;; 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 lab3-starter) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #f #t none #f () #f)))
; Design a world program word-processor that renders the characters that the user
; types in black text on a yellow background.

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

; word-processor : String -> Boolean
; Runs a simplified word processor, returns #true if all went okay
 
(define (word-processor initial-str)
  (big-bang initial-str
    [to-draw draw-wp]
    [on-key key-wp]))

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; to-draw event handler:

(define BACKGROUND (empty-scene 600 100 "yellow"))
(define WORD-SIZE 16)
(define WORD-COLOR "black")

; draw-wp : String -> Image
; Draws the current text buffer
(check-expect (draw-wp "hello")
              (overlay/align "left" "middle" (text "hello" WORD-SIZE WORD-COLOR) BACKGROUND))
 (check-expect (draw-wp "a")
              (overlay/align "left" "middle" (text "a" WORD-SIZE WORD-COLOR) BACKGROUND))
 
(define (draw-wp s)
  (overlay/align "left" "middle" (text s WORD-SIZE WORD-COLOR) BACKGROUND))
 
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; on-key event handler:

; key-wp : String KeyEvent -> String
; Adds key to the end of s, or removes the last letter if backspace is entered
(check-expect (key-wp "e" "") "e")
(check-expect (key-wp "l" "e") "le")
(check-expect (key-wp "le" "\b") "l")
 
(define (key-wp s key)
  (cond [(string=? key "\b") (remove-last s)]
        [else (string-append s key)]))
 
;; Helper function for key-wp:

; remove-last : String -> String
; Removes the last letter of the supplied string
(check-expect (remove-last "") "")
(check-expect (remove-last "apple") "appl")
 
(define (remove-last s)
  (cond [(string=? s "") s]
        [else (substring s 0 (sub1 (string-length s)))]))