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fix: reset halfmove counter on non-pawn captures #24

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Merged
Disservin merged 2 commits into from
Jul 9, 2026
Merged

fix: reset halfmove counter on non-pawn captures #24

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d30d966
fix: reset halfmove counter on non-pawn captures
MatthewDBTodd Jul 8, 2026
22bdde0
simplify by re-ordering the halfm reset checks
MatthewDBTodd Jul 9, 2026
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62 changes: 55 additions & 7 deletions src/chess/position.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -160,6 +160,13 @@ impl Position {
// clear piece from start
self.remove_piecetype(self.stm, pt, from);

// Update halfmove clock
if pt == PieceType::Pawn {
self.halfm = 0;
} else {
self.halfm += 1;
}

// capture piece
if mv.mtype() != MoveType::Castle {
let captured = self.piece_at(to);
Expand Down Expand Up @@ -232,13 +239,6 @@ impl Position {

// update state

// Update halfmove clock
if pt == PieceType::Pawn {
self.halfm = 0;
} else {
self.halfm += 1;
}

// Update fullmove number
if self.stm == Color::Black {
self.fullm += 1;
Expand Down Expand Up @@ -631,4 +631,52 @@ mod tests {
let pos = Position::new();
assert_eq!(pos, Position::from_fen(STARTPOS).unwrap());
}

#[test]
fn test_halfm_reset() {
let fen = "r1bqkb1r/pppp1ppp/2n2n2/1B2p3/4P3/5N2/PPPP1PPP/RNBQK2R w KQkq - 4 4";

let pos = Position::from_fen(fen).unwrap();
let mv = Move::new(
Square::from_string("b5").unwrap(),
Square::from_string("c6").unwrap(),
MoveType::Normal,
Piece::NONE,
);
let pos = pos.after_move(mv);
assert_eq!(0, pos.rule50_counter());

let pos = Position::from_fen(fen).unwrap();
let mv = Move::new(
Square::from_string("d2").unwrap(),
Square::from_string("d3").unwrap(),
MoveType::Normal,
Piece::NONE,
);
let pos = pos.after_move(mv);
assert_eq!(0, pos.rule50_counter());

let pos = Position::from_fen(fen).unwrap();
let mv = Move::new(
Square::from_string("b1").unwrap(),
Square::from_string("c3").unwrap(),
MoveType::Normal,
Piece::NONE,
);
let pos = pos.after_move(mv);
assert_eq!(5, pos.rule50_counter());

let pos = Position::from_fen(
"r1bqk2r/pppp1ppp/2n2n2/1Bb1p3/4P3/2N2N2/PPPP1PPP/R1BQK2R w KQkq - 6 5",
)
.unwrap();
let mv = Move::new(
Square::from_string("e1").unwrap(),
Square::from_string("h1").unwrap(),
MoveType::Castle,
Piece::NONE,
);
let pos = pos.after_move(mv);
assert_eq!(7, pos.rule50_counter());
}
}
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