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use crate::types::*;
impl PieceType {
#[inline(always)]
pub const fn index(self) -> usize {
match self {
PieceType::Pawn => 0,
PieceType::Knight => 1,
PieceType::Bishop => 2,
PieceType::Rook => 3,
PieceType::Queen => 4,
PieceType::King => 5,
}
}
#[inline(always)]
pub const fn from_index(i: usize) -> Self {
match i {
0 => PieceType::Pawn,
1 => PieceType::Knight,
2 => PieceType::Bishop,
3 => PieceType::Rook,
4 => PieceType::Queen,
_ => PieceType::King,
}
}
}
impl Color {
/// White = 0, Black = 1.
#[inline(always)]
pub const fn index(self) -> usize {
match self {
Color::White => 0,
Color::Black => 1,
}
}
}
/// The complete state of the chess board at a single point in time.
///
/// Piece positions are stored as twelve `u64` bitboards — one per
/// (colour, piece-type) pair. Bit *n* is set when a piece of that kind
/// occupies square *n* (a1 = 0 … h8 = 63).
///
/// All move-application logic lives in `movegen` to keep this struct simple.
#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub struct Board {
/// `bb[color_index][piece_type_index]`
///
/// color : White = 0 | Black = 1
/// piece : Pawn=0 Knight=1 Bishop=2 Rook=3 Queen=4 King=5
pub bb: [[u64; 6]; 2],
pub side_to_move: Color,
pub castling_rights: CastlingRights,
/// Target square for a possible en-passant capture; `None` if not applicable.
pub en_passant: Option<Square>,
/// Half-move clock for the 50-move rule (resets on pawn moves or captures).
pub halfmove_clock: u32,
/// Full-move number, starting at 1 and incrementing after Black's move.
pub fullmove_number: u32,
}
impl Board {
/// Bitboard of every square occupied by `color`.
#[inline(always)]
pub fn occupancy(&self, color: Color) -> u64 {
let c = color.index();
self.bb[c][0]
| self.bb[c][1]
| self.bb[c][2]
| self.bb[c][3]
| self.bb[c][4]
| self.bb[c][5]
}
/// Bitboard of every occupied square (both colours).
#[inline(always)]
pub fn all_occupancy(&self) -> u64 {
self.occupancy(Color::White) | self.occupancy(Color::Black)
}
/// Bitboard for a specific (colour, piece-type) combination.
#[inline(always)]
pub fn piece_bb(&self, color: Color, pt: PieceType) -> u64 {
self.bb[color.index()][pt.index()]
}
/// Returns the piece on `sq`, or `None` if the square is empty.
///
/// Scans all 12 bitboards (O(12)); avoid on the movegen hot path —
/// use bitboard operations directly there.
#[inline]
pub fn piece_at(&self, sq: Square) -> Option<Piece> {
let bit = 1u64 << sq;
for ci in 0..2usize {
for pi in 0..6usize {
if self.bb[ci][pi] & bit != 0 {
let color = if ci == 0 { Color::White } else { Color::Black };
return Some(Piece::new(PieceType::from_index(pi), color));
}
}
}
None
}
/// Places `piece` on `sq`, first clearing any piece already there.
#[inline]
pub fn set_piece(&mut self, sq: Square, piece: Option<Piece>) {
self.clear_square(sq);
if let Some(p) = piece {
self.bb[p.color.index()][p.piece_type.index()] |= 1u64 << sq;
}
}
/// Removes and returns whatever piece is on `sq`, or `None` if the square
/// was already empty.
#[inline]
pub fn take_piece(&mut self, sq: Square) -> Option<Piece> {
let p = self.piece_at(sq);
if let Some(ref p) = p {
self.bb[p.color.index()][p.piece_type.index()] &= !(1u64 << sq);
}
p
}
/// Returns a flat `[Option<Piece>; 64]` snapshot of the board.
