The SHA256 pool used ArrayBlockingQueue.take()/put() which acquire a lock
on every call. Profiling showed ~10µs synchronization overhead per SHA256
for ~2µs of actual hashing — the lock cost 5× more than the hash itself.
Replaced with ThreadLocal<MessageDigest> which gives each thread its own
instance with zero synchronization. MessageDigest.digest() implicitly
resets state, so no explicit reset is needed.
The pool is kept available for streaming use cases (hashStream) that need
incremental hashing with update()/digest() across multiple calls.
Only affects jvmAndroid — Apple uses CC_SHA256 directly, Linux uses
whyoleg CryptographyProvider. Both already have no pool overhead.
https://claude.ai/code/session_01BhU63WUe9AhikZxRdw3Lpg
Claude
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