70 lines
2.1 KiB
Rust
70 lines
2.1 KiB
Rust
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/*
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This file is part of chess_inator.
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chess_inator is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3 of the License, or (at your option) any later version.
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chess_inator is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
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You should have received a copy of the GNU General Public License along with chess_inator. If not, see https://www.gnu.org/licenses/.
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Copyright © 2024 dogeystamp <dogeystamp@disroot.org>
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*/
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//! Game-tree search.
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use crate::eval::{Eval, EvalInt};
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use crate::movegen::{Move, MoveGen, MoveGenType};
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use crate::Board;
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use std::cmp::max;
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/// Search the game tree to find the absolute (positive good) eval for the current player.
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fn minmax(board: &mut Board, depth: usize) -> EvalInt {
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if depth == 0 {
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let eval = board.eval();
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match board.turn {
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crate::Color::White => return eval,
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crate::Color::Black => return -eval,
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}
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}
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let mvs: Vec<_> = board.gen_moves(MoveGenType::Legal).into_iter().collect();
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let mut abs_best = EvalInt::MIN;
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for mv in mvs {
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let anti_mv = mv.make(board);
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abs_best = max(abs_best, -minmax(board, depth - 1));
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anti_mv.unmake(board);
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}
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abs_best
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}
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/// Find the best move for a position (internal interface).
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fn search(board: &mut Board) -> Option<Move> {
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const DEPTH: usize = 4;
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let mvs: Vec<_> = board.gen_moves(MoveGenType::Legal).into_iter().collect();
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// absolute eval value
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let mut best_eval = EvalInt::MIN;
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let mut best_mv: Option<Move> = None;
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for mv in mvs {
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let anti_mv = mv.make(board);
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let abs_eval = -minmax(board, DEPTH);
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if abs_eval > best_eval {
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best_eval = abs_eval;
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best_mv = Some(mv);
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}
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anti_mv.unmake(board);
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}
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best_mv
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}
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/// Find the best move.
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pub fn best_move(board: &mut Board) -> Option<Move> {
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search(board)
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}
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