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session-replay
session replay and heatmaps
FullStory, PostHog
System
Techniques · 26
Primitives · 88
session-replay
CDN and edge
cloud · medium: CDNs cache content in hundreds of PoPs near users, cutting latency and egress bills; edge compute runs small code there. Bandwidth price spreads 10x between providers.
›
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.
›
Partitioned commit log
distributed · medium: A topic split into replicated append-only partitions: key hash keeps per-key order, offsets give position, and a high watermark limits readers to fully replicated records.
›
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.
›
Session replay
graphics · medium: Record DOM snapshots and mutations to replay user sessions
Bot detection
security · medium: Combine declared identity, rate and timing, network, behaviour and integrity signals into one score per client, then map thresholds to allow, log, limit, challenge or block.
›
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.
›
Deflate
compression · low: DEFLATE/gzip: LZ77 + Huffman, the format behind HTTP gzip, zip and PNG
Sessionization
storage · low: Group events into sessions by inactivity gaps
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.
›
User agent parsing
search-sort · low: Parse user-agent strings and client hints into browser, OS and device
Source maps
compilers · low: Source map v3: VLQ-encoded mappings from generated to original positions
Hdr histogram
data-structures · low: Log-linear bucketed histograms with bounded relative error for latency percentiles
HyperLogLog
data-structures · low: Counts distinct elements in kilobytes with about 1 % error by tracking the longest run of leading zero bits in hashed values per bucket.
›
Event deduplication window
distributed · low: Keep the ids of recently accepted events for a time window and drop repeats; memory stays bounded by rotating time buckets instead of per-entry timers.
›
Browser performance apis
graphics · low: PerformanceObserver, navigation/resource timing, LCP/INP/CLS measurement
Dom event delegation
graphics · low: One listener at the root dispatching by target path; synthetic events and passive listeners
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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Stack trace fingerprinting
search-sort · low: Normalize and hash stack frames to group crashes
Mutual TLS
security · low: TLS where the client also proves a certificate key, so every connection carries a verified, rotatable machine identity.
›
Json streaming parser
storage · low: Streaming/zero-copy JSON parsing and in-place field rewriting
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.
›
HTTP caching semantics
networking · low: The rules (RFC 9111) that decide whether a stored response is still fresh, and how a cheap conditional request revalidates it when it is not.
›
+ 19 more techniques
+ 72 more primitives
System
Techniques · 26
Primitives · 88
session-replay
CDN and edge
cloud · medium: CDNs cache content in hundreds of PoPs near users, cutting latency and egress bills; edge compute runs small code there. Bandwidth price spreads 10x between providers.
›
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.
›
Partitioned commit log
distributed · medium: A topic split into replicated append-only partitions: key hash keeps per-key order, offsets give position, and a high watermark limits readers to fully replicated records.
›
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.
›
Session replay
graphics · medium: Record DOM snapshots and mutations to replay user sessions
Bot detection
security · medium: Combine declared identity, rate and timing, network, behaviour and integrity signals into one score per client, then map thresholds to allow, log, limit, challenge or block.
›
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.
›
Data masking
security · medium: Hide sensitive field values from people and systems that don't need them, by masked copies or role-based masking at read time, while keeping joins and formats working.
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Columnar storage
storage · medium: Store each column contiguously so analytic scans read only the columns they need, compress well and run as tight SIMD loops.
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Funnel analysis
storage · medium: Ordered multi-step conversion within a time window per user, computed in one pass
Inverted index
storage · medium: Map each term to a sorted, compressed list of the documents containing it; the core of full-text search and other fast set queries.
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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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Telemetry sampling
systems · medium: Head, tail and dynamic sampling of traces, events and logs to a budget
Delta debugging
compilers · medium: Ddmin minimization of failing inputs or change sets
Fault injection
distributed · medium: Inject latency, errors, kills and partitions via proxies, netem or hooks
Gradient-boosted decision trees (GB…
machine-learning · medium: Add small regression trees one at a time, each fitted to the loss gradient; histogram splits over 256 bins make it the strongest tabular default.
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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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Behavioral biometrics
security · medium: Build a per-user profile of input timing and motion, score each new session by distance to it, and feed the result into a risk score instead of a verdict.
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Device fingerprinting
security · medium: Collect stable and volatile device signals, hash the stable core for exact lookup, fuzzy-match the rest with drift tolerance, and feed the result into a risk score.
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Reputation scoring
security · medium: Turn per-entity good/bad event counts into a trust score that forgets old evidence, borrows strength from parent groups for new keys, and maps to allow, challenge or block.
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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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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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Vectorized execution
storage · medium: Query operators pass batches of about 1-2K values per column instead of single tuples, amortizing interpretation overhead and enabling tight SIMD loops.
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Syscall interception
security · medium: Intercept syscalls via ptrace, seccomp user notify or a user-space kernel
Dynamic time warping
signal-processing · medium: DTW alignment of time series and trajectories of different speed
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.
›
Deflate
compression · low: DEFLATE/gzip: LZ77 + Huffman, the format behind HTTP gzip, zip and PNG
Sessionization
storage · low: Group events into sessions by inactivity gaps
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.
›
User agent parsing
search-sort · low: Parse user-agent strings and client hints into browser, OS and device
Source maps
compilers · low: Source map v3: VLQ-encoded mappings from generated to original positions
Hdr histogram
data-structures · low: Log-linear bucketed histograms with bounded relative error for latency percentiles
HyperLogLog
data-structures · low: Counts distinct elements in kilobytes with about 1 % error by tracking the longest run of leading zero bits in hashed values per bucket.
