1 files changed, 70 insertions, 0 deletions
diff --git a/lib/rand.c b/lib/rand.c
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+/* rand.c: a simple pseudo-random number generator.
+
+Copyright (C) 1992 Free Software Foundation, Inc.
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 2, or (at your option)
+any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program; if not, write to the Free Software
+Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.  */
+
+#include "config.h"
+
+#include "rand.h"
+
+
+/* Current state of the random number generator.  ANSI C says that the
+   default should be as if `srand' was called with the value 1.  At all
+   times this will be a random number between 1 and RAND_MAX+1.  */
+static long rand_state = 2;
+
+
+/* Set the state.  It's ok if `seed' doesn't fit in a long, since we
+   check for negative values before we return anything.  On the other
+   hand, we cannot allow our state to become zero, since then we will
+   produce zero forever.  */
+
+void
+seed_rand (unsigned seed)
+{
+  rand_state = seed + 1;
+}
+
+
+/* Return a pseudo-random number based on `x' in the range [0, RAND_MAX].
+   We use the Ghostscript computation (from the function `zrand' in the
+   file `zmath.c').  */
+
+int
+k_rand ()
+{
+  /* We use an algorithm from CACM 31(10), pp. 1192-1201, October 1988. 
+     According to a posting by Ed Taft on comp.lang.postscript, Level 2
+     (Adobe) PostScript interpreters use this algorithm too:
+
+            x[n+1] = (16807 * x[n]) mod (2^31 - 1)
+     
+     The complication ensues from the fact that the multiplication may
+     lead to a 46-bit number.  */
+
+#define A 16807
+#define M 0x7fffffff
+#define Q 127773			/* M / A */
+#define R 2836				/* M % A */
+  rand_state = A * (rand_state % Q) - R * (rand_state / Q);
+  while ( rand_state <= 0 ) rand_state += M;
+  
+  /* OK, `rand_state' is our new random number between 1 and 2^31 - 1.
+     Now we have to subtract one, so that we return a number in the
+     range zero to RAND_MAX (inclusive).  We obviously must define our
+     RAND_MAX to be 2^31 - 2, which we do (in `rand.h').  */
+  return rand_state - 1;
+}