Corrected Direction arithmetic in agents.py (aimacode#348)

* added tests for Direction

* fixed Direction arithmetic error

Author: kaivalyar

agents.py

@@ -343,7 +343,7 @@ def __add__(self, heading):
         elif self.direction == self.L:
             return{
                 self.R: Direction(self.U),
-                self.L: Direction(self.L),
+                self.L: Direction(self.D),
             }.get(heading, None)
         elif self.direction == self.U:
             return{

tests/test_agents.py

@@ -0,0 +1,40 @@
+from agents import Direction
+
+def test_move_forward():
+    d = Direction("up")
+    l1 = d.move_forward((0,0))
+    assert l1 == (0,-1)
+    d = Direction(Direction.R)
+    l1 = d.move_forward((0,0))
+    assert l1 == (1,0)
+    d = Direction(Direction.D)
+    l1 = d.move_forward((0,0))
+    assert l1 == (0,1)
+    d = Direction("left")
+    l1 = d.move_forward((0,0))
+    assert l1 == (-1,0)
+    l2 = d.move_forward((1,0))
+    assert l2 == (0,0)
+
+def test_add():
+    d = Direction(Direction.U)
+    l1 = d + "right"
+    l2 = d + "left"
+    assert l1.direction == Direction.R
+    assert l2.direction == Direction.L
+    d = Direction("right")
+    l1 = d.__add__(Direction.L)
+    l2 = d.__add__(Direction.R)
+    assert l1.direction == "up"
+    assert l2.direction == "down"
+    d = Direction("down")
+    l1 = d.__add__("right")
+    l2 = d.__add__("left")
+    assert l1.direction == Direction.L
+    assert l2.direction == Direction.R
+    d = Direction(Direction.L)
+    l1 = d + Direction.R
+    l2 = d + Direction.L
+    assert l1.direction == Direction.U
+    assert l2.direction == Direction.D #fixed
+