[lttng-tools.git] / hashtable / hash.c

/*
 * Copyright (C) - Bob Jenkins, May 2006, Public Domain.
 * Copyright (C) 2011 - David Goulet <david.goulet@polymtl.ca>
 *
 * 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 <stdio.h>      /* defines printf for tests */
#include <time.h>       /* defines time_t for timings in the test */
#include <stdint.h>     /* defines uint32_t etc */
#include <sys/param.h>  /* attempt to define endianness */
#include <endian.h>    /* attempt to define endianness */
#include <string.h>
#include <assert.h>

/*
 * My best guess at if you are big-endian or little-endian.  This may
 * need adjustment.
 */
#if (defined(__BYTE_ORDER) && defined(__LITTLE_ENDIAN) && \
     __BYTE_ORDER == __LITTLE_ENDIAN) || \
    (defined(i386) || defined(__i386__) || defined(__i486__) || \
     defined(__i586__) || defined(__i686__) || defined(vax) || defined(MIPSEL))
# define HASH_LITTLE_ENDIAN 1
# define HASH_BIG_ENDIAN 0
#elif (defined(__BYTE_ORDER) && defined(__BIG_ENDIAN) && \
       __BYTE_ORDER == __BIG_ENDIAN) || \
      (defined(sparc) || defined(POWERPC) || defined(mc68000) || defined(sel))
# define HASH_LITTLE_ENDIAN 0
# define HASH_BIG_ENDIAN 1
#else
# define HASH_LITTLE_ENDIAN 0
# define HASH_BIG_ENDIAN 0
#endif

#define hashsize(n) ((uint32_t)1<<(n))
#define hashmask(n) (hashsize(n)-1)
#define rot(x,k) (((x)<<(k)) | ((x)>>(32-(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.
 *
 * Some k values for my "a-=c; a^=rot(c,k); c+=b;" arrangement that
 * satisfy this are
 *     4  6  8 16 19  4
 *     9 15  3 18 27 15
 *    14  9  3  7 17  3
 * Well, "9 15 3 18 27 15" didn't quite get 32 bits diffing
 * for "differ" defined as + with a one-bit base and a two-bit delta.  I
 * used http://burtleburtle.net/bob/hash/avalanche.html to choose
 * the operations, constants, and arrangements of the variables.
 *
 * 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 as 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.
 *
 * These constants passed:
 *  14 11 25 16 4 14 24
 *  12 14 25 16 4 14 24
 * and these came close:
 *   4  8 15 26 3 22 24
 *  10  8 15 26 3 22 24
 *  11  8 15 26 3 22 24
 */
#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); \
}

/*
 * hashword2() -- same as hashword(), but take two seeds and return two 32-bit
 * values.  pc and pb must both be nonnull, and *pc and *pb must both be
 * initialized with seeds.  If you pass in (*pb)==0, the output (*pc) will be
 * the same as the return value from hashword().
 */
static void hashword2(const uint32_t *k, size_t length,
                uint32_t *pc, uint32_t *pb)
{
        uint32_t a, b, c;

        /* Set up the internal state */
        a = b = c = 0xdeadbeef + ((uint32_t) (length << 2)) + *pc;
        c += *pb;

        while (length > 3) {
                a += k[0];
                b += k[1];
                c += k[2];
                mix(a, b, c);
                length -= 3;
                k += 3;
        }

        switch (length) {
        case 3 :
                c += k[2];
        case 2 :
                b += k[1];
        case 1 :
                a += k[0];
                final(a, b, c);
        case 0:     /* case 0: nothing left to add */
                break;
        }

        *pc = c;
        *pb = b;
}

/*
 * hashlittle() -- hash a variable-length key into a 32-bit value
 *   k       : the key (the unaligned variable-length array of bytes)
 *   length  : the length of the key, counting by bytes
 *   initval : can be any 4-byte value
 * Returns a 32-bit value.  Every bit of the key affects every bit of
 * the return value.  Two keys differing by one or two bits will have
 * totally different hash values.
 *
 * 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.  For example, if you need only 10 bits, do
 *   h = (h & hashmask(10));
 * In which case, the hash table should have hashsize(10) elements.
 *
 * If you are hashing n strings (uint8_t **)k, do it like this:
 *   for (i=0, h=0; i<n; ++i) h = hashlittle( k[i], len[i], h);
 *
 * By Bob Jenkins, 2006.  bob_jenkins@burtleburtle.net.  You may use this
 * code any way you wish, private, educational, or commercial.  It's free.
 *
 * Use for hash table lookup, or anything where one collision in 2^^32 is
 * acceptable.  Do NOT use for cryptographic purposes.
 */

static uint32_t hashlittle(const void *key, size_t length, uint32_t initval)
{
        uint32_t a,b,c;
        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;
                }

                /*
                 * "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;
                }

                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;

                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;
                }

                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;
}

/*
 * Hash function for number value.
 */
unsigned long hash_key(void *_key, size_t length, unsigned long seed)
{
    union {
        uint64_t v64;
        uint32_t v32[2];
    } v;
    union {
        uint64_t v64;
        uint32_t v32[2];
    } key;

    assert(length == sizeof(unsigned long));
    v.v64 = (uint64_t) seed;
    key.v64 = (uint64_t) _key;
    hashword2(key.v32, 2, &v.v32[0], &v.v32[1]);
    return v.v64;
}

/*
 * Hash function for string.
 */
unsigned long hash_key_str(void *key, size_t length, unsigned long seed)
{
        return hashlittle(key, length, seed);
}

/*
 * Hash function compare for number value.
 */
unsigned long hash_compare_key(void *key1, size_t key1_len,
                void *key2, size_t key2_len)
{
        if (key1_len != key2_len) {
                return -1;
        }

        if (key1 == key2) {
                return 0;
        }

        return 1;
}

/*
 * Hash compare function for string.
 */
unsigned long hash_compare_key_str(void *key1, size_t key1_len,
                void *key2, size_t key2_len)
{
        if (key1_len != key2_len) {
                return -1;
        }

        if (strncmp(key1, key2, key1_len) == 0) {
                return 0;
        }

        return 1;
}