/* Hash tables.
Copyright (C) 2000, 2001 Free Software Foundation, Inc.
-This file is part of Wget.
+This file is part of GNU Wget.
-This program is free software; you can redistribute it and/or modify
+GNU Wget is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or (at
your option) any later version.
-This program is distributed in the hope that it will be useful,
+GNU Wget is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
-along with this program; if not, write to the Free Software
-Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. */
+along with Wget; if not, write to the Free Software
+Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+
+In addition, as a special exception, the Free Software Foundation
+gives permission to link the code of its release of Wget with the
+OpenSSL project's "OpenSSL" library (or with modified versions of it
+that use the same license as the "OpenSSL" library), and distribute
+the linked executables. You must obey the GNU General Public License
+in all respects for all of the code used other than "OpenSSL". If you
+modify this file, you may extend this exception to your version of the
+file, but you are not obligated to do so. If you do not wish to do
+so, delete this exception statement from your version. */
+
+/* With -DSTANDALONE, this file can be compiled outside Wget source
+ tree. To test, also use -DTEST. */
#ifdef HAVE_CONFIG_H
# include <config.h>
#endif
-#ifdef HAVE_STRING_H
-# include <string.h>
-#else
-# include <strings.h>
-#endif /* HAVE_STRING_H */
+#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
+#include <string.h>
+#include <limits.h>
-#include "wget.h"
-#include "utils.h"
-
-#include "hash.h"
-
-#ifdef STANDALONE
-# undef xmalloc
-# undef xrealloc
-# undef xfree
-
-# define xmalloc malloc
-# define xrealloc realloc
+#ifndef STANDALONE
+/* Get Wget's utility headers. */
+# include "wget.h"
+# include "utils.h"
+#else
+/* Make do without them. */
+# define xnew(x) xmalloc (sizeof (x))
+# define xnew_array(type, x) xmalloc (sizeof (type) * (x))
+# define xmalloc malloc /* or something that exits
+ if not enough memory */
# define xfree free
+# define countof(x) (sizeof (x) / sizeof ((x)[0]))
+# define TOLOWER(x) ('A' <= (x) && (x) <= 'Z' ? (x) - 32 : (x))
#endif
-/* INTERFACE:
-
- Hash tables are an implementation technique used to implement
- mapping between objects. Provided a good hashing function is used,
- they guarantee constant-time access and storing of information.
- Duplicate keys are not allowed.
-
- The basics are all covered. hash_table_new creates a hash table,
- and hash_table_destroy deletes it. hash_table_put establishes a
- mapping between a key and a value. hash_table_get retrieves the
- value that corresponds to a key. hash_table_contains queries
- whether a key is stored in a table at all. hash_table_remove
- removes a mapping that corresponds to a key. hash_table_map allows
- you to map through all the entries in a hash table.
- hash_table_clear clears all the entries from the hash table.
-
- The number of mappings in a table is not limited, except by the
- amount of memory. As you add new elements to a table, it regrows
- as necessary. If you have an idea about how many elements you will
- store, you can provide a hint to hash_table_new().
-
- The hashing and equality functions are normally provided by the
- user. For the special (and frequent) case of hashing strings, you
- can use the pre-canned make_string_hash_table(), which provides an
- efficient string hashing function, and a string equality wrapper
- around strcmp().
-
- When specifying your own hash and test functions, make sure the
- following holds true:
-
- - The test function returns non-zero for keys that are considered
- "equal", zero otherwise.
+#include "hash.h"
- - The hash function returns a number that represents the
- "distinctness" of the object. In more precise terms, it means
- that for any two objects that test "equal" under the test
- function, the hash function MUST produce the same result.
+/* INTERFACE:
- This does not mean that each distinct object must produce a
- distinct value, only that non-distinct objects must produce the
- same values! For instance, a hash function that returns 0 for
- any given object is a perfectly valid (albeit extremely bad) hash
- function. A hash function that hashes a string by adding up all
- its characters is another example of a valid (but quite bad) hash
- function.
-
- The above stated rule is quite easy to enforce. For example, if
- your testing function compares strings case-insensitively, all
- your function needs to do is lower-case the string characters
- before calculating a hash. That way you have easily guaranteed
- that case differences will not result in a different hash.
