hash.c

#include "hash.h"

unsigned int hash(char *str)
{
    unsigned int ret = 5381;
    char c;
    while ((c = *str++)) // Stupid bitching GCC wall
    {
        ret = ((ret << 5) + ret) ^ c;
    }
    return ret;
}

struct hash_map *hash_map_create()
{
    struct hash_map *ret = malloc(sizeof(struct hash_map));
    memset(ret, 0, sizeof(struct hash_map));
    return ret;
}

void hash_map_destroy(struct hash_map *map)
{
    for (unsigned int i = 0; i < 65536; i++)
    {
        if (map->entries[i].count > 0)
        {
            for (unsigned int j = 0; j < map->entries[i].count; j++)
            {
                free(map->entries[i].items[j]);
            }
            free(map->entries[i].items);
        }
    }
    free(map);
}

void hash_map_insert(struct hash_map *map, char *key, char *value)
{
    unsigned int hkey = hash(key) & (65536 - 1);
    struct hash_map_entry *entry = &map->entries[hkey];
    entry->count += 1;
    entry->items = realloc(entry->items, sizeof(struct hash_map_item*) * entry->count); // Performance? Wut is dat?
    struct hash_map_item *item = malloc(sizeof(struct hash_map_item));
    item->key = key;
    item->value = value;
    entry->items[entry->count - 1] = item;
}

void hash_map_update(struct hash_map *map, char *key, char *value)
{
    unsigned int hkey = hash(key) & (65536 - 1);
    struct hash_map_entry *entry = &map->entries[hkey];
    if (entry->count == 0) return;
    for (unsigned int i = 0; i < entry->count; i++)
    {
        if (strcmp(entry->items[i]->key, key) == 0) 
        {
            entry->items[i]->value = value;
            return;
        }
    }
}

char *hash_map_get(struct hash_map *map, char *key)
{
    unsigned int hkey = hash(key) & (65536 - 1);
    struct hash_map_entry *entry = &map->entries[hkey];
    if (entry->count == 0) return NULL;
    for (unsigned int i = 0; i < entry->count; i++)
    {
        if (strcmp(entry->items[i]->key, key) == 0) return entry->items[i]->value;
    }
    return NULL;
}

hash.h

#ifndef _HASH_H_
#define _HASH_H_

#include <stdlib.h>
#include <string.h>

unsigned int hash(char*);

struct hash_map_item {
    char *key;
    char *value;
};

struct hash_map_entry {
    int count;
    struct hash_map_item **items;
};

struct hash_map {
    struct hash_map_entry entries[65536];
};

struct hash_map *hash_map_create();

void hash_map_destroy(struct hash_map *map);

void hash_map_insert(struct hash_map *map, char *key, char *value);

void hash_map_update(struct hash_map *map, char *key, char *value);

char *hash_map_get(struct hash_map *map, char *key);

#endif // _HASH_H_

main.c

#include <stdio.h>

#include "hash.h"

void print_value(struct hash_map *map, char *key);

int main(int argc, char **argv)
{
    struct hash_map *map = hash_map_create();
    print_value(map, "key");
    hash_map_insert(map, "key", "value");
    print_value(map, "key");
    hash_map_update(map, "key", "new value");
    print_value(map, "key");
    hash_map_destroy(map);
    return 0;
}

void print_value(struct hash_map *map, char *key)
{
    char *value = hash_map_get(map, key);
    if (value == NULL) printf("Key not found.\n");
    else printf("Value of '%s': '%s'.\n", key, value);
}

Makefile

CC=gcc 
CFLAGS=-Wall -std=c99 -ggdb
main: main.o hash.o

clean:
	rm -f main main.o hash.o