;; 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-intermediate-lambda-reader.ss" "lang")((modname main) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #f #t none #f () #f)))
; 2018-11-21 - Made by Claudio Maggioni - Tommaso Rodolfo Masera

; easybf
(require 2htdp/batch-io)
(require 2htdp/image)
(require 2htdp/universe)

; A Byte is an Int between 0 and 255
; Interpretation: a byte in decimal notation.

; A Tape is a NEList<Byte>
; Interpretation: a tape in brainf*ck's Turing machine.

; A DataPointer (DP) is a NonNegInt
; Interpretation: a data pointer in the brainf*ck language in a tape.

; A Program is a String of:
; - ">" (tape-right)
; - "<" (tape-left)
; - "+" (add1)
; - "-" (sub1)
; - "." (out)
; - ","
; - "["
; - "]"
; Interpretation: the brainf*ck program.

; A InstructionPointer (IP) is a NonNegInt
; Interpretation: a pointer to the instruction to execute.

; A World is a (make-world tape dp output program ip) where:
; - tape: Tape
; - dp: DataPointer
; - tape-len: Nat
; - output: String
; - program: Program
; - ip: InstructionPointer
; Interpretation: the current state of execution of a brainf*ck program.
(define-struct world [tape dp tape-len output program ip])

; Template function for World
(define (fn-for-world w)
  (... (world-tape w)
       (world-dp w)
       (world-tape-len w)
       (world-output w)
       (world-program w)
       (world-ip w)))

; string->program: String -> Program
; Given a string, returns a b(world-dp-len w)f program without any invalid character
(define (string->program s)
  (local [; valid-char: 1String -> Boolean
          ; Given a valid-char, returns #t if the character is a valid bf
          ; instruction.
          (define (valid-char? s)
            (member? s '(">" "<" "+" "-" "," "." "[" "]")))]
         (implode (filter valid-char? (explode s)))))

; Tests for string->program
(check-expect (string->program "hello") "")
(check-expect (string->program "+world50-[]") "+-[]")
(check-expect (string->program "") "")

; add1-byte: Byte -> Byte
; Given a byte, returns the byte+1 simulating overflows
(define (add1-byte b)
  (modulo (add1 b) 256))

; Tests for add1-byte
(check-expect (add1-byte 255) 0)
(check-expect (add1-byte 254) 255)

; sub1-byte: Byte -> Byte
; Given a byte, returns the byte-1 simulating underflows
(define (sub1-byte b)
  (cond [(zero? b) 255]
        [else (sub1 b)]))

; Tests for sub1-byte
(check-expect (sub1-byte 0) 255)
(check-expect (sub1-byte 1) 0)

; program->world: Program -> World
; Given a program, returns the corresponding initial world state.
(define (program->world p)
  (make-world (cons 0 '()) 0 1 "" p 0))

; Tests for program->world
(check-expect (program->world "[->+<]") (make-world (list 0) 0 1 "" "[->+<]" 0))

; tape-help: Tape DP (Byte -> Byte) -> Tape
; Given a tape and a data pointer, returns the same tape with the data in the
; location of the data pointer altered by the function `alter`.
(define (tape-help tape dp alter)
  (cond [(zero? dp) (cons (alter (first tape)) (rest tape))]
        [else (cons (first tape) (tape-help (rest tape) (sub1 dp) alter))]))

; Tests for tape-help
(check-expect (tape-help (list 0) 0 add1-byte) (list 1))
(check-expect (tape-help (list 0 1 2 3) 2 sub1-byte) (list 0 1 1 3))

; exec-add1: World -> World
; Given a world, returns a new world with the + instruction executed
(define (exec-add1 w)
  (make-world (tape-help (world-tape w) (world-dp w) add1-byte)
              (world-dp w)
              (world-tape-len w)
              (world-output w)
              (world-program w)
              (add1 (world-ip w))))

; exec-sub1: World -> World
; Given a world, returns a new world with the - instruction executed
(define (exec-sub1 w)
  (make-world (tape-help (world-tape w) (world-dp w) sub1-byte)
              (world-dp w)
              (world-tape-len w)
              (world-output w)
              (world-program w)
              (add1 (world-ip w))))

; exec-tape-left: World -> World
; Given a world, returns a new world with the < instruction executed
(define (exec-tape-left w)
  (if (zero? (world-dp w))
      (error "Can't access negative tape positions")
      (make-world (world-tape w)
                  (sub1 (world-dp w))
                  (world-tape-len w)
                  (world-output w)
                  (world-program w)
                  (add1 (world-ip w)))))

; exec-tape-right: World -> World
; Given a world, returns a new world with the > instruction executed
(define (exec-tape-right w)
  (local [(define end-of-tape (= (world-dp w) (sub1 (world-tape-len w))))]
    (make-world
     (if end-of-tape
         (cons (world-tape w) 0)
         (world-tape w)) 
     (add1 (world-dp w))
     (if end-of-tape
         (add1 (world-tape-len w))
         (world-tape-len w))
     (world-output w)
     (world-program w)
     (add1 (world-ip w)))))

; exec-out: World -> World
; Given a world, returns a new world with the . instruction executed
(define (exec-out w)
  (make-world
   (world-tape w)
   (world-dp w)
   (world-tape-len w)
   (string-append
    (world-output w)
    (list->string
     (list (integer->char (list-ref (world-tape w) (world-dp w))))))
   (world-program w)
   (add1 (world-ip w))))

; find-first: 1String String Nat -> Nat
(define (find-first char string start)
  (cond [(string=? (substring string start (add1 start)) char) start]
        [else (find-first char string (add1 start))]))

; exec-loop-start: World -> World
; Given a world, returns a new world with the [ instruction executed
(define (exec-loop-start w)
  (local [(define jump (zero? (list-ref (world-tape w) (world-dp w))))]
    (make-world
     (world-tape w)
     (world-dp w)
     (world-tape-len w)
     (world-output w)
     (world-program w)
     (add1 (if jump
         (find-first "]" (world-program w) (world-ip w))
         (world-ip w))))))

; Tests for exec-out
(check-expect (exec-out (make-world (list 50) 0 1 "" ".[->+<]" 0))
              (make-world (list 50) 0 1 "2" ".[->+<]" 1))
(check-expect (exec-out (make-world (list 65) 0 1 "" ".[->+<]" 0))
              (make-world (list 65) 0 1 "A" ".[->+<]" 1))

; main: Filename -> World
; Given a Filename, return the last world state of execution of the
; corresponding bf program in the file
(define (main file)
  (program->world (string->program (read-file file))))

; TODO:
; - big-bang:
; - - - - - - inital state (make-world (list 0) 0 "" <program> 0)
; - - - - - - on-tick fetch, decode, execute
; - - - - - - on-key for ","
; - - - - - - (on-mouse for stepper)
; - - - - - - (draw-scene for output and tape)