-
- - (optional) Choose the hash function to get as good "spreading" as
- possible. A good hash function will react to even a small change
- in its input with a completely different resulting hash.
- Finally, don't make your hash function extremely slow, because
- you're then defeating the purpose of hashing.
+ Hash tables are a technique used to implement mapping between
+ objects with near-constant-time access and storage. The table
+ associates keys to values, and a value can be very quickly
+ retrieved by providing the key. Fast lookup tables are typically
+ implemented as hash tables.
+
+ The entry points are
+ hash_table_new -- creates the table.
+ hash_table_destroy -- destroys the table.
+ hash_table_put -- establishes or updates key->value mapping.
+ hash_table_get -- retrieves value of key.
+ hash_table_get_pair -- get key/value pair for key.
+ hash_table_contains -- test whether the table contains key.
+ hash_table_remove -- remove the key->value mapping for key.
+ hash_table_map -- iterate through table mappings.
+ hash_table_clear -- clear hash table contents.
+ hash_table_count -- return the number of entries in the table.
+
+ The hash table grows internally as new entries are added and is not
+ limited in size, except by available memory. The table doubles
+ with each resize, which ensures that the amortized time per
+ operation remains constant.
+
+ By default, tables created by hash_table_new consider the keys to
+ be equal if their pointer values are the same. You can use
+ make_string_hash_table to create tables whose keys are considered
+ equal if their string contents are the same. In the general case,
+ the criterion of equality used to compare keys is specified at
+ table creation time with two callback functions, "hash" and "test".
+ The hash function transforms the key into an arbitrary number that
+ must be the same for two equal keys. The test function accepts two
+ keys and returns non-zero if they are to be considered equal.
Note that neither keys nor values are copied when inserted into the
hash table, so they must exist for the lifetime of the table. This
/* IMPLEMENTATION:
- All the hash mappings (key-value pairs of pointers) are stored in a
- contiguous array. The position of each mapping is determined by
- applying the hash function to the key: location = hash(key) % size.
- If two different keys end up on the same position, the collision is
- resolved by placing the second mapping at the next empty place in
- the array following the occupied place. This method of collision
- resolution is called "linear probing".
-
- There are more advanced collision resolution mechanisms (quadratic
- probing, double hashing), but we don't use them because they
- involve more non-sequential access to the array, and therefore
- worse cache behavior. Linear probing works well as long as the
- fullness/size ratio is kept below 75%. We make sure to regrow or
- rehash the hash table whenever this threshold is exceeded.
-
- Collisions make deletion tricky because finding collisions again
- relies on new empty spots not being created. That's why
- hash_table_remove is careful to rehash the mappings that follow the
- deleted one. */
+ The hash table is implemented as an open-addressed table with
+ linear probing collision resolution.
+
+ The above means that all the hash entries (pairs of pointers, key
+ and value) are stored in a contiguous array. The position of each
+ mapping is determined by the hash value of its key and the size of
+ the table: location := hash(key) % size. If two different keys end
+ up on the same position (collide), the one that came second is
+ placed at the next empty position following the occupied place.
+ This collision resolution technique is called "linear probing".
+
+ There are more advanced collision resolution methods (quadratic
+ probing, double hashing), but we don't use them because they incur
+ more non-sequential access to the array, which results in worse CPU
+ cache behavior. Linear probing works well as long as the
+ count/size ratio (fullness) is kept below 75%. We make sure to
+ grow and rehash the table whenever this threshold is exceeded.
+
+ Collisions make deletion tricky because clearing a position
+ followed by a colliding entry would make the position seem empty
+ and the colliding entry not found. One solution is to leave a
+ "tombstone" instead of clearing the entry, and another is to
+ carefully rehash the entries immediately following the deleted one.
