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; This file is part of "NOVA":  NOVA = search + COCOMO tools
; Copyright, 2008, Tim Menzies tim@menzies.us
;
; NOVA 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 3 of the License, or
; (at your option) any later version.
;
; NOVA 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
; a long with NOVA.  If not, see <http://www.gnu.org/licenses/>.
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(in-package :wvu-lib.tricks)

(defparameter *seed0* 10013)
(defparameter *seed*  *seed0*)

(defun reset-seed (&optional seed) (setf *seed* (or seed *seed0*)))

(defun park-miller-randomizer ()
  "The Park-Miller multiplicative congruential randomizer (CACM, October 88,
   Page 1195).  Creates pseudorandom floating-point numbers in the range
   0 < x <= 1."
  (let ((multiplier 16807d0)  ; 16807 = (expt 7 5)
	(modulus 2147483647d0))  ; 2147483647 = (- (expt 2 31) 1)
    (let ((temp (* multiplier *seed*)))
      (setf *seed* (mod temp modulus))
      (/ *seed* modulus))))

(defun my-random (n)
  "Returns a pseudorandom floating-point number in the range 0 <= x < n."
  (let ((x (park-miller-randomizer)))

    ;; x is in the range 0 < x <= 1
    ;; subtract from 1 to end up in the range 0 <= x < 1
    (* n (- 1 x))))

(defun my-random-int (n)
  "Returns a pseudorandom integer in the range 0 <= x <= n - 1."
  (let ((x (my-random 1)))
    (floor (* n x))))