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Comment by schoen | Hacker News Reader

A user who is banned asked in this thread why you can't have an unlimited-length game where kings just move back and forth. The answer is

https://en.wikipedia.org/wiki/Fifty-move_rule

https://en.wikipedia.org/wiki/Threefold_repetition

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All that the 50-move and threefold repetition rules do is permit one player to claim a draw; the game may continue if both players wish to play on. (It has happened, particularly threefold repetition where the repetitions are spaced far apart and the players don't have much time.)

There are actually new FIDE rules as of 2014 or so, where after 75 moves, or a fivefold repetition, the game is immediately declared drawn without the need for a player to take action.

This is a good point in that it means players could have collaborated to deliberately achieve an arbitrarily long game (and indeed, in some of the long reported games it looks like nobody chose to claim a draw for a while). I guess such collaboration isn't sportsmanlike in a competitive setting, but it wouldn't have been precluded by the rules.

Thanks for that clarification.

At the highest levels it probably does imply collaboration, but where I play at (much) lower levels, I might very reasonably choose not to take a draw because I think my opponent is likely to make a mistake that will result in a win for me. My opponent may reasonably think the same about me.

No, if no one claims a draw, it can continue forever.

In perfect play either black or white resigns in their first move or agrees to a draw.

But in in-perfect play you could get a situation where it plays until heat death. You at the very least could wait for a bit flip on the memory of the opposing player that might let you win.

Of course two Kings can't checkmate each other. If the board bit flipped, well that would be cheating / not a proper game. So in this specific case it'd be agreeed to a draw without the 50 move/repetition rule by the most basic of programs since checkmate is not possible given any play. But legally they wouldn't have to, unlike the above scenario when it can make logical sense to continue.

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