;;; PLT Scheme Science Collection
;;; random-distributions/bernoulli.ss
;;; Copyright (c) 2004 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 module implements the bernoulli distribution.  It is based on
;;; the Random Number Distribution in the GNU Scientific Library.
;;;
;;; Version  Date      Description
;;; 1.0.0    09/28/04  Marked as ready for Release 1.0.  Added
;;;                    contracts for functions.  (Doug Williams)

(module bernoulli mzscheme
  
  (require (lib "contract.ss"))
  
  (provide/contract
   (random-bernoulli
    (case-> (-> random-source? (real-in 0.0 1.0) (integer-in 0 1))
            (-> (real-in 0.0 1.0) (integer-in 0 1))))
   (bernoulli-pdf
    (-> integer? (real-in 0.0 1.0) (real-in 0.0 1.0)))
   (bernoulli-cdf
    (-> integer? (real-in 0.0 1.0) (real-in 0.0 1.0))))
  
  (require "../random-source.ss")
   
  ;; random-bernoulli: random-source x real -> integer
  ;; random-bernoulli: real -> integer
  ;;
  ;; The bernoulli distribution has the form:
  ;; prob(0) = 1-p, prob(1) = p
  (define random-bernoulli
    (case-lambda
      ((r p)
       (let ((u (random-uniform r)))
         (if (< u p) 1 0)))
      ((p)
       (random-bernoulli (current-random-source) p))))
  
  ;; bernoulli-pdf: integer x real -> real
  ;;
  ;; This function computes the probability density p(x) at x for a
  ;; Bernoulli distribution with probability p.
  (define (bernoulli-pdf k p)
    (cond ((= k 0) (- 1.0 p))
          ((= k 1) p)
          (else 0.0)))
  
  ;; bernoulli-cdf: integer x real -> real
  ;;
  ;; This function computes the cummulative density d(x) at x for a
  ;;; Bernoulli distribution with probability p.
  (define (bernoulli-cdf k p)
    (cond ((= k 0) (- 1.0 p))
          ((= k 1) 1.0)
          (else 0.0)))
  
)