X-Git-Url: https://git.distorted.org.uk/u/mdw/catacomb/blobdiff_plain/ba6e6b64033b1f9de49feccb5c9cd438354481f7..0f00dc4c8eb47e67bc0f148c2dd109f73a451e0a:/rand/lcrand.h

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+/* -*-c-*-
+ *
+ * Simple linear congruential generator
+ *
+ * (c) 1999 Straylight/Edgeware
+ */
+
+/*----- Licensing notice --------------------------------------------------*
+ *
+ * This file is part of Catacomb.
+ *
+ * Catacomb is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Library General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * Catacomb 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 Library General Public License for more details.
+ *
+ * You should have received a copy of the GNU Library General Public
+ * License along with Catacomb; if not, write to the Free
+ * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
+ * MA 02111-1307, USA.
+ */
+
+/*----- Notes on the linear congruential generator ------------------------*
+ *
+ * This pseudorandom number generator is simple, but has absolutely no
+ * cryptographic strength whatever.  It may be used whenever random numbers
+ * are required but cryptographic strength is not, for example when
+ * generating numbers for use in primality tests.  To be honest, it's not
+ * even particularly fast, although a certain amount of effort has been
+ * expended on making it better than awfully slow.  To put things in
+ * perspective, it can't quite spit bytes out as fast as OFB DES.  (Then
+ * again, bytes aren't its natural output format.)  Its main use is probably
+ * seeding a Fibonacci generator.
+ *
+ * There exists a fixed-point input @LCRAND_FIXEDPT@ -- when fed to the
+ * generator it comes straight back out again.  All other inputs less than
+ * the modulus are part of the same sequence of period %$p - 1$%.
+ *
+ * The generator has been tested for its statistical properties.  George
+ * Marsaglia's Diehard tests give it a reasonably clean bill of health.
+ *
+ * The modulus %$p$% is chosen as the largest prime number less than
+ * %$2^{32}$%.  The multiplier %$a$% and additive constant %$c$% are based on
+ * the decimal expansions of %$\pi$% and %$e$%, with the additional
+ * restriction that the multiplier must be a primitive element modulo %$p$%.
+ * The fixed point value is determined as %$c / (1 - a) \bmod p$%.
+ */
+
+#ifndef CATACOMB_LCRAND_H
+#define CATACOMB_LCRAND_H
+
+#ifdef __cplusplus
+  extern "C" {
+#endif
+
+/*----- Header files ------------------------------------------------------*/
+
+#include <mLib/bits.h>
+
+#ifndef CATACOMB_GRAND_H
+#  include "grand.h"
+#endif
+
+/*----- Constants ---------------------------------------------------------*/
+
+#define LCRAND_P 4294967291u		/* Modulus for the generator */
+#define LCRAND_A 314159265u		/* Multiplier (primitive mod @p@) */
+#define LCRAND_C 271828183u		/* Additive constant */
+
+#define LCRAND_FIXEDPT 3223959250u	/* Fixed point (only bad input) */
+
+/*----- Functions provided ------------------------------------------------*/
+
+/* --- @lcrand@ --- *
+ *
+ * Arguments:	@uint32 x@ = seed value
+ *
+ * Returns:	New state of the generator.
+ *
+ * Use:		Steps the generator.  Returns %$ax + c \bmod p$%.
+ */
+
+extern uint32 lcrand(uint32 /*x*/);
+
+/* --- @lcrand_range@ --- *
+ *
+ * Arguments:	@uint32 *x@ = pointer to seed value (updated)
+ *		@uint32 m@ = limit allowable
+ *
+ * Returns:	A uniformly distributed pseudorandom integer in the interval
+ *		%$[0, m)$%.
+ */
+
+extern uint32 lcrand_range(uint32 */*x*/, uint32 /*m*/);
+
+/* --- @lcrand_create@ --- *
+ *
+ * Arguments:	@uint32 x@ = initial seed
+ *
+ * Returns:	Pointer to a generic generator.
+ *
+ * Use:		Constructs a generic generator interface over a linear
+ *		congruential generator.
+ */
+
+extern grand *lcrand_create(uint32 /*x*/);
+
+/*----- That's all, folks -------------------------------------------------*/
+
+#ifdef __cplusplus
+  }
+#endif
+
+#endif