›
Event deduplication window
distributed · low: Keep the ids of recently accepted events for a time window and drop repeats; memory stays bounded by rotating time buckets instead of per-entry timers.
›
Browser performance apis
graphics · low: PerformanceObserver, navigation/resource timing, LCP/INP/CLS measurement
Dom event delegation
graphics · low: One listener at the root dispatching by target path; synthetic events and passive listeners
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.
›
Stack trace fingerprinting
search-sort · low: Normalize and hash stack frames to group crashes
Mutual TLS
security · low: TLS where the client also proves a certificate key, so every connection carries a verified, rotatable machine identity.
›
Json streaming parser
storage · low: Streaming/zero-copy JSON parsing and in-place field rewriting
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.
›
HTTP caching semantics
networking · low: The rules (RFC 9111) that decide whether a stored response is still fresh, and how a cheap conditional request revalidates it when it is not.
›
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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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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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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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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Zero-copy I/O
networking · low: Move bytes between files, sockets and NICs by passing page references instead of copying through user space: sendfile, splice, MSG_ZEROCOPY, kTLS.
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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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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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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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Snapshot delta compression
game-networking · low: Encode each client's snapshot as the difference from the last snapshot that client acknowledged, so unchanged entities and fields cost zero to a few bits.
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Proof of work challenge
security · low: Client puzzles (hashcash-style) that cost a client CPU to raise abuse cost, with adjustable difficulty
Tls fingerprinting
security · low: JA3/JA4 fingerprints of ClientHello parameters to identify clients
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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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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Sanitizers
security · low: ASan/UBSan/TSan shadow memory and instrumentation to catch memory bugs
Format-preserving encryption
security · low: Encrypt a digit or character string into another string of the same length and alphabet with a keyed Feistel network, reversible with the key and a context tweak, without a lookup table.
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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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Bit packing and PFOR
compression · low: Store blocks of integers with exactly b bits each and patch rare outliers separately (PFOR); the fastest integer codec family, billions of values per second.
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Delta encoding
compression · low: Store differences between consecutive values instead of the values, turning sorted IDs, timestamps and counters into small numbers that pack into few bits.
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Dictionary encoding
compression · low: Map values to small integer codes via a dictionary for compact, comparable columns
Zone maps
storage · low: Per-block min/max (and sparse primary) indexes to skip data during scans
Roaring bitmaps
compression · low: Compressed set of 32-bit integers in 64K-value chunks, each a sorted array, bitmap or run list; fast intersections, the standard for bitmap indexes.
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Window functions
storage · low: ROW_NUMBER, rank, running and frame aggregates via sort and segment trees
Varint (LEB128)
compression · low: Variable-length integers in 7-bit groups with a continuation bit; small numbers take 1 byte, the wire format of Protobuf, WebAssembly and DWARF.
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BM25 ranking
search-sort · low: The standard lexical relevance score: IDF-weighted term frequency with saturation (k1) and length normalization (b); default in Lucene, Elasticsearch and SQLite FTS5.
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Sorted-list intersection
search-sort · low: Intersect sorted id lists by merging, galloping from the short list into the long one, and skipping whole compressed blocks, shortest list first.
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Copy on write snapshots
storage · low: Point-in-time snapshots by sharing pages/extents and copying only on write
Hash based sampling
data-structures · low: Deterministic sampling by hashing an entity id so all its events are kept or dropped together
Reservoir sampling
data-structures · low: Uniform k-sample from a stream of unknown length (Algorithm R/L)
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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Failure bisection
search-sort · low: Binary search over commits, batches or inputs to isolate a failure
Transparent interception
networking · low: Redirect traffic to a local proxy (iptables TPROXY/REDIRECT, eBPF) without app changes
Quantile sketches
data-structures · low: Mergeable quantile sketches: t-digest, DDSketch, HDR histograms
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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Regularization
machine-learning · low: Techniques that trade training fit for generalization: weight decay, dropout, early stopping, label smoothing and data mixing, with the exact update rules and defaults.
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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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Isolation forest
machine-learning · low: Score each record by how few random axis-aligned splits it takes to isolate it in a small forest of random trees; anomalies are few and different, so they isolate fast.
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Robust statistics
statistics · low: Location and scale estimates (median, MAD, trimmed mean, Huber M-estimator) that stay correct when a minority of the data is garbage.
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Fuzzy hashing
security · low: Similarity-preserving file hashes (ssdeep CTPH, TLSH) to find variants of known files
Time decay scoring
search-sort · low: Scoring items by recency with exponential decay or half-life, combined with relevance
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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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
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
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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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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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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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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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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Bitstream serialization
game-networking · low: A bit writer and reader over a byte buffer that stores each field in exactly the bits its range needs: booleans in 1 bit, bounded integers in ceil(log2(span+1)) bits.
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Reliable UDP
game-networking · low: Transport over UDP: each packet carries a sequence number, an ack and a 32-bit ack history; only chosen messages are made reliable.
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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.
›
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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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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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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Unicode text tokenization for search
search-sort · low: Turn multilingual text into index terms: segment words (UAX 29 style, CJK bigrams), normalize, case-fold and accent-fold, identically at index and query time.
›
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
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