Tim Starling
136 linhas
4.2 KiB
C++
136 linhas
4.2 KiB
C++
/*
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+----------------------------------------------------------------------+
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| HipHop for PHP |
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+----------------------------------------------------------------------+
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| Copyright (c) 2010-2013 Facebook, Inc. (http://www.facebook.com) |
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| Copyright (c) 1997-2010 The PHP Group |
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+----------------------------------------------------------------------+
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| This source file is subject to version 3.01 of the PHP license, |
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| that is bundled with this package in the file LICENSE, and is |
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| available through the world-wide-web at the following url: |
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| http://www.php.net/license/3_01.txt |
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| If you did not receive a copy of the PHP license and are unable to |
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| obtain it through the world-wide-web, please send a note to |
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| license@php.net so we can mail you a copy immediately. |
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+----------------------------------------------------------------------+
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*/
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/**
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* FurcHash -- a consistent hash function using a binary decision tree.
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* Based on an algorithm by Mark Rabkin with two changes:
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* 1) Uses MurmurHash64A to hash the original key and to generate
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* additional bits by recursively rehashing.
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* 2) The original recursive algorithm for the decision tree has been
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* made iterative.
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*
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* Assumes that |m| is 8 million or less (2^FURC_SHIFT). Making FURC_SHIFT
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* bigger also makes furc_hash modestly slower.
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*
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* Performance is in the sub-500ns range to over 100,000 shards with 13-byte
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* keys. This version of furc_hash is fairly insensitive to key length since
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* additional bits are generated by re-hashing the initial MurmurHash64A.
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*/
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#include <assert.h>
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#include <stdint.h>
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#include <sys/time.h>
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#include <stdio.h>
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#include <string.h>
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#include <math.h>
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#include "hphp/runtime/ext/hash/hash_murmur.h"
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namespace HPHP {
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///////////////////////////////////////////////////////////////////////////////
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/* Maximum number tries for in-range result before just returning 0. */
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#define MAX_TRIES 32
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/* Gap in bit index per try; limits us to 2^FURC_SHIFT shards. Making this
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* larger will sacrifice a modest amount of performance.
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*/
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#define FURC_SHIFT 23
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/* Size of cache for hash values; should be > MAXTRIES * (FURCSHIFT + 1) */
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#define FURC_CACHE_SIZE 1024
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/**
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* furc_get_bit -- the bitstream generator
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*
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* Given a key and an index, provides a pseudorandom bit dependent on both.
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* Caches hash values; a NULL key clears the cache.
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*/
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static uint32_t furc_get_bit(const char* const key, const size_t len,
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const uint32_t idx, uint64_t* hash,
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int32_t* old_ord_p) {
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int32_t ord = (idx >> 6);
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int n;
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if (key == NULL) {
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*old_ord_p = -1;
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return 0;
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}
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if (*old_ord_p < ord) {
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for (n = *old_ord_p + 1; n <= ord; n++) {
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hash[n] = ((n == 0)
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? murmur_hash_64A(key, len, SEED)
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: murmur_rehash_64A(hash[n-1]));
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}
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*old_ord_p = ord;
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}
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return (hash[ord] >> (idx&0x3f)) & 0x1;
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}
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inline uint32_t furc_maximum_pool_size(void) {
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return (1 << FURC_SHIFT);
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}
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uint32_t furc_hash_internal(const char* const key, const size_t len, const uint32_t m) {
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uint32_t tries;
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uint32_t d;
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uint32_t num;
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uint32_t i;
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uint32_t a;
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uint64_t hash[FURC_CACHE_SIZE];
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int32_t old_ord;
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assert(m <= furc_maximum_pool_size());
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if (m <= 1) {
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return 0;
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}
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furc_get_bit(NULL, 0, 0, hash, &old_ord);
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for (d = 0; m > (1ul << d); d++)
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;
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a = d;
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for (tries = 0; tries < MAX_TRIES; tries++) {
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while (!furc_get_bit(key, len, a, hash, &old_ord)) {
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if (--d == 0) {
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return 0;
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}
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a = d;
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}
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a += FURC_SHIFT;
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num = 1;
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for (i = 0; i < d-1; i++) {
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num = (num << 1) | furc_get_bit(key, len, a, hash, &old_ord);
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a += FURC_SHIFT;
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}
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if (num < m) {
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return num;
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}
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}
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// Give up; return 0, which is a legal value in all cases.
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return 0;
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}
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///////////////////////////////////////////////////////////////////////////////
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}
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