03-1-falling-cat.rkt

;; The first three lines of this file were inserted by DrRacket. They record metadata
;; about the language level of this file in a form that our tools can easily process.
#reader(lib "htdp-beginner-reader.ss" "lang")((modname 03-1-falling-cat) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #f #t none #f () #f)))
;; falling cat.  
;; A cat falls from the top of the scene.
;; The user can pause/unpause the cat with the space bar.

;; start with (main 0)

(require rackunit)
(require "extras.rkt")

(require 2htdp/universe)
(require 2htdp/image)


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;; MAIN FUNCTION.

;; main : Integer -> World
;; GIVEN: the initial y-position of the cat
;; EFFECT: runs the simulation, starting with the cat falling
;; RETURNS: the final state of the world
(define (main initial-pos)
  (big-bang (make-world initial-pos false)
            (on-tick world-after-tick 0.5)
            (on-key world-after-key-event)
            (on-draw world-to-scene)))

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;; CONSTANTS

(define CAT-IMAGE (bitmap "cat.png"))

;; how fast the cat falls, in pixels/tick
(define CATSPEED 8)

;; dimensions of the canvas
(define CANVAS-WIDTH 200)
(define CANVAS-HEIGHT 400)
(define EMPTY-CANVAS (empty-scene CANVAS-WIDTH CANVAS-HEIGHT))
(define CAT-X-COORD (/ CANVAS-WIDTH 2))

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;;; DATA DEFINITIONS

(define-struct world (pos paused?))
;; A World is a (make-world Integer Boolean)
;; Interpretation: 
;; pos describes how far the cat has fallen, in pixels. 
;; paused? describes whether or not the cat is paused.

;; template:
;; world-fn : World -> ??
;(define (world-fn w)
;  (... (world-pos w) (world-paused? w)))

;;examples of worlds, for testing
(define unpaused-world-at-20 (make-world 20 false))  
(define paused-world-at-20   (make-world 20 true))
(define unpaused-world-at-28 (make-world 28 false))  
(define paused-world-at-28   (make-world 28 true))

;; help function for key event
;; is-pause-key-event? : KeyEvent -> Boolean
;; GIVEN: a KeyEvent
;; RETURNS: true iff the KeyEvent represents a pause instruction
(define (is-pause-key-event? ke)
  (key=? ke " "))

;; examples for testing
(define pause-key-event " ")
(define non-pause-key-event "q")   

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;; world-after-tick : World -> World
;; GIVEN: a world w
;; RETURNS: the world that should follow w after a tick.

;; EXAMPLES: 
;; cat falling:
;; (world-after-tick unpaused-world-at-20) = unpaused-world-at-28
;; cat paused:
;; (world-after-tick paused-world-at-20) = paused-world-at-20

;; STRATEGY: Use template for World on w

(define (world-after-tick w)
  (if (world-paused? w)
      w
      (make-world (+ (world-pos w) CATSPEED)
                  (world-paused? w))))

;; tests:
(begin-for-test
  (check-equal? 
    (world-after-tick unpaused-world-at-20) 
    unpaused-world-at-28
    "in unpaused world, the cat should fall CATSPEED pixels and world should still be unpaused")

  (check-equal? 
    (world-after-tick paused-world-at-20)
    paused-world-at-20
    "in paused world, cat should be unmoved"))


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;; world-to-scene : World -> Scene
;; RETURNS: a Scene that portrays the given world.
;; EXAMPLE: (world-to-scene (make-world 20 ??))
;;          = (place-image CAT-IMAGE CAT-X-COORD 20 EMPTY-CANVAS)
;; STRATEGY: Use template for World on w

(define (world-to-scene w)
  (place-image CAT-IMAGE CAT-X-COORD
               (world-pos w)
               EMPTY-CANVAS))

;; tests

;; an image showing the cat at Y = 20
;; check this visually to make sure it's what you want
(define image-at-20 (place-image CAT-IMAGE CAT-X-COORD 20 EMPTY-CANVAS))

;; note: these only test whether world-to-scene calls place-image properly.
;; it doesn't check to see whether that's the right image!
;; these are not very good test strings!
(begin-for-test
  (check-equal? 
    (world-to-scene unpaused-world-at-20)
    image-at-20
    "test of (world-to-scene unpaused-world-at-20)")

  (check-equal?
    (world-to-scene paused-world-at-20)
    image-at-20
    "test of (world-to-scene paused-world-at-20)"))


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;; world-after-key-event : World KeyEvent -> World
;; GIVEN: a world w and a keyevent kev
;; RETURNS: the world that should follow the given world
;; after the given key event.
;; on space, toggle paused?-- ignore all others
;; EXAMPLES: see tests below
;; STRATEGY: Cases on whether the kev is a pause event
(define (world-after-key-event w kev)
  (if (is-pause-key-event? kev)
    (world-with-paused-toggled w)
    w))


;; world-with-paused-toggled : World -> World
;; RETURNS: a world just like the given one, but with paused? toggled
;; STRATEGY: Use template for World on w
(define (world-with-paused-toggled w)
  (make-world
   (world-pos w)
   (not (world-paused? w))))

;; for world-after-key-event, we need 4 tests: a paused world, and an
;; unpaused world, and a pause-key-event and a non-pause key event.

(begin-for-test
  (check-equal?
    (world-after-key-event paused-world-at-20 pause-key-event)
    unpaused-world-at-20
    "after pause key, a paused world should become unpaused")

  (check-equal?
    (world-after-key-event unpaused-world-at-20 pause-key-event)
    paused-world-at-20
    "after pause key, an unpaused world should become paused")

  (check-equal?
    (world-after-key-event paused-world-at-20 non-pause-key-event)
    paused-world-at-20
    "after a non-pause key, a paused world should be unchanged")

  (check-equal?
    (world-after-key-event unpaused-world-at-20 non-pause-key-event)
    unpaused-world-at-20
    "after a non-pause key, an unpaused world should be unchanged"))