(svn r11454) -Fix: the CHANCE16 functions were biased; a 32768 in 65536 chance was really a 32769 in 65536 chance.

1 files changed, 11 insertions, 4 deletions

diff --git a/src/macros.h b/src/macros.h
index 95af44123..cb2839c90 100644
--- a/src/macros.h
+++ b/src/macros.h
@@ -476,7 +476,7 @@ template<typename T> static inline bool IS_INSIDE_1D(const T x, const int base,
  * @param b The denominator of the fraction, must of course not be null
  * @return True in (a/b) percent
  */
-#define CHANCE16(a, b) ((uint16)Random() <= (uint16)((65536 * (a)) / (b)))
+#define CHANCE16(a, b) CHANCE16I(a, b, Random())
 
 /**
  * Flips a coin with a given probability and saves the randomize-number in a variable.
@@ -484,18 +484,25 @@ template<typename T> static inline bool IS_INSIDE_1D(const T x, const int base,
  * This macro uses the same parameters as the CHANCE16 marco. The third parameter
  * must be a variable the randomize-number from Random() is saved in.
  *
+ * The low 16 bits of r will already be used and can therefor not be passed to
+ * CHANCE16I. One can only send the high 16 bits to CHANCE16I.
+ *
  * @param a The numerator of the fraction, see CHANCE16
  * @param b The denominator of the fraction, see CHANCE16
  * @param r The variable to save the randomize-number from Random()
  * @return True in (a/b) percent
  */
-#define CHANCE16R(a, b, r) ((uint16)(r = Random()) <= (uint16)((65536 * (a)) / (b)))
+#define CHANCE16R(a, b, r) CHANCE16I(a, b, r = Random())
 
 /**
  * Checks if a given randomize-number is below a given probability.
  *
  * This macro is used to check if the given probability by the fraction of (a/b)
- * is greater than the given randomize-number v.
+ * is greater than low 16 bits of the given randomize-number v.
+ *
+ * Do not use this function twice on the same random 16 bits as it will yield
+ * the same result. One can use a random number for two calls to CHANCE16I,
+ * where one call sends the low 16 bits and the other the high 16 bits.
  *
  * @param a The numerator of the fraction, see CHANCE16
  * @param b The denominator of the fraction, see CHANCE16
@@ -504,7 +511,7 @@ template<typename T> static inline bool IS_INSIDE_1D(const T x, const int base,
  */
 static inline bool CHANCE16I(const uint a, const uint b, const uint32 r)
 {
-	return (uint16)r <= (uint16)((65536 * a) / b);
+	return (uint16)r < (uint16)((65536 * a) / b);
 }