hashfn.c

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/*-------------------------------------------------------------------------
 *
 * hashfn.c
 *      Generic hashing functions, and hash functions for use in dynahash.c
 *      hashtables
 *
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/common/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_bytes() or its variant hash_bytes_uint32(),
 *    or the wrappers hash_any() and hash_uint32 defined in hashfn.h.
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "common/hashfn.h"
#include "port/pg_bitutils.h"
 
 
/*
 * This hash function was written by Bob Jenkins
 * (bob_jenkins@burtleburtle.net), and superficially adapted
 * for PostgreSQL by Neil Conway. For more information on this
 * hash function, see http://burtleburtle.net/bob/hash/doobs.html,
 * or Bob's article in Dr. Dobb's Journal, Sept. 1997.
 *
 * In the current code, we have adopted Bob's 2006 update of his hash
 * function to fetch the data a word at a time when it is suitably aligned.
 * This makes for a useful speedup, at the cost of having to maintain
 * four code paths (aligned vs unaligned, and little-endian vs big-endian).
 * It also uses two separate mixing functions mix() and final(), instead
 * of a slower multi-purpose function.
 */
 
/* Get a bit mask of the bits set in non-uint32 aligned addresses */
#define UINT32_ALIGN_MASK (sizeof(uint32) - 1)
 
#define rot(x,k) pg_rotate_left32(x, k)
 
/*----------
 * mix -- mix 3 32-bit values reversibly.
 *
 * This is reversible, so any information in (a,b,c) before mix() is
 * still in (a,b,c) after mix().
 *
 * If four pairs of (a,b,c) inputs are run through mix(), or through
 * mix() in reverse, there are at least 32 bits of the output that
 * are sometimes the same for one pair and different for another pair.
 * This was tested for:
 * * pairs that differed by one bit, by two bits, in any combination
 *   of top bits of (a,b,c), or in any combination of bottom bits of
 *   (a,b,c).
 * * "differ" is defined as +, -, ^, or ~^.  For + and -, I transformed
 *   the output delta to a Gray code (a^(a>>1)) so a string of 1's (as
 *   is commonly produced by subtraction) look like a single 1-bit
 *   difference.
 * * the base values were pseudorandom, all zero but one bit set, or
 *   all zero plus a counter that starts at zero.
 *
 * This does not achieve avalanche.  There are input bits of (a,b,c)
 * that fail to affect some output bits of (a,b,c), especially of a.  The
 * most thoroughly mixed value is c, but it doesn't really even achieve
 * avalanche in c.
 *
 * This allows some parallelism.  Read-after-writes are good at doubling
 * the number of bits affected, so the goal of mixing pulls in the opposite
 * direction from the goal of parallelism.  I did what I could.  Rotates
 * seem to cost as much as shifts on every machine I could lay my hands on,
 * and rotates are much kinder to the top and bottom bits, so I used rotates.
 *----------
 */
#define mix(a,b,c) \
{ \
  a -= c;  a ^= rot(c, 4);  c += b; \
  b -= a;  b ^= rot(a, 6);  a += c; \
  c -= b;  c ^= rot(b, 8);  b += a; \
  a -= c;  a ^= rot(c,16);  c += b; \
  b -= a;  b ^= rot(a,19);  a += c; \
  c -= b;  c ^= rot(b, 4);  b += a; \
}
 
