynomial MAC by what primitive you already have.
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d-ary heap
data-structures · low: Array heap with d children per node (d = 4 typical): half the binary heap's depth, siblings in one cache line, faster priority queues.
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Top-k selection
search-sort · low: Keep the k best of n items without sorting all n - bounded min-heap for streams, quickselect for arrays, threshold pruning to skip most items.
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Kalman filter
robotics · low: Optimal recursive estimator for a linear system with Gaussian noise: fuses a motion model with noisy measurements into a state estimate plus its covariance.
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Recursive-descent parsing
compilers · low: Hand-written top-down parser with one function per grammar rule; simple, fast, with full control over error messages and recovery.
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Aho-Corasick multi-pattern matching
search-sort · low: Find all occurrences of many fixed patterns in one pass over the text, in time independent of the pattern count, using a trie with failure links.
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SIMD substring search
search-sort · low: Find bytes or substrings by comparing 16-64 haystack positions per instruction against the needle's first and last byte, then verifying the rare candidates.
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LRU and CLOCK
systems · low: LRU evicts the least recently used entry via hash map plus linked list; CLOCK approximates it with a reference bit, so hits take no lock.
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AEAD (AES-GCM, ChaCha20-Poly1305)
security · low: Authenticated encryption that encrypts and detects tampering in one call; the only kind of symmetric encryption you should use.
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Cryptographic hashes (SHA-2, BLAKE3)
security · low: Collision-resistant fingerprints of data; SHA-256 for interoperability, BLAKE3 for speed, HMAC/HKDF for keyed uses.
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ECDH key exchange (X25519, P-256)
security · low: Each side multiplies the other's public curve point by its own scalar and both get the same point; always validate the peer key and run the result through a KDF.
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HKDF key derivation (extract-and-ex…
security · low: HMAC-based two-step key derivation: extract a uniform pseudorandom key from any secret, then expand it into as many context-bound keys as you need.
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X509 certificates
security · low: Certificate chains, path validation, SANs, revocation and short-lived issuance
Wilson score interval
statistics · low: Wilson lower bound for ranking by up/down votes and proportions with few samples
Bayesian rate estimation
statistics · low: Estimate a conversion or event rate from few observations with a Beta-Binomial or Gamma-Poisson update: two counters, an honest interval, and P(B beats A) directly.
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Linear predictive coding (LPC)
media · low: Predict each sample from the previous p; code only the small prediction error plus p coefficients. Core of FLAC, speech codecs and the Opus SILK layer.
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Ebpf
hardware-os · low: Verified in-kernel programs on hooks (kprobes, XDP, tc) with maps
Ptrace process control
hardware-os · low: Ptrace: attach, read and write registers and memory, single-step, intercept signals
Seccomp bpf
security · low: Syscall filtering with seccomp BPF programs
Loglikelihood scoring
machine-learning · low: Scoring choices by summed token log-probabilities, normalization by length, perplexity computation
fsync, O_DSYNC and O_DIRECT
hardware-os · low: What fsync, fdatasync, O_DSYNC, O_DIRECT and F_FULLFSYNC really guarantee on Linux and macOS, and the exact call sequences for crash-safe files.
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S3-FIFO
systems · low: Three FIFO queues (small, main, ghost) that evict one-hit wonders quickly and keep reused objects; low miss ratios with no list moves on hits.
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Control-flow graph
compilers · low: Split a linear instruction list into basic blocks and connect them by jumps; the graph every optimizer, SSA builder and dataflow pass walks.
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Fencing tokens
distributed · low: A number that grows with every lock grant and is checked by the protected resource, so a paused ex-holder's late write is rejected.
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Segmented log
distributed · low: Split an append-only log into files named by their first offset, write only the newest, and find any record by binary search plus a sparse index.
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BFS and DFS
graphs · low: The two basic graph traversals; BFS gives hop distances and levels, DFS gives discovery/finish order for cycles, components and topological order.
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Minimum spanning tree
graphs · low: Cheapest set of edges connecting all vertices; Kruskal sorts edges and joins components with union-find, Prim grows one tree, Boruvka merges in parallel rounds.
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Union-find
graphs · low: Disjoint-set forest with union by size and path compression; merges sets and answers same-set queries in effectively constant time.
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CRC32C checksums
hardware-os · low: The Castagnoli CRC-32 with dedicated instructions on x86 (SSE4.2) and ARMv8. It is the standard fast integrity check for storage blocks, logs and network frames.
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SIMD programming
hardware-os · low: Process 4-16 lanes per instruction with AVX2, AVX-512 or NEON via Rust std::arch, with runtime dispatch and a portable fallback.
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Loss functions
machine-learning · low: The scalar a model minimizes: cross-entropy for classes and tokens, MSE or Huber for values, each with a stable form and simple gradient.
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Text normalization
machine-learning · low: Rewrite messy text into one canonical form for matchers and models, or into spoken words for speech output (and back), keeping a map to the original offsets.
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Media containers
media · low: MP4/ISO BMFF, fragmented MP4/CMAF, Matroska and MPEG-TS mux/demux
Fast Fourier transform
numerics · low: Computes the DFT in O(n log n) by recursively splitting into even and odd halves; the basis of fast convolution and spectral methods.
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L-BFGS
numerics · low: Quasi-Newton minimizer that approximates the inverse Hessian from the last m gradient differences via the two-loop recursion; O(m·n) memory and work per step.
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LU and Cholesky factorization
numerics · low: Factor A = PLU (any square matrix, partial pivoting) or A = LLᵀ (SPD) once in O(n³), then solve each right-hand side in O(n²).
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Sparse matrices (CSR)
numerics · low: Compressed sparse row stores only nonzeros as row pointers, column indices and values; SpMV streams them once, so it is memory-bandwidth bound.
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Scalar quantization
search-sort · low: Per-dimension int8/binary quantization of vectors with rescoring
Constant time code
security · low: Writing secret-dependent code without branches or memory lookups on secrets; constant-time compare and select
Csprng
security · low: OS-backed cryptographically secure random numbers (getrandom), and when a fast PRNG is not enough
JSON Web Token (JWT)
security · low: A signed, URL-safe token carrying claims; the verifier's job is the checklist (pinned algorithm, key by kid, exp, aud, iss), not the format.
›
Biquad filters
signal-processing · low: Second-order IIR sections (RBJ cookbook): low/high pass, peaking EQ; stable cascades
IIR and FIR filters
signal-processing · low: Two ways to shape a sampled signal's spectrum: FIR (finite taps, always stable, exact linear phase) or IIR (feedback, a few coefficients, cheap but phase-distorting).
›
Optimistic concurrency control
storage · low: Run transactions without locks, record what they read, and validate at commit that nothing changed; fastest when conflicts are rare.
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Slab allocator
systems · low: Pre-carve pages into equal-size slots per size class with an in-place free list: O(1) alloc and free, no external fragmentation, cache-warm reuse.
›
Copy for AI
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