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bloom.c: add the murmur3 hash implementation

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In preparation for computing changed paths Bloom filters,
implement the Murmur3 hash algorithm as described in [1].
It hashes the given data using the given seed and produces
a uniformly distributed hash value.

[1] https://en.wikipedia.org/wiki/MurmurHash#Algorithm

Helped-by: Derrick Stolee <dstolee@microsoft.com>
Helped-by: Szeder Gábor <szeder.dev@gmail.com>
Reviewed-by: Jakub Narębski <jnareb@gmail.com>
Signed-off-by: Garima Singh <garima.singh@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Garima Singh
2020-03-30 00:31:24 +00:00
committed by Junio C Hamano
parent 3be7efcafc
commit f52207a45c
7 changed files with 133 additions and 0 deletions
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73
bloom.c Normal file
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#include "git-compat-util.h"
#include "bloom.h"
static uint32_t rotate_left(uint32_t value, int32_t count)
{
uint32_t mask = 8 * sizeof(uint32_t) - 1;
count &= mask;
return ((value << count) | (value >> ((-count) & mask)));
}
/*
* Calculate the murmur3 32-bit hash value for the given data
* using the given seed.
* Produces a uniformly distributed hash value.
* Not considered to be cryptographically secure.
* Implemented as described in https://en.wikipedia.org/wiki/MurmurHash#Algorithm
*/
uint32_t murmur3_seeded(uint32_t seed, const char *data, size_t len)
{
const uint32_t c1 = 0xcc9e2d51;
const uint32_t c2 = 0x1b873593;
const uint32_t r1 = 15;
const uint32_t r2 = 13;
const uint32_t m = 5;
const uint32_t n = 0xe6546b64;
int i;
uint32_t k1 = 0;
const char *tail;
int len4 = len / sizeof(uint32_t);
uint32_t k;
for (i = 0; i < len4; i++) {
uint32_t byte1 = (uint32_t)data[4*i];
uint32_t byte2 = ((uint32_t)data[4*i + 1]) << 8;
uint32_t byte3 = ((uint32_t)data[4*i + 2]) << 16;
uint32_t byte4 = ((uint32_t)data[4*i + 3]) << 24;
k = byte1 | byte2 | byte3 | byte4;
k *= c1;
k = rotate_left(k, r1);
k *= c2;
seed ^= k;
seed = rotate_left(seed, r2) * m + n;
}
tail = (data + len4 * sizeof(uint32_t));
switch (len & (sizeof(uint32_t) - 1)) {
case 3:
k1 ^= ((uint32_t)tail[2]) << 16;
/*-fallthrough*/
case 2:
k1 ^= ((uint32_t)tail[1]) << 8;
/*-fallthrough*/
case 1:
k1 ^= ((uint32_t)tail[0]) << 0;
k1 *= c1;
k1 = rotate_left(k1, r1);
k1 *= c2;
seed ^= k1;
break;
}
seed ^= (uint32_t)len;
seed ^= (seed >> 16);
seed *= 0x85ebca6b;
seed ^= (seed >> 13);
seed *= 0xc2b2ae35;
seed ^= (seed >> 16);
return seed;
}