RankCut

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RankCut,
a probability based depth reduction technique introduced by Yew Jin Lim and Wee Sun Lee in 2006 ¹. It estimates the probability of discovering a better move later in the search by using the relative frequency of such cases for various states during the search. These probabilities are pre-computed off-line using several self-play games. RankCut can then reduce search effort by performing a shallow search when the probability of a better move appearing is below a certain threshold. RankCut requires good move ordering and fail-soft to work well. Further elaborated by Yew Jin Lim in his 2007 Ph.D. thesis ², RankCut was successfully implemented with Crafty and Toga II.

RankCut Pseudocode

³

RankCutReSearch = false;

int RankCut(State & state, int α, int β, int depth) {
  if ((depth == 0) || isTerminal(state))
    return Evaluate(state);
  pruneRest = false;
  score = −∞;
  while (move = NextMove(state) ) {
    r = 0;
    features = determineFeatures(state);
    if (pruneRest || (probability(features) < threshold) ) {
      r = depthReduction(state);
      pruneRest = true;
    }
    score = −RankCut(successor(state, move), −β, −α, depth−1−r);
    if (RankCutReSearch && (score > α) && pruneRest)
      score = −RankCut(successor(state, move), −β, −α, depth−1);
    if (score ≥ β )
      break;
    if (score > α) {
      pruneRest = false;
      α = score;
    }
  }
  return score;
}

Crafty

In the case of Crafty 19.19, The probability computation considers following features:

• Depth
• In Check
• Move Extended
• Move Number
• Number of times Best Move changed
• Difference between the Score of the current Best Move from Alpha (discretized to 7 intervals)
• Difference between the Score of the Last Move from the current Best Move (discretized to 7 intervals)
• Phase of the Move Generation (History Moves or Remaining Moves)

See also

• Late Move Reductions
• ProbCut

Publications

• Yew Jin Lim, Wee Sun Lee (2006). RankCut - A Domain Independent Forward Pruning Method for Games. AAAI 2006, pdf
• Yew Jin Lim (2007). On Forward Pruning in Game-Tree Search. Ph.D. thesis, National University of Singapore, pdf

External Links

• Ranking from Wikipedia
• Pruning from Wikipedia

References