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| -rw-r--r-- | Documentation/technical/api-hashmap.txt | 309 |
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diff --git a/Documentation/technical/api-hashmap.txt b/Documentation/technical/api-hashmap.txt deleted file mode 100644 index ccc634bbd7..0000000000 --- a/Documentation/technical/api-hashmap.txt +++ /dev/null @@ -1,309 +0,0 @@ -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. |