RankCut

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RankCut, a probability based depth reduction technique introduced by Yew Jin Lim and Wee Sun Lee in 2006. 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 , RankCut was successfully implemented with Crafty and Toga II.

=RankCut Code= 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

=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

=References= Up one level