Simple hashtable in C

hashtable.c

#include "hash.h"

#include <stdlib.h>

static unsigned index(Hashtable *hashtable, void *key) {
    unsigned i = (unsigned) key % hashtable->size;
    while (hashtable->keys[i] && hashtable->keys[i] != key) {
        if (++i >= hashtable->size) {
            i = 0;
        }
    }

    return i;
}

Hashtable *hashtable_create(size_t size) {
    Hashtable *h = calloc(1, sizeof(Hashtable));
    h->size = size;
    h->keys = calloc(size, sizeof(void *));
    h->values = calloc(size, sizeof(void *));

    return h;
}

void hashtable_insert(Hashtable *hashtable, void *key, void *value) {
    unsigned i = index(hashtable, key);
    hashtable->keys[i] = key;
    hashtable->values[i] = value;
}

void *hashtable_lookup(Hashtable *hashtable, void *key) {
    return hashtable->values[index(hashtable, key)];
}

hashtable.h

#pragma once

#include <stddef.h>

typedef struct {
    size_t size;
    void **keys;
    void **values;
} Hashtable;
 
Hashtable *hashtable_create(size_t size);

void hashtable_insert(Hashtable *hashtable, void *key, void *value);

void *hashtable_lookup(Hashtable *hashtable, void *key);