ftp://ftp.smr.ru/pub/0/FreeBSD/releases/distfiles/xscreensaver-3.16.tar.gz

[xscreensaver] / utils / yarandom.c

diff --git a/utils/yarandom.c b/utils/yarandom.c
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+/* yarandom.c -- Yet Another Random Number Generator.
+
+   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."
+
+   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.
+ */
+
+
+#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"
+
+
+/* The following 'random' numbers are taken from CRC, 18th Edition, page 622.
+   Each array element was taken from the corresponding line in the table,
+   except that a[0] was from line 100. 8s and 9s in the table were simply
+   skipped. The high order digit was taken mod 4.
+ */
+#define VectorSize 55
+static unsigned int a[VectorSize] = {
+ 035340171546, 010401501101, 022364657325, 024130436022, 002167303062, /*  5 */
+ 037570375137, 037210607110, 016272055420, 023011770546, 017143426366, /* 10 */
+ 014753657433, 021657231332, 023553406142, 004236526362, 010365611275, /* 14 */
+ 007117336710, 011051276551, 002362132524, 001011540233, 012162531646, /* 20 */
+ 007056762337, 006631245521, 014164542224, 032633236305, 023342700176, /* 25 */
+ 002433062234, 015257225043, 026762051606, 000742573230, 005366042132, /* 30 */
+ 012126416411, 000520471171, 000725646277, 020116577576, 025765742604, /* 35 */
+ 007633473735, 015674255275, 017555634041, 006503154145, 021576344247, /* 40 */
+ 014577627653, 002707523333, 034146376720, 030060227734, 013765414060, /* 45 */
+ 036072251540, 007255221037, 024364674123, 006200353166, 010126373326, /* 50 */
+ 015664104320, 016401041535, 016215305520, 033115351014, 017411670323  /* 55 */
+};
+
+static int i1, i2;
+
+unsigned int
+ya_random (void)
+{
+  register int ret = a[i1] + a[i2];
+  a[i1] = ret;
+  if (++i1 >= VectorSize) i1 = 0;
+  if (++i2 >= VectorSize) i2 = 0;
+  return ret;
+}
+
+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);
+#else
+      gettimeofday(&tp);
+#endif
+      /* ignore overflow */
+      seed = (999*tp.tv_sec) + (1001*tp.tv_usec) + (1003 * getpid());
+    }
+
+  a[0] += seed;
+  for (i = 1; i < VectorSize; i++)
+    {
+      seed = a[i-1]*1001 + seed*999;
+      a[i] += seed;
+    }
+
+  i1 = a[0] % VectorSize;
+  i2 = (i1 + 024) % VectorSize;
+}