Chess: fix checkmate and add draw #2017
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hemberger
commented
Feb 8, 2025
hemberger
commented
- Checkmate no longer incorrectly happens when there are valid moves to block check.
- Two simple draw conditions are now checked to properly end the game in a draw:
- Insufficient material (just two kings)
- Stalemate
Previously, the condition tested was that if the King couldn't make a legal move, then it was checkmate. This is wrong. Now, we check if any piece can make a legal move (since a non-King piece could block the checkmate). Also moved the `isCheckmated` method from ChessGame to Board, since it only depends on the board state (modulo player turn), not the game itself.
Introduce the concept of a Draw into the Chess library. Clarify the Board::isCheckmated method to properly distinguish between checkmate and stalemate (both occur when no valid moves can be made by a player). Handle ending the game and updating the HOF when a Draw occurs. Note that we do not handle the following Draw conditions: * Draw by repetition * Draw by choice * Draw by insufficient material (beyond just two Kings) These are more complex and would require significant changes. Implementation note: The Board::clear method has been added to simplify setting up test board states. It adds some undesired complexity to the class, because setting an arbitrary position requires input about the castling and en passant statuses. Exposing this interface can lead to invalid game states (e.g. no Rook, but the castling state is still set). I considered adding a FEN parser, but decided it was too much work just to set up a couple test board states.
Codecov ReportAttention: Patch coverage is
Additional details and impacted files@@ Coverage Diff @@
## main #2017 +/- ##
============================================
+ Coverage 31.74% 31.94% +0.20%
- Complexity 4205 4226 +21
============================================
Files 125 125
Lines 12815 12855 +40
============================================
+ Hits 4068 4107 +39
- Misses 8747 8748 +1 ☔ View full report in Codecov by Sentry. |
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