Countermove+Heuristic

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The **Countermove Heuristic** was introduced by Jos Uiterwijk in 1992, which shows some similarities with the killer-, refutation- and history heuristic. This heuristic assumes that many moves have a "natural" response, irrespective of the actual position, and is easy to implement either with a 64 * 64 butterfly table or alternatively a more memory friendly 6 * 64 array for each side to move, indexed by [from][to] or by [piece][to] of the previous move, containing the counter move. The nature of the countermove heuristic renders it complementary to the killer heuristic, substituting position with same distance to the root with the last move played.

=Update= This is how the counter move array is updated, if a beta-cutoff occurs: code format="cpp" if ( score >= beta ) { // cutoff if ( isNonCapture (move) ) counterMove[previousMove.from][previousMove.to] = move; ...     return score; } code

=Move Score= While assigning scores to moves for move ordering, a bonus is earned for the move which matches the countermove of the last move played: code format="cpp" if ( movelist[i].move == counterMove[previousMove.from][previousMove.to] ) movelist[i].score += counterMoveBonus; code

=Butterfly Moves= Independently of Uiterwijk's work, Dap Hartmann and Peter Kouwenhoven reported on the similar technique of Butterfly board moves at Advances in Computer Chess 6, London 1990, being different from the Butterfly heuristic. Their aim was to safe move generation by proving only legality of a butterfly move. If both transposition- and killer table fail to supply a move, then 1 in 5 times the butterfly board was able to supply a legal killer which saved the complete move generation.

=See also=
 * Butterfly Boards
 * Butterfly Heuristic
 * Counter Moves History
 * History Heuristic
 * Killer Heuristic
 * Last Best Reply
 * Refutation Table

=Publications=
 * Dap Hartmann, Peter Kouwenhoven (**1991**). //Sundry Computer Chess Topics//. Advances in Computer Chess 6
 * Jos Uiterwijk (**1992**). //The Countermove Heuristic//. ICCA Journal, Vol. 15, No. 1
 * Jos Uiterwijk (**1992**). //Memory Efficiency in some Heuristics//. ICCA Journal, Vol. 15, No. 2
 * Eric Thé (**1992**). //[|An analysis of move ordering on the efficiency of alpha-beta search]//. Master's thesis, McGill University
 * Marcel van Kervinck (**2002**). //The design and implementation of the Rookie 2.0 Chess Playing Program//. Masters Thesis, [|pdf]

=Forum Posts=

2005 ...

 * [|Counter move heuristic] by Alvaro Jose Povoa Cardoso, CCC, August 30, 2005

2010 ...
> [|Re: History pruning / move ordering question] by Joona Kiiski, CCC, May 12, 2013
 * [|The LBR move ordering heuristic] by Steven Edwards, CCC, March 26, 2011 » Last Best Reply
 * [|Move ordering idea (old and new?)] by Daniel Homan, CCC, Aug 09, 2012
 * [|Re: History pruning / move ordering question] by Don Dailey, CCC, May 10, 2013

2015 ...

 * [|countermove heuristic] by Folkert van Heusden, CCC, December 31, 2016
 * [|Storing Countermoves by Ply] by Jason Fernandez, CCC, September 08, 2017

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