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/*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing,
* software distributed under the License is distributed on an
* "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
* KIND, either express or implied. See the License for the
* specific language governing permissions and limitations
* under the License.
*/
/* $Rev$ $Date$ */
#ifndef tuscany_hash_hpp
#define tuscany_hash_hpp
/**
* Fast hash functions.
*/
#include <apr_hash.h>
#include <apr_memcache.h>
namespace tuscany
{
/**
* Apache apr-util CRC32 hash function.
*
* See srclib/apr-util/memcache/apr_memcache.c from the Apache HTTPD
* source tree. Reproducing the comments from apr_memcache.c here:
*
* The crc32 functions and data were originally written by Spencer
* Garrett <srg@quick.com> and were gleaned from the PostgreSQL source
* tree at contrib/ltree/crc32.[ch] and from FreeBSD at
* src/usr.bin/cksum/crc32.c.
*/
const unsigned int crc32hash(const char* data, const size_t len) {
return (unsigned int)apr_memcache_hash_default(NULL, data, len);
}
/**
* Apache apr tables default hash function.
*
* See srclib/apr/tables/apr_hash.c from the Apache HTTPD source tree.
* Reproducing the comments from apr_hash.c here:
*
* This is the popular `times 33' hash algorithm which is used by
* perl and also appears in Berkeley DB. This is one of the best
* known hash functions for strings because it is both computed
* very fast and distributes very well.
*
* The originator may be Dan Bernstein but the code in Berkeley DB
* cites Chris Torek as the source. The best citation I have found
* is "Chris Torek, Hash function for text in C, Usenet message
* <27038@mimsy.umd.edu> in comp.lang.c , October, 1990." in Rich
* Salz's USENIX 1992 paper about INN which can be found at
* <http://citeseer.nj.nec.com/salz92internetnews.html>.
*
* The magic of number 33, i.e. why it works better than many other
* constants, prime or not, has never been adequately explained by
* anyone. So I try an explanation: if one experimentally tests all
* multipliers between 1 and 256 (as I did while writing a low-level
* data structure library some time ago) one detects that even
* numbers are not useable at all. The remaining 128 odd numbers
* (except for the number 1) work more or less all equally well.
* They all distribute in an acceptable way and this way fill a hash
* table with an average percent of approx. 86%.
*
* If one compares the chi^2 values of the variants (see
* Bob Jenkins ``Hashing Frequently Asked Questions'' at
* http://burtleburtle.net/bob/hash/hashfaq.html for a description
* of chi^2), the number 33 not even has the best value. But the
* number 33 and a few other equally good numbers like 17, 31, 63,
* 127 and 129 have nevertheless a great advantage to the remaining
* numbers in the large set of possible multipliers: their multiply
* operation can be replaced by a faster operation based on just one
* shift plus either a single addition or subtraction operation. And
* because a hash function has to both distribute good _and_ has to
* be very fast to compute, those few numbers should be preferred.
*
* -- Ralf S. Engelschall <rse@engelschall.com>
*/
const unsigned int times33hash(const char* data, const size_t len) {
apr_ssize_t l = len;
return apr_hashfunc_default(data, &l);
}
/**
* A very fast, non-cryptographic hash suitable for general hash-based
* lookup. See http://murmurhash.googlepages.com/ for more details.
*
* Original code by Austin Appleby, released to the public domain and under
* the MIT license.
*
* Compiles down to ~52 instructions on x86.
* Passes chi^2 tests for practically all keysets & bucket sizes.
* Excellent avalanche behavior. Maximum bias is under 0.5%.
* Passes Bob Jenkin's frog.c torture-test. No collisions possible for 4 byte
* keys, no small 1 to 7 bit differentials.
*/
const unsigned int murmurhash(const char* key, const size_t klen) {
unsigned int len = (unsigned int)klen;
const unsigned int seed = 0;
// 'm' and 'r' are mixing constants generated offline.
// They're not really 'magic', they just happen to work well.
const unsigned int m = 0x5bd1e995;
const int r = 24;
// Initialize the hash to a 'random' value
unsigned int h = seed ^ len;
// Mix 4 bytes at a time into the hash
const unsigned char* data = (const unsigned char*)key;
while(len >= 4) {
unsigned int k = *(unsigned int*)(void*)data;
k *= m;
k ^= k >> r;
k *= m;
h *= m;
h ^= k;
data += 4;
len -= 4;
}
// Handle the last few bytes of the input array
switch(len) {
case 3: h ^= data[2] << 16;
case 2: h ^= data[1] << 8;
case 1: h ^= data[0];
h *= m;
};
// Do a few final mixes of the hash to ensure the last few
// bytes are well-incorporated.
h ^= h >> 13;
h *= m;
h ^= h >> 15;
return h;
}
/**
* An endian and alignment neutral, but half the speed, version of
* the murmur hash.
*/
const unsigned int portablemurmurhash(const char* key, const size_t klen) {
unsigned int len = (unsigned int)klen;
const unsigned int seed = 0;
// 'm' and 'r' are mixing constants generated offline.
// They're not really 'magic', they just happen to work well.
const unsigned int m = 0x5bd1e995;
const int r = 24;
// Initialize the hash to a 'random' value
unsigned int h = seed ^ len;
// Mix 4 bytes at a time into the hash
const unsigned char* data = (const unsigned char *)key;
while(len >= 4) {
unsigned int k;
k = data[0];
k |= data[1] << 8;
k |= data[2] << 16;
k |= data[3] << 24;
k *= m;
k ^= k >> r;
k *= m;
h *= m;
h ^= k;
data += 4;
len -= 4;
}
// Handle the last few bytes of the input array
switch(len) {
case 3: h ^= data[2] << 16;
case 2: h ^= data[1] << 8;
case 1: h ^= data[0];
h *= m;
};
// Do a few final mixes of the hash to ensure the last few
// bytes are well-incorporated.
h ^= h >> 13;
h *= m;
h ^= h >> 15;
return h;
}
const unsigned int hashselect(const unsigned int hash, const unsigned int max) {
return hash % max;
}
}
#endif /* tuscany_hash_hpp */
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