[PATCH 07/11] libihash: use an integer hash function on the keys

[Justus Winter]

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