Pruning in today's top engines

[Ralph Stoesser]

Rxe5 will not be pruned, but the effective search depth for the lines starting with Rxe5 will be massively reduced. Given enough time Stockfish would find that move. It's similar to us humans. When we see a diagram position together with the hint that there is a tactical shot, we will find the solution much easier compared to the situation when we are playing a game on our own. In a chess game there is so much more to think about than tactical shots. If we only would look for tactical shots in a tournament game, it would hurt our playing strength. Same is true for engines. Houdini 3 has a tactical mode that is designed to solve tactical positions, but the playing strength is reduced in this mode.

[zullil]

But I'll now present one specific example from a common test case.

[d]4R3/P7/2bB2p1/r3pk2/2p2p1P/2P2P1K/6P1/8 w - - 0 54

Here, a latest Stockfish build after 10 hours at depth 53/89 proposes Re7 with an evaluation of 3.37. If I then show it Rxe5, it takes only 2 seconds and depth 31/49 to see that Rxe5 is much better.

It is interesting to force Stockfish to search more broadly, by setting MultiPV to a value above 1. For example, here's SF searching your position to a depth of 30, with MultiPV=5:

Code: Select all

info depth 30 seldepth 55 score cp 402 nodes 386322789 nps 7201602 time 53644 multipv 1 pv e8e5 a5e5 d6e5 c6d5 e5c7 d5c6 h3h2 c6d5 h2g1 d5a8 c7f4 f5f4 g1f2 a8c6 g2g3 f4f5 f2e3 f5f6 g3g4 f6g7 f3f4 g7h6 f4f5

info depth 30 seldepth 55 score cp 284 nodes 386322789 nps 7201602 time 53644 multipv 2 pv e8e7 f5f6 h3g4 a5a2 e7e5 a2a7 e5c5 a7a6 d6f4 c6d7 g4g3 a6c6 f4e5 f6e7 c5a5 d7f5 g3f4 f5d3 g2g4 c6b6 f4g5 b6b2 h4h5 g6h5 g5h5 b2g2 h5g5 d3e2 g5f4 e2d3 a5a6 e7f7 e5d4 g2e2 f4g3 e2e6 a6a4 e6e8 f3f4 f7e6 f4f5 e6d5

info depth 30 seldepth 55 score cp 266 nodes 386322789 nps 7201602 time 53644 multipv 3 pv e8c8 c6b7 c8f8 f5e6 d6b8 e6e7 f8h8 e7f6 h8e8 b7c6 e8d8 c6b7 d8d6 f6f5 d6d7 b7c6 d7c7 c6d5 c7e7 d5e6 h3h2 f5f6 e7e8 f6f7 e8h8 f7e7 h4h5 g6h5 h8h5 a5a3 h5e5 e7d7 e5e4 e6d5 e4f4 a3c3 f4d4 d7e6 d4d5

info depth 30 seldepth 55 score cp 242 nodes 386322789 nps 7201602 time 53644 multipv 4 pv e8f8 f5e6 d6b8 c6d7 h3h2 a5a2 b8e5 a2a7 e5d4 a7a5 f8f4 a5f5 f4g4 e6f7 g4e4 d7c6 e4e1 f5h5 h2g3 h5a5 e1e2 a5b5 e2a2 f7e7 a2a7 b5b7 a7a8 b7b2 d4c5 e7e6 a8g8 e6f7 g8d8 b2e2 d8d6 c6a4 d6a6 a4d7 c5d4 d7f5 a6c6 f5d3 c6c7 f7e6

info depth 30 seldepth 55 score cp 145 nodes 386322789 nps 7201602 time 53644 multipv 5 pv a7a8r c6a8 d6c7 a5c5 e8f8 f5e6 c7e5 c5e5 f8a8 e5e3 a8a3 e6f5 a3a5 f5e6 h3g4 e3c3 g4f4 c3c1 a5g5 c4c3 f4e3 c1d1 g5c5 d1g1 g2g4 g1c1 e3d3 c1f1 d3e4 f1h1 c5c6 e6f7 c6c3 h1h4 e4f4 h4h3 c3c7 f7f6 g4g5 f6e6 f4g4 h3h1 c7g7 h1g1 g4f4

info nodes 386322789 time 53645
bestmove e8e5 ponder a5e5

That Rxe5 is best was found at depth 29, in 34 seconds.