hashtable: add hashlittle() from lookup3.c, by Bob Jenkins

Part of a patchset for bug 5324.

1 files changed, 302 insertions, 0 deletions

diff --git a/hashtable.c b/hashtable.c
index f0fbe518..238db08d 100644
--- a/hashtable.c
+++ b/hashtable.c
@@ -170,3 +170,305 @@ void *hashtable_find(struct hashtable *tbl, int64 key, int allocate_if_missing)
 	tbl->entries++;
 	return node;
 }
+
+#ifndef WORDS_BIGENDIAN
+# define HASH_LITTLE_ENDIAN 1
+# define HASH_BIG_ENDIAN 0
+#else
+# define HASH_LITTLE_ENDIAN 0
+# define HASH_BIG_ENDIAN 1
+#endif
+
+/*
+ -------------------------------------------------------------------------------
+ lookup3.c, by Bob Jenkins, May 2006, Public Domain.
+
+ These are functions for producing 32-bit hashes for hash table lookup.
+ hash_word(), 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 hash_word().  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.
+*/
+
+#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); \
+}
+
+
+/*
+ -------------------------------------------------------------------------------
+ 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
+   val2    : IN: can be any 4-byte value OUT: second 32 bit hash.
+ 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.  Note that the return value is better
+ mixed than val2, so use that first.
+
+ 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.
+ -------------------------------------------------------------------------------
+*/
+
+uint32_t hashlittle(const void *key, size_t length)
+{
+  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);
+
+  u.ptr = key;
+  if (HASH_LITTLE_ENDIAN && ((u.i & 0x3) == 0)) {
+    const uint32_t *k = (const uint32_t *)key;         /* read 32-bit chunks */
+    const uint8_t  *k8;
+
+    /*------ 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 */
+    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;
+    }
+  } 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;
+}