Optimizing quiescence search

[Hardcode3]

📌 Optimizing quiescence search

📄 Description

• Splitted legal moves generation into two, adding a legal capture moves generation. This new legal capture moves generation avoids creating all legal moves and then filtering for capture, hence resulting in a non-negligible speedup (see Additional Notes). Quiescence search now use legal capture moves generation and negamax search uses legal moves generation.

🧩 Type of Change

🚀 Performance improvement

📝 Additional Notes

Benchmark setup

A quick benchmark was run on the Kiwipete position.

Hardware used: M4 MacBook Pro with 16 Gb Unified Memory.

d
     a   b   c   d   e   f   g   h  
   +---+---+---+---+---+---+---+---+
 8 | r |   |   |   | k |   |   | r | 8
   +---+---+---+---+---+---+---+---+
 7 | p |   | p | p | q | p | b |   | 7
   +---+---+---+---+---+---+---+---+
 6 | b | n |   |   | p | n | p |   | 6
   +---+---+---+---+---+---+---+---+
 5 |   |   |   | P | N |   |   |   | 5
   +---+---+---+---+---+---+---+---+
 4 |   | p |   |   | P |   |   |   | 4
   +---+---+---+---+---+---+---+---+
 3 |   |   | N |   |   | Q |   | p | 3
   +---+---+---+---+---+---+---+---+
 2 | P | P | P | B | B | P | P | P | 2
   +---+---+---+---+---+---+---+---+
 1 | R |   |   |   | K |   |   | R | 1
   +---+---+---+---+---+---+---+---+
     a   b   c   d   e   f   g   h  

FEN notation: r3k2r/p1ppqpb1/bn2pnp1/3PN3/1p2P3/2N2Q1p/PPPBBPPP/R3K2R w - - 0 1
Zobrist hash: 7132688269263558591

Benchmark results

Experiment (depth 1)	Total nodes	Negamax nodes	Quiescence nodes	Total time (s)	Nodes per second
Before	34'038'939	47	34'038'892	12.0681	2'820'582
After	34'038'939	47	34'038'892	8.07602	4'214'818
Result	=	=	=	-3.99208	+ 1'394'236

Experiment (depth 2)	Total nodes	Negamax nodes	Quiescence nodes	Total time (s)	Nodes per second
Before	80'460'423	610	80'459'813	28.2208	2'851'100
After	80'460'423	610	80'459'813	18.656	4'312'841
Result	=	=	=	-9.5648	+ 1'461'741

Still, most of the nodes explored here are from the quiescence search. This number could be drastically reduced using for instance Static Exchange Evaluation.