On 02.07.2013 20:33, Bruce Evans wrote:
> I checked the values returned by rand(). The ACM part works as
> intended, so it never returns RAND_MAX. It also never returns 0. So
> the distribution of values in the documented range [0, RAND_MAX] is
> very non-uniform. It is uniform in [1, RAND_MAX - 1]. To use this
> algorithm for rand(), 1 should have been subtracted, giving a range
> of [0, 0x7ffffffe].
Do you mean [0, 0x7ffffffd] (assuming 1 subtracted)?
See attached patch.
I don't see compatibility problems at least from POSIX specs point of
view - they don't say something specific about RAND_MAX.
--
http://ache.vniz.net/
bitcoin:13fGiNutKNHcVSsgtGQ7bQ5kgUKgEQHn7N
--- stdlib.h.bak 2013-07-03 00:08:25.000000000 +0400
+++ stdlib.h 2013-07-03 00:10:17.000000000 +0400
@@ -69,7 +69,7 @@
#define EXIT_FAILURE 1
#define EXIT_SUCCESS 0
-#define RAND_MAX 0x7fffffff
+#define RAND_MAX 0x7ffffffd
__BEGIN_DECLS
#ifdef _XLOCALE_H_
--- rand.c.bak 2013-07-03 00:08:00.000000000 +0400
+++ rand.c 2013-07-03 00:11:33.000000000 +0400
@@ -75,7 +75,8 @@
x = 16807 * lo - 2836 * hi;
if (x < 0)
x += 0x7fffffff;
- return ((*ctx = x) % ((u_long)RAND_MAX + 1));
+ *ctx = x;
+ return (x - 1);
#endif /* !USE_WEAK_SEEDING */
}
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