The hashmap API is a generic implementation of hash-based key-value mappings.
Data Structures
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struct hashmap -
The hash table structure. Members can be used as follows, but should not be modified directly:
The
sizemember keeps track of the total number of entries (0 means the hashmap is empty).tablesizeis the allocated size of the hash table. A non-0 value indicates that the hashmap is initialized. It may also be useful for statistical purposes (i.e.size / tablesizeis the current load factor).cmpfnstores the comparison function specified inhashmap_init(). In advanced scenarios, it may be useful to change this, e.g. to switch between case-sensitive and case-insensitive lookup. -
struct hashmap_entry -
An opaque structure representing an entry in the hash table, which must be used as first member of user data structures. Ideally it should be followed by an int-sized member to prevent unused memory on 64-bit systems due to alignment.
The
hashmember is the entry’s hash code and thenextmember points to the next entry in case of collisions (i.e. if multiple entries map to the same bucket). -
struct hashmap_iter -
An iterator structure, to be used with hashmap_iter_* functions.
Types
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int (*hashmap_cmp_fn)(const void *entry, const void *entry_or_key, const void *keydata) -
User-supplied function to test two hashmap entries for equality. Shall return 0 if the entries are equal.
This function is always called with non-NULL
entry/entry_or_keyparameters that have the same hash code. When looking up an entry, thekeyandkeydataparameters to hashmap_get and hashmap_remove are always passed as second and third argument, respectively. Otherwise,keydatais NULL.
Functions
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unsigned int strhash(const char *buf) -
unsigned int strihash(const char *buf) -
unsigned int memhash(const void *buf, size_t len) -
unsigned int memihash(const void *buf, size_t len) -
Ready-to-use hash functions for strings, using the FNV-1 algorithm (see http://www.isthe.com/chongo/tech/comp/fnv).
strhashandstrihashtake 0-terminated strings, whilememhashandmemihashoperate on arbitrary-length memory.strihashandmemihashare case insensitive versions. -
unsigned int sha1hash(const unsigned char *sha1) -
Converts a cryptographic hash (e.g. SHA-1) into an int-sized hash code for use in hash tables. Cryptographic hashes are supposed to have uniform distribution, so in contrast to
memhash(), this just copies the firstsizeof(int)bytes without shuffling any bits. Note that the results will be different on big-endian and little-endian platforms, so they should not be stored or transferred over the net. -
void hashmap_init(struct hashmap *map, hashmap_cmp_fn equals_function, size_t initial_size) -
Initializes a hashmap structure.
mapis the hashmap to initialize.The
equals_functioncan be specified to compare two entries for equality. If NULL, entries are considered equal if their hash codes are equal.If the total number of entries is known in advance, the
initial_sizeparameter may be used to preallocate a sufficiently large table and thus prevent expensive resizing. If 0, the table is dynamically resized. -
void hashmap_free(struct hashmap *map, int free_entries) -
Frees a hashmap structure and allocated memory.
mapis the hashmap to free.If
free_entriesis true, each hashmap_entry in the map is freed as well (using stdlib’s free()). -
void hashmap_entry_init(void *entry, unsigned int hash) -
Initializes a hashmap_entry structure.
entrypoints to the entry to initialize.hashis the hash code of the entry.The hashmap_entry structure does not hold references to external resources, and it is safe to just discard it once you are done with it (i.e. if your structure was allocated with xmalloc(), you can just free(3) it, and if it is on stack, you can just let it go out of scope).
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void *hashmap_get(const struct hashmap *map, const void *key, const void *keydata) -
Returns the hashmap entry for the specified key, or NULL if not found.
mapis the hashmap structure.keyis a hashmap_entry structure (or user data structure that starts with hashmap_entry) that has at least been initialized with the proper hash code (viahashmap_entry_init).If an entry with matching hash code is found,
keyandkeydataare passed tohashmap_cmp_fnto decide whether the entry matches the key. -
void *hashmap_get_from_hash(const struct hashmap *map, unsigned int hash, const void *keydata) -
Returns the hashmap entry for the specified hash code and key data, or NULL if not found.
