Changelog of @hackage/data-sketches-core 0.2.0.0

Changelog for data-sketches-core

KLL Sketch (DataSketches.Quantiles.KLL.Internal): Quantiles sketch with additive error bounds. Implemented entirely in C for performance — 26-41x faster than the initial Haskell version, faster than Java DataSketches at small-to-medium N.
HyperLogLog (DataSketches.Distinct.HyperLogLog.Internal): Cardinality estimation using hash-based registers and harmonic mean. C implementation with ldexp-based estimate (replacing the Haskell (^^) which compiled to Integer exponentiation).
Theta Sketch (DataSketches.Distinct.Theta.Internal): Distinct counting with set operations (union, intersection, difference). C implementation with sorted entries for O(log k) duplicate checks.
Count-Min Sketch (DataSketches.Frequencies.CountMin.Internal): Frequency estimation. C implementation with Lemire fast modular reduction (128-bit widening multiply instead of 64-bit division).

Off-by-one in getCountWithCriterion: When spaceAtBottom=False, the binary search upper bound was count_ instead of count_ - 1, causing reads past the active buffer region into uninitialized memory. This produced intermittent incorrect rank calculations in LowRanksAreAccurate mode.

C implementations (cbits/): KLL, HLL, CountMin, and Theta sketches are implemented as C structs with all operations in C. Zero GC pressure, zero monadic bind overhead, zero dictionary passing.
Packed mutable fields: REQ sketch's ReqSketch, ReqCompactor, and DoubleBuffer pack all mutable scalar fields into single MutableByteArrays sized to fit within one 64-byte cache line.
INLINE pragmas: Added to all URef operations, DoubleBuffer field accessors, and hot-path functions to eliminate dictionary passing overhead.
C optimization details:
- Cached total_capacity in KLL struct (avoids recomputing pow(2/3, depth) per insert)
- Insertion sort for small arrays (≤32 elements) in KLL compaction
- Branchless register update and merge in HLL
- Lemire fast modular reduction in CountMin (128-bit multiply, ~4 cycles vs ~30 for division)
- Binary search for duplicate detection in Theta (O(log k) vs O(k))
- xoshiro256++ RNG (32 bytes state vs mwc-random's 1032 bytes)