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hashmap: migrate documentation from Documentation/technical into header 

While at it, clarify the use of `key`, `keydata`, `entry_or_key` as well
as documenting the new data pointer for the compare function.

Rework the example.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Stefan Beller 2017-06-30 12:14:07 -07:00 коммит произвёл Junio C Hamano
Родитель 3da492f808
Коммит 1ecbf31d02
2 изменённых файлов: 317 добавлений и 342 удалений
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309
Documentation/technical/api-hashmap.txt
Просмотреть файл

@ -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.

350
hashmap.h
Просмотреть файл

@ -3,17 +3,123 @@
/*
* Generic implementation of hash-based key-value mappings.
* See Documentation/technical/api-hashmap.txt.
*
* An example that maps long to a string:
* For the sake of the example this allows to lookup exact values, too
* (i.e. it is operated as a set, the value is part of the key)
* -------------------------------------
*
* struct hashmap map;
* struct long2string {
* struct hashmap_entry ent; // must be the first member!
* long key;
* char value[FLEX_ARRAY]; // be careful with allocating on stack!
* };
*
* #define COMPARE_VALUE 1
*
* static int long2string_cmp(const struct long2string *e1,
* const struct long2string *e2,
* const void *keydata, const void *userdata)
* {
* char *string = keydata;
* unsigned *flags = (unsigned*)userdata;
*
* if (flags & COMPARE_VALUE)
* return !(e1->key == e2->key) || (keydata ?
* strcmp(e1->value, keydata) : strcmp(e1->value, e2->value));
* else
* return !(e1->key == e2->key);
* }
*
* int main(int argc, char **argv)
* {
* long key;
* char *value, *action;
*
* unsigned flags = ALLOW_DUPLICATE_KEYS;
*
* hashmap_init(&map, (hashmap_cmp_fn) long2string_cmp, &flags, 0);
*
* while (scanf("%s %l %s", action, key, value)) {
*
* if (!strcmp("add", action)) {
* struct long2string *e;
* e = malloc(sizeof(struct long2string) + strlen(value));
* hashmap_entry_init(e, memhash(&key, sizeof(long)));
* e->key = key;
* memcpy(e->value, value, strlen(value));
* hashmap_add(&map, e);
* }
*
* if (!strcmp("print_all_by_key", action)) {
* flags &= ~COMPARE_VALUE;
*
* struct long2string k;
* hashmap_entry_init(&k, memhash(&key, sizeof(long)));
* k.key = key;
*
* struct long2string *e = hashmap_get(&map, &k, NULL);
* if (e) {
* printf("first: %l %s\n", e->key, e->value);
* while (e = hashmap_get_next(&map, e))
* printf("found more: %l %s\n", e->key, e->value);
* }
* }
*
* if (!strcmp("has_exact_match", action)) {
* flags |= COMPARE_VALUE;
*
* struct long2string *e;
* e = malloc(sizeof(struct long2string) + strlen(value));
* hashmap_entry_init(e, memhash(&key, sizeof(long)));
* e->key = key;
* memcpy(e->value, value, strlen(value));
*
* printf("%s found\n", hashmap_get(&map, e, NULL) ? "" : "not");
* }
*
* if (!strcmp("has_exact_match_no_heap_alloc", action)) {
* flags |= COMPARE_VALUE;
*
* struct long2string e;
* hashmap_entry_init(e, memhash(&key, sizeof(long)));
* e.key = key;
*
* printf("%s found\n", hashmap_get(&map, e, value) ? "" : "not");
* }
*
* if (!strcmp("end", action)) {
* hashmap_free(&map, 1);
* break;
* }
* }
* }
*/
/* FNV-1 functions */
/*
* 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.
*/
extern unsigned int strhash(const char *buf);
extern unsigned int strihash(const char *buf);
extern unsigned int memhash(const void *buf, size_t len);
extern unsigned int memihash(const void *buf, size_t len);
extern unsigned int memihash_cont(unsigned int hash_seed, const void *buf, size_t len);
/*
* 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.
*/
static inline unsigned int sha1hash(const unsigned char *sha1)
{
/*
@ -25,90 +131,255 @@ static inline unsigned int sha1hash(const unsigned char *sha1)
return hash;
}
/* data structures */
/*
* struct hashmap_entry is 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.
*/
struct hashmap_entry {
/*
* next points to the next entry in case of collisions (i.e. if
* multiple entries map to the same bucket)
*/
struct hashmap_entry *next;
/* entry's hash code */
unsigned int hash;
};
/*
* 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` and `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 `entry_or_key` and third
* argument `keydata`, respectively. Otherwise, `keydata` is NULL.
*
* When it is too expensive to allocate a user entry (either because it is
* large or varialbe sized, such that it is not on the stack), then the
* relevant data to check for equality should be passed via `keydata`.
* In this case `key` can be a stripped down version of the user key data
* or even just a hashmap_entry having the correct hash.
*
* The `hashmap_cmp_fn_data` entry is the pointer given in the init function.
*/
typedef int (*hashmap_cmp_fn)(const void *hashmap_cmp_fn_data,
const void *entry, const void *entry_or_key,
const void *keydata);
/*
* struct hashmap is the hash table structure. Members can be used as follows,
* but should not be modified directly.
*/
struct hashmap {
struct hashmap_entry **table;
/* Stores the comparison function specified in `hashmap_init()`. */
hashmap_cmp_fn cmpfn;
const void *cmpfn_data;
unsigned int size, tablesize, grow_at, shrink_at;
unsigned disallow_rehash : 1;
};
struct hashmap_iter {
struct hashmap *map;
struct hashmap_entry *next;
