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fitness-tracking

fitness tracking

Strava

SystemTechniques · 6Primitives · 33 fitness-tracking Fan-out on writedistributed · medium: Append each new item id to every follower's precomputed, capped list so a read is one fetch; pull the few huge authors at read time instead.›Incremental sync cursorsdistributed · 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.›Map matching hmmgeometry · medium: HMM/Viterbi map matching of GPS traces onto the road graphDynamic time warpingsignal-processing · medium: DTW alignment of time series and trajectories of different speedDurable job queuedistributed · 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.›LSM-treestorage · medium: Buffer writes in memory, flush them as immutable sorted files and merge in the background; turns random writes into sequential I/O.›Geodesic distance (haversine, Vince…geometry · low: Shortest-path distance between two lat/lon points on the Earth: haversine on a sphere (about 0.5% error), Vincenty or Karney on the WGS84 ellipsoid (sub-millimetre).›R-treegeometry · low: A balanced tree of nested bounding boxes that indexes rectangles and points, answering window and nearest-neighbour queries by pruning whole subtrees.›Delta encodingcompression · low: Store differences between consecutive values instead of the values, turning sorted IDs, timestamps and counters into small numbers that pack into few bits.›Gorilla XOR compressioncompression · low: Time-series codec: delta-of-delta timestamps plus XOR of consecutive float bit patterns, storing only changed bits; about 1.4 bytes per point.›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.›Skip listdata-structures · low: Sorted linked list with random express lanes; O(log n) search, simple inserts, and easy lock-free or single-writer concurrent reads.›Line simplificationgeometry · low: Douglas-Peucker and Visvalingam polyline simplificationPoint in polygongeometry · low: Ray-casting/winding tests and indexed polygon lookup for reverse geocodingKalman filterrobotics · 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.›IIR and FIR filterssignal-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).›Range partitioningdistributed · low: Cut the sorted key space into contiguous half-open ranges kept in a boundary map; split hot or big ranges, merge small ones, move ranges between nodes.›Message sequence numbersdistributed · 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".›Cursor paginationnetworking · low: Keyset/opaque-cursor pagination for stable incremental reads from APIs and tablesWrite-ahead logstorage · 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.›Dijkstra's algorithmgraphs · low: Single-source shortest paths with non-negative edge weights by settling vertices in order of distance with a priority queue.›Convolutional codes and the Viterbi…signal-processing · low: Streaming error-correcting code: each output bit is a parity of the last K input bits; the Viterbi decoder finds the most likely path through a 2^(K-1)-state trellis.›+ 17 more primitives

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