bloom

Bloom Filter implementation written in C

Bloom Filters are a probabilistic data structure that allows for the storage and look up of elements. The data stored in a Bloom Filter is not retrievable. Once data is 'inserted', data can be checked to see if it likely has been seen or if it definitely has not. Bloom Filters guarantee a 0% False Negative rate with a pre-selected false positive rate.

To use the library, copy the src/bloom.h and src/bloom.c files into your project and include it where needed.

License:

MIT 2015 - 2021

Main Features:

• Set upper bound number of elements and desired false positive rate; the system will determine number of hashes and number of bits required
• Custom hashing algorithms support
• Import and export either as file or as hex string
  • Keeps everything but the hashing algorithm
  • Hex can be used if needing to store as a string
  • File base can be loaded either on disk or into memory
• Ability to read Bloom Filter on disk instead of in memory if needed
• Add or check for presence in the filter by using either the string or hashes
  • Using hashes can be used to check many similar Bloom Filters while only needing to hash the string once
• Calculate current false positive rate
• Union and Intersection of Bloom Filters
• Calculate the Jaccard Index between two Bloom Filters
• OpenMP support for generation and lookup
  • Ensure the bloom.c file is compiled with -fopenmp along with the utilizing program

Future Enhancements

• What would the difference between two Bloom Filters signify?

Usage:

#include "bloom.h"

BloomFilter bf;
/*  elements = 10;
    false positive rate = 5% */
bloom_filter_init(&bf, 10, 0.05);
bloom_filter_add_string(&bf, "test");
if (bloom_filter_check_string(&bf, "test") == BLOOM_FAILURE) {
    printf("'test' is not in the Bloom Filter\n");
} else {
    printf("'test' is in the Bloom Filter\n");
}
if (bloom_filter_check_string(&bf, "blah") == BLOOM_FAILURE) {
    printf("'blah' is not in the Bloom Filter!\n");
} else {
    printf("'blah' is in th Bloom Filter\n");
}
bloom_filter_stats(&bf);
bloom_filter_destroy(&bf);

User Defined Hash Function Example

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <openssl/sha.h>
#include "bloom.h"

/* Example of a custom hashing function */
uint64_t* sha256_hash(int num_hashes, char* str) {
    uint64_t* results = calloc(num_hashes, sizeof(uint64_t));
    unsigned char digest[SHA256_DIGEST_LENGTH];
    int i;
    for (i = 0; i < num_hashes; i++) {
        SHA256_CTX sha256_ctx;
        SHA256_Init(&sha256_ctx);
        if (i == 0) {
            SHA256_Update(&sha256_ctx, str, strlen(str));
        } else {
            SHA256_Update(&sha256_ctx, digest, SHA256_DIGEST_LENGTH);
        }
        SHA256_Final(digest, &sha256_ctx);
        results[i] = (uint64_t)* (uint64_t* )digest;
    }
    return results;
}

BloomFilter bf;
/*  elements = 10;
    false positive rate = 5%
    custom hashing algorithm = sha256_hash function */
bloom_filter_init_alt(&bf, 10, 0.05, &sha256_hash);
bloom_filter_add_string(&bf, "test");
if (bloom_filter_check_string(&bf, "test") == BLOOM_FAILURE) {
    printf("'test' is not in the Bloom Filter\n");
} else {
    printf("'test' is in the Bloom Filter\n");
}
if (bloom_filter_check_string(&bf, "blah") == BLOOM_FAILURE) {
    printf("'blah' is not in the Bloom Filter!\n");
} else {
    printf("'blah' is in th Bloom Filter\n");
}
bloom_filter_stats(&bf);
bloom_filter_destroy(&bf);

Required Compile Flags:

-lm

Backward Compatible Hash Function

To use the older bloom filters (v1.8.2 or lower) that utilized the default hashing algorithm, then change use the following code as the hash function:

/* NOTE: The caller will free the results */
static uint64_t* original_default_hash(unsigned int num_hashes, const char* str) {
    uint64_t *results = (uint64_t*)calloc(num_hashes, sizeof(uint64_t));
    char key[17] = {0}; // largest value is 7FFF,FFFF,FFFF,FFFF
    results[0] = __fnv_1a(str);
    for (unsigned int i = 1; i < num_hashes; ++i) {
        sprintf(key, "%" PRIx64 "", results[i-1]);
        results[i] = old_fnv_1a(key);
    }
    return results;
}

static uint64_t old_fnv_1a(const char* key) {
    // FNV-1a hash (http://www.isthe.com/chongo/tech/comp/fnv/)
    int i, len = strlen(key);
    uint64_t h = 14695981039346656073ULL; // FNV_OFFSET 64 bit
    for (i = 0; i < len; ++i){
            h = h ^ (unsigned char) key[i];
            h = h * 1099511628211ULL; // FNV_PRIME 64 bit
    }
    return h;
}

If using only older Bloom Filters, then you can update the // FNV_OFFSET 64 bit to use 14695981039346656073ULL