nhash.c

/*
-------------------------------------------------------------------------------
lookup3.c, by Bob Jenkins, May 2006, Public Domain.

These are functions for producing 32-bit hashes for hash table lookup.
hashword(), hashlittle(), hashlittle2(), hashbig(), mix(), and final()
are externally useful functions.  Routines to test the hash are included
if SELF_TEST is defined.  You can use this free for any purpose.  It's in
the public domain.  It has no warranty.

You probably want to use hashlittle().  hashlittle() and hashbig()
hash byte arrays.  hashlittle() is is faster than hashbig() on
little-endian machines.  Intel and AMD are little-endian machines.
On second thought, you probably want hashlittle2(), which is identical to
hashlittle() except it returns two 32-bit hashes for the price of one.
You could implement hashbig2() if you wanted but I haven't bothered here.

If you want to find a hash of, say, exactly 7 integers, do
  a = i1;  b = i2;  c = i3;
  mix(a,b,c);
  a += i4; b += i5; c += i6;
  mix(a,b,c);
  a += i7;
  final(a,b,c);
then use c as the hash value.  If you have a variable length array of
4-byte integers to hash, use hashword().  If you have a byte array (like
a character string), use hashlittle().  If you have several byte arrays, or
a mix of things, see the comments above hashlittle().

Why is this so big?  I read 12 bytes at a time into 3 4-byte integers,
then mix those integers.  This is fast (you can do a lot more thorough
mixing with 12*3 instructions on 3 integers than you can with 3 instructions
on 1 byte), but shoehorning those bytes into integers efficiently is messy.
-------------------------------------------------------------------------------
*/

#include "nhash.h"

uint32_t nhash(const void *key, int length, uint32_t initval) {
  uint32_t a, b, c; /* internal state */
  union {
    const void *ptr;
    size_t i;
  } u; /* needed for Mac Powerbook G4 */

  /* Set up the internal state */
  a = b = c = 0xdeadbeef + ((uint32_t)length) + initval;

  u.ptr = key;
  if (HASH_LITTLE_ENDIAN && ((u.i & 0x3) == 0)) {
    const uint32_t *k = (const uint32_t *)key; /* read 32-bit chunks */

    /*------ all but last block: aligned reads and affect 32 bits of (a,b,c) */
    while (length > 12) {
      a += k[0];
      b += k[1];
      c += k[2];
      mix(a, b, c);
      length -= 12;
      k += 3;
    }

/*----------------------------- handle the last (probably partial) block */
/*
 * "k[2]&0xffffff" actually reads beyond the end of the string, but
 * then masks off the part it's not allowed to read.  Because the
 * string is aligned, the masked-off tail is in the same word as the
 * rest of the string.  Every machine with memory protection I've seen
 * does it on word boundaries, so is OK with this.  But VALGRIND will
 * still catch it and complain.  The masking trick does make the hash
 * noticably faster for short strings (like English words).
 */
#ifndef VALGRIND

    switch (length) {
    case 12:
      c += k[2];
      b += k[1];
      a += k[0];
      break;
    case 11:
      c += k[2] & 0xffffff;
      b += k[1];
      a += k[0];
      break;
    case 10:
      c += k[2] & 0xffff;
      b += k[1];
      a += k[0];
      break;
    case 9:
      c += k[2] & 0xff;
      b += k[1];
      a += k[0];
      break;
    case 8:
      b += k[1];
      a += k[0];
      break;
    case 7:
      b += k[1] & 0xffffff;
      a += k[0];
      break;
    case 6:
      b += k[1] & 0xffff;
      a += k[0];
      break;
    case 5:
      b += k[1] & 0xff;
      a += k[0];
      break;
    case 4:
      a += k[0];
      break;
    case 3:
      a += k[0] & 0xffffff;
      break;
    case 2:
      a += k[0] & 0xffff;
      break;
    case 1:
      a += k[0] & 0xff;
      break;
    case 0:
      return c; /* zero length strings require no mixing */
    }

