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So I am implementing an intelligent chess agent but I can't get the checkmate working.

I was wondering if there is a program that takes the chessboard notation and returns a result of yes or no to whether it is a checkmate or not.

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6 Answers 6

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I imagine any chess engine must have such function. Find an open source one in your programming language of choice and draw inspiration from there.

I often use the python-chess package for simple chess programming and data mining tasks, as it offers a good API with many useful functions (available here: https://pypi.python.org/pypi/python-chess). For your purposes, it has a function:

is_checkmate() Checks if the current position is a checkmate.

The package is open source, so you can see its code, too.

You could also try implementing the function yourself! Stockfish, for example, has a simple, efficient implementation - it produces a list of legal moves, and if the list is empty and the king is in check, then it clearly is a checkmate. See the code: https://github.com/mcostalba/Stockfish/blob/master/src/search.cpp

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  • First of all thank you for your answer.I've tried implementing it myself(using the same principle as Stockfish) but since i use a pre-existing gui and game mechanics(my focus is to construct an AI) some of its functions are wrong and the list is empty when it shouldn't be.So i wanted to find some other way of checkmate evaluation. Mar 8, 2016 at 13:32
  • Note that python-chess is licensed under GPL-3.0, so if you use the library, you are required to make your project open source, too. (At least that is how I interpret GPL-3.0.) Jan 11 at 0:51
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It's actually quite easy to check for a checkmate:

  • Can I move out of mate?
  • Can I block mate?
  • Can I take the attacker?

The king only has 8 spots to move. You should already have a list of squares under attack from enemy pieces, just check if that square is in that list.

You should have a list of squares that are movable. Check if one of those blocks every attacker.

You can easily tell who is attacking the king by checking each enemy piece's attacked squares for the king's square. If there's only one attacker, and that piece's squares is in one of your piece's attacked squares, you're all set.

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  • I actually tried to implement the method you suggest above but the pre-existing gui as well as the game mechanics seem to have a peculiar bug that doesn't compute all the possible moves correctly.That's why i ask for a program that takes the chessboard notation as input and decides if it is checkmate or not. Mar 8, 2016 at 18:43
  • And temper "can I attack the attacker" and "can I block mate" with "... with an unpinned piece."
    – Tony Ennis
    Mar 10, 2016 at 12:20
  • What you say is easy seems to me like it can easily become quite difficult once you start to implement it. Can I move out of mate? Sure, but you need to know which squares are attacked. Can I block mate? Sure, you can block the attack with another piece, but you may open up check from another opposite color piece, so you need to know which pieces are pinned to the king and not use them to block. Can I take the attacker? Sure, but you need to know which pieces are pinned to the king for this one too, and if you take with the king, you need to know whether the captured piece is protected. Jan 11 at 0:46
  • @HelloGoodbye You're overthinking it - you make your list of available moves (which is simple) and test the move to see if you are still in check (which is also simple).
    – corsiKa
    Jan 21 at 19:06
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What I did in my program was to only look for checkmates if the king was in check. I only generate pseudo-legal moves in my move generation function so I simply play out all the moves generated and check if the king still is in check. If this is the case I undo that move and delete it from my list of moves. This is repeated until I find a legal move or the list is exhausted. When the list is exhausted I know it must be checkmate.

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Most engines will read a pgn. To find if it is checkmate you have to play through the game. From your comments I think you want the program to do that automatically. None of the engines that I have looked at have this capability.

My first suggestion is to rewrite your code to remove your bug(s).

The other is to search for the pgn for (#)the mate symbol.

https://www.cs.kent.ac.uk/people/staff/djb/pgn-extract/help.html pgn extract claims to have a match criteria for checkmate and stalemate and many others.

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From Chess Programming Wiki, this article (https://www.chessprogramming.org/Checkmate) explains some rules that you can use to implement checkmate detection.

1) double check - only the king can move on this situation. So, if the king has no valid moves, you have a checkmate.

2) single check - in this situation, the king may move out of check position, or if the attacker piece is a knight or a pawn, you should detect if they may be captured. For the attacking from queen, rook and bishop, they could be captured or some opposite piece should be placed between the attacked king and the attacker.

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To start the above, you may create a function that lists the pieces that are attacking the king. If the list is empty, no check. If the list has a single pice, a single check. And, finnaly, if the list has two pieces, you have a double check.

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If you are implementing either min-max search or alpha–beta search for a chess engine, one very simple (but rather inefficient) way of checking for checkmate is to consider the king a normal but very valuable piece, search two plys ahead from the current position and see whether the opponent can capture your king (which they will if they can if the king is valuable enough). If they can capture the king, the current position is a terminal position (the game is over).

Besides, if you in the current position switch the active color to your opponent’s color and search one ply ahead and the opponent can capture your king, you are in check in the current position.

If you are not in check and the position is terminal (because of a king capture in two plys), the position is a stalemate. If you are in check and the position is terminal, the position is a checkmate.

When doing these one and two ply searches, you need to ignore the threefold and fivefold repetition rules, as well as the fifty-move and seventyfive-move rules.

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