apr_hash.c

Apache V2.0.15 Alpha For Linuxhttpd-2_0_15-alpha.tar.Z

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#include "apr_private.h"

#include "apr_general.h"
#include "apr_pools.h"

#include "apr_hash.h"

#if APR_HAVE_STDLIB_H
#include <stdlib.h>
#endif
#if APR_HAVE_STRING_H
#include <string.h>
#endif


/*
 * The internal form of a hash table.
 *
 * The table is an array indexed by the hash of the key; collisions
 * are resolved by hanging a linked list of hash entries off each
 * element of the array. Although this is a really simple design it
 * isn't too bad given that pools have a low allocation overhead.
 */

typedef struct apr_hash_entry_t apr_hash_entry_t;

struct apr_hash_entry_t {
    apr_hash_entry_t	*next;
    int			 hash;
    const void		*key;
    apr_size_t		 klen;
    const void		*val;
};

/*
 * The size of the array is always a power of two. We use the maximum
 * index rather than the size so that we can use bitwise-AND for
 * modular arithmetic.
 * The count of hash entries may be greater depending on the chosen
 * collision rate.
 */
struct apr_hash_t {
    apr_pool_t		*pool;
    apr_hash_entry_t   **array;
    int                  count, max;
};
#define INITIAL_MAX 15 /* tunable == 2^n - 1 */

/*
 * Data structure for iterating through a hash table.
 *
 * We keep a pointer to the next hash entry here to allow the current
 * hash entry to be freed or otherwise mangled between calls to
 * apr_hash_next().
 */
struct apr_hash_index_t {
    apr_hash_t	       *ht;
    apr_hash_entry_t   *this, *next;
    int                 index;
};


/*
 * Hash creation functions.
 */

static apr_hash_entry_t **alloc_array(apr_hash_t *ht, int max)
{
   return apr_pcalloc(ht->pool, sizeof(*ht->array) * (max + 1));
}

APR_DECLARE(apr_hash_t *) apr_hash_make(apr_pool_t *pool)
{
    apr_hash_t *ht;
    ht = apr_palloc(pool, sizeof(apr_hash_t));
    ht->pool = pool;
    ht->count = 0;
    ht->max = INITIAL_MAX;
    ht->array = alloc_array(ht, ht->max);
    return ht;
}


/*
 * Hash iteration functions.
 */

APR_DECLARE(apr_hash_index_t *) apr_hash_next(apr_hash_index_t *hi)
{
    hi->this = hi->next;
    while (!hi->this) {
	if (hi->index > hi->ht->max)
	    return NULL;
	hi->this = hi->ht->array[hi->index++];
    }
    hi->next = hi->this->next;
    return hi;
}

APR_DECLARE(apr_hash_index_t *) apr_hash_first(apr_hash_t *ht)
{
    apr_hash_index_t *hi;
    hi = apr_palloc(ht->pool, sizeof(*hi));
    hi->ht = ht;
    hi->index = 0;
    hi->this = NULL;
    hi->next = NULL;
    return apr_hash_next(hi);
}

APR_DECLARE(void) apr_hash_this(apr_hash_index_t *hi,
			       const void **key,
			       apr_size_t *klen,
			       void **val)
{
    if (key)  *key  = hi->this->key;
    if (klen) *klen = hi->this->klen;
    if (val)  *val  = (void *)hi->this->val;
}


/*
 * Expanding a hash table
 */

static void expand_array(apr_hash_t *ht)
{
    apr_hash_index_t *hi;
    apr_hash_entry_t **new_array;
    int new_max;
    int i;

    new_max = ht->max * 2 + 1;
    new_array = alloc_array(ht, new_max);
    for (hi = apr_hash_first(ht); hi; hi = apr_hash_next(hi)) {
	i = hi->this->hash & new_max;
	hi->this->next = new_array[i];
	new_array[i] = hi->this;
    }	       
    ht->array = new_array;
    ht->max = new_max;
}

