netsurf/utils/bloom.c

/*
 * Copyright 2013 Rob Kendrick <rjek@netsurf-browser.org>
 *
 * This file is part of NetSurf, http://www.netsurf-browser.org/
 *
 * NetSurf 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; version 2 of the License.
 *
 * NetSurf 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/>.
 */

/** \file
 * Trivial bloom filter */

#include <stdlib.h>
#include "utils/bloom.h"

/**
 * Hash a string, returning a 32bit value.  The hash algorithm used is
 * Fowler Noll Vo - a very fast and simple hash, ideal for short strings.
 * See http://en.wikipedia.org/wiki/Fowler_Noll_Vo_hash for more details.
 *
 * \param  datum   The string to hash.
 * \param  len	   size_t of data length.
 * \return The calculated hash value for the datum.
 */

static inline uint32_t fnv(const char *datum, size_t len)
{
	uint32_t z = 0x811c9dc5;
	
	if (datum == NULL)
		return 0;

	while (len--) {
		z *= 0x01000193;
		z ^= *datum++;
	}

	return z;
}

struct bloom_filter {
	size_t size;
	uint32_t items;
	uint8_t filter[];
};

struct bloom_filter *bloom_create(size_t size)
{
	struct bloom_filter *r = calloc(sizeof(*r) + size, 1);
        
	if (r == NULL)
		return NULL;
        
	r->size = size;
        
	return r;
}

void bloom_destroy(struct bloom_filter *b)
{
        free(b);
}

void bloom_insert_str(struct bloom_filter *b, const char *s, size_t z)
{
	uint32_t hash = fnv(s, z);
	bloom_insert_hash(b, hash);
}

void bloom_insert_hash(struct bloom_filter *b, uint32_t hash)
{
	unsigned int index = hash % (b->size << 3);
	unsigned int byte_index = index >> 3;
	unsigned int bit_index = index & 7;

	b->filter[byte_index] |= (1 << bit_index);
	b->items++;
}

bool bloom_search_str(struct bloom_filter *b, const char *s, size_t z)
{
	uint32_t hash = fnv(s, z);
	return bloom_search_hash(b, hash);
}

bool bloom_search_hash(struct bloom_filter *b, uint32_t hash)
{
	unsigned int index = hash % (b->size << 3);
	unsigned int byte_index = index >> 3;
	unsigned int bit_index = index & 7;
	
	return (b->filter[byte_index] & (1 << bit_index)) != 0;
}

uint32_t bloom_items(struct bloom_filter *b)
{
	return b->items;
}

#ifdef TEST_RIG

#include <stdio.h>
#include <string.h>
#include <assert.h>

int main(int argc, char *arg[])
{
	struct bloom_filter *b = bloom_create(8192);
	FILE *dict = fopen("/usr/share/dict/words", "r");
	char buf[BUFSIZ];
	int false_positives = 0, total = 0;
	
	for (int i = 0; i < 8192; i++) {
		fscanf(dict, "%s", buf);
		printf("adding %s\n", buf);
		bloom_insert_str(b, buf, strlen(buf));
	}
	
	printf("adding NetSurf\n");
	
	bloom_insert_str(b, "NetSurf", 7);
	printf("checking NetSurf (should be true)\n");
	assert(bloom_search_str(b, "NetSurf", 7));
	
	fseek(dict, 0, SEEK_SET);
	
	for (int i = 0; i < 8192; i++) {
		fscanf(dict, "%s", buf);
		printf("checking %s (should be true)\n", buf);
		assert(bloom_search_str(b, buf, strlen(buf)));
		
		total++;
	}
	
	for (int i = 0; i < 8192; i++) {
		fscanf(dict, "%s", buf);
		printf("checking %s (should be false)\n", buf);
		if (bloom_search_str(b, buf, strlen(buf)) == true)
			false_positives++;
		total++;
	}
	
	printf("false positives: %d of %d, %f%%\n",
		false_positives, total,
		((float)false_positives / total) * 100);
	
	fclose(dict);
	bloom_destroy(b);
	
	return 0;
}

#endif /* TEST_RIG */