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//! Defines the `MoveGenerator` trait.
use std::mem::uninitialized;
use std::cmp::max;
use uci::SetOption;
use board::*;
use moves::*;
use value::*;
use evaluator::Evaluator;
use bitsets::*;
use utils::BoardGeometry;
/// A trait for move generators.
///
/// A `MoveGenerator` holds a chess position and can:
///
/// * Calculate Zobrist hashes.
///
/// * Find which pieces and pawns attack a given square.
///
/// * Find if the side to move is in check.
///
/// * Generate all legal moves, or a subset of all legal moves in the
/// current position.
///
/// * Generate a null move.
///
/// * Verify if a random move digest represents a proper move.
///
/// * Play a selected move and take it back.
///
/// * Provide a static evaluator bound to the current position.
///
/// * Perform static exchange evaluation for the generated moves.
///
/// **Important note:** `MoveGenerator` is unaware of repeating
/// positions and rule-50.
pub trait MoveGenerator: Clone + SetOption + Send + 'static {
/// The type of static evaluator that the implementation works
/// with.
type Evaluator: Evaluator;
/// Creates a new instance, consuming the supplied `Board`
/// instance.
///
/// Returns `Err(IllegalBoard)` if the position is illegal.
fn from_board(board: Board) -> Result<Self, IllegalBoard>;
/// Returns a reference to the underlying `Board` instance.
fn board(&self) -> &Board;
/// Returns the Zobrist hash value for the underlying `Board`
/// instance.
///
/// Zobrist hashing is a technique to transform a board position
/// into a number of a fixed length, with an equal distribution
/// over all possible numbers, invented by Albert Zobrist. The key
/// property of this method is that two similar positions generate
/// entirely different hash numbers.
///
/// **Important note:** This method will be relatively slow if the
/// implementation calculates the hash value "from
/// scratch". Inspect the implementation before using `hash` in
/// time-critical paths. (See `do_move`.)
fn hash(&self) -> u64;
/// Returns a bitboard with all pieces and pawns that attack a
/// given square.
fn attacks_to(&self, square: Square) -> Bitboard;
/// Returns if the side to move is in check.
fn is_check(&self) -> bool;
/// Generates all legal moves, possibly including some
/// pseudo-legal moves too.
///
/// The moves are added to `moves`. All generated moves with
/// pieces other than the king will be legal. Some of the
/// generated king's moves may be illegal because the destination
/// square is under attack. This arrangement has two important
/// advantages:
///
/// * `do_move` can do its work without knowing the set of
/// checkers and pinned pieces, so there is no need to keep
/// those around.
///
/// * A beta cut-off may make the verification that king's
/// destination square is not under attack unnecessary, thus
/// saving time.
///
/// The initial move score for the generated moves is
/// *unspecified*.
///
/// **Note:** A pseudo-legal move is a move that is otherwise
/// legal, except it might leave the king in check.
fn generate_all<T: AddMove>(&self, moves: &mut T);
/// Generates moves for the quiescence search.
///
/// The moves are added to `moves`. This method always generates a
/// **subset** of the moves generated by `generate_all`:
///
/// * If the king is in check, all legal moves are included.
///
/// * Captures and pawn promotions to queen are always included.
///
/// * If `generate_checks` is `true`, moves that give check are
/// included too. Discovered checks and checks given by castling
/// can be omitted for speed.
fn generate_forcing<T: AddMove>(&self, generate_checks: bool, moves: &mut T);
/// Returns a null move.
///
/// "Null move" is a pseudo-move that changes only the side to
/// move. It is sometimes useful to include a speculative null
/// move in the search tree so as to achieve more aggressive
/// pruning. Null moves are represented as king's moves for which
/// the origin and destination squares are the same.
fn null_move(&self) -> Move;
/// Verifies if the supplied move digest represents a proper move.
///
/// If a move `m` exists that would be generated by `generate_all`
/// if called for the current position on the board, and for that
/// move `m.digest()` equals the supplied move digest, this method
/// will return `Some(m)`. Otherwise it will return `None`. This
/// is useful when playing moves from the transposition table,
/// without calling `generate_all`.
fn try_move_digest(&self, move_digest: MoveDigest) -> Option<Move>;
/// Plays a move on the board.
///
/// It the move leaves the king in check, `None` is returned
/// without updating the board. Otherwise, the board is updated
/// and an `u64` value is returned, which should be XOR-ed with
/// board's old Zobrist hash value to obtain board's new Zobrist
/// hash value. The move passed to this method **must** have been
/// generated by `generate_all`, `generate_forcing`,
/// `try_move_digest`, or `null_move` methods for the current
/// position on the board.
fn do_move(&mut self, m: Move) -> Option<u64>;
/// Takes back last played move.
///
/// The move passed to this method **must** be the last move passed
/// to `do_move`.
fn undo_move(&mut self, m: Move);
/// Returns a reference to a static evaluator bound to the current
/// position.
fn evaluator(&self) -> &Self::Evaluator;
/// Returns the likely evaluation change (material) to be lost or
/// gained as a result of a given move.
