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I Made a Lightweight C Alternative to std::unordered_map

·635 words·3 mins
Author
Lazarus Overlook
Software developer, writer, ideogrammatologist
RolandMarchand/hashmap.h

Type-safe hashmap type generator for C

C
20
0

hashmap.h - Type-Safe Hash Tables for C
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A production-ready, macro-based hashmap library to generate type-safe hash tables with separate chaining.

#include "hashmap.h"

/* Arguments: struct name, func. prefix, key type, value type, hash func (opt), compare func (opt) */
HASHMAP_DECLARE(IntMap, int_map, int, int, NULL, NULL)
HASHMAP_DEFINE(IntMap, int_map, int, int, NULL, NULL)

int main(void) {
	IntMap map = { 0 };

	int_map_insert(&map, 10, 42);
	int_map_insert(&map, 20, 7);

	int value;
	if (int_map_get(&map, 10, &value)) {
		printf("Found: %d\n", value);  /* 42 */
	}

	printf("Size: %zu\n", map.size);   /* 2 */

	int_map_free(&map);
}

Features
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  • Type-safe: Generate hashmaps for any key-value pair types
  • Portable: C89 compatible, tested on GCC/Clang/MSVC/ICX across x86/x86_64/ARM64
  • Flexible: Custom hash and comparison functions, or use built-in defaults
  • Efficient: Separate chaining with 0.75 load factor, power-of-2 growth
  • Configurable: Custom allocators, null-pointer policies
  • Zero dependencies: Just standard C library

Quick Start
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For String Keys (Common Case)
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  1. Include the header and declare your hashmap:
/* my_hashmap.h */
#include "hashmap.h"

/* Automatically uses strcmp and FNV-1a string hash */
HASHMAP_DECLARE_STRING(UserMap, user_map, int)
  1. Define the implementation (usually in a .c file):
#include "my_hashmap.h"

HASHMAP_DEFINE_STRING(UserMap, user_map, int)
  1. Use it:
UserMap users = {0};

user_map_insert(&users, "alice", 100);
user_map_insert(&users, "bob", 200);

int score;
if (user_map_get(&users, "alice", &score)) {
	printf("Alice's score: %d\n", score);
}

user_map_remove(&users, "bob", NULL);
user_map_free(&users);

For Custom Key Types
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/* my_hashmap.h */
#include "hashmap.h"

unsigned long my_hash_func(int key);
int my_compare_func(int a, int b);

/* Pass NULL for hash/compare to use default FNV-1a/memcmp */
HASHMAP_DECLARE(IntMap, int_map, int, float, my_hash_func, my_compare_func)

/* my_hashmap.c */
#include "my_hashmap.h"

HASHMAP_DEFINE(IntMap, int_map, int, float, my_hash_func, my_compare_func)

unsigned long my_hash_func(int key) {
	return (unsigned long)key * 2654435761UL;
}

int my_compare_func(int a, int b) {
	return a - b;
}

Alternatively, if you plan to use your hashmap within a single file:

#include "hashmap.h"

HASHMAP_DECLARE_STRING(MyMap, my_map, int)
HASHMAP_DEFINE_STRING(MyMap, my_map, int)

Important: Pointer Keys vs. Content
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By default, keys are compared by value, not content. If your key is a char * and you pass NULL for the comparison function, the pointer addresses will be compared, not the string contents.

/* WRONG: Compares pointer addresses, not string content */
HASHMAP_DECLARE(BadMap, bad_map, char *, int, NULL, NULL)

/* CORRECT: Uses strcmp and fnv1a_32_str (comes with library) to compare string content */
HASHMAP_DECLARE(GoodMap, good_map, const char *, int, good_map_fnv1a_32_str, strcmp)

/* BEST: For string keys, use the STRING variant */
HASHMAP_DECLARE_STRING(BestMap, best_map, int)

API Overview
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  • hashmap_insert(map, key, value) - Insert or update (returns 1 if overwritten, 0 if new)
  • hashmap_get(map, key, &out) - Retrieve value (returns 1 if found, 0 otherwise)
  • hashmap_has(map, key) - Check if key exists
  • hashmap_remove(map, key, &out) - Remove key-value pair (returns 1 if removed, 0 if not found)
  • hashmap_iterate(map, context) - Iterate over all pairs using callback
  • hashmap_duplicate(dest, src) - Deep copy hashmap
  • hashmap_clear(map) - Remove all elements (keeps capacity)
  • hashmap_free(map) - Deallocate memory

Iteration
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Set the iteration_callback field and call hashmap_iterate():

int print_pair(const char *key, int value, void *context) {
	printf("%s: %d\n", key, value);
	return 1;  /* Continue iterating (return 0 to stop) */
}

StringMap map = {0};
map.iteration_callback = print_pair;

string_map_insert(&map, "one", 1);
string_map_insert(&map, "two", 2);

string_map_iterate(&map, NULL);  /* Pass context if needed */

Configuration
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Define before including the library:

#define HASHMAP_NO_PANIC_ON_NULL 1    /* Return silently on NULL instead of panic */
#define HASHMAP_REALLOC my_realloc    /* Custom allocator */
#define HASHMAP_FREE my_free          /* Custom deallocator */

Testing
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mkdir build
cmake -S . -B build/ -DCMAKE_BUILD_TYPE=Debug
cd build
make test

Tests cover insert/remove operations, collision handling, growth, iteration, and edge cases.

Contribution
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Contributors and library hackers should work on hashmap.in.h instead of hashmap.h. It is a version of the library with hardcoded types and function names. To generate the final library from it, run libgen.py.

License
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This library is licensed under the BSD Zero license.