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So I played many games online where I am winning but then lose on time. Sometimes the game is drawn (when there is no sequence of legal moves that would produce checkmate) and sometimes I lose (when there is such a sequence). The decision the website takes happens instantly. I am wondering, how do they determine whether the game is a draw or not? If they use brute force, then the decision would take longer to arrive at.

5

As far as I know, every chess website simply looks at the opponents material to determine if they have mating material or not. It doesn't matter even if mate is possible.

The ruleset you are referring to are the FIDE rules, and for simplicity chess websites do not use these rules because the percentage of positions where self-mate is possible is too low.

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This comes from python-chess:

def is_insufficient_material(self):
    """Checks for a draw due to insufficient mating material."""
    # Enough material to mate.
    if self.pawns or self.rooks or self.queens:
        return False

    # A single knight or a single bishop.
    if popcount(self.occupied) <= 3:
        return True

    # More than a single knight.
    if self.knights:
        return False

    # All bishops on the same color.
    if self.bishops & BB_DARK_SQUARES == 0:
        return True
    elif self.bishops & BB_LIGHT_SQUARES == 0:
        return True
    else:
        return False
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    if self.knights or self.rooks or self.queens or self.kings: return False - It is not at all obvious what this line is doing. I mean, I would ordinarily assume that it's checking whether you have one of the listed pieces - but then it would return false if you had a king, which makes no sense at all. – D M Jun 30 '18 at 15:40
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    That doesn't seem to make sense either; it returns an immediate False if the king has a legal move? – D M Jun 30 '18 at 19:58
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    Not sure where you got that source file, but that or self.kings isn't there anymore in the current version of python-chess. And those variables are bitmasks showing where each piece is, so it just means "if there's any king on the board". – Federico Poloni Jul 1 '18 at 11:16
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    @FedericoPoloni yeah, I think I took the wrong chess variant, not the original chess. thank you for the correction, replaced the code with the current version! – lenik Jul 1 '18 at 11:42
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    Thank you, can you please explain what this code does? I have no knowledge of python at all. – Wais Kamal Jul 3 '18 at 16:06

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