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Home * Search * Alpha-Beta * Fail-Soft

Window of Perception [1]

Fail-Soft is a term related to an Alpha-Beta like search. Returned scores might be outside the bounds:


In his 1983 paper Another optimization of alpha-beta search [2], John Philip Fishburn introduced Fail-Soft Alpha-Beta as an improvement of Fail-Hard without any extra work [3]. Fail-Soft has the reputation for searching less nodes than Fail-Hard, but might also require some care regarding to search instability issues in conjunction with transposition tables and various pruning-, reduction- and extension techniques.

Mate Scores

In Chrilly Donninger's initial null move pruning implementation there was a deep search extension [4], if the null move was refuted by a mate attack, thus relying on Fail-Soft of a null window search, where many "random" moves may refute the null-move with or without score in the mate range.

See also


Forum Posts

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External Links

Lineup: Mike Ratledge, John Marshall, Karl Jenkins, Roy Babbington, Allan Holdsworth
Lineup: Ray Russell, Ian Carr, Brian Smith, Karl Jenkins, John Marshall, Roy Babbington


  1. Window of Perception (Jump Start Century 21 and move on) by Dominic01, May 19, 2013, Wikimedia Commons
  2. John Philip Fishburn (1983). Another optimization of alpha-beta search. SIGART Bulletin, Issue 84, pdf
  3. Jean-Christophe Weill (1991). Experiments With the NegaC* Search - An Alternative for Othello Endgame Search. Heuristic Programming in AI 2
  4. Chrilly Donninger (1993). Null Move and Deep Search: Selective-Search Heuristics for Obtuse Chess Programs. ICCA Journal, Vol. 16, No. 3

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