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;;; random-maker: constructs a random-number generator
;;; Copyright (c) 2001  John David Stone
;;; 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 of the License, 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.,
;;; 51 Franklin Street - Fifth Floor, Boston, MA  02110-1301, USA.
;;; John David Stone
;;; Department of Mathematics and Computer Science
;;; Grinnell College
;;; Grinnell, Iowa 50112
;;; stone@cs.grinnell.edu
;;; created July 10, 1995
;;; last revised March 23, 2001
;;; A call to the RANDOM-MAKER procedure presented here yields a
;;; dynamically constructed procedure that acts as a random-number
;;; generator.  When the dynamically constructed procedure is invoked with
;;; no arguments, it returns a pseudo-random real value evenly distributed
;;; in the range [0.0, 1.0); when it is invoked with one argument (which
;;; should be a positive integer N), it returns a pseudo-random integer
;;; value evenly distributed in the range [0, N); when it is invoked with
;;; two arguments, the first of which should be a positive integer and the
;;; second the symbol RESET, it changes the seed of the random-number
;;; generator to the value of the first argument.
;;; The generator employs the linear-congruential method, and specifically
;;; uses a choice of multiplier that was proposed as a standard by Stephen
;;; K. Park _et al._ in ``Technical correspondence,'' _Communications of
;;; the ACM_ 36 (1993), number 7, 108--110.
;; an arbitrarily chosen birthday
"random: unrecognized message";; another arbitrarily chosen birthday
;;; The RANDOM procedure added at the end shows how to call
;;; RANDOM-MAKER to get a random-number generator with a specific seed.
;;; The random-number generator itself is invoked as described above, by
;;; such calls as (RANDOM), to get a real number between 0 and 1, and
;;; (RANDOM N), to get an integer in the range from 0 to N - 1.
;;; The location of the binding of SEED -- inside the body of RANDOM-MAKER,
;;; but outside the LAMBDA-expression that denotes the dynamically
;;; allocated procedure -- ensures that the storage location containing the
;;; seed will be different for each invocation of RANDOM-MAKER (so that
;;; every generator that is constructed will have an independently settable
;;; seed), yet inaccessible except through invocations to the dynamically
;;; allocated procedure itself.  In effect, random-number generators in
;;; this implementation constitute an abstract data type with the
;;; constructor RANDOM-MAKER and exactly three operations, corresponding to
;;; the three possible arities of a call to the generator.
;;; When calling this procedure, the programmer must supply an initial
;;; value for the seed.  This should be an integer (if it is not, an
;;; arbitrary default seed is silently substituted).  The value supplied is
;;; forced into the range (0, MODULUS], since it is an invariant of the
;;; procedure that the seed must always be in this range.
;;; To obtain an initial seed that is likely to be different each time a
;;; new generator is constructed, use some combination of the program's
;;; running time and the wall-clock time.  (Most Scheme implementations
;;; provide procedures that return one or both of these quantities.  For
;;; instance, in SCM, the call
;;;
;;;    (RANDOM-MAKER (+ (* 100000 (GET-INTERNAL-RUN-TIME)) (CURRENT-TIME)))
;;;
;;; yields a generator with an effectively random seed.)