http://www.jwz.org/xscreensaver/xscreensaver-5.11.tar.gz

[xscreensaver] / utils / yarandom.c

diff --git a/utils/yarandom.c b/utils/yarandom.c
index 228b81caa31524843ec87a40c7d8797ad481c185..6d4a32385b4cb94fbbce4a068d240de9c9496f6b 100644 (file)
--- a/utils/yarandom.c
+++ b/utils/yarandom.c
@@ -1,20 +1,68 @@
-/* ya_random -- Yet Another Random Number Generator.
+/* yarandom.c -- Yet Another Random Number Generator.
+ * Copyright (c) 1997-2010 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.
+
+   ---------------------------
+   Note: xlockmore 4.03a10 uses this very simple RNG:
+
+       if ((seed = seed % 44488 * 48271 - seed / 44488 * 3399) < 0)
+         seed += 2147483647;
+       return seed-1;
+
+   of which it says
+
+       ``Dr. Park's algorithm published in the Oct. '88 ACM  "Random Number
+         Generators: Good Ones Are Hard To Find" His version available at
+         ftp://cs.wm.edu/pub/rngs.tar Present form by many authors.''
+
+   Karlton says: ``the usual problem with that kind of RNG turns out to
+   be unexepected short cycles for some word lengths.''
+
+   Karlton's RNG is faster, since it does three adds and two stores, while the
+   xlockmore RNG does two multiplies, two divides, three adds, and one store.
+
+   Compiler optimizations make a big difference here:
+       gcc -O:     difference is 1.2x.
+       gcc -O2:    difference is 1.4x.
+       gcc -O3:    difference is 1.5x.
+       SGI cc -O:  difference is 2.4x.
+       SGI cc -O2: difference is 2.4x.
+       SGI cc -O3: difference is 5.1x.
+   Irix 6.2; Indy r5k; SGI cc version 6; gcc version 2.7.2.1.
  */
 
-#include <unistd.h>   /* for getpid() */
+
+#ifdef HAVE_CONFIG_H
+# include "config.h"
+#endif
+
+#ifdef HAVE_UNISTD_H
+# include <unistd.h>  /* for getpid() */
+#endif
 #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.
    Each array element was taken from the corresponding line in the table,
@@ -38,35 +86,54 @@ static unsigned int a[VectorSize] = {
 
 static int i1, i2;
 
-unsigned int ya_random()
+unsigned int
+ya_random (void)
 {
   register int ret = a[i1] + a[i2];
   a[i1] = ret;
-  i1 = i1 >= VectorSize ? 0 : i1 + 1;
-  i2 = i2 >= VectorSize ? 0 : i2 + 1;
+  if (++i1 >= VectorSize) i1 = 0;
+  if (++i2 >= VectorSize) i2 = 0;
   return ret;
 }
 
-void ya_rand_init(seed)
-   register unsigned int seed;
+void
+ya_rand_init(unsigned int seed)
 {
   int i;
   if (seed == 0)
     {
       struct timeval tp;
+#ifdef GETTIMEOFDAY_TWO_ARGS
       struct timezone tzp;
       gettimeofday(&tp, &tzp);
-      /* ignore overflow */
-      seed = (999*tp.tv_sec) + (1001*tp.tv_usec) + (1003 * getpid());
+#else
+      gettimeofday(&tp);
+#endif
+      /* 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 = (999 * 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;
 }