hash: Improve double hashing

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].

diff --git a/src/wayland-hash.c b/src/wayland-hash.c
index 1ec6be4..01ccd7c 100644 (file)
--- a/src/wayland-hash.c
+++ b/src/wayland-hash.c
@@ -176,9 +176,7 @@ hash_table_search(struct wl_hash_table *ht, uint32_t hash)
                        return entry;
                }
 
-               double_hash = hash % ht->rehash;
-               if (double_hash == 0)
-                       double_hash = 1;
+               double_hash = 1 + hash % ht->rehash;
 
                hash_address = (hash_address + double_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;
                }
 
-               double_hash = hash % ht->rehash;
-               if (double_hash == 0)
-                       double_hash = 1;
+               double_hash = 1 + hash % ht->rehash;
 
                hash_address = (hash_address + double_hash) % ht->size;
        } while (hash_address != hash % ht->size);