+ We use the latter method because it results in less bookkeeping and
+ faster retrieval at the (slight) expense of deletion. */
+
+/* Maximum allowed fullness: when hash table's fullness exceeds this
+ value, the table is resized. */
+#define HASH_MAX_FULLNESS 0.75
+
+/* The hash table size is multiplied by this factor (and then rounded
+ to the next prime) with each resize. This guarantees infrequent
+ resizes. */
+#define HASH_RESIZE_FACTOR 2
struct mapping {
void *key;
void *value;
};
+typedef unsigned long (*hashfun_t) (const void *);
+typedef int (*testfun_t) (const void *, const void *);
+
struct hash_table {
- unsigned long (*hash_function) (const void *);
- int (*test_function) (const void *, const void *);
+ hashfun_t hash_function;
+ testfun_t test_function;
- int size; /* size of the array */
- int count; /* number of non-empty, non-deleted
- fields. */
+ struct mapping *mappings; /* pointer to the table entries. */
+ int size; /* size of the array. */
+ int count; /* number of non-empty entries. */
int resize_threshold; /* after size exceeds this number of
entries, resize the table. */
-
- struct mapping *mappings;
+ int prime_offset; /* the offset of the current prime in
+ the prime table. */
};
-#define EMPTY_MAPPING_P(mp) ((mp)->key == NULL)
-#define NEXT_MAPPING(mp, mappings, size) (mp == mappings + (size - 1) \
- ? mappings : mp + 1)
+/* We use the all-bits-set constant (INVALID_PTR) marker to mean that
+ a mapping is empty. It is unaligned and therefore illegal as a
+ pointer. INVALID_PTR_BYTE (0xff) is the one-byte value used to
+ initialize the mappings array as empty.
+
+ The all-bits-set value is a better choice than NULL because it
+ allows the use of NULL/0 keys. Since the keys are either integers
+ or pointers, the only key that cannot be used is the integer value
+ -1. This is acceptable because it still allows the use of
+ nonnegative integer keys. */
+
+#define INVALID_PTR ((void *) ~0UL)
+#ifndef UCHAR_MAX
+# define UCHAR_MAX 0xff
+#endif
+#define INVALID_PTR_BYTE UCHAR_MAX
+
+#define NON_EMPTY(mp) ((mp)->key != INVALID_PTR)
+#define MARK_AS_EMPTY(mp) ((mp)->key = INVALID_PTR)
+
+/* "Next" mapping is the mapping after MP, but wrapping back to
+ MAPPINGS when MP would reach MAPPINGS+SIZE. */
+#define NEXT_MAPPING(mp, mappings, size) (mp != mappings + (size - 1) \
+ ? mp + 1 : mappings)
+/* Loop over non-empty mappings starting at MP. */
#define LOOP_NON_EMPTY(mp, mappings, size) \
- for (; !EMPTY_MAPPING_P (mp); mp = NEXT_MAPPING (mp, mappings, size))
+ for (; NON_EMPTY (mp); mp = NEXT_MAPPING (mp, mappings, size))
+
+/* Return the position of KEY in hash table SIZE large, hash function
+ being HASHFUN. */
+#define HASH_POSITION(key, hashfun, size) ((hashfun) (key) % size)
-#define HASH_POSITION(ht, key) (ht->hash_function (key) % ht->size)
+/* Find a prime near, but greather than or equal to SIZE. The primes
+ are looked up from a table with a selection of primes convenient
+ for this purpose.
-/* Find a prime near, but greather than or equal to SIZE. */
+ PRIME_OFFSET is a minor optimization: it specifies start position
+ for the search for the large enough prime. The final offset is
+ stored in the same variable. That way the list of primes does not
+ have to be scanned from the beginning each time around. */
static int
-prime_size (int size)
+prime_size (int size, int *prime_offset)
{
- static const unsigned long primes [] = {
- 19, 29, 41, 59, 79, 107, 149, 197, 263, 347, 457, 599, 787, 1031,
+ static const int primes[] = {
+ 13, 19, 29, 41, 59, 79, 107, 149, 197, 263, 347, 457, 599, 787, 1031,
1361, 1777, 2333, 3037, 3967, 5167, 6719, 8737, 11369, 14783,
19219, 24989, 32491, 42257, 54941, 71429, 92861, 120721, 156941,
204047, 265271, 344857, 448321, 582821, 757693, 985003, 1280519,
10445899, 13579681, 17653589, 22949669, 29834603, 38784989,
50420551, 65546729, 85210757, 110774011, 144006217, 187208107,
243370577, 316381771, 411296309, 534685237, 695090819, 903618083,
- 1174703521, 1527114613, 1985248999, 2580823717UL, 3355070839UL
+ 1174703521, 1527114613, 1837299131, 2147483647
};
int i;
- for (i = 0; i < ARRAY_SIZE (primes); i++)
+
+ for (i = *prime_offset; i < countof (primes); i++)
if (primes[i] >= size)
- return primes[i];
- /* huh? */
- return size;
+ {
+ /* Set the offset to the next prime. That is safe because,
+ next time we are called, it will be with a larger SIZE,
+ which means we could never return the same prime anyway.
