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Add regression coverage for low-bit hash dispersion

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copilot-swe-agent[bot] 2026-07-14 01:42:41 +00:00 • committed by GitHub
parent 0452362e9f
commit 55a5af1f93
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1 changed files with 11 additions and 2 deletions
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13
src/test/chashtable.cpp
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@ -176,17 +176,26 @@ static void tst6() {
static void tst_combine_hash_low_bits() {
constexpr unsigned num_buckets = 1 << 12;
constexpr unsigned mask = num_buckets - 1;
constexpr unsigned seed = 0x12345678u;
constexpr unsigned alignment_shift = 12;
// This threshold is intentionally conservative: with 65,536 (2^16) samples over
// 4,096 (2^12) buckets we expect near-complete coverage under good mixing;
// requiring >=73% still robustly detects low-bit clustering while keeping
// the test tolerant.
constexpr unsigned min_covered_buckets = (num_buckets * 73) / 100;
std::array<bool, num_buckets> seen{};
unsigned num_seen = 0;
for (unsigned i = 0; i < (1u << 16); ++i) {
unsigned h = combine_hash(i << 12, 0x12345678u);
// Probe low-bit dispersion for aligned first components against a fixed
// non-zero second component.
unsigned h = combine_hash(i << alignment_shift, seed);
unsigned b = h & mask;
if (!seen[b]) {
seen[b] = true;
++num_seen;
}
}
ENSURE(num_seen > 3000);
ENSURE(num_seen > min_covered_buckets);
}
void tst_chashtable() {