http://www.ibiblio.org/pub/historic-linux/ftp-archives/sunsite.unc.edu/Sep-29-1996...

[xscreensaver] / utils / yarandom.c

diff --git a/utils/yarandom.c b/utils/yarandom.c
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+/* ya_random -- 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.
+ */
+
+#include <unistd.h>   /* for getpid() */
+#include <sys/time.h> /* for gettimeofday() */
+
+
+/* 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()
+{
+  register int ret = a[i1] + a[i2];
+  a[i1] = ret;
+  i1 = i1 >= VectorSize ? 0 : i1 + 1;
+  i2 = i2 >= VectorSize ? 0 : i2 + 1;
+  return ret;
+}
+
+void ya_rand_init(seed)
+   register unsigned int seed;
+{
+  int i;
+  if (seed == 0)
+    {
+      struct timeval tp;
+      struct timezone tzp;
+      gettimeofday(&tp, &tzp);
+      /* 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;
+}