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hash: Improve double hashing

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Instead of artificially introducing collisions in the step value by
replacing 0 with 1 (which causes the value 1 to have twice the
frequency of any other value), the step value can simply be computed
as an uniformly distributed value in the range [1, rehash], extremes
included.

This is safe because any step value smaller than the hash modulus is
co-prime with it, hence induces an orbit which includes every integer
in [0, size - 1].
This commit is contained in:
Andrea Canciani 2011-11-14 10:24:47 +01:00 • committed by Kristian Høgsberg
parent e742dcc9ed
commit 3175e91efa
1 changed files with 2 additions and 6 deletions
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8
src/wayland-hash.c
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@ -176,9 +176,7 @@ hash_table_search(struct wl_hash_table *ht, uint32_t hash)
return entry; return entry;
} }
double_hash = hash % ht->rehash; double_hash = 1 + hash % ht->rehash;
if (double_hash == 0)
double_hash = 1;
hash_address = (hash_address + double_hash) % ht->size; hash_address = (hash_address + double_hash) % ht->size;
} while (hash_address != hash % ht->size); } while (hash_address != hash % ht->size);
@ -277,9 +275,7 @@ wl_hash_table_insert(struct wl_hash_table *ht, uint32_t hash, void *data)
return 0; return 0;
} }
double_hash = hash % ht->rehash; double_hash = 1 + hash % ht->rehash;
if (double_hash == 0)
double_hash = 1;
hash_address = (hash_address + double_hash) % ht->size; hash_address = (hash_address + double_hash) % ht->size;
} while (hash_address != hash % ht->size); } while (hash_address != hash % ht->size);