hashmap API
===========

The hashmap API is a generic implementation of hash-based key-value mappings.

Data Structures
---------------

`struct hashmap`::

        The hash table structure. Members can be used as follows, but should
        not be modified directly:
+
The `size` member keeps track of the total number of entries (0 means the
hashmap is empty).
+
`tablesize` is 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 / tablesize` is the current load factor).
+
`cmpfn` stores the comparison function specified in `hashmap_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 `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).
+
`strhash` and `strihash` take 0-terminated strings, while `memhash` and
`memihash` operate on arbitrary-length memory.
+
`strihash` and `memihash` are 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 first `sizeof(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.
+
`map` is the hashmap to initialize.
+
The `equals_function` can 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_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.
+
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, unsigned int hash)`::

        Initializes a hashmap_entry structure.
+
`entry` points to the entry to initialize.
+
`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.
+
`map` is the hashmap structure.
+
`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`).
+
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_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`).
+
`map` is the hashmap structure.
+
`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).
+
`map` is the hashmap structure.
+
`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.
+
`map` is the hashmap structure.
+
`entry` is the entry to add or replace.
+
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.
+
`map` is the hashmap structure.
+
`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`).
+
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.
+
`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.
------------
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.