Fix non-canonical hash after null moves#1095
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codedeliveryservice merged 1 commit intoJul 7, 2026
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increment_stack toggles the castling key out on every ply, which is the remove-old-rights half of the pair that make_move completes when it updates castling rights. make_null_move goes through increment_stack but never toggles it back, so the hash after a null move is left off by the castling term: a position reached via a null move hashes differently from the same position parsed from FEN. Move the castling toggle out of the shared increment_stack into make_move, which owns both halves. Null moves now leave castling untouched, matching the standard convention (Stockfish, Viridithas). STC Elo | 3.45 +- 2.88 (95%) SPRT | 8.0+0.08s Threads=1 Hash=16MB LLR | 2.92 (-2.25, 2.89) [-2.75, 0.25] Games | N: 14716 W: 3733 L: 3587 D: 7396 Penta | [43, 1679, 3779, 1803, 54] https://recklesschess.space/test/15402/ LTC Elo | 2.57 +- 2.47 (95%) SPRT | 40.0+0.40s Threads=1 Hash=64MB LLR | 2.91 (-2.25, 2.89) [-2.75, 0.25] Games | N: 17832 W: 4442 L: 4310 D: 9080 Penta | [7, 2002, 4764, 2138, 5] https://recklesschess.space/test/15403/ Bench: 2931707
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increment_stack toggles the castling key out on every ply, which is the remove-old-rights half of the pair that make_move completes when it updates castling rights. make_null_move goes through increment_stack but never toggles it back, so the hash after a null move is left off by the castling term: a position reached via a null move hashes differently from the same position parsed from FEN.
Move the castling toggle out of the shared increment_stack into make_move, which owns both halves. Null moves now leave castling untouched, matching the standard convention (Stockfish, Viridithas).
STC
Elo | 3.45 +- 2.88 (95%)
SPRT | 8.0+0.08s Threads=1 Hash=16MB
LLR | 2.92 (-2.25, 2.89) [-2.75, 0.25]
Games | N: 14716 W: 3733 L: 3587 D: 7396
Penta | [43, 1679, 3779, 1803, 54]
https://recklesschess.space/test/15402/
LTC
Elo | 2.57 +- 2.47 (95%)
SPRT | 40.0+0.40s Threads=1 Hash=64MB
LLR | 2.91 (-2.25, 2.89) [-2.75, 0.25]
Games | N: 17832 W: 4442 L: 4310 D: 9080
Penta | [7, 2002, 4764, 2138, 5]
https://recklesschess.space/test/15403/
Bench: 2931707