/*----------
 * final -- final mixing of 3 32-bit values (a,b,c) into c
 *
 * Pairs of (a,b,c) values differing in only a few bits will usually
 * produce values of c that look totally different.  This was tested for
 * * pairs that differed by one bit, by two bits, in any combination
 *   of top bits of (a,b,c), or in any combination of bottom bits of
 *   (a,b,c).
 * * "differ" is defined as +, -, ^, or ~^.  For + and -, I transformed
 *   the output delta to a Gray code (a^(a>>1)) so a string of 1's (as
 *   is commonly produced by subtraction) look like a single 1-bit
 *   difference.
 * * the base values were pseudorandom, all zero but one bit set, or
 *   all zero plus a counter that starts at zero.
 *
 * The use of separate functions for mix() and final() allow for a
 * substantial performance increase since final() does not need to
 * do well in reverse, but is does need to affect all output bits.
 * mix(), on the other hand, does not need to affect all output
 * bits (affecting 32 bits is enough).  The original hash function had
 * a single mixing operation that had to satisfy both sets of requirements
 * and was slower as a result.
 *----------
 */
#define final(a,b,c) \
{ \
  c ^= b; c -= rot(b,14); \
  a ^= c; a -= rot(c,11); \
  b ^= a; b -= rot(a,25); \
  c ^= b; c -= rot(b,16); \
  a ^= c; a -= rot(c, 4); \
  b ^= a; b -= rot(a,14); \
  c ^= b; c -= rot(b,24); \
}
 
/*
 * hash_bytes() -- hash a variable-length key into a 32-bit value
 *      k       : the key (the unaligned variable-length array of bytes)
 *      len     : the length of the key, counting by bytes
 *
 * Returns a uint32 value.  Every bit of the key affects every bit of
 * the return value.  Every 1-bit and 2-bit delta achieves avalanche.
 * About 6*len+35 instructions. The best hash table sizes are powers
 * of 2.  There is no need to do mod a prime (mod is sooo slow!).
 * If you need less than 32 bits, use a bitmask.
 *
 * This procedure must never throw elog(ERROR); the ResourceOwner code
 * relies on this not to fail.
 *
 * Note: we could easily change this function to return a 64-bit hash value
 * by using the final values of both b and c.  b is perhaps a little less
 * well mixed than c, however.
 */
uint32
hash_bytes(const unsigned char *k, int keylen)
{
    uint32      a,
                b,
                c,
                len;
 
    /* Set up the internal state */
    len = keylen;
    a = b = c = 0x9e3779b9 + len + 3923095;
 
    /* If the source pointer is word-aligned, we use word-wide fetches */
    if (((uintptr_t) k & UINT32_ALIGN_MASK) == 0)
    {
        /* Code path for aligned source data */
        const uint32 *ka = (const uint32 *) k;
 
        /* handle most of the key */
        while (len >= 12)
        {
            a += ka[0];
            b += ka[1];
            c += ka[2];
            mix(a, b, c);
            ka += 3;
            len -= 12;
        }
 
        /* handle the last 11 bytes */
        k = (const unsigned char *) ka;
#ifdef WORDS_BIGENDIAN
        switch (len)
        {
            case 11:
                c += ((uint32) k[10] << 8);
                /* fall through */
            case 10:
                c += ((uint32) k[9] << 16);
                /* fall through */
            case 9:
                c += ((uint32) k[8] << 24);
                /* fall through */
            case 8:
                /* the lowest byte of c is reserved for the length */
                b += ka[1];
                a += ka[0];
                break;
            case 7:
                b += ((uint32) k[6] << 8);
                /* fall through */
            case 6:
                b += ((uint32) k[5] << 16);
                /* fall through */
            case 5:
                b += ((uint32) k[4] << 24);
                /* fall through */
            case 4:
                a += ka[0];
                break;
            case 3:
                a += ((uint32) k[2] << 8);
                /* fall through */
            case 2:
                a += ((uint32) k[1] << 16);
                /* fall through */
            case 1:
                a += ((uint32) k[0] << 24);
                /* case 0: nothing left to add */
        }
#else                           /* !WORDS_BIGENDIAN */
        switch (len)
        {
            case 11:
                c += ((uint32) k[10] << 24);
                /* fall through */
            case 10:
                c += ((uint32) k[9] << 16);
                /* fall through */
            case 9:
                c += ((uint32) k[8] << 8);
                /* fall through */
            case 8:
                /* the lowest byte of c is reserved for the length */
                b += ka[1];
                a += ka[0];
                break;
            case 7:
                b += ((uint32) k[6] << 16);
                /* fall through */
            case 6:
                b += ((uint32) k[5] << 8);
                /* fall through */
            case 5:
                b += k[4];
                /* fall through */
            case 4:
                a += ka[0];
                break;
            case 3:
                a += ((uint32) k[2] << 16);
                /* fall through */
            case 2:
                a += ((uint32) k[1] << 8);
                /* fall through */
            case 1:
                a += k[0];
                /* case 0: nothing left to add */
        }
#endif                          /* WORDS_BIGENDIAN */
    }
    else
    {
        /* Code path for non-aligned source data */
 