///
/// This method performs static exchange evaluation (SEE). It
/// examines the consequence of a series of exchanges on the
/// destination square after a given move. A positive returned
/// value indicates a "winning" move. For example, "PxQ" will
/// always be a win, since the pawn side can choose to stop the
/// exchange after its pawn is recaptured, and still be ahead. SEE
/// is just an evaluation calculated without actually trying moves
/// on the board, and therefore the returned value might be
/// incorrect.
///
/// The move passed to this method must have been generated by
/// `generate_all`, `generate_forcing`, `try_move_digest`, or
/// `null_move` methods for the current position on the board.
fn evaluate_move(&self, m: Move) -> Value {
debug_assert!(m.played_piece() < PIECE_NONE);
debug_assert!(m.captured_piece() <= PIECE_NONE);
const PIECE_VALUES: [Value; 8] = [10000, 975, 500, 325, 325, 100, 0, 0];
unsafe {
let mut piece = m.played_piece();
let captured_piece = m.captured_piece();
// Try not to waste CPU cycles when the played piece is
// less valuable than the captured piece.
if piece > captured_piece {
return *PIECE_VALUES.get_unchecked(captured_piece);
}
// This is the square on which all the action takes place.
let exchange_square = m.dest_square();
let color: &[Bitboard; 2] = &self.board().pieces.color;
let piece_type: &[Bitboard; 6] = &self.board().pieces.piece_type;
let file_sliders = piece_type[QUEEN] | piece_type[ROOK];
let diag_sliders = piece_type[QUEEN] | piece_type[BISHOP];
let geometry = BoardGeometry::get();
let behind_blocker: &[Bitboard; 64] = geometry
.squares_behind_blocker
.get_unchecked(exchange_square);
// These variables (along with `piece`) will be updated on each capture:
let mut us = self.board().to_move;
let mut depth = 0;
let mut orig_square_bb = 1 << m.orig_square();
let mut attackers_and_defenders = self.attacks_to(exchange_square);
// The `gain` array will hold the total material gained at
// each `depth`, from the viewpoint of the side that made the
// last capture (`us`).
let mut gain: [Value; 34] = uninitialized();
gain[0] = if m.move_type() == MOVE_PROMOTION {
piece = Move::piece_from_aux_data(m.aux_data());
PIECE_VALUES[captured_piece] + PIECE_VALUES[piece] - PIECE_VALUES[PAWN]
} else {
*PIECE_VALUES.get_unchecked(captured_piece)
};
// Examine the possible exchanges, fill the `gain` array.
'exchange: while orig_square_bb != 0 {
let current_gain = *gain.get_unchecked(depth);
// Store a speculative value that will be used if the
// captured piece happens to be defended.
let speculative_gain: &mut Value = gain.get_unchecked_mut(depth + 1);
*speculative_gain = *PIECE_VALUES.get_unchecked(piece) - current_gain;
if max(-current_gain, *speculative_gain) < 0 {
// The side that made the last capture wins even if
// the captured piece happens to be defended. So, we
// stop here to save precious CPU cycles. Note that
// here we may happen to return an incorrect SEE
// value, but the sign will be correct, which is by
// far the most important information.
break;
}
// Register that capturing piece's origin square is now vacant.
attackers_and_defenders &= !orig_square_bb;
// Consider adding new attackers/defenders, now that
// capturing piece's origin square is vacant.
let behind = self.board().occupied &
*behind_blocker.get_unchecked(bsf(orig_square_bb));
if behind & (file_sliders | diag_sliders) != 0 && piece != KING {
attackers_and_defenders |=
match behind & file_sliders &
geometry.attacks_from_unsafe(ROOK, exchange_square, behind) {
0 => {
// Not a file slider, possibly a diagonal slider.
behind & diag_sliders &
geometry.attacks_from_unsafe(BISHOP, exchange_square, behind)
}
bb => {
// A file slider.
bb
}
};
}
// Change the side to move.
us ^= 1;
// Find the next piece to enter the exchange. (The least
// valuable piece belonging to the side to move.)
let candidates = attackers_and_defenders & *color.get_unchecked(us);
if candidates != 0 {
for p in (KING..PIECE_NONE).rev() {
let bb = candidates & piece_type[p];
if bb != 0 {
depth += 1;
piece = p;
orig_square_bb = lsb(bb);
continue 'exchange;
}
}
}
break 'exchange;
}
// Negamax the `gain` array for the final static exchange
// evaluation. (The `gain` array actually represents an unary
// tree, at each node of which the player can either continue
// the exchange or back off.)
while depth > 0 {
*gain.get_unchecked_mut(depth - 1) = -max(-*gain.get_unchecked(depth - 1),
*gain.get_unchecked(depth));
depth -= 1;
}
gain[0]
}
}
}