;;; PLT Scheme Science Collection
;;; random-distributions/flat.ss
;;; Copyright (c) 2004-2007 M. Douglas Williams
;;;
;;; This library is free software; you can redistribute it and/or
;;; modify it under the terms of the GNU Lesser General Public
;;; License as published by the Free Software Foundation; either
;;; version 2.1 of the License, or (at your option) any later version.
;;;
;;; This library 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
;;; Lesser General Public License for more details.
;;;
;;; You should have received a copy of the GNU Lesser General Public
;;; License along with this library; if not, write to the Free
;;; Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
;;; 02111-1307 USA.
;;;
;;; -------------------------------------------------------------------
;;;
;;; This code implements a flat (uniform) distribution.  It is based on
;;; the Random Number Distributions in the GNU Scientific Library.
;;;
;;; Version  Date      Description
;;; 0.9.0    08/06/04  This is the initial release of the flat
;;;                    distribution routines. (Doug Williams)
;;; 1.0.0    09/28/04  Marked as ready for Release 1.0.  Added
;;;                    contracts for functions.  (Doug Williams)
;;; 2.0.0    11/19/07  Added unchecked versions of functions and
;;;                    getting ready for PLT Scheme 4.0 (Doug Williams)

(module flat mzscheme
  
  (require (lib "contract.ss"))
  
  (provide
   (rename random-flat unchecked-random-flat)
   (rename flat-pdf unchecked-flat-pdf)
   (rename flat-cdf unchecked-flat-cdf))
  
  (provide/contract
   (random-flat
    (case-> (->r ((r random-source?)
                  (a real?)
                  (b (>/c a)))
                 real?)
            (->r ((a real?)
                  (b (>/c a)))
                 real?)))
   (flat-pdf
    (->r ((x real?)
          (a real?)
          (b (>/c a)))
         (>=/c 0.0)))
   (flat-cdf
    (->r ((x real?)
          (a real?)
          (b (>=/c a)))
         (real-in 0.0 1.0))))
  
  (require "../random-source.ss")
  
  ;; random-flat: random-source x real x real -> real
  ;; random-flat: real x real -> real
  ;;
  ;; This function returns a random variate from the flat (uniform)
  ;; distribution from a to b.
  (define random-flat
    (case-lambda
      ((r a b)
       (let ((u (unchecked-random-uniform r)))
         (+ (* a (- 1 u)) (* b u))))
      ((a b)
       (random-flat (current-random-source) a b))))
  
  ;; flat-pdf: real x real x real-> real
  ;;
  ;; This function computes the probability density p(x) at x for a
  ;; flat (uniform) distribution from a to b.
  (define (flat-pdf x a b)
    (if (< x a)
        ;; x < a
        0
        (if (<= x b)
            ;; a <= x <= b
            (/ 1.0 (- b a))
            ;; x > b
            0)))
  
  ;; flat-cdf: real x real x real-> real
  ;;
  ;; This function computes the cummulative density d(x) at x for a
  ;; flat (uniform) distribution from a to b.
  (define (flat-cdf x a b)
    (if (< x a)
        ;; x < a
        0
        (if (<= x b)
            ;; a <= x <= c
            (/ (- x a) (- b a))
            ;; x > b
            1)))
  
)