Default moves

changes the behaviour for default movement from "always go up" to repeat your
last move.

lib/battle_snake/move.ex

@@ -15,21 +15,43 @@ defmodule BattleSnake.Move do
     snake_id: reference
   }
 
+  @up %Point{x: 0, y: -1}
   @spec up() :: Point.t
-  def up, do: %Point{x: 0, y: -1}
+  def up, do: @up
 
+  @down %Point{x: 0, y: 1}
   @spec down() :: Point.t
-  def down, do: %Point{x: 0, y: 1}
+  def down, do: @down
 
+  @right %Point{x: 1, y: 0}
   @spec right() :: Point.t
-  def right, do: %Point{x: 1, y: 0}
+  def right, do: @right
 
+  @left %Point{x: -1, y: 0}
   @spec left() :: Point.t
-  def left, do: %Point{x: -1, y: 0}
+  def left, do: @left
 
   @spec default_move() :: Point.t
   def default_move(), do: %Move{move: "up"}
 
+  def default_move(snake) do
+    do_default_move(snake.coords)
+  end
+
+  defp do_default_move([v1, v2 | _]),
+    do: Point.sub(v1, v2)
+    |> from_point
+    |> do_default_move
+
+  defp do_default_move({:ok, move}),
+    do: move
+
+  defp do_default_move(:error),
+    do: default_move()
+
+  defp do_default_move(_),
+    do: do_default_move(:error)
+
   @spec moves() :: [Point.t]
   def moves do
     [
@@ -53,4 +75,19 @@ defmodule BattleSnake.Move do
       "right" -> right()
     end
   end
+
+  def from_point(point) do
+    case point do
+      @up ->
+        {:ok, %Move{move: "up"}}
+      @down ->
+        {:ok, %Move{move: "down"}}
+      @left ->
+        {:ok, %Move{move: "left"}}
+      @right ->
+        {:ok, %Move{move: "right"}}
+      _ ->
+        :error
+    end
+  end
 end

lib/battle_snake/movement.ex

@@ -91,7 +91,7 @@ defmodule BattleSnake.Movement do
     #{inspect e, pretty: true}
     """
 
-    move = Move.default_move()
+    move = Move.default_move(snake)
 
     {move, snake}
   end

test/battle_snake/move_test.exs

@@ -1,5 +1,6 @@
 defmodule BattleSnake.MoveTest  do
   use BattleSnake.Case, async: true
+  use BattleSnake.Point
 
   alias BattleSnake.{
     Move,
@@ -12,4 +13,27 @@ defmodule BattleSnake.MoveTest  do
         %Move{move: "up"} |> Move.to_point
     end
   end
+
+  describe "Move.default_move(snake) when the last move is undefined" do
+    test "goes up" do
+      s1 = build(:snake, coords: [p(0, 0), p(0, 0)])
+      s2 = build(:snake, coords: [p(0, 0), p(10, 10)])
+      s3 = build(:snake, coords: [])
+
+      for snake <- [s1, s2, s3] do
+        move = Move.default_move(snake)
+        assert move == %Move{move: "up"}
+      end
+    end
+  end
+
+  describe "Move.default_move(snake)" do
+    test "repeats the previous move" do
+      snake = build(:snake, coords: [p(5, 5), p(6, 5), p(7, 5)])
+      move = Move.default_move(snake)
+      assert move == %Move{move: "left"}
+      new_snake = BattleSnake.Snake.move(snake, move)
+      assert new_snake.coords == [p(4, 5), p(5, 5), p(6, 5)]
+    end
+  end
 end