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From 7849d265ce00e936df1a186647253cd2231d1c90 Mon Sep 17 00:00:00 2001
From: Justus Winter <4winter@informatik.uni-hamburg.de>
Date: Thu, 8 May 2014 15:45:00 +0200
Subject: [PATCH 6/6] libihash: use an integer hash function on the keys
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 | 23 +++++++++++++++++++++--
1 file changed, 21 insertions(+), 2 deletions(-)
diff --git a/libihash/ihash.c b/libihash/ihash.c
index d670fee..1de4c35 100644
--- a/libihash/ihash.c
+++ b/libihash/ihash.c
@@ -81,6 +81,25 @@ static const unsigned int ihash_nsizes = (sizeof ihash_sizes
/ sizeof ihash_sizes[0]);
+/* Integer hashing follows Thomas Wang's paper about his 32/64-bits
+ mix functions :
+ - http://www.concentric.net/~Ttwang/tech/inthash.htm */
+static inline uint32_t
+hash_int32(uint32_t n, unsigned int bits)
+{
+ uint32_t hash;
+
+ hash = n;
+ hash = ~hash + (hash << 15);
+ hash ^= (hash >> 12);
+ hash += (hash << 2);
+ hash ^= (hash >> 4);
+ hash += (hash << 3) + (hash << 11);
+ hash ^= (hash >> 16);
+
+ return hash >> (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 +130,7 @@ find_index (hurd_ihash_t ht, hurd_ihash_key_t key)
unsigned int up_idx;
unsigned int down_idx;
- idx = key % ht->size;
+ idx = hash_int32 (key, 32) % ht->size;
if (ht->items[idx].value == _HURD_IHASH_EMPTY || ht->items[idx].key == key)
return idx;
@@ -264,7 +283,7 @@ 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 = hash_int32 (key, 32) % ht->size;
first_free = idx;
if (ht->items[idx].value != _HURD_IHASH_EMPTY && ht->items[idx].key != key)
--
2.0.0.rc0
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