From 3a421ca213153261baaf900657cf3df85e447409 Mon Sep 17 00:00:00 2001
From: Dan Barber <github@danbarber.me>
Date: Mon, 12 Mar 2018 14:58:08 -0400
Subject: [PATCH] Knight moves

---
 lib/chess/moves.ex               |  5 +++--
 lib/chess/moves/knight.ex        | 15 +++++++++++++++
 test/chess/moves/knight_test.exs | 29 +++++++++++++++++++++++++++++
 3 files changed, 47 insertions(+), 2 deletions(-)
 create mode 100644 lib/chess/moves/knight.ex
 create mode 100644 test/chess/moves/knight_test.exs

diff --git a/lib/chess/moves.ex b/lib/chess/moves.ex
index c61f03b..b863d9e 100644
--- a/lib/chess/moves.ex
+++ b/lib/chess/moves.ex
@@ -2,8 +2,9 @@ defmodule Chess.Moves do
   @moduledoc false
 
   alias Chess.Moves.Pawn
-  alias Chess.Moves.Rook
   alias Chess.Moves.Bishop
+  alias Chess.Moves.Knight
+  alias Chess.Moves.Rook
   alias Chess.Moves.Queen
 
   def available(board, {file, rank}) do
@@ -17,7 +18,7 @@ defmodule Chess.Moves do
       %{"type" => "bishop"} ->
         Bishop.moves(board, {file, rank})
       %{"type" => "knight"} ->
-        []
+        Knight.moves(board, {file, rank})
       %{"type" => "king"} ->
         []
       %{"type" => "queen"} ->
diff --git a/lib/chess/moves/knight.ex b/lib/chess/moves/knight.ex
new file mode 100644
index 0000000..51ffe1d
--- /dev/null
+++ b/lib/chess/moves/knight.ex
@@ -0,0 +1,15 @@
+defmodule Chess.Moves.Knight do
+  @moduledoc false
+
+  def moves(_board, {file, rank}) do
+    patterns
+    |> Enum.map(fn ({fv, rv}) -> {file + fv, rank + rv} end)
+    |> Enum.reject(fn ({file, rank}) ->
+      file < 0 || rank < 0 || file > 7 || rank > 7
+    end)
+  end
+
+  defp patterns do
+    [{1, 2}, {2, 1}, {-1, 2}, {-2, 1}, {1, -2}, {2, -1}, {-1, -2}, {-2, -1}]
+  end
+end
diff --git a/test/chess/moves/knight_test.exs b/test/chess/moves/knight_test.exs
new file mode 100644
index 0000000..76e5309
--- /dev/null
+++ b/test/chess/moves/knight_test.exs
@@ -0,0 +1,29 @@
+defmodule Chess.Moves.KnightTest do
+  use Chess.DataCase
+
+  alias Chess.Moves
+
+  test "knights move in an L shape" do
+    board = %{"4,5" => %{"type" => "knight", "colour" => "white"}}
+    moves = Moves.available(board, {4, 5})
+
+    expected_moves = Enum.sort([
+      {3, 7}, {5, 7}, {6, 6}, {6, 4}, {5, 3}, {3, 3}, {2, 4}, {2, 6},
+    ])
+    assert Enum.sort(moves) == expected_moves
+  end
+
+  test "knights cannot move beyond the edges of the board" do
+    board = %{"0,0" => %{"type" => "knight", "colour" => "white"}}
+    moves = Moves.available(board, {0, 0})
+
+    expected_moves = Enum.sort([
+      {1, 2}, {2, 1}
+    ])
+    assert Enum.sort(moves) == expected_moves
+  end
+
+  def board do
+    Chess.Board.default
+  end
+end