moparisthebest
/
wget
mirror of https://github.com/moparisthebest/wget
1
0
Fork 0
Code Issues Releases Wiki Activity

[svn] If hash/test functions aren't given, default to identity hash tables.

This commit is contained in:
hniksic 2003-09-22 07:32:17 -07:00
parent 566289ea9d
commit c2ffc02ab3
3 changed files with 95 additions and 43 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
src/ChangeLog
View File

@ -1,3 +1,9 @@
2003-09-22 Hrvoje Niksic <hniksic@xemacs.org>
* hash.c (ptrhash): New function.
(ptrcmp): Ditto.
(hash_table_new): Default to identity hash table.
2003-09-22 Hrvoje Niksic <hniksic@xemacs.org>
* safe-ctype.h (_sch_test): The cast of BIT to unsigned char was

129
src/hash.c
View File

@ -60,31 +60,32 @@ so, delete this exception statement from your version. */
/* 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.
mapping between objects. Assuming a good hashing function is used,
they provide near-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.
This file defines the following entry points: 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().
The hashing and equality functions depend on the type of key and
are normally provided by the user. For the special (and frequent)
case of using string keys, 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
When specifying your hash and test functions, make sure the
following holds true:
- The test function returns non-zero for keys that are considered
@ -109,11 +110,12 @@ so, delete this exception statement from your version. */
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.
- If you care about performance, choose a hash function with 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 the hash function itself
overly slow, because you'll be incurring a non-negligible
overhead to reads and writes to the hash table.
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
@ -125,16 +127,16 @@ so, delete this exception statement from your version. */
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".
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 (hash collision), 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 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
probing, double hashing), but we don't use them because they incur
more non-sequential access to the array, which results in 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.
@ -227,7 +229,11 @@ prime_size (int size, int *prime_offset)
being a prime number.
Consequently, if you wish to start out with a "small" table which
will be regrown as needed, specify INITIAL_SIZE 0. */
will be regrown as needed, specify INITIAL_SIZE 0.
If HASH_FUNCTION is not provided, identity table is assumed,
i.e. key pointers are compared as keys. If you want strings with
equal contents to hash the same, use make_string_hash_table. */
struct hash_table *
hash_table_new (int initial_size,
@ -237,8 +243,8 @@ hash_table_new (int initial_size,
struct hash_table *ht
= (struct hash_table *)xmalloc (sizeof (struct hash_table));
ht->hash_function = hash_function;
ht->test_function = test_function;
ht->hash_function = hash_function ? hash_function : ptrhash;
ht->test_function = test_function ? test_function : ptrcmp;
ht->prime_offset = 0;
ht->size = prime_size (initial_size, &ht->prime_offset);
@ -497,10 +503,11 @@ hash_table_count (struct hash_table *ht)
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.
======== */
/*
* Support for hash tables whose keys are strings.
*
*/
/* 31 bit hash function. Taken from Gnome's glib, modified to use
standard C types.
@ -537,10 +544,11 @@ make_string_hash_table (int initial_size)
return hash_table_new (initial_size, string_hash, string_cmp);
}
/* ========
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. */
@ -574,10 +582,45 @@ make_nocase_string_hash_table (int initial_size)
return hash_table_new (initial_size, string_hash_nocase, string_cmp_nocase);
}
#if 0
/* If I ever need it: hashing of integers. */
/* Hashing of pointers. Used for hash tables that are keyed by
pointer identity. (Common Lisp calls them EQ hash tables, and Java
calls them IdentityHashMaps.) */
unsigned int
unsigned long
ptrhash (const void *ptr)
{
unsigned long key = (unsigned long)ptr;
key += (key << 12);
key ^= (key >> 22);
key += (key << 4);
key ^= (key >> 9);
key += (key << 10);
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;
}
int
ptrcmp (const void *ptr1, const void *ptr2)
{
return ptr1 == ptr2;
}
#if 0
/* Currently unused: hashing of integers. */
unsigned long
inthash (unsigned int key)
{
key += (key << 12);

3
src/hash.h
View File

@ -69,4 +69,7 @@ int string_cmp PARAMS ((const void *, const void *));
struct hash_table *make_string_hash_table PARAMS ((int));
struct hash_table *make_nocase_string_hash_table PARAMS ((int));
unsigned long ptrhash PARAMS ((const void *));
int ptrcmp PARAMS ((const void *, const void *));
#endif /* HASH_H */