+ (If that is not the case, the caller can simply reset
+ *prime_offset.) */
+ *prime_offset = i + 1;
+ return primes[i];
+ }
+
+ abort ();
}
-/* Create a hash table of INITIAL_SIZE with hash function
- HASH_FUNCTION and test function TEST_FUNCTION. INITIAL_SIZE will
- be rounded to the next prime, so you don't have to worry about it
- being a prime number.
+static int cmp_pointer (const void *, const void *);
+
+/* Create a hash table with hash function HASH_FUNCTION and test
+ function TEST_FUNCTION. The table is empty (its count is 0), but
+ pre-allocated to store at least ITEMS items.
+
+ ITEMS is the number of items that the table can accept without
+ needing to resize. It is useful when creating a table that is to
+ be immediately filled with a known number of items. In that case,
+ the regrows are a waste of time, and specifying ITEMS correctly
+ will avoid them altogether.
+
+ Note that hash tables grow dynamically regardless of ITEMS. The
+ only use of ITEMS is to preallocate the table and avoid unnecessary
+ dynamic regrows. Don't bother making ITEMS prime because it's not
+ used as size unchanged. To start with a small table that grows as
+ needed, simply specify zero ITEMS.
- Consequently, if you wish to start out with a "small" table which
- will be regrown as needed, specify INITIAL_SIZE 0. */
+ If hash and test callbacks are not specified, identity mapping is
+ assumed, i.e. pointer values are used for key comparison. (Common
+ Lisp calls such tables EQ hash tables, and Java calls them
+ IdentityHashMaps.) If your keys require different comparison,
+ specify hash and test functions. For easy use of C strings as hash
+ keys, you can use the convenience functions make_string_hash_table
+ and make_nocase_string_hash_table. */
struct hash_table *
-hash_table_new (int initial_size,
+hash_table_new (int items,
unsigned long (*hash_function) (const void *),
int (*test_function) (const void *, const void *))
{
- struct hash_table *ht
- = (struct hash_table *)xmalloc (sizeof (struct hash_table));
+ int size;
+ struct hash_table *ht = xnew (struct hash_table);
- ht->hash_function = hash_function;
- ht->test_function = test_function;
+ ht->hash_function = hash_function ? hash_function : hash_pointer;
+ ht->test_function = test_function ? test_function : cmp_pointer;
- ht->size = prime_size (initial_size);
- ht->resize_threshold = ht->size * 3 / 4;
+ /* If the size of struct hash_table ever becomes a concern, this
+ field can go. (Wget doesn't create many hashes.) */
+ ht->prime_offset = 0;
- ht->count = 0;
+ /* Calculate the size that ensures that the table will store at
+ least ITEMS keys without the need to resize. */
+ size = 1 + items / HASH_MAX_FULLNESS;
+ size = prime_size (size, &ht->prime_offset);
+ ht->size = size;
+ ht->resize_threshold = size * HASH_MAX_FULLNESS;
+ /*assert (ht->resize_threshold >= items);*/
- ht->mappings = xmalloc (ht->size * sizeof (struct mapping));
- memset (ht->mappings, '\0', ht->size * sizeof (struct mapping));
+ ht->mappings = xnew_array (struct mapping, ht->size);
+
+ /* Mark mappings as empty. We use 0xff rather than 0 to mark empty
+ keys because it allows us to use NULL/0 as keys. */
+ memset (ht->mappings, INVALID_PTR_BYTE, size * sizeof (struct mapping));
+
+ ht->count = 0;
return ht;
}
xfree (ht);
}
-/* The heart of almost all functions in this file -- find the mapping
- whose KEY is equal to key, using linear probing. Returns the
- mapping that matches KEY, or NULL if none matches. */
+/* The heart of most functions in this file -- find the mapping whose
+ KEY is equal to key, using linear probing. Returns the mapping
+ that matches KEY, or the first empty mapping if none matches. */
static inline struct mapping *
-find_mapping (struct hash_table *ht, const void *key)
+find_mapping (const struct hash_table *ht, const void *key)
{
struct mapping *mappings = ht->mappings;
int size = ht->size;
- struct mapping *mp = mappings + HASH_POSITION (ht, key);
- int (*equals) (const void *, const void *) = ht->test_function;
+ struct mapping *mp = mappings + HASH_POSITION (key, ht->hash_function, size);
+ testfun_t equals = ht->test_function;
LOOP_NON_EMPTY (mp, mappings, size)
if (equals (key, mp->key))
- return mp;
- return NULL;
+ break;
+ return mp;
}
/* Get the value that corresponds to the key KEY in the hash table HT.
