/* yarandom.c -- Yet Another Random Number Generator.
+ * Copyright (c) 1997-2014 by Jamie Zawinski <jwz@jwz.org>
+ *
+ * Permission to use, copy, modify, distribute, and sell this software and its
+ * documentation for any purpose is hereby granted without fee, provided that
+ * the above copyright notice appear in all copies and that both that
+ * copyright notice and this permission notice appear in supporting
+ * documentation. No representations are made about the suitability of this
+ * software for any purpose. It is provided "as is" without express or
+ * implied warranty.
+ */
- The unportable mess that is rand(), random(), drand48() and friends led me
+/* The unportable mess that is rand(), random(), drand48() and friends led me
to ask Phil Karlton <karlton@netscape.com> what the Right Thing to Do was.
He responded with this. It is non-cryptographically secure, reasonably
random (more so than anything that is in any C library), and very fast.
I don't understand how it works at all, but he says "look at Knuth,
Vol. 2 (original edition), page 26, Algorithm A. In this case n=55,
- k=20 and m=2^32."
+ k=24 and m=2^32."
So there you have it.
#include <sys/time.h> /* for gettimeofday() */
#include "yarandom.h"
+# undef ya_rand_init
/* The following 'random' numbers are taken from CRC, 18th Edition, page 622.
#else
gettimeofday(&tp);
#endif
- /* ignore overflow */
- seed = (999*tp.tv_sec) + (1001*tp.tv_usec) + (1003 * getpid());
+ /* Since the multiplications will have a larger effect on the
+ upper bits than the lower bits, after every addition in the
+ seed, perform a bitwise rotate by an odd number, resulting
+ in a better distribution of randomness throughout the bits.
+ -- Brian Carlson, 2010.
+ */
+#define ROT(X,N) (((X)<<(N)) | ((X)>>((sizeof(unsigned int)*8)-(N))))
+ seed = (999U * (unsigned int) tp.tv_sec);
+ seed = ROT (seed, 11);
+ seed += (1001 * tp.tv_usec);
+ seed = ROT (seed, 7);
+ seed += (1003 * getpid());
+ seed = ROT (seed, 13);
}
a[0] += seed;
for (i = 1; i < VectorSize; i++)
{
- seed = a[i-1]*1001 + seed*999;
+ seed = seed*999;
+ seed = ROT (seed, 9);
+ seed += a[i-1]*1001;
+ seed = ROT (seed, 15);
a[i] += seed;
}
i1 = a[0] % VectorSize;
- i2 = (i1 + 024) % VectorSize;
+ i2 = (i1 + 24) % VectorSize;
}