If pawn advancement alone (more precisely: promotion distance) is really insufficient for an effective search in KPK positions when bitbase support is present (I doubt that) then of course you can add king advancement. But here I don't think you need such a complex condition like "at most 3 ranks in front of the pawn", since we are only talking about bitbase-won positions, and within the (usually small) set of won positions with identical promotion distance it is probably optimal to have the friendly king as close as possible to the promotion square. If that means to be four ranks in front of the pawn then so be it, as long as it is still won! I wouldn't even care about the enemy king's location in that case.hgm wrote:It depends on how detailed you want to indicate progress. Advance of the Pawn represents progress, but it could take 8 ply to force a winning advance. So if you will always search at least 8 ply that is OK. If you also want the engine to make progress when it searches less than 8 ply, you would have to define 'partial goals' based on the King position. Like also awarding points for advancing the King (e.g. 4 times less than for advancing the Pawn) to at most 3 ranks in front of the Pawn.
Sven