/* 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 */ //! Position evaluation. use crate::{Board, Color, Piece, Square, N_COLORS, N_PIECES, N_SQUARES}; use core::cmp::max; use core::ops::Index; /// Signed centipawn type. /// /// Positive is good for White, negative good for Black. pub type EvalInt = i16; pub trait Eval { /// Evaluate a position and assign it a score. /// /// Negative for Black advantage and positive for White. fn eval(&self) -> EvalInt; } pub(crate) mod eval_score { //! Opaque "score" counters to be used in the board. use super::{EvalInt, PST_ENDGAME, PST_MIDGAME}; use crate::{ColPiece, Square}; /// Internal score-keeping for a board. /// /// This is kept in order to efficiently update evaluation with moves. #[derive(PartialEq, Eq, PartialOrd, Ord, Clone, Copy, Default, Debug)] pub struct EvalScores { /// Middle-game perspective evaluation of this board. pub midgame: EvalScore, /// End-game perspective evaluation of this board. pub endgame: EvalScore, /// Non-pawn/king piece count, used to determine when the endgame has begun. pub min_maj_pieces: u8, } impl EvalScores { /// Add/remove the value of a piece based on the PST. /// /// Use +1 as sign to add, -1 to delete. fn change_piece(&mut self, pc: ColPiece, sq: Square, sign: i8) { assert!(sign == 1 || sign == -1); let tables = [ (&mut self.midgame, PST_MIDGAME), (&mut self.endgame, PST_ENDGAME), ]; for (phase, pst) in tables { phase.score += pst[pc.pc][pc.col][sq] * EvalInt::from(pc.col.sign() * sign); } use crate::Piece::*; if matches!(pc.pc, Rook | Queen | Knight | Bishop) { match sign { -1 => self.min_maj_pieces -= 1, 1 => self.min_maj_pieces += 1, _ => panic!(), } } } /// Remove the value of a piece on a square. pub fn del_piece(&mut self, pc: ColPiece, sq: Square) { self.change_piece(pc, sq, -1); } /// Add the value of a piece on a square. pub fn add_piece(&mut self, pc: ColPiece, sq: Square) { self.change_piece(pc, sq, 1); } } /// Score from a given perspective (e.g. midgame, endgame). #[derive(PartialEq, Eq, PartialOrd, Ord, Clone, Copy, Default, Debug)] pub struct EvalScore { /// Signed score. pub score: EvalInt, } } /// The main piece-square-table (PST) type that assigns scores to pieces on given squares. /// /// This is the main source of positional knowledge, as well as the ability to count material. pub struct Pst([PstPiece; N_PIECES]); /// A PST for a specific piece. type PstPiece = [PstSide; N_COLORS]; /// A PST for a given piece, of a given color. type PstSide = [EvalInt; N_SQUARES]; impl Index for Pst { type Output = PstPiece; fn index(&self, index: Piece) -> &Self::Output { &self.0[index as usize] } } impl Index for PstPiece { type Output = PstSide; fn index(&self, index: Color) -> &Self::Output { &self[index as usize] } } impl Index for PstSide { type Output = EvalInt; fn index(&self, index: Square) -> &Self::Output { &self[usize::from(index)] } } #[rustfmt::skip] const