hashmap.c

Hash map implementation in C.

Features

• Open addressing using Robin Hood hashing
• Generic interface with support for variable sized items.
• Built-in SipHash or MurmurHash3 and allows for alternative algorithms.
• ANSI C (C99)
• Supports custom allocators
• Pretty darn good performance. 🚀

Example

#include <stdio.h>
#include <string.h>
#include "hashmap.h"

struct user {
    char *name;
    int age;
};

int user_compare(const void *a, const void *b, void *udata) {
    const struct user *ua = a;
    const struct user *ub = b;
    return strcmp(ua->name, ub->name);
}

bool user_iter(const void *item, void *udata) {
    const struct user *user = item;
    printf("%s (age=%d)\n", user->name, user->age);
    return true;
}

uint64_t user_hash(const void *item, uint64_t seed0, uint64_t seed1) {
    const struct user *user = item;
    return hashmap_sip(user->name, strlen(user->name), seed0, seed1);
}

int main() {
    // create a new hash map where each item is a `struct user`. The second
    // argument is the initial capacity. The third and fourth arguments are 
    // optional seeds that are passed to the following hash function.
    struct hashmap *map = hashmap_new(sizeof(struct user), 0, 0, 0, 
                                     user_hash, user_compare, NULL, NULL);

    // Here we'll load some users into the hash map. Each set operation
    // performs a copy of the data that is pointed to in the second argument.
    hashmap_set(map, &(struct user){ .name="Dale", .age=44 });
    hashmap_set(map, &(struct user){ .name="Roger", .age=68 });
    hashmap_set(map, &(struct user){ .name="Jane", .age=47 });

    struct user *user; 
    
    printf("\n-- get some users --\n");
    user = hashmap_get(map, &(struct user){ .name="Jane" });
    printf("%s age=%d\n", user->name, user->age);

    user = hashmap_get(map, &(struct user){ .name="Roger" });
    printf("%s age=%d\n", user->name, user->age);

    user = hashmap_get(map, &(struct user){ .name="Dale" });
    printf("%s age=%d\n", user->name, user->age);

    user = hashmap_get(map, &(struct user){ .name="Tom" });
    printf("%s\n", user?"exists":"not exists");

    printf("\n-- iterate over all users --\n");
    hashmap_scan(map, user_iter, NULL);

    hashmap_free(map);
}

// output:
// -- get some users --
// Jane age=47
// Roger age=68
// Dale age=44
// not exists
// 
// -- iterate over all users --
// Dale (age=44)
// Roger (age=68)
// Jane (age=47)

Functions

Basic

hashmap_new      # allocate a new hash map
hashmap_free     # free the hash map
hashmap_count    # returns the number of items in the hash map
hashmap_set      # insert or replace an existing item and return the previous
hashmap_get      # get an existing item
hashmap_delete   # delete and return an item
hashmap_clear    # clear the hash map

Iteration

hashmap_scan     # iterate over items in hash map

Hash helpers

hashmap_sip      # returns hash value for data using SipHash-2-4
hashmap_murmur   # returns hash value for data using MurmurHash3

Testing and benchmarks

$ cc -DHASHMAP_TEST hashmap.c && ./a.out              # run tests
$ cc -DHASHMAP_TEST -O3 hashmap.c && BENCH=1 ./a.out  # run benchmarks

The following benchmarks were run on my 2019 Macbook Pro (2.4 GHz 8-Core Intel Core i9) using gcc-9. The items are simple 4-byte ints. The hash function is MurmurHash3. Testing with 5,000,000 items. The (cap) results are hashmaps that are created with an inital capacity of 5,000,000.

set            5000000 ops in 0.708 secs, 142 ns/op, 7057960 op/sec, 26.84 bytes/op
get            5000000 ops in 0.303 secs, 61 ns/op, 16492723 op/sec
delete         5000000 ops in 0.486 secs, 97 ns/op, 10280873 op/sec
set (cap)      5000000 ops in 0.429 secs, 86 ns/op, 11641660 op/sec
get (cap)      5000000 ops in 0.303 secs, 61 ns/op, 16490493 op/sec
delete (cap)   5000000 ops in 0.410 secs, 82 ns/op, 12200091 op/sec

License

hashmap.c source code is available under the MIT License.