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Commit ID 108f0da4d7f993732aa2e854b8f3fa8ca6d3b46c
Author syzygy1
Date 2018-04-18 16:46:24 UTC
Tablebases root ranking This patch corrects both MultiPV behaviour and "go searchmoves" behaviour for tablebases. We change the logic of table base probing at root positions from filtering to ranking. The ranking code is much more straightforward than the current filtering code (this is a simplification), and also more versatile. If the root is a TB position, each root move is probed and assigned a TB score and a TB rank. The TB score is the Value to be displayed to the user for that move (unless the search finds a mate score), while the TB rank determines which moves should appear higher in a multi-pv search. In game play, the engine will always pick a move with the highest rank. Ranks run from -1000 to +1000: 901 to 1000 : TB win 900 : normally a TB win, in rare cases this could be a draw 1 to 899 : cursed TB wins 0 : draw -1 to -899 : blessed TB losses -900 : normally a TB loss, in rare cases this could be a draw -901 to -1000 : TB loss Normally all winning moves get rank 1000 (to let the search pick the best among them). The exception is if there has been a first repetition. In that case, moves are ranked strictly by DTZ so that the engine will play a move that lowers DTZ (and therefore cannot repeat the position a second time). Losing moves get rank -1000 unless they have relatively high DTZ, meaning they have some drawing chances. Those get ranks towards -901 (when they cross -900 the draw is certain). Closes No functional change (without tablebases).
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