PERSPECTIVE_WHITE: [usize; N_SQUARES] = [ 56, 57, 58, 59, 60, 61, 62, 63, 48, 49, 50, 51, 52, 53, 54, 55, 40, 41, 42, 43, 44, 45, 46, 47, 32, 33, 34, 35, 36, 37, 38, 39, 24, 25, 26, 27, 28, 29, 30, 31, 16, 17, 18, 19, 20, 21, 22, 23, 8, 9, 10, 11, 12, 13, 14, 15, 0, 1, 2, 3, 4, 5, 6, 7, ]; /// This perspective is also horizontally reversed so the king is on the right side. #[rustfmt::skip] const PERSPECTIVE_BLACK: [usize; N_SQUARES] = [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, ]; /// Helper to have the right board perspective in the source code. /// /// In the source code, a1 will be at the bottom left, while h8 will be at the top right, /// corresponding to how humans usually see the board. This means that a8 is index 0, and h1 is /// index 63. This function shifts it so that a1 is 0, and h8 is 63, as in our implementation. /// /// # Arguments /// * pst: Square values in centipawns. /// * base_val: The base value of the piece, which is added to every square. const fn pst_perspective( pst: PstSide, base_val: EvalInt, perspective: [usize; N_SQUARES], ) -> PstSide { let mut ret = pst; let mut i = 0; while i < N_SQUARES { let j = perspective[i]; ret[i] = pst[j] + base_val; i += 1; } ret } /// Construct PSTs for a single piece, from white's perspective. const fn make_pst(val: PstSide, base_val: EvalInt) -> PstPiece { [ pst_perspective(val, base_val, PERSPECTIVE_WHITE), pst_perspective(val, base_val, PERSPECTIVE_BLACK), ] } /// Middle-game PSTs. #[rustfmt::skip] pub const PST_MIDGAME: Pst = Pst([ // rook make_pst([ 1, 3, 2, 1, 4, 3, 2, 1, // 8 20, 20, 20, 20, 20, 20, 20, 20, // 7 1, 2, 3, 1, 2, 1, 2, 1, // 6 -1, -2, 1, 2, 1, -1, 1, -1, // 5 -1, -1, 2, -1, 1, -1, 2, 1, // 4 2, 1, 1, 0, 0, 0, 0, 0, // 3 0, 0, 0, 0, 0, 0, 0, 0, // 2 0, 0, 0, 10, 10, 5, 0, 0, // 1 // a b c d e f g h ], 500), // bishop make_pst([ 0, 0, 0, 0, 0, 0, 0, 0, // 8 0, 0, 0, 0, 0, 0, 0, 0, // 7 0, 0, 0, 0, 0, 0, 0, 0, // 6 0, 0, 0, 0, 0, 0, 0, 0, // 5 0, 0, 0, 0, 0, 0, 0, 0, // 4 0, 0, 0, 0, 0, 0, 0, 0, // 3 0, 0, 0, 0, 0, 0, 0, 0, // 2 0, 0, -10, 0, 0, -10, 0, 0, // 1 // a b c d e f g h ], 300), // knight make_pst([ -5, -5, -5, -5, -5, -5, -5, -5, // 8 -5, 0, 0, 0, 0, 0, 0, -5, // 7 -5, 1, 0, 0, 0, 0, 0, -5, // 6 -5, 2, 0, 10, 10, 0, 0, -5, // 5 -5, 0, 1, 10, 10, 0, 0, -5, // 4 -5, 2, 10, 0, 0, 10, 0, -5, // 3 -5, 1, 0, 0, 0, 0, 0, -5, // 2 -5, -5, -5, -5, -5, -5, -5, -5, // 1 // a b c d e f g h ], 300), // king make_pst([ 0, 0, 0, 0, 0, 0, 0, 0, // 8 0, 0, 0, 0, 0, 0, 0, 0, // 7 0, 0, 0, 0, 0, 0, 0, 0, // 6 0, 0, 0, 0, 0, 0, 0, 0, // 5 0, 0, 0, 0, 0, 0, 0, 0, // 4 0, 0, 0, 0, 0, 0, 0, 0, // 3 0, 0, 0, 0, 0, 0, 0, 0, // 2 0, 0, 10, 0, 0, 0, 20, 0, // 1 // a b c d e f g h ], 20_000), // queen make_pst([ 0, 0, 0, 0, 0, 0, 0, 0, // 8 0, 0, 0, 0, 0, 0, 0, 0, // 7 0, 0, 0, 0, 0, 0, 0, 0, // 6 0, 0, 0, 0, 0, 0, 0, 0, // 5 0, 0, 0, 0, 0, 0, 0, 0, // 4 0, 0, 0, 