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.
+
When `disallow_rehash` is set, automatic rehashes are prevented during inserts
and deletes.

`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)`::
`unsigned int memihash_cont(unsigned int hash_seed, 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.
+
`memihash_cont` is a variant of `memihash` that allows a computation to be
continued with another chunk of data.

`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.
+
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).

`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_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`).
+
`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_disallow_rehash(struct hashmap *map, unsigned value)`::

        Disallow/allow automatic rehashing of the hashmap during inserts
        and deletes.
+
This is useful if the caller knows that the hashmap will be accessed
by multiple threads.
+
The caller is still responsible for any necessary locking; this simply
prevents unexpected rehashing.  The caller is also responsible for properly
sizing the initial hashmap to ensure good performance.
+
A call to allow rehashing does not force a rehash; that might happen
with the next insert or delete.

`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. Note that it is
        not safe to add or remove entries to the hashmap while
        iterating.
+
`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).

`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.intern` API in Java and .NET, respectively.
        Interned strings remain valid for the entire lifetime of the process.
+
Can be used as `[x]strdup()` or `xmemdupz` replacement, 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.