Re: [PATCH 2/7] libihash: use an integer hash function on the keys

[Samuel Thibault]

Justus Winter, le Tue 13 May 2014 21:02:51 +0200, a écrit :
> Use an integer hash function to derive the index from the key.  This
> should reduce the number of collisions.

Ack.

> * 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
> 

-- 
Samuel
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 Oui, ya qu'a t'observer pour connaître le fait que tu es une feignasse.
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