add a hashtable implementation that supports O(1) removal

The existing hashtable implementation (in hash.[ch]) uses open addressing
(i.e. resolve hash collisions by distributing entries across the table).
Thus, removal is difficult to implement with less than O(n) complexity.
Resolving collisions of entries with identical hashes (e.g. via chaining)
is left to the client code.

Add a hashtable implementation that supports O(1) removal and is slightly
easier to use due to builtin entry chaining.

Supports all basic operations init, free, get, add, remove and iteration.

Also includes ready-to-use hash functions based on the public domain FNV-1
algorithm (http://www.isthe.com/chongo/tech/comp/fnv).

The per-entry data structure (hashmap_entry) is piggybacked in front of
the client's data structure to save memory. See test-hashmap.c for usage
examples.

The hashtable is resized by a factor of four when 80% full. With these
settings, average memory consumption is about 2/3 of hash.[ch], and
insertion is about twice as fast due to less frequent resizing.

Lookups are also slightly faster, because entries are strictly confined to
their bucket (i.e. no data of other buckets needs to be traversed).

Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 235 insertions, 0 deletions

diff --git a/Documentation/technical/api-hashmap.txt b/Documentation/technical/api-hashmap.txt
new file mode 100644
index 0000000000..b2280f1b0d
--- /dev/null
+++ b/Documentation/technical/api-hashmap.txt
@@ -0,0 +1,235 @@
+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).
++
+`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.
++
+`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, 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.
+[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.