/*
 * This is where we keep the details of the hash function and control
 * the maximum collision rate.
 *
 * If val is non-NULL it creates and initializes a new hash entry if
 * there isn't already one there; it returns an updatable pointer so
 * that hash entries can be removed.
 */

static apr_hash_entry_t **find_entry(apr_hash_t *ht,
				    const void *key,
				    apr_ssize_t klen,
				    const void *val)
{
    apr_hash_entry_t **hep, *he;
    const unsigned char *p;
    int hash;
    int i;

    if (klen == APR_HASH_KEY_STRING)
	klen = strlen(key);

    /*
     * This is the popular `times 33' hash algorithm which is used by
     * perl and also appears in Berkeley DB. This is one of the best
     * known hash functions for strings because it is both computed
     * very fast and distributes very well.
     *
     * The originator may be Dan Bernstein but the code in Berkeley DB
     * cites Chris Torek as the source. The best citation I have found
     * is "Chris Torek, Hash function for text in C, Usenet message
     * <[e-mail]> in comp.lang.c , October, 1990." in Rich
     * Salz's USENIX 1992 paper about INN which can be found at
     * <http://citeseer.nj.nec.com/salz92internetnews.html>.
     *
     * The magic of number 33, i.e. why it works better than many other
     * constants, prime or not, has never been adequately explained by
     * anyone. So I try an explanation: if one experimentally tests all
     * multipliers between 1 and 256 (as I did while writing a low-level
     * data structure library some time ago) one detects that even
     * numbers are not useable at all. The remaining 128 odd numbers
     * (except for the number 1) work more or less all equally well.
     * They all distribute in an acceptable way and this way fill a hash
     * table with an average percent of approx. 86%.
     *
     * If one compares the chi^2 values of the variants (see
     * Bob Jenkins ``Hashing Frequently Asked Questions'' at
     * http://burtleburtle.net/bob/hash/hashfaq.html for a description
     * of chi^2), the number 33 not even has the best value. But the
     * number 33 and a few other equally good numbers like 17, 31, 63,
     * 127 and 129 have nevertheless a great advantage to the remaining
     * numbers in the large set of possible multipliers: their multiply
     * operation can be replaced by a faster operation based on just one
     * shift plus either a single addition or subtraction operation. And
     * because a hash function has to both distribute good _and_ has to
     * be very fast to compute, those few numbers should be preferred.
     *
     *                  -- Ralf S. Engelschall <[e-mail]>
     */
    hash = 0;
    for (p = key, i = klen; i; i--, p++)
	hash = hash * 33 + *p;
    
    /* scan linked list */
    for (hep = &ht->array[hash & ht->max], he = *hep;
	 he;
	 hep = &he->next, he = *hep) {
	if (he->hash == hash &&
	    he->klen == klen &&
	    memcmp(he->key, key, klen) == 0)
	    break;
    }
    if (he || !val)
	return hep;
    /* add a new entry for non-NULL values */
    he = apr_pcalloc(ht->pool, sizeof(*he));
    he->hash = hash;
    he->key  = key;
    he->klen = klen;
    he->val  = val;
    *hep = he;
    ht->count++;
    return hep;
}

APR_DECLARE(void *) apr_hash_get(apr_hash_t *ht,
			       const void *key,
			       apr_ssize_t klen)
{
    apr_hash_entry_t *he;
    he = *find_entry(ht, key, klen, NULL);
    if (he)
	return (void *)he->val;
    else
	return NULL;
}

APR_DECLARE(void) apr_hash_set(apr_hash_t *ht,
			     const void *key,
			     apr_ssize_t klen,
			     const void *val)
{
    apr_hash_entry_t **hep;
    hep = find_entry(ht, key, klen, val);
    if (*hep) {
        if (!val) {
            /* delete entry */
            *hep = (*hep)->next;
            --ht->count;
        }
        else {
            /* replace entry */
            (*hep)->val = val;
            /* check that the collision rate isn't too high */
            if (ht->count > ht->max) {
                expand_array(ht);
            }
        }
    }
    /* else key not present and val==NULL */
}

APR_DECLARE(int) apr_hash_count(apr_hash_t *ht)
{
    return ht->count;
}