Optimised the CDB hash function, now roughly twice as fast for large strings. Most...

diff --git a/includes/Cdb_PHP.php b/includes/Cdb_PHP.php
index eb2d317..f2a044c 100644 (file)
--- a/includes/Cdb_PHP.php
+++ b/includes/Cdb_PHP.php
@@ -12,39 +12,6 @@
  * Common functions for readers and writers
  */
 class CdbFunctions {
-       /**
-        * Do a sum of 32-bit signed integers with 2's complement overflow.
-        *
-        * PHP has broken plus and minus operators, but the bitwise operators 
-        * (&, |, ^, ~, <<, >>) are all implemented as a simple wrapper around the
-        * underlying C operator. The algorithm here uses a binary view of addition
-        * to simulate 32-bit addition using 31-bit registers.
-        */
-       public static function sumWithOverflow( $a, $b ) {
-               $sum = $a + $b;
-               if ( is_float( $sum ) ) {
-                       // Use the plus operator to do a sum of the lowest 30 bits to produce a 31-bit result
-                       $lowA = $a & 0x3fffffff;
-                       $lowB = $b & 0x3fffffff;
-                       $sum = $lowA + $lowB;
-
-                       // Strip off the carry bit
-                       $carry = ($sum & 0x40000000) >> 30;
-                       $sum = $sum & 0x3fffffff;
-
-                       // Get the last two bits
-                       $highA = self::unsignedShiftRight( $a, 30 );
-                       $highB = self::unsignedShiftRight( $b, 30 );
-
-                       // Add with carry
-                       $highSum = $carry + $highA + $highB;
-
-                       // Recombine
-                       $sum = $sum | ( $highSum << 30 );
-               }
-               return $sum;
-       }
-
        /**
         * Take a modulo of a signed integer as if it were an unsigned integer.
         * $b must be less than 0x40000000 and greater than 0
@@ -72,10 +39,26 @@ class CdbFunctions {
                }
        }
 
+       /**
+        * The CDB hash function.
+        */
        public static function hash( $s ) {
                $h = 5381;
                for ( $i = 0; $i < strlen( $s ); $i++ ) {
-                       $h = self::sumWithOverflow( $h, $h << 5 ) ^ ord( $s[$i] );
+                       $h5 = $h << 5;
+                       // Do a 32-bit sum
+                       // Inlined here for speed
+                       $sum = ($h & 0x3fffffff) + ($h5 & 0x3fffffff);
+                       $h = 
+                               (
+                                       ( $sum & 0x40000000 ? 1 : 0 )
+                                       + ( $h < 0 ? 2 : 0 )
+                                       + ( $h & 0x40000000 ? 1 : 0 )
+                                       + ( $h5 < 0 ? 2 : 0 )
+                                       + ( $h5 & 0x40000000 ? 1 : 0 )
+                               ) << 30
+                               | ( $sum & 0x3fffffff );
+                       $h ^= ord( $s[$i] );
                }
                return $h;
        }