src/utils/hash.c File Reference

#include <ctype.h>
#include <sys/types.h>
#include "utils/hash.h"

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Functions
unsigned long	sdbm (const unsigned char *str, size_t len)

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Detailed Description

Author:

Georg Hopp

Unknown (find out)

Copyright © 2012 Georg Hopp

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see <http://www.gnu.org/licenses/>.

Definition in file hash.c.

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Function Documentation

unsigned long sdbm	(	const unsigned char *	str,
		size_t	len
	)

SDBM hashing algorithm:

this algorithm was created for sdbm (a public-domain reimplementation of ndbm) database library. it was found to do well in scrambling bits, causing better distribution of the keys and fewer splits. it also happens to be a good general hashing function with good distribution. the actual function is hash(i) = hash(i - 1) * 65599 + str[i]; what is included below is the faster version used in gawk. [there is even a faster, duff-device version] the magic constant 65599 was picked out of thin air while experimenting with different constants, and turns out to be a prime. this is one of the algorithms used in berkeley db (see sleepycat) and elsewhere.

Definition at line 43 of file hash.c.

{
        unsigned long hash = 0;

        for(; 0 < len; str++, len--)
                hash = tolower(*str) + (hash << 6) + (hash << 16) - hash;

        return hash;
}

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