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[PATCH 2/7] libihash: use an integer hash function on the keys
From: |
Justus Winter |
Subject: |
[PATCH 2/7] libihash: use an integer hash function on the keys |
Date: |
Tue, 13 May 2014 21:02:51 +0200 |
Use an integer hash function to derive the index from the key. This
should reduce the number of collisions.
* libihash/ihash.c (hash_int32): New function.
(find_index): Use hash_int32 on the key to derive the index.
(add_one): Likewise.
---
libihash/ihash.c | 17 +++++++++++++++--
1 file changed, 15 insertions(+), 2 deletions(-)
diff --git a/libihash/ihash.c b/libihash/ihash.c
index d670fee..71ddc26 100644
--- a/libihash/ihash.c
+++ b/libihash/ihash.c
@@ -81,6 +81,19 @@ static const unsigned int ihash_nsizes = (sizeof ihash_sizes
/ sizeof ihash_sizes[0]);
+/* This is the integer finalizer from MurmurHash3. */
+static inline uint32_t
+murmur3_mix32 (uint32_t h, unsigned int bits)
+{
+ h ^= h >> 16;
+ h *= 0x85ebca6b;
+ h ^= h >> 13;
+ h *= 0xc2b2ae35;
+ h ^= h >> 16;
+
+ return h >> (32 - bits);
+}
+
/* Return 1 if the slot with the index IDX in the hash table HT is
empty, and 0 otherwise. */
static inline int
@@ -111,7 +124,7 @@ find_index (hurd_ihash_t ht, hurd_ihash_key_t key)
unsigned int up_idx;
unsigned int down_idx;
- idx = key % ht->size;
+ idx = murmur3_mix32 (key, 32) % ht->size;
if (ht->items[idx].value == _HURD_IHASH_EMPTY || ht->items[idx].key == key)
return idx;
@@ -264,7 +277,7 @@ add_one (hurd_ihash_t ht, hurd_ihash_key_t key,
hurd_ihash_value_t value)
unsigned int idx;
unsigned int first_free;
- idx = key % ht->size;
+ idx = murmur3_mix32 (key, 32) % ht->size;
first_free = idx;
if (ht->items[idx].value != _HURD_IHASH_EMPTY && ht->items[idx].key != key)
--
2.0.0.rc0