Build your own hash table in C[3

Step 3 Hash Table APIs

In this implementations, hash table provides public APIs like get and set; Internally, hash table supports resize, iterate etc.

// hash_table.c
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include "hash_table.h"


#define INITIAL_CAPACITY 10
#define FNV_offset_basis 14695981039346656037UL
#define FNV_prime 1099511628211UL

item* create_item(const char* key, void* value) {
    item* i = malloc(sizeof(item));
    i->key = strdup(key);
    i->value = value;
    return i;
}

void free_item(item* i) {
    free((void*)i->key);
    free(i->value);
    free(i);
}

hash_table* create_ht() {
    hash_table* ht = malloc(sizeof(hash_table));
    ht->size = INITIAL_CAPACITY;
    ht->count = 0;
    ht->items = calloc(ht->size, sizeof(item*));
    return ht;
}

void free_ht(hash_table* ht) {
    for (int i=0; i<ht->size; ++i) {
        item* item = ht->items[i];
        if (item != NULL){
            free_item(item);
        }
    }
    free(ht->items);
    free(ht);
}

// https://en.wikipedia.org/wiki/Fowler–Noll–Vo_hash_function
// algorithm fnv-1a is
//     hash := FNV_offset_basis
//     for each byte_of_data to be hashed do
//         hash := hash XOR byte_of_data
//         hash := hash × FNV_prime
//     return hash


uint64_t hash(const char* key) {
    uint64_t hash = FNV_offset_basis;
    int len = strlen(key);
    for (int i=0; i<len; ++i) {
        hash ^= key[i];
        hash *= FNV_prime;
    }
    return hash;
}

void* get(hash_table* ht, const char* key){
    uint64_t index = hash(key) % ht->size;
    while (ht->items[index] != NULL){
        if (strcmp(ht->items[index]->key, key) == 0) {
            return ht->items[index]->value;
        }
        index += 1;
        if (index >= ht->size) {
            index = 0;
        }
    }
    return NULL;
}

void set(hash_table* ht, const char* key, void* value) {

    // If more than half of the capacity is filled, expand the table
    if (ht->count > ht->size / 2) {
        resize(ht);
    }

    uint64_t index = hash(key) % ht->size;

    // Look up if the key already exists, update the value if so
    while (ht->items[index] != NULL){
        if (strcmp(ht->items[index]->key, key) == 0) {
            ht->items[index]->value = value;
            return;
        }
        index += 1;
        if (index >= ht->size) {
            index = 0;
        }
    }

    // Key doesn't exist yet, add the item
    ht->items[index] = create_item(key, value);
    ht->count++;
}

size_t size(hash_table* ht){
    return ht->count;
}

void resize(hash_table* ht){
    item** new_items = calloc(ht->size * 2, sizeof(item*));
        for (int i=0; i<ht->size; ++i) {
            item* item = ht->items[i];
            if (item != NULL) {
                uint64_t new_index = hash(item->key) % (ht->size * 2);
                while (new_items[new_index] != NULL) {
                    new_index += 1;
                    if (new_index >= ht->size * 2) {
                        new_index = 0;
                    }
                }
                new_items[new_index] = item;
            }
        }
        free(ht->items);
        ht->items = new_items;
        ht->size *= 2;
}

hash_iterator create_iterator(hash_table* ht){
    hash_iterator it;
    it.ht = ht;
    it.index = 0;
    return it;
}

bool next(hash_iterator* it){
    hash_table* ht = it->ht;
    while (it->index < ht->size) {
        size_t i = it->index;
        it->index += 1;

        if (ht->items[i] != NULL) {
            item* item = ht->items[i];
            it->key = item->key;
            it->value = item->value;
            return true;
        }
    }
    return false;
}

void dump(hash_table* ht){
    if (ht == NULL) {
        return;
    }
    if (ht->count == 0) {
        printf("empty\n");
    }

    for (int i=0; i<ht->size; ++i) {
        item* item = ht->items[i];
        if (item != NULL) {
            printf("index: %d; key: %s; value: %s\n", i, item->key, (char*)item->value);
        }
    }
}


int main(int argc, char** argv) {
  hash_table* ht = create_ht();
  set(ht, "key1", "val1");
  set(ht, "key2", "val2");
  set(ht, "key3", "val3");
  set(ht, "key4", "val4");
  set(ht, "key5", "val5");
  set(ht, "key6", "val6");
  set(ht, "key7", "val7");
  set(ht, "key8", "val8");
  set(ht, "key8", "newVal8");
  dump(ht);
}

//hash_table.h
#include <stdbool.h>

// Hashtable Structure
typedef struct {
    const char* key;
    void* value;
} item;

typedef struct {
    item** items;
    int size;
    int count;
} hash_table;

typedef struct {
    hash_table* ht;
    size_t index;
    const char* key;
    void* value;
} hash_iterator;

// Public APIs:
hash_table* create_ht();

void free_ht(hash_table* ht);

void* get(hash_table* ht, const char* key);

void set(hash_table* ht, const char* key, void* value);

//void* remove(hash_table* ht, const char* key);

size_t size(hash_table* ht);

bool next(hash_iterator* it);


// Internal APIs:
uint64_t hash(const char* key);
void resize(hash_table* ht);
item* create_item(const char* key, void* value);
void free_item(item* i);

demo run:

./hash_table                                   
index: 1; key: key7; value: val7
index: 3; key: key1; value: val1
index: 4; key: key8; value: newVal8
index: 5; key: key3; value: val3
index: 8; key: key4; value: val4
index: 10; key: key6; value: val6
index: 14; key: key2; value: val2
index: 19; key: key5; value: val5