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Code Issues 1 Releases Wiki Activity

Improve pointer hashing to avoid hash collisions.

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This commit is contained in:
Calvin Rose 2022-08-26 12:18:10 -05:00
parent a575f5df36
commit 99f63a41a3
1 changed files with 11 additions and 5 deletions
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16
src/core/value.c
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@ -295,6 +295,15 @@ int janet_equals(Janet x, Janet y) {
return 1; return 1;
} }
static uint64_t murmur64(uint64_t h) {
h ^= h >> 33;
h *= 0xff51afd7ed558ccdUL;
h ^= h >> 33;
h *= 0xc4ceb9fe1a85ec53UL;
h ^= h >> 33;
return h;
}
/* Computes a hash value for a function */ /* Computes a hash value for a function */
int32_t janet_hash(Janet x) { int32_t janet_hash(Janet x) {
int32_t hash = 0; int32_t hash = 0;
@ -341,11 +350,8 @@ int32_t janet_hash(Janet x) {
default: default:
if (sizeof(double) == sizeof(void *)) { if (sizeof(double) == sizeof(void *)) {
/* Assuming 8 byte pointer (8 byte aligned) */ /* Assuming 8 byte pointer (8 byte aligned) */
uint64_t i = janet_u64(x); uint64_t i = murmur64(janet_u64(x));
uint32_t lo = (uint32_t)(i & 0xFFFFFFFF); hash = (int32_t)(i >> 32);
uint32_t hi = (uint32_t)(i >> 32);
uint32_t hilo = (hi ^ lo) * 2654435769u;
hash = (int32_t)((hilo << 16) | (hilo >> 16));
} else { } else {
/* Assuming 4 byte pointer (or smaller) */ /* Assuming 4 byte pointer (or smaller) */
uintptr_t diff = (uintptr_t) janet_unwrap_pointer(x); uintptr_t diff = (uintptr_t) janet_unwrap_pointer(x);