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Simplify is_legal #812

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protonspring wants to merge 2 commits into from
+16 −33
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Simplify is_legal #812

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befd54f
I break stuff.
protonspring Mar 24, 2026
eb9d103
Formatting.
protonspring Mar 24, 2026
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49 changes: 16 additions & 33 deletions src/board.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -347,22 +347,15 @@ impl Board {

/// Checks if the given move is legal in the current position.
pub fn is_legal(&self, mv: Move) -> bool {
if mv.is_null() {
return false;
}

let stm = self.side_to_move();
let king = self.king_square(stm);

let from = mv.from();
let to = mv.to();

if self.in_check() && king != from {
if self.checkers().is_multiple() {
return false;
}

if !mv.is_en_passant() && !(self.checkers() | between(king, self.checkers().lsb())).contains(to) {
if self.checkers().is_multiple()
|| !(mv.is_en_passant() || (self.checkers() | between(king, self.checkers().lsb())).contains(to))
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@87flowers 87flowers Mar 25, 2026 •
edited
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I don't enjoy this specific change as this makes it harder to read.

{
return false;
}
}
Expand All @@ -375,10 +368,6 @@ impl Board {
let captured = self.piece_on(to).piece_type();

if mv.is_castling() {
if king != from {
return false;
}

let kind = match to {
Square::G1 => CastlingKind::WhiteKingside,
Square::C1 => CastlingKind::WhiteQueenside,
Expand All @@ -387,25 +376,19 @@ impl Board {
_ => unreachable!(),
};

return self.castling().is_allowed(kind)
return king == from
&& 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]);
}

if king == from && self.all_threats().contains(to) {
return false;
}

if !self.us().contains(from) || self.us().contains(to) {
return false;
}

if captured != PieceType::None && (!mv.is_capture() || captured == PieceType::King) {
return false;
}

if mv.is_capture() && !mv.is_en_passant() && !self.them().contains(to) {
if (king == from && self.all_threats().contains(to))
|| !self.us().contains(from)
|| self.us().contains(to)
|| (captured != PieceType::None && (!mv.is_capture() || captured == PieceType::King))
|| (mv.is_capture() && !mv.is_en_passant() && !self.them().contains(to))
{
return false;
}

Expand All @@ -417,23 +400,23 @@ impl Board {
let diagonal = bishop_attacks(king, occupancies) & diagonal;
let orthogonal = rook_attacks(king, occupancies) & orthogonal;
return to == self.en_passant()
&& pawn_attacks(from, self.side_to_move()).contains(to)
&& pawn_attacks(from, stm).contains(to)
&& (orthogonal | diagonal).is_empty();
}

let offset = Square::UP[self.side_to_move()];
let promotion_rank = if self.side_to_move() == Color::White { Rank::R8 } else { Rank::R1 };
let offset = Square::UP[stm];
let promotion_rank = if stm == Color::White { Rank::R8 } else { Rank::R1 };

if mv.is_promotion() != (mv.to().rank() == promotion_rank) {
return false;
}

if mv.is_capture() {
return pawn_attacks(from, self.side_to_move()).contains(to) && self.them().contains(to);
return pawn_attacks(from, stm).contains(to) && self.them().contains(to);
}

if mv.is_double_push() {
return from.rank() == (if self.side_to_move() == Color::White { Rank::R2 } else { Rank::R7 })
return from.rank() == (if stm == Color::White { Rank::R2 } else { Rank::R7 })
&& from.shift(2 * offset) == to
&& !self.occupancies().contains(from.shift(offset))
&& !self.occupancies().contains(to);
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