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#![no_std]
#[macro_use]
extern crate alloc;
use alloc::{
string::{String, ToString},
vec::Vec,
};
use core::convert::TryFrom;
mod board;
pub use board::{Board, BoardBuilder};
mod square;
pub use square::{Square, EMPTY_SQUARE};
mod piece;
pub use piece::Piece;
mod position;
pub use position::*;
pub const WHITE: Color = Color::White;
pub const BLACK: Color = Color::Black;
/// The result of a move being played on the board.
#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
pub enum GameResult {
/// The game is not finished, and the game is still in play.
Continuing(Board),
/// One player, the victor, checkmated the other.
/// This stores the color of the winner.
Victory(Color),
/// The game is drawn. This can be a result of the current player
/// having no legal moves and not being in check, or because
/// both players have insufficient material on the board.
///
/// Insufficient material consists of:
/// 1. The player only has a king
/// 2. The player only has a king and a knight
/// 3. The player only has a king and two knights
/// 4. The player only has a king and a bishop
/// 5. The player only has a king and two bishops
///
/// In a regular game of chess, threefold repetition also triggers
/// a stalemate, but this engine does not have builtin support for
/// threefold repetition detection yet.
Stalemate,
/// An illegal move was made. This can include many things,
/// such as moving a piece through another piece, attempting
/// to capture an allied piece, moving non-orthogonally or
/// non-diagonally, or non-knight-like according the rules
/// governing the movement of the piece. Additionally,
/// moves that put the player in check, (for example, moving a pinned piece),
/// are also illegal.
IllegalMove(Move),
}
/// The color of a piece.
#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
pub enum Color {
White,
Black,
}
impl core::fmt::Display for Color {
fn fmt(&self, f: &mut core::fmt::Formatter) -> Result<(), core::fmt::Error> {
write!(
f,
"{}",
match self {
Self::White => "White",
Self::Black => "Black",
}
)
}
}
/// A color can be inverted using the `!` operator.
/// `!Color::White` becomes `Color::Black` and vice versa.
impl core::ops::Not for Color {
type Output = Self;
fn not(self) -> Self {
match self {
Self::White => Self::Black,
Self::Black => Self::White,
}
}
}
/// A move that can be applied to a board.
/// When applied to a board, the board assumes that the move is
/// being applied for the current turn's player.
#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
pub enum Move {
/// If the current player is white, move the king to the C1 square, and the kingside rook to
/// the D1 square. If the current player is black, however, move the king to the C8 square,
/// and the kingside rook to the D8 square.
///
/// Castling can only be performed if
/// 1. The king has not moved at all since the game began
/// 2. The respective rook (kingside or queenside) has also not moved
/// 3. The square adjacent to the king on the respective side is not threatened by an enemy piece
///
/// If all of these conditions are satisfied, castling is a legal move
QueenSideCastle,
/// If the current player is white, move the king to the G1 square, and the kingside rook to
/// the F1 square. If the current player is black, however, move the king to the G8 square,
/// and the kingside rook to the F8 square.
KingSideCastle,
/// Move a piece from one square to another.
/// This can allow the player to capture another piece, by
/// simply moving a piece to the position of an enemy piece.
///
/// Additionally, this can be used to [en-passant capture](https://en.wikipedia.org/wiki/En_passant),
/// even though the en-passant square itself does not contain any capturable pieces.
///
/// En-passant captures MUST be performed with a pawn, upon an enemy pawn
/// that has just surpassed it by move two squares. An en-passant capture
/// must also be performed the turn immediately after the enemy pawn surpasses
/// the allied pawn. After the one turn a player has to en-passant capture, the
/// en-passant square is forgotten and can no longer be used.
Piece(Position, Position),
/// When played by another player, it awards victory to the other.
Resign,
}
/// Try to parse a Move from a string.
///
/// Possible valid formats include:
/// - `"resign"`
/// - `"resigns"`
/// - `"castle queenside"`
/// - `"O-O-O"` (correct notation)
/// - `"o-o-o"` (incorrect notation, but will accept)
/// - `"0-0-0"` (incorrect notation, but will accept)
/// - `"castle kingside"`
/// - `"O-O"` (correct notation)
/// - `"o-o"` (incorrect notation, but will accept)
/// - `"0-0"` (incorrect notation, but will accept)
/// - `"e2e4"`
/// - `"e2 e4"`
/// - `"e2 to e4"`
///
/// Parsing a move such as `"knight to e4"` or `"Qxe4"` will NOT work.
impl TryFrom<String> for Move {
type Error = String;
fn try_from(repr: String) -> Result<Self, Self::Error> {
let repr = repr.trim().to_string();
Ok(match repr.as_str() {
"resign" | "resigns" => Self::Resign,
"queenside castle" | "castle queenside" | "O-O-O" | "0-0-0" | "o-o-o" => {
Self::QueenSideCastle
}
"kingside castle" | "castle kingside" | "O-O" | "0-0" | "o-o" => Self::KingSideCastle,
other => {
let words = other.split_whitespace().collect::<Vec<&str>>();
if words.len() == 1 && words[0].len() == 4 {
Self::Piece(
Position::pgn(&words[0][..2])?,
Position::pgn(&words[0][2..4])?,
)
} else if words.len() == 2 {
Self::Piece(Position::pgn(&words[0])?, Position::pgn(&words[1])?)
} else if words.len() == 3 && words[1] == "to" {
Self::Piece(Position::pgn(&words[0])?, Position::pgn(&words[2])?)
