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f52207a45c
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>
73 lines
1.7 KiB
C
73 lines
1.7 KiB
C
#include "git-compat-util.h"
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#include "bloom.h"
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static uint32_t rotate_left(uint32_t value, int32_t count)
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{
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uint32_t mask = 8 * sizeof(uint32_t) - 1;
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count &= mask;
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return ((value << count) | (value >> ((-count) & mask)));
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}
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/*
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* Calculate the murmur3 32-bit hash value for the given data
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* using the given seed.
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* Produces a uniformly distributed hash value.
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* Not considered to be cryptographically secure.
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* Implemented as described in https://en.wikipedia.org/wiki/MurmurHash#Algorithm
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*/
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uint32_t murmur3_seeded(uint32_t seed, const char *data, size_t len)
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{
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const uint32_t c1 = 0xcc9e2d51;
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const uint32_t c2 = 0x1b873593;
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const uint32_t r1 = 15;
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const uint32_t r2 = 13;
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const uint32_t m = 5;
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const uint32_t n = 0xe6546b64;
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int i;
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uint32_t k1 = 0;
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const char *tail;
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int len4 = len / sizeof(uint32_t);
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uint32_t k;
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for (i = 0; i < len4; i++) {
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uint32_t byte1 = (uint32_t)data[4*i];
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uint32_t byte2 = ((uint32_t)data[4*i + 1]) << 8;
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uint32_t byte3 = ((uint32_t)data[4*i + 2]) << 16;
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uint32_t byte4 = ((uint32_t)data[4*i + 3]) << 24;
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k = byte1 | byte2 | byte3 | byte4;
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k *= c1;
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k = rotate_left(k, r1);
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k *= c2;
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seed ^= k;
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seed = rotate_left(seed, r2) * m + n;
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}
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tail = (data + len4 * sizeof(uint32_t));
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switch (len & (sizeof(uint32_t) - 1)) {
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case 3:
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k1 ^= ((uint32_t)tail[2]) << 16;
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/*-fallthrough*/
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case 2:
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k1 ^= ((uint32_t)tail[1]) << 8;
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/*-fallthrough*/
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case 1:
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k1 ^= ((uint32_t)tail[0]) << 0;
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k1 *= c1;
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k1 = rotate_left(k1, r1);
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k1 *= c2;
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seed ^= k1;
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break;
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}
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seed ^= (uint32_t)len;
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seed ^= (seed >> 16);
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seed *= 0x85ebca6b;
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seed ^= (seed >> 13);
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seed *= 0xc2b2ae35;
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seed ^= (seed >> 16);
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return seed;
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} |