I added evaluation of king safety to MadChess 3.0 Beta. Because determining the safety of a king’s position involves examining moves that attack squares near the king, I combined it with piece mobility evaluation, which also examines moves. When examining piece mobility, keep a weighted count of attacks on squares ringing the king (16 squares in outer ring and 8 squares in inner ring) with separate weights for attacks by minor pieces, rooks, and queens.
In the following code, white moves to squares ringing the black king contribute negatively to black’s king safety. Black moves to squares ringing the white king contribute negatively to white’s king safety. This will become evident in a later code snippet that illustrates the king safety array contains negative values.
The above code encourages MadChess to block enemy pieces from “aiming” at its king. Conversely, it encourages MadChess to direct its pieces toward the enemy king for an eventual attack.
Next, add a weighted count of semi-open files (missing guard pawn) near the king (left file, king file, and right file). Evaluate this only for the middlegame, assuming open files are common in the endgame and don’t necessarily make a king’s position unsafe.
Finally, lookup the king safety score in an array. The array is calculated at engine startup using a non-linear formula.
showevalparams command displays the king safety values calculated by the above code.
PS C:\Users\Erik\Documents\Chess\Engines\MadChess\3.0> .\MadChess.Engine.exe showevalparams King Safety KingSafetyPowerPer16: 029 King Safety MgKingSafetySemiOpenFilePer8: 062 King Safety KingSafetyMinorAttackOuterRingPer8: 008 King Safety KingSafetyMinorAttackInnerRingPer8: 021 King Safety KingSafetyRookAttackOuterRingPer8: 007 King Safety KingSafetyRookAttackInnerRingPer8: 018 King Safety KingSafetyQueenAttackOuterRingPer8: 014 King Safety KingSafetyQueenAttackInnerRingPer8: 033 King Safety KingSafetyScalePer128: 043 King Safety: +000 +000 -001 -002 -004 -006 -008 -011 -014 -018 -021 -025 -030 -035 -040 -045 -051 -057 -063 -069 -076 -083 -091 -098 -106 -114 -123 -132 -141 -150 -159 -169 -179 -189 -200 -211 -222 -233 -245 -257 -269 -281 -294 -306 -319 -333 -346 -360 -374 -388 -403 -418 -433 -448 -463 -479 -495 -511 -527 -544 -561 -578 -595 -613
In addition, I simplified use of aspiration windows when searching the root position. The engine searches with a window of 200 centipawns (100 in each direction) centered around the best score from the prior search depth. If this fails high or low, adjust the aspiration window in the direction of failure by 600 centipawns. If this fails high or low, search the root position using an infinite window.
This code increased the playing strength of MadChess 3.0 Beta by 63 ELO. MadChess has crossed the 2500 ELO threshold.
|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|
Material and Piece Location
|Baseline||2018 Nov 08||58||269||2096||0|
- GitHub (hash) or Subversion (integer) source code revision
- Win At Chess position test, 3 seconds per position
- Bullet chess, 2 min / game + 1 sec / move