Bring rand.c up to date

Author: rlerdorf

ext/standard/rand.c

@@ -201,6 +201,8 @@ static inline uint32 randomMT(void)
 #define PHP_RAND_MAX RAND_MAX
 #endif
 
+/* {{{ proto void srand(int seed)
+   Seeds random number generator */
 PHP_FUNCTION(srand)
 {
 	pval *arg;
@@ -219,7 +221,10 @@ PHP_FUNCTION(srand)
 #endif
 #endif
 }
+/* }}} */
 
+/* {{{ proto void mt_srand(int seed)
+   Seeds Mersenne Twister random number generator */
 PHP_FUNCTION(mt_srand)
 {
 	pval *arg;
@@ -229,7 +234,10 @@ PHP_FUNCTION(mt_srand)
 	convert_to_long(arg);
 	seedMT(arg->value.lval);
 }
+/* }}} */
 
+/* {{{ proto int rand([int min, int max])
+   Returns a random number */
 PHP_FUNCTION(rand)
 {
 	pval *p_min=NULL, *p_max=NULL;
@@ -262,13 +270,40 @@ PHP_FUNCTION(rand)
 	return_value->value.lval = rand();
 #endif
 #endif
-
+    /*                                                                                                                    
+     * A bit of tricky math here.  We want to avoid using a modulus because                                               
+     * that simply tosses the high-order bits and might skew the distribution                                             
+     * of random values over the range.  Instead we map the range directly.                                               
+     *                                                                                                                    
+     * We need to map the range from 0...M evenly to the range a...b                                                      
+     * Let n = the random number and n' = the mapped random number
+     *                                                                                                                    
+     * Then we have: n' = a + n(b-a)/M
+     *                                                                                                                    
+     * We have a problem here in that only n==M will get mapped to b which
+     # means the chances of getting b is much much less than getting any of
+     # the other values in the range.  We can fix this by increasing our range
+     # artifically and using:
+     #
+     #               n' = a + n(b-a+1)/M
+     *
+     # Now we only have a problem if n==M which would cause us to produce a                                               
+     # number of b+1 which would be bad.  So we bump M up by one to make sure
+     # this will never happen, and the final algorithm looks like this:
+     #
+     #               n' = a + n(b-a+1)/(M+1) 
+     *
+     * -RL
+     */
 	if (p_min && p_max) { /* implement range */
 		return_value->value.lval = p_min->value.lval +
-			(int)((double)p_max->value.lval * return_value->value.lval/(PHP_RAND_MAX+(double)p_min->value.lval));
+			(int)((double)(p_max->value.lval - p_min->value.lval + 1) * return_value->value.lval/(PHP_RAND_MAX+1.0));     
 	}
 }
+/* }}} */
 
+/* {{{ proto int mt_rand([int min, int max])
+   Returns a random number from Mersenne Twister */
 PHP_FUNCTION(mt_rand)
 {
 	pval *p_min=NULL, *p_max=NULL;
@@ -304,16 +339,23 @@ PHP_FUNCTION(mt_rand)
 
 	if (p_min && p_max) { /* implement range */
 		return_value->value.lval = p_min->value.lval +
-			(int)((double)p_max->value.lval * return_value->value.lval/(PHP_RAND_MAX+(double)p_min->value.lval));
+			(int)((double)(p_max->value.lval - p_min->value.lval + 1) * return_value->value.lval/(PHP_RAND_MAX+1.0));
 	}
 }
+/* }}} */
 
+/* {{{ proto int getrandmax(void)
+   Returns the maximum value a random number can have */
 PHP_FUNCTION(getrandmax)
 {
 	return_value->type = IS_LONG;
 	return_value->value.lval = PHP_RAND_MAX;
 }
+/* }}} */
+
 
+/* {{{ proto int mt_getrandmax(void)
+   Returns the maximum value a random number from Mersenne Twister can have */
 PHP_FUNCTION(mt_getrandmax)
 {
 	return_value->type = IS_LONG;
@@ -323,7 +365,7 @@ PHP_FUNCTION(mt_getrandmax)
 	 */
   	return_value->value.lval = 2147483647;	/* 2^^31 */
 }
-
+/* }}} */
 /*
  * Local variables:
  * tab-width: 4