        /* handle most of the key */
        while (len >= 12)
        {
#ifdef WORDS_BIGENDIAN
            a += (k[3] + ((uint32) k[2] << 8) + ((uint32) k[1] << 16) + ((uint32) k[0] << 24));
            b += (k[7] + ((uint32) k[6] << 8) + ((uint32) k[5] << 16) + ((uint32) k[4] << 24));
            c += (k[11] + ((uint32) k[10] << 8) + ((uint32) k[9] << 16) + ((uint32) k[8] << 24));
#else                           /* !WORDS_BIGENDIAN */
            a += (k[0] + ((uint32) k[1] << 8) + ((uint32) k[2] << 16) + ((uint32) k[3] << 24));
            b += (k[4] + ((uint32) k[5] << 8) + ((uint32) k[6] << 16) + ((uint32) k[7] << 24));
            c += (k[8] + ((uint32) k[9] << 8) + ((uint32) k[10] << 16) + ((uint32) k[11] << 24));
#endif                          /* WORDS_BIGENDIAN */
            mix(a, b, c);
            k += 12;
            len -= 12;
        }
 
        /* handle the last 11 bytes */
#ifdef WORDS_BIGENDIAN
        switch (len)
        {
            case 11:
                c += ((uint32) k[10] << 8);
                /* fall through */
            case 10:
                c += ((uint32) k[9] << 16);
                /* fall through */
            case 9:
                c += ((uint32) k[8] << 24);
                /* fall through */
            case 8:
                /* the lowest byte of c is reserved for the length */
                b += k[7];
                /* fall through */
            case 7:
                b += ((uint32) k[6] << 8);
                /* fall through */
            case 6:
                b += ((uint32) k[5] << 16);
                /* fall through */
            case 5:
                b += ((uint32) k[4] << 24);
                /* fall through */
            case 4:
                a += k[3];
                /* fall through */
            case 3:
                a += ((uint32) k[2] << 8);
                /* fall through */
            case 2:
                a += ((uint32) k[1] << 16);
                /* fall through */
            case 1:
                a += ((uint32) k[0] << 24);
                /* case 0: nothing left to add */
        }
#else                           /* !WORDS_BIGENDIAN */
        switch (len)
        {
            case 11:
                c += ((uint32) k[10] << 24);
                /* fall through */
            case 10:
                c += ((uint32) k[9] << 16);
                /* fall through */
            case 9:
                c += ((uint32) k[8] << 8);
                /* fall through */
            case 8:
                /* the lowest byte of c is reserved for the length */
                b += ((uint32) k[7] << 24);
                /* fall through */
            case 7:
                b += ((uint32) k[6] << 16);
                /* fall through */
            case 6:
                b += ((uint32) k[5] << 8);
                /* fall through */
            case 5:
                b += k[4];
                /* fall through */
            case 4:
                a += ((uint32) k[3] << 24);
                /* fall through */
            case 3:
                a += ((uint32) k[2] << 16);
                /* fall through */
            case 2:
                a += ((uint32) k[1] << 8);
                /* fall through */
            case 1:
                a += k[0];
                /* case 0: nothing left to add */
        }
#endif                          /* WORDS_BIGENDIAN */
    }
 
    final(a, b, c);
 
    /* report the result */
    return c;
}
 
/*
 * hash_bytes_extended() -- hash into a 64-bit value, using an optional seed
 *      k       : the key (the unaligned variable-length array of bytes)
 *      len     : the length of the key, counting by bytes
 *      seed    : a 64-bit seed (0 means no seed)
 *
 * Returns a uint64 value.  Otherwise similar to hash_bytes.
 */
uint64
hash_bytes_extended(const unsigned char *k, int keylen, uint64 seed)
{
    uint32      a,
                b,
                c,
                len;
 
