feat: basic basic search

src/bin/engine.rs

@@ -15,6 +15,7 @@ Copyright © 2024 dogeystamp <dogeystamp@disroot.org>
 
 use chess_inator::fen::FromFen;
 use chess_inator::movegen::{FromUCIAlgebraic, Move, MoveGen, MoveGenType, ToUCIAlgebraic};
+use chess_inator::search::best_move;
 use chess_inator::Board;
 use std::io;
 
@@ -87,8 +88,7 @@ fn cmd_position(mut tokens: std::str::SplitWhitespace<'_>) -> Board {
 
 /// Play the game.
 fn cmd_go(mut _tokens: std::str::SplitWhitespace<'_>, board: &mut Board) {
-    let mvs: Vec<_> = board.gen_moves(MoveGenType::Legal).into_iter().collect();
-    let chosen = mvs.first();
+    let chosen = best_move(board);
     match chosen {
         Some(mv) => println!("bestmove {}", mv.to_uci_algebraic()),
         None => println!("bestmove 0000"),

src/eval.rs

@@ -18,7 +18,7 @@ use crate::{Board, Color, N_PIECES};
 /// Signed centipawn type.
 ///
 /// Positive is good for White, negative good for Black.
-type EvalInt = i16;
+pub type EvalInt = i16;
 
 pub trait Eval {
     /// Evaluate a position and assign it a score.

src/lib.rs

@@ -19,6 +19,7 @@ use std::str::FromStr;
 pub mod eval;
 pub mod fen;
 pub mod movegen;
+pub mod search;
 
 use crate::fen::{FromFen, ToFen, START_POSITION};
 

src/movegen.rs

@@ -96,7 +96,7 @@ pub struct AntiMove {
 
 impl AntiMove {
     /// Undo the move.
-    fn unmake(self, pos: &mut Board) {
+    pub fn unmake(self, pos: &mut Board) {
         pos.move_piece(self.dest, self.src);
         pos.half_moves = self.half_moves;
         pos.castle = self.castle;

src/search.rs

@@ -0,0 +1,69 @@
+/*
+
+This file is part of chess_inator.
+
+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.
+
+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.
+
+You should have received a copy of the GNU General Public License along with chess_inator. If not, see https://www.gnu.org/licenses/.
+
+Copyright © 2024 dogeystamp <dogeystamp@disroot.org>
+*/
+
+//! Game-tree search.
+
+use crate::eval::{Eval, EvalInt};
+use crate::movegen::{Move, MoveGen, MoveGenType};
+use crate::Board;
+use std::cmp::max;
+
+/// Search the game tree to find the absolute (positive good) eval for the current player.
+fn minmax(board: &mut Board, depth: usize) -> EvalInt {
+    if depth == 0 {
+        let eval = board.eval();
+        match board.turn {
+            crate::Color::White => return eval,
+            crate::Color::Black => return -eval,
+        }
+    }
+
+    let mvs: Vec<_> = board.gen_moves(MoveGenType::Legal).into_iter().collect();
+
+    let mut abs_best = EvalInt::MIN;
+
+    for mv in mvs {
+        let anti_mv = mv.make(board);
+        abs_best = max(abs_best, -minmax(board, depth - 1));
+        anti_mv.unmake(board);
+    }
+
+    abs_best
+}
+
+/// Find the best move for a position (internal interface).
+fn search(board: &mut Board) -> Option<Move> {
+    const DEPTH: usize = 4;
+    let mvs: Vec<_> = board.gen_moves(MoveGenType::Legal).into_iter().collect();
+
+    // absolute eval value
+    let mut best_eval = EvalInt::MIN;
+    let mut best_mv: Option<Move> = None;
+
+    for mv in mvs {
+        let anti_mv = mv.make(board);
+        let abs_eval = -minmax(board, DEPTH);
+        if abs_eval > best_eval {
+            best_eval = abs_eval;
+            best_mv = Some(mv);
+        }
+        anti_mv.unmake(board);
+    }
+
+    best_mv
+}
+
+/// Find the best move.
+pub fn best_move(board: &mut Board) -> Option<Move> {
+    search(board)
+}