REFACTOR: introduce Direction enum and extract wrapping logic

lib/mars_rover.dart

@@ -1,57 +1,72 @@
+enum Direction {
+  north('N'),
+  east('E'),
+  south('S'),
+  west('W');
+
+  final String code;
+  const Direction(this.code);
+
+  static Direction fromCode(String code) {
+    return Direction.values.firstWhere((d) => d.code == code);
+  }
+
+  Direction turnLeft() {
+    return Direction.values[(index + 3) % 4];
+  }
+
+  Direction turnRight() {
+    return Direction.values[(index + 1) % 4];
+  }
+}
+
 class Rover {
   int x;
   int y;
-  String direction;
+  Direction _direction;
   final int plateauWidth;
   final int plateauHeight;
 
   Rover({
     required this.x,
     required this.y,
-    required this.direction,
+    required String direction,
     this.plateauWidth = 100,
     this.plateauHeight = 100,
-  });
+  }) : _direction = Direction.fromCode(direction);
+
+  String get direction => _direction.code;
 
   void turnLeft() {
-    const leftTurns = {
-      'N': 'W',
-      'W': 'S',
-      'S': 'E',
-      'E': 'N',
-    };
-    direction = leftTurns[direction]!;
+    _direction = _direction.turnLeft();
   }
 
   void turnRight() {
-    const rightTurns = {
-      'N': 'E',
-      'E': 'S',
-      'S': 'W',
-      'W': 'N',
-    };
-    direction = rightTurns[direction]!;
+    _direction = _direction.turnRight();
   }
 
   void moveForward() {
-    switch (direction) {
-      case 'N':
-        y = (y + 1) % (plateauHeight + 1);
+    switch (_direction) {
+      case Direction.north:
+        y = _wrapCoordinate(y + 1, plateauHeight);
         break;
-      case 'E':
-        x = (x + 1) % (plateauWidth + 1);
+      case Direction.east:
+        x = _wrapCoordinate(x + 1, plateauWidth);
         break;
-      case 'S':
-        y = (y - 1) % (plateauHeight + 1);
-        if (y < 0) y += plateauHeight + 1;
+      case Direction.south:
+        y = _wrapCoordinate(y - 1, plateauHeight);
         break;
-      case 'W':
-        x = (x - 1) % (plateauWidth + 1);
-        if (x < 0) x += plateauWidth + 1;
+      case Direction.west:
+        x = _wrapCoordinate(x - 1, plateauWidth);
         break;
     }
   }
 
+  int _wrapCoordinate(int value, int max) {
+    final wrapped = value % (max + 1);
+    return wrapped < 0 ? wrapped + max + 1 : wrapped;
+  }
+
   void execute(String commands) {
     for (var i = 0; i < commands.length; i++) {
       final command = commands[i];