    /* Set up the internal state */
    len = keylen;
    a = b = c = 0x9e3779b9 + len + 3923095;
 
    /* If the seed is non-zero, use it to perturb the internal state. */
    if (seed != 0)
    {
        /*
         * In essence, the seed is treated as part of the data being hashed,
         * but for simplicity, we pretend that it's padded with four bytes of
         * zeroes so that the seed constitutes a 12-byte chunk.
         */
        a += (uint32) (seed >> 32);
        b += (uint32) seed;
        mix(a, b, c);
    }
 
    /* If the source pointer is word-aligned, we use word-wide fetches */
    if (((uintptr_t) k & UINT32_ALIGN_MASK) == 0)
    {
        /* Code path for aligned source data */
        const uint32 *ka = (const uint32 *) k;
 
        /* handle most of the key */
        while (len >= 12)
        {
            a += ka[0];
            b += ka[1];
            c += ka[2];
            mix(a, b, c);
            ka += 3;
            len -= 12;
        }
 
        /* handle the last 11 bytes */
        k = (const unsigned char *) ka;
#ifdef WORDS_BIGENDIAN
        switch (len)
        {
            case 11:
                c += ((uint32) k[10] << 8);
                /* fall through */
            case 10:
                c += ((uint32) k[9] << 16);
                /* fall through */
            case 9:
                c += ((uint32) k[8] << 24);
                /* fall through */
            case 8:
                /* the lowest byte of c is reserved for the length */
                b += ka[1];
                a += ka[0];
                break;
            case 7:
                b += ((uint32) k[6] << 8);
                /* fall through */
            case 6:
                b += ((uint32) k[5] << 16);
                /* fall through */
            case 5:
                b += ((uint32) k[4] << 24);
                /* fall through */
            case 4:
                a += ka[0];
                break;
            case 3:
                a += ((uint32) k[2] << 8);
                /* fall through */
            case 2:
                a += ((uint32) k[1] << 16);
                /* fall through */
            case 1:
                a += ((uint32) k[0] << 24);
                /* case 0: nothing left to add */
        }
#else                           /* !WORDS_BIGENDIAN */
        switch (len)
        {
            case 11:
                c += ((uint32) k[10] << 24);
                /* fall through */
            case 10:
                c += ((uint32) k[9] << 16);
                /* fall through */
            case 9:
                c += ((uint32) k[8] << 8);
                /* fall through */
            case 8:
                /* the lowest byte of c is reserved for the length */
                b += ka[1];
                a += ka[0];
                break;
            case 7:
                b += ((uint32) k[6] << 16);
                /* fall through */
            case 6:
                b += ((uint32) k[5] << 8);
                /* fall through */
            case 5:
                b += k[4];
                /* fall through */
            case 4:
                a += ka[0];
                break;
            case 3:
                a += ((uint32) k[2] << 16);
                /* fall through */
            case 2:
                a += ((uint32) k[1] << 8);
                /* fall through */
            case 1:
                a += k[0];
                /* case 0: nothing left to add */
        }
#endif                          /* WORDS_BIGENDIAN */
    }
    else
    {
        /* Code path for non-aligned source data */
 
        /* handle most of the key */
        while (len >= 12)
        {
#ifdef WORDS_BIGENDIAN
            a += (k[3] + ((uint32) k[2] << 8) + ((uint32) k[1] << 16) + ((uint32) k[0] << 24));
            b += (k[7] + ((uint32) k[6] << 8) + ((uint32) k[5] << 16) + ((uint32) k[4] << 24));
            c += (k[11] + ((uint32) k[10] << 8) + ((uint32) k[9] << 16) + ((uint32) k[8] << 24));
#else                           /* !WORDS_BIGENDIAN */
            a += (k[0] + ((uint32) k[1] << 8) + ((uint32) k[2] << 16) + ((uint32) k[3] << 24));
            b += (k[4] + ((uint32) k[5] << 8) + ((uint32) k[6] << 16) + ((uint32) k[7] << 24));
            c += (k[8] + ((uint32) k[9] << 8) + ((uint32) k[10] << 16) + ((uint32) k[11] << 24));
#endif                          /* WORDS_BIGENDIAN */
            mix(a, b, c);
            k += 12;
            len -= 12;
        }
 