#else /* make valgrind happy */
    const uint8_t *k8;

    k8 = (const uint8_t *)k;
    switch (length) {
    case 12:
      c += k[2];
      b += k[1];
      a += k[0];
      break;
    case 11:
      c += ((uint32_t)k8[10]) << 16; /* fall through */
    case 10:
      c += ((uint32_t)k8[9]) << 8; /* fall through */
    case 9:
      c += k8[8]; /* fall through */
    case 8:
      b += k[1];
      a += k[0];
      break;
    case 7:
      b += ((uint32_t)k8[6]) << 16; /* fall through */
    case 6:
      b += ((uint32_t)k8[5]) << 8; /* fall through */
    case 5:
      b += k8[4]; /* fall through */
    case 4:
      a += k[0];
      break;
    case 3:
      a += ((uint32_t)k8[2]) << 16; /* fall through */
    case 2:
      a += ((uint32_t)k8[1]) << 8; /* fall through */
    case 1:
      a += k8[0];
      break;
    case 0:
      return c;
    }

#endif /* !valgrind */

  } else if (HASH_LITTLE_ENDIAN && ((u.i & 0x1) == 0)) {
    const uint16_t *k = (const uint16_t *)key; /* read 16-bit chunks */
    const uint8_t *k8;

    /*--------------- all but last block: aligned reads and different mixing */
    while (length > 12) {
      a += k[0] + (((uint32_t)k[1]) << 16);
      b += k[2] + (((uint32_t)k[3]) << 16);
      c += k[4] + (((uint32_t)k[5]) << 16);
      mix(a, b, c);
      length -= 12;
      k += 6;
    }

    /*----------------------------- handle the last (probably partial) block */
    k8 = (const uint8_t *)k;
    switch (length) {
    case 12:
      c += k[4] + (((uint32_t)k[5]) << 16);
      b += k[2] + (((uint32_t)k[3]) << 16);
      a += k[0] + (((uint32_t)k[1]) << 16);
      break;
    case 11:
      c += ((uint32_t)k8[10]) << 16; /* fall through */
    case 10:
      c += k[4];
      b += k[2] + (((uint32_t)k[3]) << 16);
      a += k[0] + (((uint32_t)k[1]) << 16);
      break;
    case 9:
      c += k8[8]; /* fall through */
    case 8:
      b += k[2] + (((uint32_t)k[3]) << 16);
      a += k[0] + (((uint32_t)k[1]) << 16);
      break;
    case 7:
      b += ((uint32_t)k8[6]) << 16; /* fall through */
    case 6:
      b += k[2];
      a += k[0] + (((uint32_t)k[1]) << 16);
      break;
    case 5:
      b += k8[4]; /* fall through */
    case 4:
      a += k[0] + (((uint32_t)k[1]) << 16);
      break;
    case 3:
      a += ((uint32_t)k8[2]) << 16; /* fall through */
    case 2:
      a += k[0];
      break;
    case 1:
      a += k8[0];
      break;
    case 0:
      return c; /* zero length requires no mixing */
    }

  } else { /* need to read the key one byte at a time */
    const uint8_t *k = (const uint8_t *)key;

    /*--------------- all but the last block: affect some 32 bits of (a,b,c) */
    while (length > 12) {
      a += k[0];
      a += ((uint32_t)k[1]) << 8;
      a += ((uint32_t)k[2]) << 16;
      a += ((uint32_t)k[3]) << 24;
      b += k[4];
      b += ((uint32_t)k[5]) << 8;
      b += ((uint32_t)k[6]) << 16;
      b += ((uint32_t)k[7]) << 24;
      c += k[8];
      c += ((uint32_t)k[9]) << 8;
      c += ((uint32_t)k[10]) << 16;
      c += ((uint32_t)k[11]) << 24;
      mix(a, b, c);
      length -= 12;
      k += 12;
    }

    /*-------------------------------- last block: affect all 32 bits of (c) */
    switch (length) /* all the case statements fall through */
    {
    case 12:
      c += ((uint32_t)k[11]) << 24;
    case 11:
      c += ((uint32_t)k[10]) << 16;
    case 10:
      c += ((uint32_t)k[9]) << 8;
    case 9:
      c += k[8];
    case 8:
      b += ((uint32_t)k[7]) << 24;
    case 7:
      b += ((uint32_t)k[6]) << 16;
    case 6:
      b += ((uint32_t)k[5]) << 8;
    case 5:
      b += k[4];
    case 4:
      a += ((uint32_t)k[3]) << 24;
    case 3:
      a += ((uint32_t)k[2]) << 16;
    case 2:
      a += ((uint32_t)k[1]) << 8;
    case 1:
      a += k[0];
      break;
    case 0:
      return c;
    }
  }

  final(a, b, c);
  return c;
}