Add tests and add validity check to labyrinth dimensions.

src/main/java/ch/fritteli/labyrinth/model/Direction.java

@@ -2,21 +2,22 @@ package ch.fritteli.labyrinth.model;
 
 public enum Direction {
     TOP,
+    RIGHT,
     BOTTOM,
-    LEFT,
-    RIGHT;
+    LEFT;
 
     public Direction invert() {
         switch (this) {
             case TOP:
                 return BOTTOM;
-            case LEFT:
-                return RIGHT;
             case RIGHT:
                 return LEFT;
             case BOTTOM:
                 return TOP;
+            case LEFT:
+                return RIGHT;
+            default:
+                throw new IllegalStateException("Programming error: Not all enum values covered in enum Direction#invert()!");
         }
-        throw new IllegalStateException("Programming error: Not all enum values covered in enum Direction#invert()!");
     }
 }

src/main/java/ch/fritteli/labyrinth/model/Labyrinth.java

@@ -28,10 +28,13 @@ public class Labyrinth {
     }
 
     public Labyrinth(final int width, final int height, final long randomSeed) {
+        if (width <= 0 || height <= 0) {
+            throw new IllegalArgumentException("widht and height must be >=1");
+        }
         this.width = width;
         this.height = height;
         this.randomSeed = randomSeed;
-        this.random = new Random(this.randomSeed);
+        this.random = new Random(randomSeed);
         this.field = new Tile[width][height];
         this.start = new Position(0, 0);
         this.end = new Position(this.width - 1, this.height - 1);