;;; PLT Scheme Science Collection
;;; random-distributions.geometric.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 modules implements geometric distributions.  It is based on
;;; the Random Number Distributions 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 geometric mzscheme
  
  (require (lib "contract.ss"))
  
  (provide/contract
   (random-geometric
    (case-> (-> random-source? (real-in 0.0 1.0) integer?)
            (-> (real-in 0.0 1.0) integer?)))
   (geometric-pdf
    (-> integer? (real-in 0.0 1.0) (>=/c 0.0))))
  
  (require "../random-source.ss")
  
  ;; random-geometric: random-source x real -> integer
  ;; random-geometric: real -> integer
  ;;
  ;; This functions returns a random variate from a geometric
  ;; distribution with probability p.
  (define random-geometric
    (case-lambda
      ((r p)
       (let ((u (random-uniform r)))
         (if (= p 1.0)
             1
             (+ (inexact->exact (truncate (/ (log u)
                                             (log (- 1.0 p)))))
                1))))
      ((p)
       (random-geometric (current-random-source) p))))
  
  ;; geometric-pdf: integer x real -> real
  ;;
  ;; This function computes the probability density p(x) at x for a
  ;; geometric distribution with probability p.
  (define (geometric-pdf k p)
    (cond ((= k 0)
           0.0)
          ((= k 1)
           p)
          (else
           (* p (expt (- 1.0 p) (- k 1.0))))))
  
)