fix: consider threath not legal move for in_check?

if i got this right, a king is still in check, even if the
potential move would leave your own king in check.

lib/board.rb

@@ -33,6 +33,13 @@ def legal_target_positions_for(position)
     end.flatten
   end
 
+  def threatening_positions_for(position)
+    piece = get(position).piece
+    piece.move_types.map do |type|
+      type.new(self, position).threatening_positions
+    end.flatten
+  end
+
   def move(origin, target)
     move_type = find_move_type(origin, target)
     current_move = move_type.new(self, origin, target)
@@ -54,14 +61,13 @@ def in_check?(color)
     opposite_color = color == :black ? :white : :black
     return false unless king_square(color)
 
-    (squares_occupied_by(opposite_color) - [king_square(opposite_color)]).any? do |square|
-      legal_target_positions_for(square.position).any? { |position| position == king_square(color).position }
+    squares_occupied_by(opposite_color).any? do |square|
+      threatening_positions_for(square.position).any? { |position| position == king_square(color).position }
     end
   end
 
   def in_checkmate?(color)
-    king_square = find_king(color)
-    return false unless king_square
+    return false unless king_square(color)
     return false unless in_check?(color)
 
     squares_occupied_by(color).map do |square|
@@ -100,7 +106,6 @@ def create_squares
   def king_square(color)
     @squares.find { |square| square.piece == King.new(color) }
   end
-  alias find_king king_square
 
   def find_move_type(origin, target)
     origin = Position.parse(origin)

lib/move/base.rb

@@ -27,6 +27,13 @@ def legal_target_positions
       positions
     end
 
+    # to check for check, an attack move that leaves your own king in check, is still relevant
+    def threatening_positions
+      positions = position_candidates
+      positions -= target_positions_that_are_occupied_by_friend
+      positions
+    end
+
     private
 
     def legal_target_positions_in_line(positions)
@@ -57,11 +64,11 @@ def occupied_by_enemy?(target_position)
     end
 
     def target_positions_that_are_occupied_by_friend
-      position_candidates.select { |target_position| occupied_by_friend?(target_position) }
+      position_candidates.compact.select { |target_position| occupied_by_friend?(target_position) }
     end
 
     def target_positions_that_leave_check_for_own_king
-      position_candidates.select do |target_position|
+      position_candidates.compact.select do |target_position|
         temp_board = Marshal.load(Marshal.dump(@board))
         temp_board.move(position, target_position)
         temp_board.in_check?(piece.color)

lib/piece/pawn.rb

@@ -20,7 +20,7 @@ def black_move_types
 
   def white_move_types
     [
-      Move::Move::OneUp,
+      Move::OneUp,
       (Move::TwoUp unless moved?),
       Move::UpDiagonalCapture
     ].compact

test/piece/king_test.rb

@@ -17,5 +17,10 @@ def test_black_king_can_castle
       assert_equal Rook.new(:black), board.get("C8").piece
       assert_equal King.new(:black), board.get("B8").piece
     end
+
+    def test_king_cannot_castle_if_fields_are_threatened
+      board = Board.from_fen("r2qkbnr/p2ppppp/8/8/8/8/P2PPPPP/2RQKBNR w Kkq - 0 1")
+      assert !board.legal_target_positions_for("E8").include?(Position.parse("B8"))
+    end
   end
 end