#lang sicp

(define (stream-car stream) 
  (car stream))
(define (stream-cdr stream) 
  (force (cdr stream)))

(define (stream-enumerate-interval low high)
  (if (> low high)
      the-empty-stream
      (cons-stream
       low
       (stream-enumerate-interval (+ low 1)
                                  high))))

(define (stream-for-each proc s)
  (if (stream-null? s)
      'done
      (begin
        (proc (stream-car s))
        (stream-for-each proc
                         (stream-cdr s)))))

(define (display-stream s)
  (stream-for-each display-line s))

(define (display-line x)
  (newline)
  (display x))

(define (stream-ref s n)
  (if (= n 0)
      (stream-car s)
      (stream-ref (stream-cdr s) (- n 1))))

(define (stream-map proc . argstreams)
  (if (stream-null? (car argstreams))
      the-empty-stream
      (cons-stream
       (apply proc (map stream-car argstreams))
       (apply stream-map (cons proc (map stream-cdr argstreams))))))

(define (stream-filter pred stream)
  (cond ((stream-null? stream)
         the-empty-stream)
        ((pred (stream-car stream))
         (cons-stream
          (stream-car stream)
          (stream-filter
           pred
           (stream-cdr stream))))
        (else (stream-filter
               pred
               (stream-cdr stream)))))

(define sum 0)

(define (accum x)
  (set! sum (+ x sum))
  sum)

(define seq 
  (stream-map 
   accum 
   (stream-enumerate-interval 1 20)))
;; 1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 66, 78, 91, 105, 120, 136, 153, 171, 190, 210

(define y (stream-filter even? seq))
;; Evens: 6, 10, 28, 36, 66, 78, 120, 136, 190, 210

(define z 
  (stream-filter 
   (lambda (x) 
     (= (remainder x 5) 0)) seq))
;; Divisible by 5: 10, 15, 45, 55, 105, 120, 190, 210

(stream-ref y 7)
;; 136, the seventh even triangular number
(display-stream z)
;; Prints 10, 15, 45, 55, 105, 120, 190, 210