[svn-r12661] Description:

    Add 'loookup3' checksum routine and switch to using it for metadata
checksums - it's just as "strong" as the CRC32 and about 40% faster in general
(with some compiler optimizations, it's nearly as fast as the fletcher-32
algorithm).

Tested on:
    Linux/32 2.6 (chicago)
    Linux/64 2.6 (chicago2)

2 files changed, 183 insertions, 10 deletions

diff --git a/src/H5checksum.c b/src/H5checksum.c
index fd2a075..98260ef 100644
--- a/src/H5checksum.c
+++ b/src/H5checksum.c
@@ -97,7 +97,8 @@ static hbool_t H5_crc_table_computed = FALSE;
  * Note #3:     The algorithm below differs from that given in the Wikipedia
  *              page by copying the data into 'sum1' in a more portable way
  *              and also by initializing 'sum1' and 'sum2' to 0 instead of
- *              0xffff (for backward compatibility reasons, mostly).
+ *              0xffff (for backward compatibility reasons with earlier
+ *              HDF5 fletcher32 I/O filter routine, mostly).
  *
  * Return:	32-bit fletcher checksum of input buffer (can't fail)
  *
@@ -250,6 +251,184 @@ H5_checksum_crc(const void *_data, size_t len)
     FUNC_LEAVE_NOAPI(H5_checksum_crc_update((uint32_t)0xffffffffL, (const uint8_t *)_data, len) ^ 0xffffffffL)
 } /* end H5_checksum_crc() */
 
+/*
+-------------------------------------------------------------------------------
+H5_lookup3_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 H5_lookup3_rot(x,k) (((x)<<(k)) ^ ((x)>>(32-(k))))
+#define H5_lookup3_mix(a,b,c) \
+{ \
+  a -= c;  a ^= H5_lookup3_rot(c, 4);  c += b; \
+  b -= a;  b ^= H5_lookup3_rot(a, 6);  a += c; \
+  c -= b;  c ^= H5_lookup3_rot(b, 8);  b += a; \
+  a -= c;  a ^= H5_lookup3_rot(c,16);  c += b; \
+  b -= a;  b ^= H5_lookup3_rot(a,19);  a += c; \
+  c -= b;  c ^= H5_lookup3_rot(b, 4);  b += a; \
+}
+
+/*
+-------------------------------------------------------------------------------
+H5_lookup3_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 H5_lookup3_final(a,b,c) \
+{ \
+  c ^= b; c -= H5_lookup3_rot(b,14); \
+  a ^= c; a -= H5_lookup3_rot(c,11); \
+  b ^= a; b -= H5_lookup3_rot(a,25); \
+  c ^= b; c -= H5_lookup3_rot(b,16); \
+  a ^= c; a -= H5_lookup3_rot(c,4);  \
+  b ^= a; b -= H5_lookup3_rot(a,14); \
+  c ^= b; c -= H5_lookup3_rot(b,24); \
+}
+
+/*
+-------------------------------------------------------------------------------
+H5_checksum_lookup3() -- 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.
+-------------------------------------------------------------------------------
+*/
+
+uint32_t
+H5_checksum_lookup3(const void *key, size_t length /*, uint32_t initval */)
+{
+    const uint8_t *k = (const uint8_t *)key;
+    uint32_t a, b, c;           /* internal state */
+
+    FUNC_ENTER_NOAPI_NOINIT_NOFUNC(H5_checksum_lookup3)
+
+    /* Sanity check */
+    HDassert(key);
+    HDassert(length > 0);
+
+    /* Set up the internal state */
+    a = b = c = 0xdeadbeef + ((uint32_t)length) /* + initval */;
+
+    /*--------------- 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;
+      H5_lookup3_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 : goto done;
+    }
+
+    H5_lookup3_final(a, b, c);
+
+done:
+    FUNC_LEAVE_NOAPI(c)
+} /* end H5_checksum_lookup3() */
+
 
 /*-------------------------------------------------------------------------
  * Function:	H5_checksum_metadata
@@ -268,8 +447,6 @@ H5_checksum_crc(const void *_data, size_t len)
 uint32_t
 H5_checksum_metadata(const void *data, size_t len)
 {
-    uint32_t chksum;            /* Checksum value to return */
-
     FUNC_ENTER_NOAPI_NOINIT_NOFUNC(H5_checksum_metadata)
 
     /* Sanity check */
@@ -277,12 +454,7 @@ H5_checksum_metadata(const void *data, size_t len)
     HDassert(len > 0);
 
     /* Choose the appropriate checksum routine */
-    /* (use Fletcher's checksum for "larger" buffers and CRC for "shorter" ones) */
-    if(len < 256)
-        chksum = H5_checksum_crc(data, len);
-    else
-        chksum = H5_checksum_fletcher32(data, len);
-
-    FUNC_LEAVE_NOAPI(chksum)
+    /* (use Bob Jenkin's "lookup3" algorithm for all buffer sizes) */
+    FUNC_LEAVE_NOAPI(H5_checksum_lookup3(data, len))
 } /* end H5_checksum_metadata() */
 
diff --git a/src/H5private.h b/src/H5private.h
index e9beabd..254c2eb 100644
--- a/src/H5private.h
+++ b/src/H5private.h
@@ -1449,6 +1449,7 @@ H5_DLL int H5Z_term_interface(void);
 /* Checksum functions */
 H5_DLL uint32_t H5_checksum_fletcher32(const void *data, size_t len);
 H5_DLL uint32_t H5_checksum_crc(const void *data, size_t len);
+H5_DLL uint32_t H5_checksum_lookup3(const void *data, size_t len);
 H5_DLL uint32_t H5_checksum_metadata(const void *data, size_t len);
 
 /* Functions for debugging */