alt Hacker Newsis there some reason to implement it as a time limit instead of iterations or something else deterministic? it being affected by CPU speed or machine load seems obvious.
or whatever makes sense if “iterations” isn’t a thing, I know nothing about chess algorithms
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A time limit is also deterministic in some sense. Level settings used to be mainly time based, because computers at lower settings were no serious competition to decent players, but you don't necessarily want to wait for 30 seconds each move, so there were more casual and more serious levels.
Limiting the search depth is much more deterministic. At lower levels, it has hilarious results, and is pretty good at emulating beginning players (who know the rules, but have a limited skill of calculating moves ahead).
One problem with fixed search depth is that I think most good engines prefer to use dynamic search depth (where they sense that some positions need to be searched a bit deeper to reach a quiescent point), so they will be handicapped with a fix depth.
It’s simpler. Chess is a search through the space of possible moves, looking for a move that’s estimated to be better than the best move you’ve seen so far.
The search is by depth of further moves, and “better” is a function of heuristics (explicit or learned) on the resulting board positions, because most of the time you can’t be sure a move will inevitably result in a win or a loss.
So any particular move evaluation might take more or less time before the algorithm gives up on it—or chooses it as the new winner. To throw a consistent amount of compute at each move, the simple thing to do is give the engine consistent amounts of time per move.