Add pawn taking moves

lib/chess/moves/pawn.ex

@@ -2,11 +2,33 @@ defmodule Chess.Moves.Pawn do
   @moduledoc false
 
   def moves(board, {file, rank}) do
+    normal_moves(board, {file, rank}) ++
+      taking_moves(board, {file, rank})
+  end
+
+  defp normal_moves(board, {file, rank}) do
     board["#{file},#{rank}"]
     |> Map.get("colour")
     |> _moves(board, {file, rank})
   end
 
+  defp taking_moves(board, {file, rank}) do
+    colour =
+      board["#{file},#{rank}"]
+      |> Map.get("colour")
+
+    colour
+    |> _taking_moves(board, {file, rank}, patterns(colour))
+  end
+
+  defp patterns("white") do
+    [{-1, 1}, {1, 1}]
+  end
+
+  defp patterns("black") do
+    [{-1, -1}, {1, -1}]
+  end
+
   defp _moves("white", board, {file, rank}) do
     cond do
       obstruction?(board, {file, rank + 1}) ->
@@ -29,6 +51,21 @@ defmodule Chess.Moves.Pawn do
     end
   end
 
+  def _taking_moves(_colour, _board, {_file, _rank}, []), do: []
+  def _taking_moves(colour, board, {file, rank}, [{fv, rv} | moves]) do
+    move_square = {file + fv, rank + rv}
+    if can_take_piece?(colour, board, move_square) do
+      [move_square | _taking_moves(colour, board, {file, rank}, moves)]
+    else
+      _taking_moves(colour, board, {file, rank}, moves)
+    end
+  end
+
+  defp can_take_piece?(colour, board, {file, rank}) do
+    piece = board["#{file},#{rank}"]
+    piece && piece["colour"] != colour
+  end
+
   defp obstruction?(board, {file, rank}) do
     board
     |> Map.has_key?("#{file},#{rank}")

test/chess/moves/pawn_test.exs

@@ -66,6 +66,28 @@ defmodule Chess.Moves.PawnTest do
     assert moves == expected_moves
   end
 
+  test "white pawn can take an opponents piece" do
+    board = %{
+      "4,2" => %{"type" => "pawn", "colour" => "white"},
+      "5,3" => %{"type" => "pawn", "colour" => "black"},
+    }
+    moves = Pawn.moves(board, {4, 2})
+
+    expected_moves = [{4, 3}, {5, 3}]
+    assert moves == expected_moves
+  end
+
+  test "black pawn can take an opponents piece" do
+    board = %{
+      "6,6" => %{"type" => "pawn", "colour" => "black"},
+      "5,5" => %{"type" => "pawn", "colour" => "white"},
+    }
+    moves = Pawn.moves(board, {6, 6})
+
+    expected_moves = [{6, 5}, {6, 4}, {5, 5}]
+    assert moves == expected_moves
+  end
+
   def default_board do
     Chess.Board.default
   end