From 4c53eb7bc0c06ec620afd0f92546b2f556e3f824 Mon Sep 17 00:00:00 2001
From: grepfuldead <299977258+grepfuldead@users.noreply.github.com>
Date: Mon, 6 Jul 2026 17:48:48 +0300
Subject: [PATCH] Fix is_legal accepting non-canonical castling encodings

is_legal derives the castling kind from to.file() == File::G alone and
never checks that to is the actual landing square, so with castling
rights and a clear path it accepts (king_from, arbitrary to, Castling)
for 63 bogus encodings per position. Playing one hits the unreachable
in get_castling_rook. Search is unaffected (UCI and the TT path only
see canonical encodings), but islegalperft feeds every accepted
encoding to make_move and currently panics on any position with
castling rights:

    position fen r3k2r/p1ppqpb1/bn2pnp1/3PN3/1p2P3/2N2Q1p/PPPBBPPP/R3K2R w KQkq - 0 1
    islegalperft 1  ->  panicked: entered unreachable code

Require to == kind.landing_square(), making is_legal an exact mirror
of generate_all_moves and restoring the diagnostic (kiwipete
islegalperft 1 now completes: 48). Matches the convention in Stockfish,
where special-move validity is generated-set membership.

No functional change.

Bench: 2526370
---
 src/board.rs | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/board.rs b/src/board.rs
index 311942f98..7791f579e 100644
--- a/src/board.rs
+++ b/src/board.rs
@@ -363,7 +363,8 @@ impl Board {
             if mv.is_castling() {
                 let kind = CastlingKind::KINDS[stm][(to.file() == File::G) as usize];
 
-                return self.castling().is_allowed(kind)
+                return to == kind.landing_square()
+                    && self.castling().is_allowed(kind)
                     && (self.castling_path[kind] & self.occupancies()).is_empty()
                     && (self.castling_threat[kind] & self.all_threats()).is_empty()
                     && !self.pinned(stm).contains(self.castling_rooks[kind]);