; in-lecture examples
; CS 131 - Week 2, Lecture 2

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

(define WIDTH 200)
(define HEIGHT 150)
(define BACKDROP
        (place-image (circle 30 "solid" "gold")
                     (- WIDTH 50)
                     (/ HEIGHT 3)
                     (empty-scene WIDTH HEIGHT)))
BACKDROP

(define (triple a-number)   ; <-- this is the function header
  (* 3 a-number)            ; <-- this is the function body
)
(triple 25)
(triple WIDTH)

; the basic function syntax
; (define (desired-name param-name param-name ...)
;     expression
; )

; let's say I want MANY purple stars of many sizes
; SO -- I'm going to write a function to do so

; I'm going to write a signature for my function-to-be

; signature: purple-star: number -> image

; now, I'm going to write a purpose statement that
;   DESCRIBES what the function expects and DESCRIBES
;   what ir produces

; purpose: expects a desired enclosing pentagon side length
;    in pixels, and produces a purple star of that size

;(define (purple-star size)
;  ...
;)

; NOW -- write some specific examples of what our
;   function should do -- write them as check-expect tests

(check-expect (purple-star 27)
              (star 27 "solid" "purple"))

(check-expect (purple-star 46)
              (star 46 "solid" "purple"))

; now complete the function body

(define (purple-star size)
  (star size "solid" "purple")
)

(purple-star 30)
(purple-star 4)

; the universe teach-pack has a big-bang operation
;    that can animate scenes...

; it needs an initial "state" or world --
;    this can be a simple as a natural number,
;    and that's what we'll start with

; it needs a function that takes the current state,
;    and produces the state of the next clock tick

; signature: get-1-bigger: number -> number
; purpose: expects a number, and produces the number
;     that is 1 bigger

(define (get-1-bigger a-number)
  (+ a-number 1)
)

; note -- this check-expect was written BEFORE get-1-bigger's
;    body was filled in!

(check-expect (get-1-bigger 15) 16)
(get-1-bigger 45)

; and, I need a function that gives me the current
;    scene based on the current state

; I decide I want a scene with a purple star whose
;    size is based on the current state in the middle
;    of a scene

; signature: draw-star-scene: number -> scene
; purpose: expects the current state of the universe,
;    and produces a scene with a purple star of that
;    size in the middle of the scene

(define (draw-star-scene a-number)
  (place-image
                   (purple-star a-number)
                   (/ WIDTH 2)
                   (/ HEIGHT 2)
                   BACKDROP)
)

; note -- this check-expect was written BEFORE 
;    draw-star-scene's body was filled in!

(check-expect (draw-star-scene 15)
              (place-image
                   (purple-star 15)
                   (/ WIDTH 2)
                   (/ HEIGHT 2)
                   BACKDROP))

(check-expect (draw-star-scene 47)
              (place-image
                   (purple-star 47)
                   (/ WIDTH 2)
                   (/ HEIGHT 2)
                   BACKDROP))

(draw-star-scene 15)

; in the SIMPLEST kind of call to the big-bang function,
; big-bang needs 
;   ...an initial world or state, 
;      (in the simplest case, a number will do),
;
;   ... a call to on-tick with the name of a function 
;       that expects a state of the world and produces what ;       the world should be after the next clock tick
;       (here, it just needs to expect a number and produce
;       a number) -- this will be called for each clock
;       tick.
;       *   optionally, give on-tick a 2nd expression,
;           how often the clock should tick (0.5 for every
;           half second, 1/56 for every 1/56th of a second,
;           etc.)
;
;    ...a call to on-draw with the name of a function
;       that expects a state of the world and produces
;       the scene as it should be at that state of the world
;       -- this will be called after each clock tick and
;       the resulting scene displayed
;
; and, finally, the big-bang expression produces the
;    final state of its world when the depicted animation
;    is stopped
; PLEASE remember to stop it! It *can* overload and crash
;    DrRacket...

(big-bang 0
          (on-tick get-1-bigger)
          (on-draw draw-star-scene))

; what would happen if both the star's x-coordinate and size
;    are BOTH determined by the current state?

; signature: draw-star-scene2: number -> scene
; purpose: expects the current state of the universe,
;    and produces a scene with a purple star of that
;    size centered at (state, HEIGHT/2)

(define (draw-star-scene2 a-number)
  (place-image
                   (purple-star a-number)
                   a-number
                   (/ HEIGHT 2)
                   BACKDROP)
)


(check-expect (draw-star-scene2 15)
              (place-image
                   (purple-star 15)
                   15
                   (/ HEIGHT 2)
                   BACKDROP))
(check-expect (draw-star-scene2 47)
              (place-image
                   (purple-star 47)
                   47
                   (/ HEIGHT 2)
                   BACKDROP))
(draw-star-scene2 15)

(big-bang 0
          (on-tick get-1-bigger 1/56)
          (on-draw draw-star-scene2))