;    Copyright (C) 2006  Will M. Farr <farr@mit.edu>
;
;    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.


(module streams mzscheme
  (require (only (lib "1.ss" "srfi") any))
  
  (provide stream? stream-car stream-cdr stream-ref stream-cons 
           (rename my-stream stream) stream-map stream-filter stream-take
           stream-converged? stream-take-converged)
    
  (define-struct stream (a b) #f)
  
  (define stream-car stream-a)
  
  (define (stream-cdr s)
    (force (stream-b s)))
  
  (define (stream-ref s n)
    (if (= n 0) 
        (stream-car s)
        (stream-ref (stream-cdr s) (- n 1))))
  
  (define-syntax stream-cons
    (syntax-rules ()
      ((cons-stream car cdr)
       (make-stream car (delay cdr)))))
  
  (define-syntax my-stream
    (syntax-rules ()
      ((stream a)
       (stream-cons a ()))
      ((stream a b)
       (stream-cons a b))
      ((stream a b ...)
       (stream-cons a (my-stream b ...)))))       

  (define (stream-map f . ss)
    (stream-cons (apply f (map stream-car ss)) (apply stream-map f (map stream-cdr ss))))
  
  (define (stream-for-each f . ss)
    (apply f (map stream-car ss))
    (stream-for-each f (map stream-cdr ss)))
  
  (define (stream-filter test? s)
    (let ((s1 (stream-car s)))
      (if (test? s1)
          (stream-cons s1 (stream-filter test? (stream-cdr s)))
          (stream-filter test? (stream-cdr s)))))
  
  (define (stream-take n s)
    (if (= n 0)
        '()
        (cons (stream-car s) (stream-take (- n 1) (stream-cdr s)))))
  
  (define (stream-converged? s)
    (= (stream-car s) (stream-ref s 1)))
  
  (define (stream-take-converged s)
    (if (stream-converged? s)
        (stream-car s)
        (stream-take-converged (stream-cdr s)))))