feat: basic basic eval

src/eval.rs

@@ -0,0 +1,72 @@
+/*
+
+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>
+*/
+
+//! Position evaluation.
+
+use crate::{Board, Color, N_PIECES};
+
+/// Signed centipawn type.
+///
+/// Positive is good for White, negative good for Black.
+type EvalInt = i16;
+
+pub trait Eval {
+    /// Evaluate a position and assign it a score.
+    fn eval(&self) -> EvalInt;
+}
+
+impl Eval for Board {
+    fn eval(&self) -> EvalInt {
+        use crate::Piece::*;
+        let mut score: EvalInt = 0;
+
+        // scores in centipawns for each piece
+        let material_score: [EvalInt; N_PIECES] = [
+            500,   // rook
+            300,   // bishop
+            300,   // knight
+            20000, // king
+            900,   // queen
+            100,   // pawn
+        ];
+
+        for pc in [Rook, Queen, Pawn, Knight, Bishop, King] {
+            let tally_white = self.pl(Color::White).board(pc).0.count_ones();
+            let tally_black = self.pl(Color::Black).board(pc).0.count_ones();
+            let tally =
+                EvalInt::try_from(tally_white).unwrap() - EvalInt::try_from(tally_black).unwrap();
+
+            score += material_score[pc as usize] * tally;
+        }
+
+        score
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::fen::FromFen;
+
+    /// Sanity check.
+    #[test]
+    fn test_eval() {
+        let board1 = Board::from_fen("4k3/8/8/8/8/8/PPPPPPPP/RNBQKBNR w KQ - 0 1").unwrap();
+        let eval1 = board1.eval();
+        let board2 = Board::from_fen("rnbqkbnr/pppppppp/8/8/8/8/8/4K3 w kq - 0 1").unwrap();
+        let eval2 = board2.eval();
+
+        assert!(eval1 > 0, "got eval {eval1}");
+        assert!(eval2 < 0, "got eval {eval2}");
+    }
+}

src/lib.rs

@@ -16,6 +16,7 @@ Copyright © 2024 dogeystamp <dogeystamp@disroot.org>
 use std::fmt::Display;
 use std::str::FromStr;
 
+pub mod eval;
 pub mod fen;
 pub mod movegen;