Added doc for each bf instruction

main.rkt

@@ -18,11 +18,11 @@
 ; 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)
 ; - ","
 ; - "["
 ; - "]"
@@ -34,22 +34,24 @@
 ; 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 output program ip])
+(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 bf program without any invalid character
+; 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
@@ -85,17 +87,17 @@
 ; program->world: Program -> World
 ; Given a program, returns the corresponding initial world state.
 (define (program->world p)
-  (make-world (cons 0 '()) 0 "" p 0))
+  (make-world (cons 0 '()) 0 1 "" p 0))
 
 ; Tests for program->world
-(check-expect (program->world "[->+<]") (make-world (list 0) 0 "" "[->+<]" 0))
+(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)))]))
+        [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))
@@ -106,19 +108,88 @@
 (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
+; 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