cig/map.c

#include "cig.h"
#include <assert.h>

#define FLAG_FREE       (-1)
#define FLAG_GRAVESTONE (-2)

static inline unsigned int mapping_cap(unsigned int capacity) {
	return capacity * 2;
}

typedef struct internal_map_sizes {
	int bytes;
	int real_cap_of_items_arr;
} internal_map_sizes_t;

internal_map_sizes_t internal_map_sizes(unsigned int capacity, unsigned int item_size) {
	const size_t cap_of_index_arr =
		sizeof(int) * mapping_cap(capacity);
	const size_t INC = ALIGN_OF(int);
	const size_t SIZE = item_size * capacity;
	const size_t cap_of_items_arr = (SIZE + INC - 1) / INC;
	const size_t bytes =
		sizeof(map_header_t) + cap_of_items_arr + cap_of_index_arr;
	return (internal_map_sizes_t){
		.bytes=bytes,
		.real_cap_of_items_arr=cap_of_items_arr,
	};
}

void *map_create_func(
	map_create_func_args_t args
) {
	internal_map_sizes_t sizes = internal_map_sizes(
		args.initial_capacity,
		args.item_size
	);
	map_header_t *header =
		allocator_alloc_func(
			args.allocator,
			sizes.bytes,
			args.file,
			args.line
		);

	header->n_items = 0;
	header->capacity = args.initial_capacity;
	header->item_size = args.item_size;
	header->key_size = args.key_size;
	header->allocator = args.allocator;
	header->hash = args.hash;
	header->equals = args.equals;
	header->mapping_arr = (int*) &header->bytes[sizes.real_cap_of_items_arr];
	header->mapping_capacity = mapping_cap(args.initial_capacity);

	for ( int i = 0; i < header->mapping_capacity; i++ ) {
		header->mapping_arr[i] = FLAG_FREE;
	}

	return header->bytes;
}

void *map_grow_func(void *this, const char *file, int line) {
	map_header_t *header = PTR_FROM_FIELD_PTR(map_header_t, bytes, this);
	int new_n_items = header->n_items + 1;

	if (header->capacity < new_n_items) {
		allocator_t allocator = header->allocator;
		unsigned int new_capacity = 1;
		if (header->capacity > 0) {
			new_capacity = header->capacity * 2;
		}
		internal_map_sizes_t new_size = internal_map_sizes(
			new_capacity,
			header->item_size
		);
		header = allocator_resize_func(allocator, header, new_size.bytes, file, line);
		header->mapping_arr = (int*) &header->bytes[new_size.real_cap_of_items_arr];
		header->mapping_capacity = mapping_cap(new_capacity);
		header->capacity=new_capacity;
		
		// first we have to clear the entire mapping array
		for ( unsigned int i = 0; i < header->mapping_capacity; i++ ) {
			header->mapping_arr[i] = FLAG_FREE;
		}

		// Then we have to re-hash and map all the items in the existing map.
		// The reason for re-hashing is that all hashes are moduloed by the
		// size of the mapping array, so the resulting hash will likely change.
		for ( unsigned int i = 0; i < header->n_items; i++ ) {
			void *pair = &this[i*header->item_size];
			// TODO: make the map_pair_hash more private? would be nice to just
			// pass in the header, and I don't really think it is needed
			// directly by the user of the library.
			unsigned int mapping_index = map_pair_hash(this, pair);
			header->mapping_arr[mapping_index] = i;
		}
	}

	header->n_items = new_n_items;
	return header->bytes;
}

void map_shrink_func(void *this, const char *file, int line) {
	if (map_len(this) <= 0) {
		fprintf(
			stderr,
			"%s:%d: tried to shrink map of size 0.\n",
			file,
			line
		);
		exit(1);
	}
	map_header_t *header = PTR_FROM_FIELD_PTR(map_header_t, bytes, this);
	header->n_items--;
	static_assert(0, "Hash the key at the removed item to find it in the mapping array, and set it to the gravestone value.");
}

uint32_t fnv_1a(const uint8_t *bytes, const unsigned int size) {
	// Yanked from: https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function
	const uint32_t PRIME = 0x01000193;
	const uint32_t OFFSET = 0x811c9dc5;
	uint32_t hash = OFFSET;
	for (int i = 0; i < size; i++) {
		uint32_t byte = (uint32_t)bytes[i];
		hash = hash ^ byte;
		hash = hash * PRIME;
	}
	return hash;
}

static void *internal_cig_key_ptr_from_pair_ptr(void *this, void *pair) {
	map_header_t *header = PTR_FROM_FIELD_PTR(map_header_t, bytes, this);
	return &pair[header->key_offset];
}

// TODO: just make these internal probably
bool map_pair_key_equals(void *this, void *pair1, void *pair2) {
}

unsigned int map_pair_hash(void *this, void *pair) {
	map_header_t *header = PTR_FROM_FIELD_PTR(map_header_t, bytes, this);
	void *key = internal_cig_key_ptr_from_pair_ptr(this, pair);

	// find the initial index based on hash
	uint32_t index;
	if (header->hash != NULL) {
		index = header->hash(key) % header->mapping_capacity;
	} else {
		index = fnv_1a(key, header->key_size) % header->mapping_capacity;
	}

	int existing_index = header->mapping_arr[index];
	while (!(existing_index == FLAG_FREE || existing_index == FLAG_GRAVESTONE)) {
		index = fnv_1a(
			(const uint8_t *) &index,
			sizeof(index)
		) % header->mapping_capacity;
		int existing_index = header->mapping_arr[index];
	}
	return (unsigned int) index;
}

int map_len(void *this) {
	if (this == NULL) {
		return 0;
	}
	map_header_t *header = PTR_FROM_FIELD_PTR(map_header_t, bytes, this);
	return header->n_items;
}

int map_cap(void *this) {
	if (this == NULL) {
		return 0;
	}
	map_header_t *header = PTR_FROM_FIELD_PTR(map_header_t, bytes, this);
	return header->capacity;
}

#undef FLAG_FREE
#undef FLAG_GRAVESTONE