Add basic pawn move generation

lib/chess/moves.ex

@@ -0,0 +1,14 @@
+defmodule Chess.Moves do
+  @moduledoc false
+
+  alias Chess.Moves.Pawn
+
+  def available(board, {file, rank}) do
+    piece = board["#{file},#{rank}"]
+
+    case piece do
+      %{type: :pawn} ->
+        Pawn.moves(board, {file, rank})
+    end
+  end
+end

lib/chess/moves/pawn.ex

@@ -0,0 +1,20 @@
+defmodule Chess.Moves.Pawn do
+  @moduledoc false
+
+  def moves(board, {file, rank}) do
+    piece = board["#{file},#{rank}"]
+
+    case piece do
+      %{colour: :white} ->
+        case rank do
+          1 -> [{file, rank + 1}, {file, rank + 2}]
+          _ -> [{file, rank + 1}]
+        end
+      %{colour: :black} ->
+        case rank do
+          6 -> [{file, rank - 1}, {file, rank - 2}]
+          _ -> [{file, rank - 1}]
+        end
+    end
+  end
+end

test/chess/moves/pawn_test.exs

@@ -0,0 +1,39 @@
+defmodule Chess.Moves.PawnTest do
+  use Chess.DataCase
+
+  alias Chess.Moves.Pawn
+
+  test "white pawn can move forward one or two spaces" do
+    moves = Pawn.moves(board(), {4, 1})
+
+    expected_moves = [{4, 2}, {4, 3}]
+    assert moves == expected_moves
+  end
+
+  test "black pawn can move forward one or two spaces" do
+    moves = Pawn.moves(board(), {4, 6})
+
+    expected_moves = [{4, 5}, {4, 4}]
+    assert moves == expected_moves
+  end
+
+  test "white pawn not on starting square can move forward one space" do
+    board = %{"4,2" => %{type: :pawn, colour: :white}}
+    moves = Pawn.moves(board, {4, 2})
+
+    expected_moves = [{4, 3}]
+    assert moves == expected_moves
+  end
+
+  test "black pawn not on starting square can move forward one space" do
+    board = %{"4,5" => %{type: :pawn, colour: :black}}
+    moves = Pawn.moves(board, {4, 5})
+
+    expected_moves = [{4, 4}]
+    assert moves == expected_moves
+  end
+
+  def board do
+    Chess.Board.default
+  end
+end