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author | rubidium <rubidium@openttd.org> | 2007-11-17 23:16:41 +0000 |
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committer | rubidium <rubidium@openttd.org> | 2007-11-17 23:16:41 +0000 |
commit | 80a7a917308a1cd422923e953ced86d66503906f (patch) | |
tree | 6875b2043f6f455b0745dccacf6c5c4897db8dfb /src/macros.h | |
parent | 32202be4528f18fbe7eaa03bf2585f5fd8179882 (diff) | |
download | openttd-80a7a917308a1cd422923e953ced86d66503906f.tar.xz |
(svn r11454) -Fix: the CHANCE16 functions were biased; a 32768 in 65536 chance was really a 32769 in 65536 chance.
Diffstat (limited to 'src/macros.h')
-rw-r--r-- | src/macros.h | 15 |
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); } |