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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, table_size - 1].
This commit is contained in:
Andrea Canciani 2011-08-01 18:18:31 +02:00
parent c5405f7324
commit 02665975d3
1 changed files with 5 additions and 13 deletions
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18
src/cairo-hash.c
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@ -72,7 +72,7 @@
* prime chosen to be a little more than double the high water mark for a
* given arrangement, so the tables should remain < 50% full. The table
* size makes for the "first" hash modulus; a second prime (2 less than the
* first prime) serves as the "second" hash modulus, which is co-prime and
* first prime) serves as the "second" hash modulus, which is smaller and
* thus guarantees a complete permutation of table indices.
*
* This structure, and accompanying table, is borrowed/modified from the
@ -218,9 +218,7 @@ _cairo_hash_table_lookup_unique_key (cairo_hash_table_t *hash_table,
return entry;
i = 1;
step = key->hash % hash_table->arrangement->rehash;
if (step == 0)
step = 1;
step = 1 + key->hash % hash_table->arrangement->rehash;
do {
idx += step;
if (idx >= table_size)
@ -324,9 +322,7 @@ _cairo_hash_table_lookup (cairo_hash_table_t *hash_table,
return NULL;
i = 1;
step = key->hash % hash_table->arrangement->rehash;
if (step == 0)
step = 1;
step = 1 + key->hash % hash_table->arrangement->rehash;
do {
idx += step;
if (idx >= table_size)
@ -381,9 +377,7 @@ _cairo_hash_table_random_entry (cairo_hash_table_t *hash_table,
return entry;
i = 1;
step = hash % hash_table->arrangement->rehash;
if (step == 0)
step = 1;
step = 1 + hash % hash_table->arrangement->rehash;
do {
idx += step;
if (idx >= table_size)
@ -455,9 +449,7 @@ _cairo_hash_table_lookup_exact_key (cairo_hash_table_t *hash_table,
return entry;
i = 1;
step = key->hash % hash_table->arrangement->rehash;
if (step == 0)
step = 1;
step = 1 + key->hash % hash_table->arrangement->rehash;
do {
idx += step;
if (idx >= table_size)