function. */
void *
-hash_table_get (struct hash_table *ht, const void *key)
+hash_table_get (const struct hash_table *ht, const void *key)
{
struct mapping *mp = find_mapping (ht, key);
- if (mp)
+ if (NON_EMPTY (mp))
return mp->value;
else
return NULL;
value. Returns non-zero on success. */
int
-hash_table_get_pair (struct hash_table *ht, const void *lookup_key,
+hash_table_get_pair (const struct hash_table *ht, const void *lookup_key,
void *orig_key, void *value)
{
struct mapping *mp = find_mapping (ht, lookup_key);
-
- if (mp)
+ if (NON_EMPTY (mp))
{
if (orig_key)
*(void **)orig_key = mp->key;
/* Return 1 if HT contains KEY, 0 otherwise. */
int
-hash_table_contains (struct hash_table *ht, const void *key)
+hash_table_contains (const struct hash_table *ht, const void *key)
{
- return find_mapping (ht, key) != NULL;
+ struct mapping *mp = find_mapping (ht, key);
+ return NON_EMPTY (mp);
}
/* Grow hash table HT as necessary, and rehash all the key-value
static void
grow_hash_table (struct hash_table *ht)
{
+ hashfun_t hasher = ht->hash_function;
struct mapping *old_mappings = ht->mappings;
struct mapping *old_end = ht->mappings + ht->size;
struct mapping *mp, *mappings;
int newsize;
- newsize = prime_size (ht->size * 2);
+ newsize = prime_size (ht->size * HASH_RESIZE_FACTOR, &ht->prime_offset);
#if 0
- printf ("growing from %d to %d\n", ht->size, newsize);
+ printf ("growing from %d to %d; fullness %.2f%% to %.2f%%\n",
+ ht->size, newsize,
+ 100.0 * ht->count / ht->size,
+ 100.0 * ht->count / newsize);
#endif
ht->size = newsize;
- ht->resize_threshold = newsize * 3 / 4;
+ ht->resize_threshold = newsize * HASH_MAX_FULLNESS;
- mappings = xmalloc (ht->size * sizeof (struct mapping));
- memset (mappings, '\0', ht->size * sizeof (struct mapping));
+ mappings = xnew_array (struct mapping, newsize);
+ memset (mappings, INVALID_PTR_BYTE, newsize * sizeof (struct mapping));
ht->mappings = mappings;
for (mp = old_mappings; mp < old_end; mp++)
- if (!EMPTY_MAPPING_P (mp))
+ if (NON_EMPTY (mp))
{
- struct mapping *new_mp = mappings + HASH_POSITION (ht, mp->key);
- /* We don't need to call test function and worry about
- collisions because all the keys come from the hash table
- and are therefore guaranteed to be unique. */
+ struct mapping *new_mp;
+ /* We don't need to test for uniqueness of keys because they
+ come from the hash table and are therefore known to be
+ unique. */
+ new_mp = mappings + HASH_POSITION (mp->key, hasher, newsize);
LOOP_NON_EMPTY (new_mp, mappings, newsize)
;
*new_mp = *mp;
void
hash_table_put (struct hash_table *ht, const void *key, void *value)
{
- struct mapping *mappings = ht->mappings;
- int size = ht->size;
- int (*equals) (const void *, const void *) = ht->test_function;
-
- struct mapping *mp = mappings + HASH_POSITION (ht, key);
+ struct mapping *mp = find_mapping (ht, key);
+ if (NON_EMPTY (mp))
+ {
+ /* update existing item */
+ mp->key = (void *)key; /* const? */
+ mp->value = value;
+ return;
+ }
- LOOP_NON_EMPTY (mp, mappings, size)
- if (equals (key, mp->key))
- {
- mp->key = (void *)key; /* const? */
- mp->value = value;
- return;
- }
+ /* If adding the item would make the table exceed max. fullness,
+ grow the table first. */
+ if (ht->count >= ht->resize_threshold)
+ {