mapis the hashmap structure.hashis the hash code of the entry to look up.If an entry with matching hash code is found,
keydatais passed tohashmap_cmp_fnto decide whether the entry matches the key. Theentry_or_keyparameter points to a bogus hashmap_entry structure that should not be used in the comparison. -
void *hashmap_get_next(const struct hashmap *map, const void *entry) -
Returns the next equal hashmap entry, or NULL if not found. This can be used to iterate over duplicate entries (see
hashmap_add).mapis the hashmap structure.entryis the hashmap_entry to start the search from, obtained via a previous call tohashmap_getorhashmap_get_next. -
void hashmap_add(struct hashmap *map, void *entry) -
Adds a hashmap entry. This allows to add duplicate entries (i.e. separate values with the same key according to hashmap_cmp_fn).
mapis the hashmap structure.entryis the entry to add. -
void *hashmap_put(struct hashmap *map, void *entry) -
Adds or replaces a hashmap entry. If the hashmap contains duplicate entries equal to the specified entry, only one of them will be replaced.
mapis the hashmap structure.entryis the entry to add or replace.Returns the replaced entry, or NULL if not found (i.e. the entry was added).
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void *hashmap_remove(struct hashmap *map, const void *key, const void *keydata) -
Removes a hashmap entry matching the specified key. If the hashmap contains duplicate entries equal to the specified key, only one of them will be removed.
mapis the hashmap structure.keyis a hashmap_entry structure (or user data structure that starts with hashmap_entry) that has at least been initialized with the proper hash code (viahashmap_entry_init).If an entry with matching hash code is found,
keyandkeydataare passed tohashmap_cmp_fnto decide whether the entry matches the key.Returns the removed entry, or NULL if not found.
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void hashmap_iter_init(struct hashmap *map, struct hashmap_iter *iter) -
void *hashmap_iter_next(struct hashmap_iter *iter) -
void *hashmap_iter_first(struct hashmap *map, struct hashmap_iter *iter) -
Used to iterate over all entries of a hashmap.
hashmap_iter_initinitializes ahashmap_iterstructure.hashmap_iter_nextreturns the next hashmap_entry, or NULL if there are no more entries.hashmap_iter_firstis a combination of both (i.e. initializes the iterator and returns the first entry, if any). -
const char *strintern(const char *string) -
const void *memintern(const void *data, size_t len) -
Returns the unique, interned version of the specified string or data, similar to the
String.internAPI in Java and .NET, respectively. Interned strings remain valid for the entire lifetime of the process.Can be used as
[x]strdup()orxmemdupzreplacement, except that interned strings / data must not be modified or freed.Interned strings are best used for short strings with high probability of duplicates.
Uses a hashmap to store the pool of interned strings.
Usage example
Here’s a simple usage example that maps long keys to double values.
struct hashmap map;
struct long2double {
struct hashmap_entry ent; /* must be the first member! */
long key;
double value;
};
static int long2double_cmp(const struct long2double *e1, const struct long2double *e2, const void *unused)
{
return !(e1->key == e2->key);
}
void long2double_init(void)
{
hashmap_init(&map, (hashmap_cmp_fn) long2double_cmp, 0);
}
void long2double_free(void)
{
hashmap_free(&map, 1);
}
static struct long2double *find_entry(long key)
{
struct long2double k;
hashmap_entry_init(&k, memhash(&key, sizeof(long)));
k.key = key;
return hashmap_get(&map, &k, NULL);
}
double get_value(long key)
{
struct long2double *e = find_entry(key);
return e ? e->value : 0;
}
void set_value(long key, double value)
{
struct long2double *e = find_entry(key);
if (!e) {
e = malloc(sizeof(struct long2double));
hashmap_entry_init(e, memhash(&key, sizeof(long)));
e->key = key;
hashmap_add(&map, e);
}
e->value = value;
}
Using variable-sized keys
The hashmap_entry_get and hashmap_entry_remove functions expect an ordinary
hashmap_entry structure as key to find the correct entry. If the key data is
variable-sized (e.g. a FLEX_ARRAY string) or quite large, it is undesirable
to create a full-fledged entry structure on the heap and copy all the key data
into the structure.
In this case, the keydata parameter can be used to pass
variable-sized key data directly to the comparison function, and the key
parameter can be a stripped-down, fixed size entry structure allocated on the
stack.
See test-hashmap.c for an example using arbitrary-length strings as keys.