0, 0, 0, 0, 0, // 3 0, 0, 0, 0, 0, 0, 0, 0, // 2 0, 0, 0, 0, 0, 0, 0, 0, // 1 // a b c d e f g h ], 900), // pawn make_pst([ 10, 10, 10, 10, 10, 10, 10, 10, // 8 9, 9, 9, 9, 9, 9, 9, 9, // 7 8, 8, 8, 8, 8, 8, 8, 8, // 6 7, 7, 7, 8, 8, 7, 7, 7, // 5 6, 6, 6, 6, 6, 6, 6, 6, // 4 2, 2, 2, 4, 4, 0, 2, 0, // 3 0, 0, 0, 0, 0, 0, 0, 0, // 2 0, 0, 0, 0, 0, 0, 0, 0, // 1 // a b c d e f g h ], 100), ]); #[rustfmt::skip] pub const PST_ENDGAME: Pst = Pst([ // rook make_pst([ 0, 0, 0, 0, 0, 0, 0, 0, // 8 0, 0, 0, 0, 0, 0, 0, 0, // 7 1, 2, 3, 1, 2, 1, 2, 1, // 6 1, 2, 1, 2, 1, 1, 1, 1, // 5 1, 1, 2, 1, 1, 1, 2, 1, // 4 2, 1, 1, 0, 0, 0, 0, 0, // 3 0, 0, 0, 0, 0, 0, 0, 0, // 2 0, 0, 0, 0, 0, 0, 0, 0, // 1 // a b c d e f g h ], 500), // bishop make_pst([ 0, 0, 0, 0, 0, 0, 0, 0, // 8 0, 0, 0, 0, 0, 0, 0, 0, // 7 0, 0, 0, 0, 0, 0, 0, 0, // 6 0, 0, 0, 0, 0, 0, 0, 0, // 5 0, 0, 0, 0, 0, 0, 0, 0, // 4 0, 0, 0, 0, 0, 0, 0, 0, // 3 0, 0, 0, 0, 0, 0, 0, 0, // 2 0, 0, 0, 0, 0, 0, 0, 0, // 1 // a b c d e f g h ], 300), // knight make_pst([ -5, -5, -5, -5, -5, -5, -5, -5, // 8 -5, 0, 0, 0, 0, 0, 0, -5, // 7 -5, 0, 0, 0, 0, 0, 0, -5, // 6 -5, 0, 0, 0, 0, 0, 0, -5, // 5 -5, 0, 0, 0, 0, 0, 0, -5, // 4 -5, 0, 0, 0, 0, 0, 0, -5, // 3 -5, 0, 0, 0, 0, 0, 0, -5, // 2 -5, -5, -5, -5, -5, -5, -5, -5, // 1 // a b c d e f g h ], 300), // king make_pst([ -50, -50, -50, -50, -50, -50, -50, -50, // 8 -50, -10, -10, -10, -10, -10, -10, -50, // 7 -50, -10, 0, 0, 0, 0, -10, -50, // 6 -50, -10, 0, 4, 4, 0, -10, -50, // 5 -50, -10, 0, 4, 4, 0, -10, -50, // 4 -50, -10, 0, 0, 0, 0, -10, -50, // 3 -50, -10, -10, -10, -10, -10, -10, -50, // 2 -50, -50, -50, -50, -50, -50, -50, -50, // 1 // a b c d e f g h ], 20_000), // queen make_pst([ 0, 0, 0, 0, 0, 0, 0, 0, // 8 0, 0, 0, 0, 0, 0, 0, 0, // 7 0, 0, 0, 0, 0, 0, 0, 0, // 6 0, 0, 0, 0, 0, 0, 0, 0, // 5 0, 0, 0, 0, 0, 0, 0, 0, // 4 0, 0, 0, 0, 0, 0, 0, 0, // 3 0, 0, 0, 0, 0, 0, 0, 0, // 2 0, 0, 0, 0, 0, 0, 0, 0, // 1 // a b c d e f g h ], 900), // pawn make_pst([ 10, 10, 10, 10, 10, 10, 10, 10, // 8 139, 139, 139, 139, 139, 139, 139, 139, // 7 138, 138, 138, 138, 138, 138, 138, 108, // 6 67, 67, 67, 68, 68, 67, 67, 67, // 5 36, 36, 36, 36, 36, 36, 36, 36, // 4 32, 32, 32, 34, 34, 30, 32, 30, // 3 0, 0, 0, 0, 0, 0, 0, 0, // 2 0, 0, 0, 0, 0, 0, 0, 0, // 1 // a b c d e f g h ], 100), ]); impl Eval for Board { fn eval(&self) -> EvalInt { // we'll define endgame as the moment when there are 7 non pawn/king pieces left on the // board in total. // // `phase` will range from 7 (game start) to 0 (endgame). let phase = i32::from(self.eval.min_maj_pieces.saturating_sub(7)); let eval = i32::from(self.eval.midgame.score) * phase / 7 + i32::from(self.eval.endgame.score) * max(7 - phase, 0) / 7; eval.try_into().unwrap() } } #[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} ({:?})", board1.eval); assert!(eval2 < 0, "got eval {eval2} ({:?})", board2.eval); } }