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Fix #625 - no fancy mixing in number hasing 

Just hash upper 32 bits with lower 32 bits. Trying to get too fancy
was causing slowdowns in very trivial cases. Assuming that all
combinations of 64 bits in a double are equally likely (suspect but
probably not that incorrect), the obvious method of xoring the top
32 bits with the lower 32 bits gives a uniform distribution.
This commit is contained in:
Calvin Rose 2021-02-04 19:37:11 -06:00
parent 54c66ecfc0
commit b168b0758a
1 changed files with 4 additions and 12 deletions
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16
src/core/value.c
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@ -307,18 +307,10 @@ int32_t janet_hash(Janet x) {
hash = janet_struct_hash(janet_unwrap_struct(x));
break;
case JANET_NUMBER: {
double num = janet_unwrap_number(x);
if (isnan(num) || isinf(num) || num == 0) {
hash = 0;
} else {
hash = (int32_t)num;
hash = ((hash >> 16) ^ hash) * 0x45d9f3b;
hash = ((hash >> 16) ^ hash) * 0x45d9f3b;
hash = (hash >> 16) ^ hash;
uint32_t lo = (uint32_t)(janet_u64(x) & 0xFFFFFFFF);
hash ^= lo + 0x9e3779b9 + (hash << 6) + (hash >> 2);
}
uint64_t i = janet_u64(x);
uint32_t lo = (uint32_t)(i & 0xFFFFFFFF);
uint32_t hi = (uint32_t)(i >> 32);
hash = (int32_t)(hi ^ (lo >> 3));
break;
}
case JANET_ABSTRACT: {