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Improve the bucket distribution of SDL_HashTable

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SDL_HashID does no hashing, which isn't good if the lower bits of the key aren't evenly distributed.
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Brick
2024-07-09 19:16:58 +01:00
parent d604555142
commit 40ed098ce8
1 changed files with 8 additions and 3 deletions
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11
src/SDL_hashtable.c
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@@ -32,6 +32,7 @@ struct SDL_HashTable
{ {
SDL_HashItem **table; SDL_HashItem **table;
Uint32 table_len; Uint32 table_len;
int hash_shift;
SDL_bool stackable; SDL_bool stackable;
void *data; void *data;
SDL_HashTable_HashFn hash; SDL_HashTable_HashFn hash;
@@ -46,8 +47,9 @@ SDL_HashTable *SDL_CreateHashTable(void *data, const Uint32 num_buckets, const S
{ {
SDL_HashTable *table; SDL_HashTable *table;
// num_buckets must be a power of two so we get a solid block of bits to mask hash values against. // num_buckets must be a power of two so we can derive the bucket index with just a bitshift.
if ((num_buckets == 0) || ((num_buckets & (num_buckets - 1)) != 0)) { // Need at least two buckets, otherwise hash_shift would be 32, which is UB!
if ((num_buckets < 2) || !SDL_HasExactlyOneBitSet32(num_buckets)) {
SDL_SetError("num_buckets must be a power of two"); SDL_SetError("num_buckets must be a power of two");
return NULL; return NULL;
} }
@@ -64,6 +66,7 @@ SDL_HashTable *SDL_CreateHashTable(void *data, const Uint32 num_buckets, const S
} }
table->table_len = num_buckets; table->table_len = num_buckets;
table->hash_shift = 32 - SDL_MostSignificantBitIndex32(num_buckets);
table->stackable = stackable; table->stackable = stackable;
table->data = data; table->data = data;
table->hash = hashfn; table->hash = hashfn;
@@ -74,7 +77,9 @@ SDL_HashTable *SDL_CreateHashTable(void *data, const Uint32 num_buckets, const S
static SDL_INLINE Uint32 calc_hash(const SDL_HashTable *table, const void *key) static SDL_INLINE Uint32 calc_hash(const SDL_HashTable *table, const void *key)
{ {
return table->hash(key, table->data) & (table->table_len - 1); // Mix the bits together, and use the highest bits as the bucket index.
const Uint32 BitMixer = 0x9E3779B1u;
return (table->hash(key, table->data) * BitMixer) >> table->hash_shift;
} }