git/hash.h

#ifndef HASH_H
#define HASH_H

/*
 * These are some simple generic hash table helper functions.
 * Not necessarily suitable for all users, but good for things
 * where you want to just keep track of a list of things, and
 * have a good hash to use on them.
 *
 * It keeps the hash table at roughly 50-75% free, so the memory
 * cost of the hash table itself is roughly
 *
 *	3 * 2*sizeof(void *) * nr_of_objects
 *
 * bytes.
 *
 * FIXME: on 64-bit architectures, we waste memory. It would be
 * good to have just 32-bit pointers, requiring a special allocator
 * for hashed entries or something.
 */
struct hash_table_entry {
	unsigned int hash;
	void *ptr;
};

struct hash_table {
	unsigned int size, nr;
	struct hash_table_entry *array;
};

extern void *lookup_hash(unsigned int hash, const struct hash_table *table);
extern void **insert_hash(unsigned int hash, void *ptr, struct hash_table *table);
extern int for_each_hash(const struct hash_table *table, int (*fn)(void *, void *), void *data);
extern void free_hash(struct hash_table *table);

static inline void init_hash(struct hash_table *table)
{
	table->size = 0;
	table->nr = 0;
	table->array = NULL;
}

static inline void preallocate_hash(struct hash_table *table, unsigned int elts)
{
	assert(table->size == 0 && table->nr == 0 && table->array == NULL);
	table->size = elts * 2;
	table->array = xcalloc(sizeof(struct hash_table_entry), table->size);
}

#endif