Bug 589: make random selection slightly more random. PHP's

mt_rand() function has a maximum value of 2^31-1, which
is small enough that duplicate values can occur due to the
Birthday paradox, e.g. on the English Wikipedia. To fix
this, add a wfRandom() function that calls mt_rand()
twice to get the desired amount of randomness.

includes/GlobalFunctions.php

@@ -124,6 +124,22 @@ function wfSeedRandom() {
 	}
 }
 
+/**
+ * Get a random decimal value between 0 and 1, in a way
+ * not likely to give duplicate values for any realistic
+ * number of articles.
+ *
+ * @return string
+ */
+function wfRandom() {
+	# The maximum random value is "only" 2^31-1, so get two random
+	# values to reduce the chance of dupes
+	$max = mt_getrandmax();
+	$rand = number_format( mt_rand() * mt_rand()
+		/ $max / $max, 12, '.', '' );
+	return $rand;
+}
+
 /**
  * We want / and : to be included as literal characters in our title URLs.
  * %2F in the page titles seems to fatally break for some reason.

includes/SpecialRandompage.php

@@ -20,10 +20,9 @@ function wfSpecialRandompage() {
 	#
 	# Using a literal constant means the whole thing gets optimized on
 	# the index, and the comparison is both fast and fair.
-	$rand = mt_rand() / mt_getrandmax();
 
 	# interpolation and sprintf() can muck up with locale-specific decimal separator
-	$randstr = number_format( $rand, 12, ".", "" );
+	$randstr = wfRandom();
 
 	$db =& wfGetDB( DB_SLAVE );
 	$use_index = $db->useIndexClause( 'cur_random' );

includes/SpecialUndelete.php

@@ -167,7 +167,7 @@ function undelete( $timestamps ) {
 			$redirect = MagicWord::get( MAG_REDIRECT );
 			$redir = $redirect->matchStart( $text ) ? 1 : 0;
 
-			$rand = number_format( mt_rand() / mt_getrandmax(), 12, '.', '' );
+			$rand = wfRandom();
 			$dbw->insertArray( 'cur', array(
 				'cur_id' => $dbw->nextSequenceValue( 'cur_cur_id_seq' ),
 				'cur_namespace' => $namespace,

includes/Title.php

@@ -1441,7 +1441,7 @@ function moveTo( &$nt, $auth = true ) {
 		$now = $dbw->timestamp();
 		$won = wfInvertTimestamp( wfTimestamp(TS_MW,$now) );
 		wfSeedRandom();
-		$rand = number_format( mt_rand() / mt_getrandmax(), 12, '.', '' );
+		$rand = wfRandom();
 
 		# Rename cur entry
 		$dbw->updateArray( 'cur',