///
/// Index `n` corresponds to square `n` (a1 = 0 … h8 = 63). This is
/// primarily used by code that needs to iterate over all pieces without
/// working directly with bitboards.
pub fn squares(&self) -> [Option<Piece>; 64] {
let mut arr = [None; 64];
for sq in 0u8..64 {
arr[sq as usize] = self.piece_at(sq);
}
arr
}
/// Square of the king of `color`, found in O(1) via `trailing_zeros`.
///
/// Returns `None` only in an illegal position where the king is missing.
#[inline]
pub fn king_square(&self, color: Color) -> Option<Square> {
let bb = self.bb[color.index()][PieceType::King.index()];
if bb == 0 {
None
} else {
Some(bb.trailing_zeros() as Square)
}
}
/// Clears all bitboards at `sq`.
///
/// We scan all 12 BBs unconditionally rather than calling `piece_at` first —
/// both approaches are O(12), but the branchless AND-mask loop avoids the
/// early-return overhead and is friendlier to the branch predictor on the
/// hot path (e.g. inside `set_piece` during FEN parsing or move application).
#[inline]
pub(crate) fn clear_square(&mut self, sq: Square) {
let mask = !(1u64 << sq);
for ci in 0..2usize {
for pi in 0..6usize {
self.bb[ci][pi] &= mask;
}
}
}
/// Returns a board set up in the standard chess starting position.
pub fn starting_position() -> Self {
let mut b = Self {
bb: [[0u64; 6]; 2],
side_to_move: Color::White,
castling_rights: CastlingRights::all(),
en_passant: None,
halfmove_clock: 0,
fullmove_number: 1,
};
let w = Color::White.index();
let k = Color::Black.index();
b.bb[w][PieceType::Pawn.index()] = 0x000000000000FF00; // rank 2
b.bb[w][PieceType::Knight.index()] = 0x0000000000000042; // b1, g1
b.bb[w][PieceType::Bishop.index()] = 0x0000000000000024; // c1, f1
b.bb[w][PieceType::Rook.index()] = 0x0000000000000081; // a1, h1
b.bb[w][PieceType::Queen.index()] = 0x0000000000000008; // d1
b.bb[w][PieceType::King.index()] = 0x0000000000000010; // e1
b.bb[k][PieceType::Pawn.index()] = 0x00FF000000000000; // rank 7
b.bb[k][PieceType::Knight.index()] = 0x4200000000000000; // b8, g8
b.bb[k][PieceType::Bishop.index()] = 0x2400000000000000; // c8, f8
b.bb[k][PieceType::Rook.index()] = 0x8100000000000000; // a8, h8
b.bb[k][PieceType::Queen.index()] = 0x0800000000000000; // d8
b.bb[k][PieceType::King.index()] = 0x1000000000000000; // e8
b
}
/// Returns the piece-placement field of the FEN string (rank 8 … rank 1).
pub fn fen_piece_placement(&self) -> String {
let mut out = String::with_capacity(72);
for rank in (0..8u8).rev() {
let mut empty: u8 = 0;
for file in 0..8u8 {
match self.piece_at(rank * 8 + file) {
None => empty += 1,
Some(p) => {
if empty > 0 {
out.push((b'0' + empty) as char);
empty = 0;
}
out.push(p.to_fen_char());
}
}
}
if empty > 0 {
out.push((b'0' + empty) as char);
}
if rank > 0 {
out.push('/');
}
}
out
}
/// A compact position key for threefold-repetition detection.
///
/// Includes piece placement, side to move, castling rights, and en passant
/// target — matching the fields that define a unique chess position.
pub fn position_key(&self) -> String {
let ep = self
.en_passant
.map(square_name)
.unwrap_or_else(|| "-".to_string());
format!(
"{} {} {} {}",
self.fen_piece_placement(),
self.side_to_move.to_char(),
self.castling_rights.to_fen_str(),
ep
)
}
}