From 2c8f2c7bcdefb81e837bb676c51fbc323be9ec2b Mon Sep 17 00:00:00 2001
From: Dan Barber <github@danbarber.me>
Date: Tue, 13 Mar 2018 22:20:48 -0400
Subject: [PATCH] Pawn can be blocked

---
 lib/chess/moves/pawn.ex        | 39 +++++++++++++++++++++++-----------
 test/chess/moves/pawn_test.exs | 39 +++++++++++++++++++++++++++++++---
 2 files changed, 63 insertions(+), 15 deletions(-)

diff --git a/lib/chess/moves/pawn.ex b/lib/chess/moves/pawn.ex
index 8419a21..67bae47 100644
--- a/lib/chess/moves/pawn.ex
+++ b/lib/chess/moves/pawn.ex
@@ -2,19 +2,34 @@ defmodule Chess.Moves.Pawn do
   @moduledoc false
 
   def moves(board, {file, rank}) do
-    piece = board["#{file},#{rank}"]
+    board["#{file},#{rank}"]
+    |> _moves(board, {file, rank})
+  end
 
-    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
+  defp _moves(%{"colour" => "white"}, board, {file, rank}) do
+    cond do
+      obstruction?(board, {file, rank + 1}) -> []
+      rank == 1 -> [
+        {file, rank + 1} |
+        _moves(%{"colour" => "white"}, board, {file, rank + 1})
+      ]
+      true -> [{file, rank + 1}]
     end
   end
+
+  defp _moves(%{"colour" => "black"}, board, {file, rank}) do
+    cond do
+      obstruction?(board, {file, rank - 1}) -> []
+      rank == 6 -> [
+        {file, rank - 1} |
+        _moves(%{"colour" => "black"}, board, {file, rank - 1})
+      ]
+      true -> [{file, rank - 1}]
+    end
+  end
+
+  defp obstruction?(board, {file, rank}) do
+    board
+    |> Map.has_key?("#{file},#{rank}")
+  end
 end
diff --git a/test/chess/moves/pawn_test.exs b/test/chess/moves/pawn_test.exs
index f70d03d..9f5ce0e 100644
--- a/test/chess/moves/pawn_test.exs
+++ b/test/chess/moves/pawn_test.exs
@@ -4,14 +4,14 @@ defmodule Chess.Moves.PawnTest do
   alias Chess.Moves.Pawn
 
   test "white pawn can move forward one or two spaces" do
-    moves = Pawn.moves(board(), {4, 1})
+    moves = Pawn.moves(default_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})
+    moves = Pawn.moves(default_board(), {4, 6})
 
     expected_moves = [{4, 5}, {4, 4}]
     assert moves == expected_moves
@@ -33,7 +33,40 @@ defmodule Chess.Moves.PawnTest do
     assert moves == expected_moves
   end
 
-  def board do
+  test "pawn is blocked from moving two squares by another piece" do
+    board = %{
+      "4,1" => %{"type" => "pawn", "colour" => "white"},
+      "4,3" => %{"type" => "pawn", "colour" => "black"},
+    }
+    moves = Pawn.moves(board, {4, 1})
+
+    expected_moves = [{4, 2}]
+    assert moves == expected_moves
+  end
+
+  test "pawn is blocked from moving one or two squares by another piece" do
+    board = %{
+      "4,1" => %{"type" => "pawn", "colour" => "white"},
+      "4,2" => %{"type" => "pawn", "colour" => "black"},
+    }
+    moves = Pawn.moves(board, {4, 1})
+
+    expected_moves = []
+    assert moves == expected_moves
+  end
+
+  test "pawn is blocked from moving one square by another piece" do
+    board = %{
+      "4,2" => %{"type" => "pawn", "colour" => "white"},
+      "4,3" => %{"type" => "pawn", "colour" => "black"},
+    }
+    moves = Pawn.moves(board, {4, 2})
+
+    expected_moves = []
+    assert moves == expected_moves
+  end
+
+  def default_board do
     Chess.Board.default
   end
 end