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[svn] Minor optimization of prime_size.

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Published in <sxswuu2p6pm.fsf@florida.munich.redhat.com>.
This commit is contained in:
hniksic 2002-05-17 18:48:39 -07:00
parent 83dc077b17
commit a4abdda23d
2 changed files with 50 additions and 13 deletions
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10
src/ChangeLog
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@ -1,3 +1,13 @@
2002-05-16 Hrvoje Niksic <hniksic@arsdigita.com>
* hash.c (prime_size): Store the offset of the prime number in the
prime table. When searching, start with the given offset.
(hash_table_new): Pass the pointer to ht->prime_offset to
prime_size.
(grow_hash_table): Ditto.
(prime_size): Make 13 the first prime to make empty hash tables
slightly smaller.
2002-05-16 Ian Abbott <abbotti@mev.co.uk>
* recur.c (download_child_p): Minor optimization to avoid an

53
src/hash.c
View File

@ -42,6 +42,9 @@ Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. */
# define xmalloc malloc
# define xrealloc realloc
# define xfree free
# undef TOLOWER
# define TOLOWER(x) ('A' <= (x) && (x) <= 'Z' ? (x) - 32 : (x))
#endif
/* INTERFACE:
@ -145,8 +148,10 @@ struct hash_table {
int resize_threshold; /* after size exceeds this number of
entries, resize the table. */
int prime_offset; /* the offset of the current prime in
the prime table. */
struct mapping *mappings;
struct mapping *mappings; /* the array of mapping pairs. */
};
#define EMPTY_MAPPING_P(mp) ((mp)->key == NULL)
@ -163,13 +168,21 @@ struct hash_table {
#define HASH_POSITION(ht, key) (ht->hash_function (key) % ht->size)
/* Find a prime near, but greather than or equal to SIZE. */
/* Find a prime near, but greather than or equal to SIZE. Of course,
the primes are not calculated, but looked up from a table. The
table does not contain all primes in range, just a selection useful
for this purpose.
PRIME_OFFSET is a micro-optimization: if specified, it starts the
search for the prime number beginning with the specific offset in
the prime number table. The final offset is stored in the same
variable. */
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,
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,
@ -180,12 +193,22 @@ prime_size (int size)
1174703521, 1527114613, 1985248999,
(unsigned long)0x99d43ea5, (unsigned long)0xc7fa5177
};
int i;
for (i = 0; i < ARRAY_SIZE (primes); i++)
int i = *prime_offset;
for (; i < ARRAY_SIZE (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 ();
return 0;
}
/* Create a hash table of INITIAL_SIZE with hash function
@ -207,10 +230,11 @@ hash_table_new (int initial_size,
ht->hash_function = hash_function;
ht->test_function = test_function;
ht->size = prime_size (initial_size);
ht->prime_offset = 0;
ht->size = prime_size (initial_size, &ht->prime_offset);
ht->resize_threshold = ht->size * 3 / 4;
ht->count = 0;
ht->count = 0;
ht->mappings = xmalloc (ht->size * sizeof (struct mapping));
memset (ht->mappings, '\0', ht->size * sizeof (struct mapping));
@ -302,9 +326,12 @@ grow_hash_table (struct hash_table *ht)
struct mapping *mp, *mappings;
int newsize;
newsize = prime_size (ht->size * 2);
newsize = prime_size (ht->size * 2, &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,
(double)100 * ht->count / ht->size,
(double)100 * ht->count / newsize);
#endif
ht->size = newsize;