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a0f4271497
This function was designed for haproxy while testing other functions in the past. Initially it was not planned to be used given the not very interesting numbers it showed on real URL data : it is not as smooth as the other ones. But later tests showed that the other ones are extremely sensible to the server count and the type of input data, especially DJB2 which must not be used on numeric input. So in fact this function is still a generally average performer and it can make sense to merge it in the end, as it can provide an alternative to sdbm+avalanche or djb2+avalanche for consistent hashing or when hashing on numeric data such as a source IP address or a visitor identifier in a URL parameter.
89 lines
2.1 KiB
C
89 lines
2.1 KiB
C
/*
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* Hash function implementation
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*
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* See mailing list thread on "Consistent hashing alternative to sdbm"
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* http://marc.info/?l=haproxy&m=138213693909219
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*
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* Copyright 2000-2010 Willy Tarreau <w@1wt.eu>
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License
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* as published by the Free Software Foundation; either version
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* 2 of the License, or (at your option) any later version.
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*
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*/
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#include <common/hash.h>
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unsigned long hash_wt6(const char *key, int len)
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{
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unsigned h0 = 0xa53c965aUL;
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unsigned h1 = 0x5ca6953aUL;
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unsigned step0 = 6;
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unsigned step1 = 18;
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for (; len > 0; len--) {
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unsigned int t;
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t = ((unsigned int)*key);
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key++;
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h0 = ~(h0 ^ t);
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h1 = ~(h1 + t);
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t = (h1 << step0) | (h1 >> (32-step0));
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h1 = (h0 << step1) | (h0 >> (32-step1));
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h0 = t;
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t = ((h0 >> 16) ^ h1) & 0xffff;
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step0 = t & 0x1F;
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step1 = t >> 11;
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}
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return h0 ^ h1;
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}
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unsigned long hash_djb2(const char *key, int len)
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{
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unsigned long hash = 5381;
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/* the hash unrolled eight times */
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for (; len >= 8; len -= 8) {
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hash = ((hash << 5) + hash) + *key++;
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hash = ((hash << 5) + hash) + *key++;
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hash = ((hash << 5) + hash) + *key++;
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hash = ((hash << 5) + hash) + *key++;
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hash = ((hash << 5) + hash) + *key++;
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hash = ((hash << 5) + hash) + *key++;
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hash = ((hash << 5) + hash) + *key++;
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hash = ((hash << 5) + hash) + *key++;
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}
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switch (len) {
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case 7: hash = ((hash << 5) + hash) + *key++; /* fallthrough... */
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case 6: hash = ((hash << 5) + hash) + *key++; /* fallthrough... */
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case 5: hash = ((hash << 5) + hash) + *key++; /* fallthrough... */
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case 4: hash = ((hash << 5) + hash) + *key++; /* fallthrough... */
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case 3: hash = ((hash << 5) + hash) + *key++; /* fallthrough... */
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case 2: hash = ((hash << 5) + hash) + *key++; /* fallthrough... */
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case 1: hash = ((hash << 5) + hash) + *key++; break;
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default: /* case 0: */ break;
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}
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return hash;
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}
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unsigned long hash_sdbm(const char *key, int len)
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{
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unsigned long hash = 0;
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int c;
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while (len--) {
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c = *key++;
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hash = c + (hash << 6) + (hash << 16) - hash;
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}
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return hash;
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}
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