From 235653359c6b04151bd5648e331b020321a55fd4 Mon Sep 17 00:00:00 2001
From: tommi27 <tommi99@hotmail.it>
Date: Mon, 3 Dec 2018 16:54:41 +0100
Subject: [PATCH] fixed matching bracket bug

---
 README.md       |  4 +---
 interpreter.rkt | 53 +++++++++++++++++++++++++++++++++----------------
 2 files changed, 37 insertions(+), 20 deletions(-)

diff --git a/README.md b/README.md
index 2bf3c19..3b046ca 100644
--- a/README.md
+++ b/README.md
@@ -1,4 +1,4 @@
-# easybf / DrBrainf\*ck
+# DrBrainf\*ck
 
 This is the repo for the PF1 final project of _Claudio Maggioni_ and 
 _Tommaso Rodolfo Masera_.
@@ -15,7 +15,5 @@ Run the file `gui.rkt` using *DrRacket* or the `racket` CLI tool.
 
 ## Bugs
 
-- `[` and `]` are not brainf\*ck compliant since the interpreter does not 
-  search for the matching bracket but it considers the nearest bracket;
 - `,` function not implemented (will crash the program if encountered);
 - No validation or user-friendly error handling.
diff --git a/interpreter.rkt b/interpreter.rkt
index ae3c36d..8f7dab5 100644
--- a/interpreter.rkt
+++ b/interpreter.rkt
@@ -230,23 +230,44 @@
 (check-expect (char-at "malusa" 2) "l")
 (check-error (char-at "another string" 300))
 
-; find-first: 1String String Nat (Nat -> Nat) -> Nat
-; Given a 1String `char`, a String `string` and a start index, returns
-; the position of the first occurrence of `char` in `string` starting from the
-; index walking the string changing the index using the `walk` function.
-(define (find-first char string start walk)
-  (cond [(string=? (substring string start (add1 start))
-                   char) start]
-        [else (find-first char string (walk start) walk)]))
+; WalkingDirection can be one of:
+; - 'forward
+; - 'backward
+; Interpretation: the direction to walk the Brainf*ck Program with.
+
+; find-matching: Program Nat WalkingDirection -> Nat
+; Given a program, a starting position in the progam and a walking direction
+; returns the position of the matching bracket waking in the direction provided.
+(define (find-matching prg start wd)
+
+  (define brkt (if (symbol=? wd 'forward) #\] #\[))
+  (define oppos-brkt (if (symbol=? wd 'forward) #\[ #\]))
+  (define upd-start (if (symbol=? wd 'forward) add1 sub1))
+  
+  ; fm-helper Nat Nat -> Nat
+  ; Given a starting position and a nest accumulator, returns the position of
+  ; the matching bracket waking in the direction provided by updating the
+  ; accumulator recursively with the current level of nesting.
+  (define (fm-helper s nest)
+    (cond
+      [(and (zero? nest) (char=? (string-ref prg s) brkt))
+       s]
+      [(char=? (string-ref prg s) oppos-brkt)
+       (fm-helper (upd-start s) (add1 nest))]
+      [(char=? (string-ref prg s) brkt)
+       (fm-helper (upd-start s) (sub1 nest))]
+      [else
+       (fm-helper (upd-start s) nest)]))
+
+  (fm-helper (upd-start start) 0))
 
 ; Tests for find-first
-(check-expect (find-first "]" "[++++++---->><-]++++]+--" 0 add1)
+(check-expect (find-matching "[++++++---->><-]++++]+--" 0 'forward)
               15)
-(check-expect (find-first "]" "[+++++][+---->><-]++++]+--" 7 add1)
+(check-expect (find-matching "[+++++][+---->><-]++++]+--" 7 'forward)
               17)
-(check-expect (find-first "[" "[+++++][+---->><-]++++]+--" 17 sub1)
-              7)
-
+(check-expect (find-matching "[+++++[]+---->><-]++++]+--" 17 'backward)
+              0)
 
 ; exec-loop-start: ProgState -> ProgState
 ; Given a ProgState, returns a new ProgState with the [ instruction executed
@@ -259,7 +280,7 @@
      (prog-state-output w)
      (prog-state-program w)
      (add1 (if jump
-         (find-first "]" (prog-state-program w) (prog-state-ip w) add1) 
+         (find-matching (prog-state-program w) (prog-state-ip w) 'forward) 
          (prog-state-ip w))))))
 
 ; Tests for exec-loop-start
@@ -281,7 +302,7 @@
      (prog-state-output w)
      (prog-state-program w)
      (add1 (if jump
-         (find-first "[" (prog-state-program w) (prog-state-ip w) sub1) 
+         (find-matching (prog-state-program w) (prog-state-ip w) 'backward) 
          (prog-state-ip w))))))
 
 ; Tests for exec-loop-end
@@ -320,5 +341,3 @@
                           "++>+++++[<+>-]++++++++[<++++++>-]<." 35))
 ; Run tests
 (test)
-
-; TODO: fix matching bracket bug