SICP_2.50-2.51

#lang sicp

(#%require sicp-pict)

(define (origin-frame frame)
  (car frame))

(define (edge1-frame frame)
  (cadr frame))

(define (edge2-frame frame)
  (caddr frame))

(define (make-vect a b)
  (cons a b))

(define (xcor-vect v)
  (car v))

(define (ycor-vect v)
  (cdr v))

(define (add-vect v1 v2)
  (make-vect (+ (xcor-vect v1)
                (xcor-vect v2))
             (+ (ycor-vect v1)
                (ycor-vect v2))))

(define (sub-vect v1 v2)
  (make-vect (- (xcor-vect v1)
                (xcor-vect v2))
             (- (ycor-vect v1)
                (ycor-vect v2))))

(define (scale-vect s v1)
  (make-vect (* s (xcor-vect v1))
             (* s (ycor-vect v1))))

(define (frame-coord-map frame)
  (lambda (v)
    (add-vect
     (origin-frame frame)
     (add-vect (scale-vect (xcor-vect v)
                           (edge1-frame frame))
               (scale-vect (ycor-vect v)
                           (edge2-frame frame))))))
;;;;;;;;;;;;;;;;;;;2.50
(define (transform-painter painter origin corner1 corner2)
  (lambda (frame)
    (let ((m (frame-coord-map frame)))
      (let ((new-origin (m origin)))
        (painter
         (make-frame new-origin
                     (sub-vect (m corner1) new-origin)
                     (sub-vect (m corner2) new-origin)))))))

(define (flip-horiz painter)
  (transform-painter painter
                     (make-vect 1.0 0.0)
                     (make-vect 0.0 0.0)
                     (make-vect 1.0 1.0)))

(define (roate90 painter)
  (transform-painter painter
                     (make-vect 1.0 0.0)
                     (make-vect 1.0 1.0)
                     (make-vect 0.0 0.0)))

(define (roate180 painter)
  (transform-painter painter
                     (make-vect 1.0 1.0)
                     (make-vect 0.0 1.0)
                     (make-vect 1.0 0.0)))

(define (roate270 painter)
  (transform-painter painter
                     (make-vect 0.0 1.0)
                     (make-vect 0.0 0.0)
                     (make-vect 1.0 1.0)))

;;;;;;;;;;;;;;;;;2.51
(define (my-below painter1 painter2)
  (let ((split-point (make-vect 0.0 0.5)))
    (let ((paint-up
           (transform-painter painter2
                               split-point
                              (make-vect 1.0 0.5)
                              (make-vect 0.0 1.0)))
          (paint-bottom
           (transform-painter painter1
                              (make-vect 0.0 0.0)
                               (make-vect 1.0 0.0)
                               split-point)))
      (lambda (frame)
        (paint-up frame)
        (paint-bottom frame)))))

(define (my-below2 painter1 painter2)
  (roate270 (beside (roate90 painter1) (roate90 painter2))))

1.理解painter是个过程接受frame参数，即在frame中画painter

2.frame-coord-map 中将一个单位正方形中的点映射到一个新框架中

3.transform-painter 则是将painter映射到一个新框架中返回的也是一个过程，也可以理解为返回一个painter