added undo test and its passing

lib/game.rb

@@ -23,8 +23,8 @@ def at(x,y)
 
   def undo
     move = @moves.pop
-    piece = at(*move[:piece][:point_a])
-    new_piece = piece.class.new(*move[:piece][:point_b])
+    piece = at(*move[:piece][:point_b])
+    new_piece = piece.class.new(*move[:piece][:point_a])
     capture = move[:capture]
 
     @board.remove(piece)

ruby-chess.gemspec

@@ -5,7 +5,7 @@ files.push('LICENSE', 'README.md', 'rakefile')
 
 Gem::Specification.new do |s|
   s.name        = 'ruby-chess'
-  s.version     = '0.0.1'
+  s.version     = '0.0.2'
 	s.date        = "#{Time.now.strftime("%Y-%m-%d")}"
 	s.homepage    = 'https://github.com/jphager2/chess'
   s.summary     = 'Chess game logic in Ruby'

test/game_test.rb

@@ -31,12 +31,6 @@ def test_can_play_black
     assert_kind_of BlackPawn, this_game.at(6,5)
   end
 
-  def test_game_can_undo_moves
-    game = white_in_check
-    2.times {game.undo}
-    assert_kind_of EmptySpace, game.at(1,3)
-  end
-
   def test_cannot_play_while_in_check
     game = white_in_check
     assert_raises(Game::IllegalMove) do

test/undo_test.rb

@@ -0,0 +1,40 @@
+require_relative '../lib/chess'
+
+require 'minitest/autorun'
+
+class UndoTest < Minitest::Test
+
+  def white_in_check
+    game = Game.new
+    game.play([4,1],[4,3])
+    game.play([3,6],[3,5])
+    game.play([6,1],[6,3])
+    game.play([2,7],[6,3])
+    game
+  end
+
+  def test_game_can_undo_moves
+    game = white_in_check
+    2.times {game.undo}
+    assert_kind_of EmptySpace, game.at(6,3)
+  end
+
+  def test_undo_adds_removed_pieces
+    game = Game.new
+    game.play([4,1],[4,3])
+    game.play([3,6],[3,4])
+    game.play([4,3],[3,4])
+    game.undo
+    assert_kind_of BlackPawn, game.at(3,4)
+  end
+
+  def test_undo_reverses_the_last_move
+    game = Game.new
+    game.play([4,1],[4,3])
+    game.play([3,6],[3,4])
+    game.play([4,3],[3,4])
+    game.undo
+    assert_kind_of WhitePawn, game.at(4,3)
+  end
+
+end