The reverse index walk dedupes seen NeedleIds with a BoomFilters bloom
filter. Its default kernel is FNV-1, whose final step only XORs the last
input byte into the hash's low bits, so keys sharing the high 7 bytes of
a big-endian NeedleId land on correlated probe positions and false
positives arrive in bursts of hundreds instead of the ~0.001 target
rate. Each false positive hides a live-latest key and inflates the
rebuilt deletion counters, which is what made
TestFastLoadingNeedleMapMetrics flake past its +256 bound in CI
(observed DeletedCount drift of 267).
Swap the kernel for xxhash, which fully diffuses the key bytes; the
observed false-positive count in 300 local runs of the test drops to
max 2 (avg 0.23).
Chris Lu
ab559bca76