Player will win with a diagonal down

lib/space.rb

@@ -33,7 +33,8 @@ def marked_by? player
     def winning_space_for? player
       completes_row_for?(player) ||
         completes_column_for?(player) ||
-          completes_diagonal_up_for?(player)
+          completes_diagonal_up_for?(player) ||
+            completes_diagonal_down_for?(player)
     end
 
     def row_neighbors
@@ -54,6 +55,12 @@ def diagonal_up_neighbors
       end
     end
 
+    def diagonal_down_neighbors
+      @board.find_spaces_excluding(self) do |space|
+        space.row == space.column
+      end
+    end
+
     private
 
     def completes_row_for? player
@@ -67,5 +74,9 @@ def completes_column_for? player
     def completes_diagonal_up_for? player
       open? && diagonal_up_neighbors.all? { |space| space.marked_by? player }
     end
+
+    def completes_diagonal_down_for? player
+      open? && diagonal_down_neighbors.all? { |space| space.marked_by? player }
+    end
   end
 end

spec/player_spec.rb

@@ -93,6 +93,32 @@ module TicTacToe
           player.move.row_and_column.should eql "2 0"
         end
       end
+
+      context "when the player can complete a diagonal down" do
+        it "finds the winning space in the top left corner" do
+          player = Player.new("O", [ ["_", "_", "_"],
+                                     ["_", "O", "_"],
+                                     ["_", "_", "O"] ])
+
+          player.move.row_and_column.should eql "0 0"
+        end
+
+        it "finds the winning space in the center" do
+          player = Player.new("O", [ ["O", "_", "_"],
+                                     ["_", "_", "_"],
+                                     ["_", "_", "O"] ])
+
+          player.move.row_and_column.should eql "1 1"
+        end
+
+        it "finds the winning space in the bottom right corner" do
+          player = Player.new("O", [ ["O", "_", "_"],
+                                     ["_", "O", "_"],
+                                     ["_", "_", "_"] ])
+
+          player.move.row_and_column.should eql "2 2"
+        end
+      end
     end
   end
 end