noobpwnftw wrote: ↑
Thu Apr 11, 2019 6:39 pm
I am not quite clear what the purpose of this is.
I believe that is used to filter known worse moves at root given WDL information.
Cfish and Stockfish rank moves if only WDL is available but also if WDL+DTZ is available. The WDL+DTZ ranking is obviously more accurate.
The reason for ranking is that SF does not want to simply play the move that minimizes DTZ (because that can lead to very strange play).
Before ranking was implemented, SF would filter out root moves that according to WDL/WDL+DTZ are "worse" than the best root move (in the sense of risking to lose half a point or more). The problem with this is that it breaks both multipv=N > 1 (if fewer than N moves remain after filtering) and searchmoves (if none of the optimal moves are among the specified moves).
Ranking scores each move on the basis of WDL/WDL+DTZ information and then sorts the moves according to their score/rank. SF's search uses this information as follows: If multipv=N and there are K>=N top-ranked moves, then the search will be limited to those K moves. If K<N, the search will first produce K lines from those K moves, then do a multipv=N-K search on the remaining moves.