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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 [1] . 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] [2] 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.


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

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:
   if ( movelist[i].move == counterMove[previousMove.from][previousMove.to] )
      movelist[i].score += counterMoveBonus;

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 [3] . 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 [4] .

See also


Forum Posts


  1. ^ Jos Uiterwijk (1992). The Countermove Heuristic. ICCA Journal, Vol. 15, No. 1
  2. ^ Jos Uiterwijk (1992). Memory Efficiency in some Heuristics. ICCA Journal, Vol. 15, No. 2
  3. ^ Dap Hartmann, Peter Kouwenhoven (1991). Sundry Computer Chess Topics. Advances in Computer Chess 6
  4. ^ Paul Lu (1990). Report on the Advances in Computer Chess 6 Conference. ICCA Journal, Vol. 13, No. 3

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