hashfn.c

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/*-------------------------------------------------------------------------
 *
 * hashfn.c
 *      Hash functions for use in dynahash.c hashtables
 *
 *
 * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/utils/hash/hashfn.c
 *
 * NOTES
 *    It is expected that every bit of a hash function's 32-bit result is
 *    as random as every other; failure to ensure this is likely to lead
 *    to poor performance of hash tables.  In most cases a hash
 *    function should use hash_any() or its variant hash_uint32().
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/hash.h"


/*
 * string_hash: hash function for keys that are NUL-terminated strings.
 *
 * NOTE: this is the default hash function if none is specified.
 */
uint32
string_hash(const void *key, Size keysize)
{
    /*
     * If the string exceeds keysize-1 bytes, we want to hash only that many,
     * because when it is copied into the hash table it will be truncated at
     * that length.
     */
    Size        s_len = strlen((const char *) key);

    s_len = Min(s_len, keysize - 1);
    return DatumGetUInt32(hash_any((const unsigned char *) key,
                                   (int) s_len));
}

/*
 * tag_hash: hash function for fixed-size tag values
 */
uint32
tag_hash(const void *key, Size keysize)
{
    return DatumGetUInt32(hash_any((const unsigned char *) key,
                                   (int) keysize));
}

/*
 * oid_hash: hash function for keys that are OIDs
 *
 * (tag_hash works for this case too, but is slower)
 */
uint32
oid_hash(const void *key, Size keysize)
{
    Assert(keysize == sizeof(Oid));
    return DatumGetUInt32(hash_uint32((uint32) *((const Oid *) key)));
}

/*
 * bitmap_hash: hash function for keys that are (pointers to) Bitmapsets
 *
 * Note: don't forget to specify bitmap_match as the match function!
 */
uint32
bitmap_hash(const void *key, Size keysize)
{
    Assert(keysize == sizeof(Bitmapset *));
    return bms_hash_value(*((const Bitmapset *const *) key));
}

/*
 * bitmap_match: match function to use with bitmap_hash
 */
int
bitmap_match(const void *key1, const void *key2, Size keysize)
{
    Assert(keysize == sizeof(Bitmapset *));
    return !bms_equal(*((const Bitmapset *const *) key1),
                      *((const Bitmapset *const *) key2));
}