aspiration window

{{Short description|Search heuristic for combinatorial games}}

An aspiration window is a heuristic used in pair with alpha-beta pruning in order to reduce search time for combinatorial games by supplying a window (or range) around an estimated score guess. Use of an aspiration window allows alpha-beta search to compete in the terms of efficiency against other pruning algorithms.{{sfn|Shams|Kaindl|Horacek|1991|p=192}}

Alpha-beta pruning achieves its performance by using cutoffs from its original range. Aspiration windows take advantage of this by supplying a smaller initial window, which increases the amount of cutoffs and therefore efficiency.{{example needed|date=March 2024}}

However, due to search instability, the score may not always be in the window range. This may lead to a costly re-search that can penalize performance.[https://web.archive.org/web/20070705134903/www.seanet.com/%7Ebrucemo/topics/aspiration.htm Bruce Moreland's Programming Topics: Aspiration Windows] Despite this, popular engines such as Stockfish still use aspiration windows.[https://github.com/official-stockfish/Stockfish/blob/46756996e7884c665da18f357208c2344a0f9374/src/search.cpp#L349 Stockfish source code - direct aspiration window mention]

The guess that aspiration windows use is usually supplied by the last iteration of iterative deepening.[http://www.frayn.net/beowulf/theory.html#aspiration Computer Chess Programming Theory: Aspiration Windows]