+ grow_hash_table (ht);
+ mp = find_mapping (ht, key);
+ }
+ /* add new item */
++ht->count;
mp->key = (void *)key; /* const? */
mp->value = value;
-
- if (ht->count > ht->resize_threshold)
- /* When table is 75% full, regrow it. */
- grow_hash_table (ht);
}
/* Remove a mapping that matches KEY from HT. Return 0 if there was
hash_table_remove (struct hash_table *ht, const void *key)
{
struct mapping *mp = find_mapping (ht, key);
- if (!mp)
+ if (!NON_EMPTY (mp))
return 0;
else
{
int size = ht->size;
struct mapping *mappings = ht->mappings;
+ hashfun_t hasher = ht->hash_function;
- mp->key = NULL;
+ MARK_AS_EMPTY (mp);
--ht->count;
/* Rehash all the entries following MP. The alternative
approach is to mark the entry as deleted, i.e. create a
- "tombstone". That makes remove faster, but leaves a lot of
+ "tombstone". That speeds up removal, but leaves a lot of
garbage and slows down hash_table_get and hash_table_put. */
mp = NEXT_MAPPING (mp, mappings, size);
LOOP_NON_EMPTY (mp, mappings, size)
{
const void *key2 = mp->key;
- struct mapping *mp_new = mappings + HASH_POSITION (ht, key2);
+ struct mapping *mp_new;
/* Find the new location for the key. */
-
+ mp_new = mappings + HASH_POSITION (key2, hasher, size);
LOOP_NON_EMPTY (mp_new, mappings, size)
if (key2 == mp_new->key)
/* The mapping MP (key2) is already where we want it (in
goto next_rehash;
*mp_new = *mp;
- mp->key = NULL;
+ MARK_AS_EMPTY (mp);
next_rehash:
;
void
hash_table_clear (struct hash_table *ht)
{
- memset (ht->mappings, '\0', ht->size * sizeof (struct mapping));
+ memset (ht->mappings, INVALID_PTR_BYTE, ht->size * sizeof (struct mapping));
ht->count = 0;
}
/* Map MAPFUN over all the mappings in hash table HT. MAPFUN is
- called with three arguments: the key, the value, and the CLOSURE.
+ called with three arguments: the key, the value, and MAPARG.
It is undefined what happens if you add or remove entries in the
hash table while hash_table_map is running. The exception is the
void
hash_table_map (struct hash_table *ht,
int (*mapfun) (void *, void *, void *),
- void *closure)
+ void *maparg)
{
struct mapping *mp = ht->mappings;
struct mapping *end = ht->mappings + ht->size;
for (; mp < end; mp++)
- if (!EMPTY_MAPPING_P (mp))
+ if (NON_EMPTY (mp))
{
void *key;
repeat:
key = mp->key;
- if (mapfun (key, mp->value, closure))
+ if (mapfun (key, mp->value, maparg))
return;
/* hash_table_remove might have moved the adjacent
mappings. */
- if (mp->key != key && !EMPTY_MAPPING_P (mp))
+ if (mp->key != key && NON_EMPTY (mp))
goto repeat;
}
}
greater than the number of elements. */
int
-hash_table_count (struct hash_table *ht)
+hash_table_count (const struct hash_table *ht)
{
return ht->count;
}
don't strictly belong to this file. However, this is as good a
place for them as any. */
-/* ========
- Support for hash tables whose keys are strings.
- ======== */
+/* Guidelines for creating custom hash and test functions:
+ - The test function returns non-zero for keys that are considered
+ "equal", zero otherwise.
+
+ - The hash function returns a number that represents the
+ "distinctness" of the object. In more precise terms, it means
+ that for any two objects that test "equal" under the test
+ function, the hash function MUST produce the same result.