unsigned int tablepos;
/* total number of entries (0 means the hashmap is empty) */
unsigned int size;
/*
* 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).
*/
unsigned int tablesize;
unsigned int grow_at;
unsigned int shrink_at;
/* See `hashmap_disallow_rehash`. */
unsigned disallow_rehash : 1;
};
/* hashmap functions */
/*
* 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.
*
* The `equals_function_data` parameter can be used to provide additional data
* (a callback cookie) that will be passed to `equals_function` each time it
* is called. This allows a single `equals_function` to implement multiple
* comparison functions.
*
* 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.
*/
extern void hashmap_init(struct hashmap *map,
hashmap_cmp_fn equals_function,
const void *equals_function_data,
size_t initial_size);
/*
* Frees a hashmap structure and allocated memory.
*
* If `free_entries` is true, each hashmap_entry in the map is freed as well
* using stdlibs free().
*/
extern void hashmap_free(struct hashmap *map, int free_entries);
/* hashmap_entry functions */
/*
* 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).
*/
static inline void hashmap_entry_init(void *entry, unsigned int hash)
{
struct hashmap_entry *e = entry;
e->hash = hash;
e->next = NULL;
}
extern void *hashmap_get(const struct hashmap *map, const void *key,
const void *keydata);
extern void *hashmap_get_next(const struct hashmap *map, const void *entry);
extern void hashmap_add(struct hashmap *map, void *entry);
extern void *hashmap_put(struct hashmap *map, void *entry);
extern void *hashmap_remove(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 user data structure that starts with hashmap_entry that has at
* least been initialized with the proper hash code (via `hashmap_entry_init`).
*
* `keydata` is a data structure that holds just enough information to check
* for equality to a given 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.
*
* 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.
*/
extern void *hashmap_get(const struct hashmap *map, const void *key,
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 of `hashmap_cmp_fn` points to a hashmap_entry
* structure that should not be used in the comparison.
*/
static inline void *hashmap_get_from_hash(const struct hashmap *map,
unsigned int hash, const void *keydata)
unsigned int hash,
const void *keydata)
{
struct hashmap_entry key;
hashmap_entry_init(&key, hash);
return hashmap_get(map, &key, keydata);
}
/*
* 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`.
*/
extern void *hashmap_get_next(const struct hashmap *map, const 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.
*/
extern void hashmap_add(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).
*/
extern void *hashmap_put(struct hashmap *map, void *entry);
/*
* 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. Returns the removed entry, or NULL if not found.
*
* Argument explanation is the same as in `hashmap_get`.
*/
extern void *hashmap_remove(struct hashmap *map, const void *key,
const void *keydata);
/*
* Returns the `bucket` an entry is stored in.
* Useful for multithreaded read access.
*/
int hashmap_bucket(const struct hashmap *map, unsigned int hash);
/*
* Disallow/allow rehashing of the hashmap.
* This is useful if the caller knows that the hashmap
* needs multi-threaded access. The caller is still
* required to guard/lock searches and inserts in a
* manner appropriate to their usage. This simply
* prevents the table from being unexpectedly re-mapped.
* This is useful if the caller knows that the hashmap needs multi-threaded
* access. The caller is still required to guard/lock searches and inserts
* in a manner appropriate to their usage. This simply prevents the table
* from being unexpectedly re-mapped.
*
* If is up to the caller to ensure that the hashmap is
* initialized to a reasonable size to prevent poor
* performance.
* It is up to the caller to ensure that the hashmap is initialized to a
* reasonable size to prevent poor performance.
*
* When value=1, prevent future rehashes on adds and deleted.
* When value=0, allow future rehahses. This DOES NOT force
* a rehash now.
* A call to allow rehashing does not force a rehash; that might happen
* with the next insert or delete.
*/
static inline void hashmap_disallow_rehash(struct hashmap *map, unsigned value)
{
map->disallow_rehash = value;
}
/* hashmap_iter functions */
/*
* 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.
*/
struct hashmap_iter {
struct hashmap *map;
struct hashmap_entry *next;
unsigned int tablepos;
};
/* Initializes a `hashmap_iter` structure. */
extern void hashmap_iter_init(struct hashmap *map, struct hashmap_iter *iter);
/* Returns the next hashmap_entry, or NULL if there are no more entries. */
extern void *hashmap_iter_next(struct hashmap_iter *iter);
/* Initializes the iterator and returns the first entry, if any. */
static inline void *hashmap_iter_first(struct hashmap *map,
struct hashmap_iter *iter)
{
@ -116,8 +387,21 @@ static inline void *hashmap_iter_first(struct hashmap *map,
return hashmap_iter_next(iter);
}
/* string interning */
/* String interning */
/*
* 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.
*/
extern const void *memintern(const void *data, size_t len);
static inline const char *strintern(const char *string)
{