        /* handle the last 11 bytes */
#ifdef WORDS_BIGENDIAN
        switch (len)
        {
            case 11:
                c += ((uint32) k[10] << 8);
                /* fall through */
            case 10:
                c += ((uint32) k[9] << 16);
                /* fall through */
            case 9:
                c += ((uint32) k[8] << 24);
                /* fall through */
            case 8:
                /* the lowest byte of c is reserved for the length */
                b += k[7];
                /* fall through */
            case 7:
                b += ((uint32) k[6] << 8);
                /* fall through */
            case 6:
                b += ((uint32) k[5] << 16);
                /* fall through */
            case 5:
                b += ((uint32) k[4] << 24);
                /* fall through */
            case 4:
                a += k[3];
                /* fall through */
            case 3:
                a += ((uint32) k[2] << 8);
                /* fall through */
            case 2:
                a += ((uint32) k[1] << 16);
                /* fall through */
            case 1:
                a += ((uint32) k[0] << 24);
                /* case 0: nothing left to add */
        }
#else                           /* !WORDS_BIGENDIAN */
        switch (len)
        {
            case 11:
                c += ((uint32) k[10] << 24);
                /* fall through */
            case 10:
                c += ((uint32) k[9] << 16);
                /* fall through */
            case 9:
                c += ((uint32) k[8] << 8);
                /* fall through */
            case 8:
                /* the lowest byte of c is reserved for the length */
                b += ((uint32) k[7] << 24);
                /* fall through */
            case 7:
                b += ((uint32) k[6] << 16);
                /* fall through */
            case 6:
                b += ((uint32) k[5] << 8);
                /* fall through */
            case 5:
                b += k[4];
                /* fall through */
            case 4:
                a += ((uint32) k[3] << 24);
                /* fall through */
            case 3:
                a += ((uint32) k[2] << 16);
                /* fall through */
            case 2:
                a += ((uint32) k[1] << 8);
                /* fall through */
            case 1:
                a += k[0];
                /* case 0: nothing left to add */
        }
#endif                          /* WORDS_BIGENDIAN */
    }
 
    final(a, b, c);
 
    /* report the result */
    return ((uint64) b << 32) | c;
}
 
/*
 * hash_bytes_uint32() -- hash a 32-bit value to a 32-bit value
 *
 * This has the same result as
 *      hash_bytes(&k, sizeof(uint32))
 * but is faster and doesn't force the caller to store k into memory.
 */
uint32
hash_bytes_uint32(uint32 k)
{
    uint32      a,
                b,
                c;
 
    a = b = c = 0x9e3779b9 + (uint32) sizeof(uint32) + 3923095;
    a += k;
 
    final(a, b, c);
 
    /* report the result */
    return c;
}
 
/*
 * hash_bytes_uint32_extended() -- hash 32-bit value to 64-bit value, with seed
 *
 * Like hash_bytes_uint32, this is a convenience function.
 */
uint64
hash_bytes_uint32_extended(uint32 k, uint64 seed)
{
    uint32      a,
                b,
                c;
 
    a = b = c = 0x9e3779b9 + (uint32) sizeof(uint32) + 3923095;
 
    if (seed != 0)
    {
        a += (uint32) (seed >> 32);
        b += (uint32) seed;
        mix(a, b, c);
    }
 
    a += k;
 
    final(a, b, c);
 
    /* report the result */
    return ((uint64) b << 32) | c;
}
 
/*
 * 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 hash_bytes((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 hash_bytes((const unsigned char *) key, (int) keysize);
}
 
/*
 * uint32_hash: hash function for keys that are uint32 or int32
 *
 * (tag_hash works for this case too, but is slower)
 */
uint32
uint32_hash(const void *key, Size keysize)
{
    Assert(keysize == sizeof(uint32));
    return hash_bytes_uint32(*((const uint32 *) key));
}