} else {
return Err(format!("invalid move format `{}`", other));
}
}
})
}
}
impl Move {
/// Try to parse a Move from a string.
///
/// Possible valid formats include:
/// - `"resign"`
/// - `"resigns"`
/// - `"castle queenside"`
/// - `"O-O-O"` (correct notation)
/// - `"o-o-o"` (incorrect notation, but will accept)
/// - `"0-0-0"` (incorrect notation, but will accept)
/// - `"castle kingside"`
/// - `"O-O"` (correct notation)
/// - `"o-o"` (incorrect notation, but will accept)
/// - `"0-0"` (incorrect notation, but will accept)
/// - `"e2e4"`
/// - `"e2 e4"`
/// - `"e2 to e4"`
///
/// Parsing a move such as `"knight to e4"` or `"Qxe4"` will NOT work.
pub fn parse(repr: String) -> Result<Self, String> {
Self::try_from(repr)
}
}
impl core::fmt::Display for Move {
fn fmt(&self, f: &mut core::fmt::Formatter) -> Result<(), core::fmt::Error> {
match self {
// Move::EnPassant(from) => write!(f, "ep {}", from),
Move::Piece(from, to) => write!(f, "{} to {}", from, to),
Move::KingSideCastle => write!(f, "O-O"),
Move::QueenSideCastle => write!(f, "O-O-O"),
Move::Resign => write!(f, "Resign"),
}
}
}
/// Evaluate a board and extract information, such as the best and worst moves.
pub trait Evaluate: Sized {
/// Get the value of the board for a given color.
/// This subtracts the opponents value, and accounts for piece positions
/// and material value.
fn value_for(&self, color: Color) -> f64;
/// Get the current player's color.
fn get_current_player_color(&self) -> Color;
/// Get the legal moves for the current player.
fn get_legal_moves(&self) -> Vec<Move>;
/// Apply a move to the board for evaluation.
fn apply_eval_move(&self, m: Move) -> Self;
/// Get the best move for the current player with `depth` number of moves
/// of lookahead.
///
/// This method returns
/// 1. The best move
/// 2. The number of boards evaluated to come to a conclusion
/// 3. The rating of the best move
///
/// It's best not to use the rating value by itself for anything, as it
/// is relative to the other player's move ratings as well.
fn get_best_next_move(&self, depth: i32) -> (Move, u64, f64) {
let legal_moves = self.get_legal_moves();
let mut best_move_value = -999999.0;
let mut best_move = Move::Resign;
let color = self.get_current_player_color();
let mut board_count = 0;
for m in &legal_moves {
let child_board_value = self.apply_eval_move(*m).minimax(
depth,
-1000000.0,
1000000.0,
false,
color,
&mut board_count,
);
if child_board_value >= best_move_value {
best_move = *m;
best_move_value = child_board_value;
}
}
(best_move, board_count, best_move_value)
}
/// Get the best move for the current player with `depth` number of moves
/// of lookahead.
///
/// This method returns
/// 1. The best move
/// 2. The number of boards evaluated to come to a conclusion
/// 3. The rating of the best move
///
/// It's best not to use the rating value by itself for anything, as it
/// is relative to the other player's move ratings as well.
fn get_worst_next_move(&self, depth: i32) -> (Move, u64, f64) {
let legal_moves = self.get_legal_moves();
let mut best_move_value = -999999.0;
let mut best_move = Move::Resign;
let color = self.get_current_player_color();
let mut board_count = 0;
for m in &legal_moves {
let child_board_value = self.apply_eval_move(*m).minimax(
depth,
-1000000.0,
1000000.0,
true,
!color,
&mut board_count,
);
if child_board_value >= best_move_value {
best_move = *m;
best_move_value = child_board_value;
}
}
(best_move, board_count, best_move_value)
}
/// Perform minimax on a certain position, and get the minimum or maximum value
/// for a board. To get the best move, you minimize the values of the possible outcomes from your
/// own position, and maximize the values of the replies made by the other player.
///
/// In other words, choose moves with the assumption that your opponent will make the
/// best possible replies to your moves. Moves that are seemingly good, but are easily countered,
/// are categorically eliminated by this algorithm.
fn minimax(
&self,
depth: i32,
mut alpha: f64,
mut beta: f64,
is_maximizing: bool,
getting_move_for: Color,
board_count: &mut u64,
) -> f64 {
*board_count += 1;
if depth == 0 {
return self.value_for(getting_move_for);
}
let legal_moves = self.get_legal_moves();
let mut best_move_value;
if is_maximizing {
best_move_value = -999999.0;
for m in &legal_moves {
let child_board_value = self.apply_eval_move(*m).minimax(
depth - 1,
alpha,
beta,
!is_maximizing,
getting_move_for,
board_count,
);
if child_board_value > best_move_value {
best_move_value = child_board_value;
}
if best_move_value > alpha {
alpha = best_move_value
}
if beta <= alpha {
return best_move_value;
}
}
} else {
best_move_value = 999999.0;
for m in &legal_moves {
let child_board_value = self.apply_eval_move(*m).minimax(
depth - 1,
alpha,
beta,
!is_maximizing,
getting_move_for,
board_count,
);
if child_board_value < best_move_value {
best_move_value = child_board_value;
}
if best_move_value < beta {
beta = best_move_value
}
if beta <= alpha {
return best_move_value;
}
}
}
best_move_value
}
}