Fail-Soft

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Fail-Soft is a term related to an Alpha-Beta like search. Returned scores might be outside the bounds: =History= In his 1983 paper Another optimization of alpha-beta search, John Philip Fishburn introduced Fail-Soft Alpha-Beta as an improvement of Fail-Hard without any extra work. 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.
 * an upper bound less than alpha at All-Nodes
 * a lower bound greater than beta at Cut-Nodes

=Mate Scores= In Chrilly Donninger's initial null move pruning implementation there was a deep search extension, 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=
 * Fail-Hard
 * Fail-High
 * Fail-Low
 * Principal Variation Search
 * NegaScout
 * NegaC*
 * MTD(f)

=Publications=
 * John Philip Fishburn (1983). Another optimization of alpha-beta search. SIGART Bulletin, Issue 84, pdf

=Forum Posts=

1995 ...

 * bounds in alpha-beta by Walter Ravenek, rgcc, June 05, 1996 » Arthur, Crafty
 * New improvement to alpha/beta + TT? by Heiner Marxen, rgcc, January 13, 1997
 * Fail-soft with PVS? by Will Singleton, CCC, March 09, 1999 » Principal Variation Search
 * What is the advantage of fail-soft? by Bas Hamstra, CCC, August 25, 1999

2000 ...

 * Fail-soft or Fail-hard ? by Teerapong Tovirat, CCC, November 04, 2000 » Fail-Hard
 * Alpha beta fail soft, pruning & hash bounds? by Steve Maughan, CCC, December 14, 2000 » Pruning, Bound
 * Fail Soft Alpha Beta & Transpositions by Orhan Öztürk, rgcc, September 3, 2003
 * (Fail soft) alpha beta by Delphi, rgcc, September 4, 2003
 * Fail soft alpha-beta by Russell Reagan, CCC, September 08, 2003
 * Is this a correct fail-soft? by Mikael Bäckman, CCC, September 21, 2003
 * fail soft question? by Daniel Shawul, Winboard Forum, December 03, 2003
 * My fail soft reduces quality of collected PV. Help needed by Volker Böhm, CCC, April 20, 2004
 * Fail-hard, fail-soft question by Dan Honeycutt, CCC, May 06, 2004

2005 ...

 * Fail Soft question by David B. Weller, CCC, February 17, 2006
 * Search questions by Sven Schüle, Winboard Forum, July 17, 2007 » Mate Distance Pruning, Principal Variation Search
 * Returning score by Fermin Serrano, CCC, June 05, 2008
 * mate detetion issue by Mike Adams, CCC, October 24, 2008 » Checkmate, Connect Four
 * fail soft vs fail hard by cyberfish, CCC, November 19, 2008
 * Return eval or upper bound? by John Merlino, CCC, October 26, 2009

2010 ...

 * First post (and FailHigh question!) by Natale Galioto, CCC, June 14, 2013 » Fail-High, Fail-Hard
 * Fail soft vs fail hard by Sergei S. Markoff, CCC, February 15, 2014 » Fail-Hard, Fail-Low, Internal Iterative Deepening
 * Implications of Lazy eval on Don Beal effect in Fail Soft by Henk van den Belt, CCC, November 19, 2014 » Lazy Evaluation
 * Multi-cut and fail-soft by Matthew R. Brades, CCC, June 30, 2016 » Multi-Cut
 * Fail Soft best practices by kickstone, OpenChess Forum, July 30, 2018

2020 ...

 * Fail hard/soft by Philippe Chevalier, CCC, January 28, 2022

=External Links=
 * Alpha–beta pruning from Wikipedia
 * Lecture notes for February 2, 1999 Variants of Alpha-Beta Search by David Eppstein
 * fail-soft - Wiktionary
 * Fail soft from Wikipedia
 * Soft Machine - Hazard Profile, 1975, YouTube Video
 * Lineup: Mike Ratledge, John Marshall, Karl Jenkins, Roy Babbington, Allan Holdsworth


 * Ian Carr's Nucleus - Song For The Bearded Lady (1970), YouTube Video
 * Lineup: Ian Carr, Brian Smith, Karl Jenkins, John Marshall, Chris Spedding, Jeff Clyne

=References= Up one Level