Bishop moves

lib/chess/moves.ex

@@ -3,6 +3,7 @@ defmodule Chess.Moves do
 
   alias Chess.Moves.Pawn
   alias Chess.Moves.Rook
+  alias Chess.Moves.Bishop
 
   def available(board, {file, rank}) do
     piece = board["#{file},#{rank}"]
@@ -13,7 +14,7 @@ defmodule Chess.Moves do
       %{"type" => "rook"} ->
         Rook.moves(board, {file, rank})
       %{"type" => "bishop"} ->
-        []
+        Bishop.moves(board, {file, rank})
       %{"type" => "knight"} ->
         []
       %{"type" => "king"} ->

lib/chess/moves/bishop.ex

@@ -0,0 +1,34 @@
+defmodule Chess.Moves.Bishop do
+  @moduledoc false
+
+  def moves(board, {file, rank}) do
+    moves_northeast(board, {file, rank}) ++
+      moves_southeast(board, {file, rank}) ++
+      moves_northwest(board, {file, rank}) ++
+      moves_southwest(board, {file, rank})
+  end
+
+  defp moves_northeast(_board, {7, _rank}), do: []
+  defp moves_northeast(_board, {_file, 7}), do: []
+  defp moves_northeast(board, {file, rank}) do
+    [{file + 1, rank + 1} | moves_northeast(board, {file + 1, rank + 1})]
+  end
+
+  defp moves_southeast(_board, {7, _rank}), do: []
+  defp moves_southeast(_board, {_file, 0}), do: []
+  defp moves_southeast(board, {file, rank}) do
+    [{file + 1, rank - 1} | moves_southeast(board, {file + 1, rank - 1})]
+  end
+
+  defp moves_northwest(_board, {0, _rank}), do: []
+  defp moves_northwest(_board, {_file, 7}), do: []
+  defp moves_northwest(board, {file, rank}) do
+    [{file - 1, rank + 1} | moves_northwest(board, {file - 1, rank + 1})]
+  end
+
+  defp moves_southwest(_board, {0, _rank}), do: []
+  defp moves_southwest(_board, {_file, 0}), do: []
+  defp moves_southwest(board, {file, rank}) do
+    [{file - 1, rank - 1} | moves_southwest(board, {file - 1, rank - 1})]
+  end
+end

test/chess/moves/bishop_test.exs

@@ -0,0 +1,32 @@
+defmodule Chess.Moves.BishopTest do
+  use Chess.DataCase
+
+  alias Chess.Moves.Bishop
+
+  test "bishops can move diagonally" do
+    board = %{"4,5" => %{"type" => "bishop", "colour" => "white"}}
+    moves = Bishop.moves(board, {4, 5})
+
+    expected_moves = Enum.sort([
+      {5, 6}, {6, 7},
+      {5, 4}, {6, 3}, {7, 2},
+      {3, 4}, {2, 3}, {1, 2}, {0, 1},
+      {3, 6}, {2, 7},
+    ])
+    assert Enum.sort(moves) == expected_moves
+  end
+
+  test "bishops cannot move further than the edge" do
+    board = %{"0,0" => %{"type" => "bishop", "colour" => "white"}}
+    moves = Bishop.moves(board, {0, 0})
+
+    expected_moves = Enum.sort([
+      {1, 1}, {2, 2}, {3, 3}, {4, 4}, {5, 5}, {6, 6}, {7, 7}
+    ])
+    assert Enum.sort(moves) == expected_moves
+  end
+
+  def board do
+    Chess.Board.default
+  end
+end