The reference port recomputed the inverse DCT cosine tables once per output
pixel — for a 32x24 decode that's ~43k redundant cos() calls. This change
lifts the tables out of the inner loop and caches them across decodes
keyed by (size, componentCount) so subsequent placeholders at the same
target size skip cosine evaluation entirely.
Additional wins:
- AC coefficients unpack into pre-sized DoubleArrays (no ArrayList<Double>
boxing, no post-hoc copy)
- truncated hashes are rejected up-front via a single length check instead
of mid-stream
- LPQA -> sRGB uses an inline branch clamp instead of a
min/max/round/coerceIn chain
Tests: 12 unit tests cover determinism across repeated decodes, cache
clearing, alpha preservation, aspect-ratio preservation both directions,
average-color drift bounds, truncated input rejection, and round-tripping
base64 vs. raw bytes. All green.
Bench: new ThumbHashBenchmark exercises opaque decode, alpha decode,
warm- and cold-cache decode, aspect-ratio probe, and the full
decodeKeepAspectRatio pipeline so the perf delta is visible on device.
Claude
63da850e3b