+
+ This does not mean that all different objects must produce
+ different values (that would be "perfect" hashing), only that
+ non-distinct objects must produce the same values! For instance,
+ a hash function that returns 0 for any given object is a
+ perfectly valid (albeit extremely bad) hash function. A hash
+ function that hashes a string by adding up all its characters is
+ another example of a valid (but quite bad) hash function.
+
+ It is not hard to make hash and test functions agree about
+ equality. For example, if the test function compares strings
+ case-insensitively, the hash function can lower-case the
+ characters when calculating the hash value. That ensures that
+ two strings differing only in case will hash the same.
+
+ - If you care about performance, choose a hash function with as
+ good "spreading" as possible. A good hash function will use all
+ the bits of the input when calculating the hash, and will react
+ to even small changes in input with a completely different
+ output. Finally, don't make the hash function itself overly
+ slow, because you'll be incurring a non-negligible overhead to
+ all hash table operations. */
+
+/*
+ * Support for hash tables whose keys are strings.
+ *
+ */
+
/* 31 bit hash function. Taken from Gnome's glib, modified to use
standard C types.
We used to use the popular hash function from the Dragon Book, but
this one seems to perform much better. */
-unsigned long
-string_hash (const void *key)
+static unsigned long
+hash_string (const void *key)
{
const char *p = key;
unsigned int h = *p;
/* Frontend for strcmp usable for hash tables. */
-int
-string_cmp (const void *s1, const void *s2)
+static int
+cmp_string (const void *s1, const void *s2)
{
return !strcmp ((const char *)s1, (const char *)s2);
}
-/* Return a hash table of initial size INITIAL_SIZE suitable to use
- strings as keys. */
+/* Return a hash table of preallocated to store at least ITEMS items
+ suitable to use strings as keys. */
struct hash_table *
-make_string_hash_table (int initial_size)
+make_string_hash_table (int items)
{
- return hash_table_new (initial_size, string_hash, string_cmp);
+ return hash_table_new (items, hash_string, cmp_string);
}
-/* ========
- Support for hash tables whose keys are strings, but which are
- compared case-insensitively.
- ======== */
+/*
+ * Support for hash tables whose keys are strings, but which are
+ * compared case-insensitively.
+ *
+ */
-/* Like string_hash, but produce the same hash regardless of the case. */
+/* Like hash_string, but produce the same hash regardless of the case. */
static unsigned long
-string_hash_nocase (const void *key)
+hash_string_nocase (const void *key)
{
const char *p = key;
unsigned int h = TOLOWER (*p);
string_cmp_nocase. */
struct hash_table *
-make_nocase_string_hash_table (int initial_size)
+make_nocase_string_hash_table (int items)
{
- return hash_table_new (initial_size, string_hash_nocase, string_cmp_nocase);
+ return hash_table_new (items, hash_string_nocase, string_cmp_nocase);
}
-#if 0
-/* If I ever need it: hashing of integers. */
+/* Hashing of numeric values, such as pointers and integers.
-unsigned int
-inthash (unsigned int key)
+ This implementation is the Robert Jenkins' 32 bit Mix Function,
+ with a simple adaptation for 64-bit values. It offers excellent
+ spreading of values and doesn't need to know the hash table size to
+ work (unlike the very popular Knuth's multiplication hash). */
+
+unsigned long
+hash_pointer (const void *ptr)
{
+ unsigned long key = (unsigned long)ptr;
key += (key << 12);
key ^= (key >> 22);
key += (key << 4);
key ^= (key >> 2);
key += (key << 7);
key ^= (key >> 12);
+#if SIZEOF_LONG > 4
+ key += (key << 44);
+ key ^= (key >> 54);
+ key += (key << 36);
+ key ^= (key >> 41);
+ key += (key << 42);
+ key ^= (key >> 34);
+ key += (key << 39);
+ key ^= (key >> 44);
+#endif
return key;
}
-#endif
+
+static int
+cmp_pointer (const void *ptr1, const void *ptr2)
+{
+ return ptr1 == ptr2;
+}
\f
-#ifdef STANDALONE
+#ifdef TEST
#include <stdio.h>
#include <string.h>
#endif
return 0;
}
-#endif
+#endif /* TEST */
projects / wget / blobdiff