Current File : //usr/share/doc/git-1.8.3.1/technical/api-hashmap.txt
hashmap API
===========
The hashmap API is a generic implementation of hash-based key-value mappings.
Data Structures
---------------
`struct hashmap`::
The hash table structure.
+
The `size` member keeps track of the total number of entries. The `cmpfn`
member is a function used to compare two entries for equality. The `table` and
`tablesize` members store the hash table and its size, respectively.
`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 `hash` member is the entry's hash code and the `next` member 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
-----
`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_key`
parameters that have the same hash code. When looking up an entry, the `key`
and `keydata` parameters to hashmap_get and hashmap_remove are always passed
as second and third argument, respectively. Otherwise, `keydata` is NULL.
Functions
---------
`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).
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`strhash` and `strihash` take 0-terminated strings, while `memhash` and
`memihash` operate on arbitrary-length memory.
+
`strihash` and `memihash` are case insensitive versions.
`void hashmap_init(struct hashmap *map, hashmap_cmp_fn equals_function, size_t initial_size)`::
Initializes a hashmap structure.
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`map` is the hashmap to initialize.
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The `equals_function` can be specified to compare two entries for equality.
If NULL, entries are considered equal if their hash codes are equal.
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If the total number of entries is known in advance, the `initial_size`
parameter 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.
+
`map` is the hashmap to free.
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If `free_entries` is true, each hashmap_entry in the map is freed as well
(using stdlib's free()).
`void hashmap_entry_init(void *entry, int hash)`::
Initializes a hashmap_entry structure.
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`entry` points to the entry to initialize.
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`hash` is the hash code of the entry.
`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.
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`map` is the hashmap structure.
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`key` is a hashmap_entry structure (or user data structure that starts with
hashmap_entry) that has at least been initialized with the proper hash code
(via `hashmap_entry_init`).
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If an entry with matching hash code is found, `key` and `keydata` are passed
to `hashmap_cmp_fn` to 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.
+
`map` is the hashmap structure.
+
`hash` is the hash code of the entry to look up.
+
If an entry with matching hash code is found, `keydata` is passed to
`hashmap_cmp_fn` to decide whether the entry matches the key. The
`entry_or_key` parameter 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`).
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`map` is the hashmap structure.
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`entry` is the hashmap_entry to start the search from, obtained via a previous
call to `hashmap_get` or `hashmap_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).
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`map` is the hashmap structure.
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`entry` is 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.
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`map` is the hashmap structure.
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`entry` is the entry to add or replace.
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Returns the replaced entry, or NULL if not found (i.e. the entry was added).
`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.
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`map` is the hashmap structure.
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`key` is a hashmap_entry structure (or user data structure that starts with
hashmap_entry) that has at least been initialized with the proper hash code
(via `hashmap_entry_init`).
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If an entry with matching hash code is found, `key` and `keydata` are
passed to `hashmap_cmp_fn` to decide whether the entry matches the key.
+
Returns the removed entry, or NULL if not found.
`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_init` initializes a `hashmap_iter` structure.
+
`hashmap_iter_next` returns the next hashmap_entry, or NULL if there are no
more entries.
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`hashmap_iter_first` is a combination of both (i.e. initializes the iterator
and returns the first entry, if any).
Usage example
-------------
Here's a simple usage example that maps long keys to double values.
[source,c]
------------
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.
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