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hash: replace hash with better function

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Add Bob Jenkins hash function to get better working hash function,
which does genarate way less colisions (especially with similar
strings).

For a comparision also a kernel function used in DM kernel is include.
While it's better then our existing one, it's still far worse,
then Bob Jenkins hash.
This commit is contained in:
Zdenek Kabelac 2021-03-08 15:18:04 +01:00
parent d602837b91
commit ff21723512
1 changed files with 74 additions and 18 deletions
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92
base/data-struct/hash.c
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@ -37,8 +37,11 @@ struct dm_hash_table {
struct dm_hash_node **slots;
};
/* Permutation of the Integers 0 through 255 */
static unsigned char _nums[] = {
#if 0 /* TO BE REMOVED */
static unsigned _hash(const void *key, unsigned len)
{
/* Permutation of the Integers 0 through 255 */
static unsigned char _nums[] = {
1, 14, 110, 25, 97, 174, 132, 119, 138, 170, 125, 118, 27, 233, 140, 51,
87, 197, 177, 107, 234, 169, 56, 68, 30, 7, 173, 73, 188, 40, 36, 65,
49, 213, 104, 190, 57, 211, 148, 223, 48, 115, 15, 2, 67, 186, 210, 28,
@ -63,23 +66,9 @@ static unsigned char _nums[] = {
44, 38, 31, 149, 135, 0, 216, 52, 63, 23, 37, 69, 39, 117, 146, 184,
163, 200, 222, 235, 248, 243, 219, 10, 152, 131, 123, 229, 203, 76, 120,
209
};
};
static struct dm_hash_node *_create_node(const void *key, unsigned len)
{
struct dm_hash_node *n = malloc(sizeof(*n) + len);
if (n) {
memcpy(n->key, key, len);
n->keylen = len;
}
return n;
}
static unsigned _hash(const void *key, unsigned len)
{
const unsigned char *str = key;
const uint8_t *str = key;
unsigned h = 0, g;
unsigned i;
@ -96,6 +85,73 @@ static unsigned _hash(const void *key, unsigned len)
return h;
}
/* In-kernel DM hashing, still lots of collisions */
static unsigned _hash_in_kernel(const char *key, unsigned len)
{
const unsigned char *str = (unsigned char *)key;
const unsigned hash_mult = 2654435387U;
unsigned hash = 0, i;
for (i = 0; i < len; ++i)
hash = (hash + str[i]) * hash_mult;
return hash;
}
#endif
#undef get16bits
#if (defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__)))
#define get16bits(d) (*((const uint16_t *) (d)))
#endif
#if !defined (get16bits)
#define get16bits(d) ((((uint32_t)(((const uint8_t *)(d))[1])) << 8)\
+(uint32_t)(((const uint8_t *)(d))[0]) )
#endif
/*
* Adapted Bob Jenkins hash to read by 2 bytes if possible.
* https://secure.wikimedia.org/wikipedia/en/wiki/Jenkins_hash_function
*
* Reduces amount of hash collisions
*/
static unsigned _hash(const void *key, unsigned len)
{
const uint8_t *str = (uint8_t*) key;
unsigned hash = 0, i;
unsigned sz = len / 2;
for(i = 0; i < sz; ++i) {
hash += get16bits(str + 2 * i);
hash += (hash << 10);
hash ^= (hash >> 6);
}
if (len & 1) {
hash += str[len - 1];
hash += (hash << 10);
hash ^= (hash >> 6);
}
hash += (hash << 3);
hash ^= (hash >> 11);
hash += (hash << 15);
return hash;
}
static struct dm_hash_node *_create_node(const void *key, unsigned len)
{
struct dm_hash_node *n = malloc(sizeof(*n) + len);
if (n) {
memcpy(n->key, key, len);
n->keylen = len;
}
return n;
}
struct dm_hash_table *dm_hash_create(unsigned size_hint)
{
size_t len;