MadChess 3.0 Beta 22002dc (Move Generation Optimization)

Rather than repeat myself, I’ll explain my recent code update by copying the text of my Pull Request #10 here:

Improved detection of pieces pinned to own king by sliding attackers. Previous implementation only found potentially pinned pieces (because the pieces were on the same file, rank, or diagonal as the sliding attacker). The new implementation finds all actually pinned pieces.

This speeds up resolution of pseudo-legal moves to legal moves by eliminating unnecessary calls to Board.IsSquareAttacked(kingSquare) in Board.IsMoveLegal method. Its benefit is limited though because many pseudo-legal moves never are examined for legality because a beta cutoff occurs before the move is searched.

Eliminated unnecessary call to Board.PlayNullMove and Board.UndoMove in Board.IsMoveLegal method. Instead of actually making a null move, the code flips side-to-move, sets a few other properties of the position, calls IsSquareAttacked(kingSquare), then restores original property values.

I experimented with eliminating the remaining call to Board.PlayMove and Board.UndoMove in Board.IsMoveLegal method to determine if this speeds up resolution of pseudo-legal moves to legal moves. I did this by intersecting pre-calculated moves from the destination (To) square to the opponent king. And by detecting pieces pinned to the opponent king by own sliding attackers. Moving such pieces in a direction other than the attacking ray creates a discovered check. Unfortunately, this code was slower than simply moving the piece and calling Board.IsSquareAttacked. Board.IsSquareAttacked uses pre-calculated moves (magic bitboards) to determine if own king is attacked after moving the piece, constituting an illegal move. Did not include in this PR.

Also, I experimented combining legality checking with playing a move in a Board.PlayMoveIfLegal method. Unfortunately this causes numerous complications with futility-pruned moves. MadChess never prunes moves that deliver check, moves aren’t known to deliver check until they’re played, so this must be detected after the fact (which complicates undoing the move and restoring board state) or detected prior to playing the move (which already has been proven slower). Ugly and prone to bugs. Did not include in this PR.

Also adjusted how nodes are counted- affecting Node Per Second (NPS) metric- to a more honest measurement. A node is counted only in…

  1. Board.PlayMove method
  2. Board.PlayNullMove method (because this changes side-to-move plus a few other properties and advances position index)

2562 +/- 16 Elo at bullet chess.


Feature Category Date Commit1 WAC2 Elo Rating3 Improvement
Move Generation Optimization Search 2021 Feb 17 22002dc 287 2562 +12
PVS and Null Move Search 2021 Feb 09 f231dac 285 2550 +20
Remove Aspiration Windows Search 2020 Dec 20 4b7963b 290 2530 +9
Time Management Search 2020 Dec 19 d143bb5 286 2521 +8
Crash Bug Search 2020 Aug 29 2d855ec 288 2513 +0
King Safety Evaluation 2020 Aug 16 6794c89 288 2513 +63
Eval Param Tuning Evaluation 2020 Jul 23 bef88d5 283 2450 +30
Late Move Pruning Search 2020 Feb 08 6f3d17a 288 2420 +29
Piece Mobility Evaluation 2020 Feb 01 5c5d4fc 282 2391 +62
Passed Pawns Evaluation 2018 Dec 27 103 279 2329 +119
Staged Move Generation Search 2018 Dec 15 93 275 2210 +39
History Heuristics Search 2018 Dec 03 84 275 2171 +28
Eval Param Tuning Evaluation 2018 Nov 24 75 272 2143 +47
Sophisticated Search
Material and Piece Location
Baseline 2018 Nov 08 58 269 2096 0
  1. GitHub commit (hash) or Subversion source code revision (integer)
  2. Win At Chess position test, 3 seconds per position
  3. Bullet chess, 2 min / game + 1 sec / move
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