feat: better endgame heuristics

it may or may not be able to deliver checkmate

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, ToUCIAlgebraic};
 use chess_inator::search::{best_line, SearchEval};
+use chess_inator::eval::{eval_metrics};
 use chess_inator::Board;
 use std::io;
 
@@ -108,6 +109,12 @@ fn cmd_go(mut _tokens: std::str::SplitWhitespace<'_>, board: &mut Board) {
     }
 }
 
+/// Print static evaluation of the position.
+fn cmd_eval(mut _tokens: std::str::SplitWhitespace<'_>, board: &mut Board) {
+    let res = eval_metrics(board);
+    println!("STATIC EVAL (negative black, positive white):\n- pst: {}\n- king distance: {} ({} distance)\n- phase: {}\n- total: {}", res.pst_eval, res.king_distance_eval, res.king_distance, res.phase, res.total_eval);
+}
+
 fn main() {
     let stdin = io::stdin();
 
@@ -137,6 +144,10 @@ fn main() {
                 "go" => {
                     cmd_go(tokens, &mut board);
                 }
+                // non-standard command.
+                "eval" => {
+                    cmd_eval(tokens, &mut board);
+                }
                 _ => ignore!(),
             }
 

src/eval.rs

@@ -14,7 +14,7 @@ Copyright © 2024 dogeystamp <dogeystamp@disroot.org>
 //! Position evaluation.
 
 use crate::{Board, Color, Piece, Square, N_COLORS, N_PIECES, N_SQUARES};
-use core::cmp::max;
+use core::cmp::{max, min};
 use core::ops::Index;
 
 /// Signed centipawn type.
@@ -306,14 +306,14 @@ pub const PST_ENDGAME: Pst = Pst([
 
     // 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
+     -100,-100, -90, -70, -70, -90,-100,-100, // 8
+     -100, -20, -15, -13, -13, -15, -20,-100, // 7
+      -90, -15,  -5,  -5,  -5,  -5, -15, -90, // 6
+      -70, -13,  -5,   4,   4,  -5, -13, -70, // 5
+      -70, -13,  -5,   4,   4,  -5, -13, -70, // 4
+      -90, -15,  -5,  -5,  -5,  -5, -15, -90, // 3
+     -100, -20, -15, -13, -13, -15, -20,-100, // 2
+     -100,-100, -90, -70, -70, -90,-100,-100, // 1
     //  a    b    c    d    e    f    g    h
     ], 20_000),
 
@@ -344,16 +344,57 @@ pub const PST_ENDGAME: Pst = Pst([
     ], 100),
 ]);
 
+/// Centipawn, signed, eval metrics.
+pub struct EvalMetrics {
+    pub pst_eval: i32,
+    /// Manhattan distance between kings.
+    pub king_distance: usize,
+    pub king_distance_eval: i32,
+    pub total_eval: i32,
+    /// game phase, from 14 (start) to 0 (end endgame).
+    pub phase: i32,
+}
+
+pub fn eval_metrics(board: &Board) -> EvalMetrics {
+    // 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 14 (game start) to 7 (endgame) to 0 (end end game).
+    let phase = i32::from(board.eval.min_maj_pieces);
+
+    // piece square tables
+    let pst_eval = i32::from(board.eval.midgame.score) * max(7, phase - 7) / 7
+        + i32::from(board.eval.endgame.score) * min(14 - phase, 7) / 7;
+
+    // Signed factor of the color with the material advantage.
+    let advantage = pst_eval / 10;
+
+    let kings1 = board[board.turn][Piece::King].into_iter();
+    let kings2 = board[board.turn.flip()][Piece::King].into_iter();
+    let king_distance = kings1
+        .zip(kings2)
+        .next()
+        .map_or(0, |(k1, k2)| k1.manhattan(k2));
+
+    // attempt to minimize king distance for checkmates
+    let king_distance_eval = -advantage * i32::try_from(king_distance).unwrap() * max(7 - phase, 0) / 100;
+
+    let eval = pst_eval + king_distance_eval;
+
+    EvalMetrics {
+        pst_eval,
+        king_distance,
+        king_distance_eval,
+        total_eval: eval,
+        phase,
+    }
+}
+
 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()
+        let res = eval_metrics(self);
+
+        res.total_eval.try_into().unwrap()
     }
 }
 

src/lib.rs

@@ -205,14 +205,14 @@ macro_rules! from_row_col_generic {
 }
 
 impl Square {
-    fn from_row_col(r: usize, c: usize) -> Result<Self, SquareError> {
+    pub fn from_row_col(r: usize, c: usize) -> Result<Self, SquareError> {
         //! Get index of square based on row and column.
         from_row_col_generic!(usize, r, c)
     }
-    fn from_row_col_signed(r: isize, c: isize) -> Result<Self, SquareError> {
+    pub fn from_row_col_signed(r: isize, c: isize) -> Result<Self, SquareError> {
         from_row_col_generic!(isize, r, c)
     }
-    fn to_row_col(self) -> (usize, usize) {
+    pub fn to_row_col(self) -> (usize, usize) {
         //! Get row, column from index
         let div = usize::from(self.0) / BOARD_WIDTH;
         let rem = usize::from(self.0) % BOARD_WIDTH;
@@ -220,19 +220,26 @@ impl Square {
         debug_assert!(rem <= 7);
         (div, rem)
     }
-    fn to_row_col_signed(self) -> (isize, isize) {
+    pub fn to_row_col_signed(self) -> (isize, isize) {
         //! Get row, column (signed) from index
         let (r, c) = self.to_row_col();
         (r.try_into().unwrap(), c.try_into().unwrap())
     }
 
     /// Vertically mirror a square.
-    fn mirror_vert(&self) -> Self {
+    pub fn mirror_vert(&self) -> Self {
         let (r, c) = self.to_row_col();
         let (nr, nc) = (BOARD_HEIGHT - 1 - r, c);
         Square::from_row_col(nr, nc)
             .unwrap_or_else(|e| panic!("mirrored square should be valid: nr {nr} nc {nc}: {e:?}"))
     }
+
+    /// Manhattan (grid-based) distance with another Square.
+    pub fn manhattan(&self, other: Self) -> usize {
+        let (r1, c1) = self.to_row_col();
+        let (r2, c2) = other.to_row_col();
+        r1.abs_diff(r2) + c1.abs_diff(c2)
+    }
 }
 
 impl Display for Square {
@@ -761,4 +768,26 @@ mod tests {
             assert_eq!(tc.flip_colors(), expect);
         }
     }
+
+    #[test]
+    fn manhattan_distance() {
+        let test_cases = [
+            ("a3", "a3", 0),
+            ("a3", "a4", 1),
+            ("a3", "b3", 1),
+            ("a3", "b4", 2),
+            ("a1", "b8", 8),
+        ];
+
+        for (sq_str1, sq_str2, expected) in test_cases {
+            let sq1 = Square::from_str(sq_str1).unwrap();
+            let sq2 = Square::from_str(sq_str2).unwrap();
+            let res = sq1.manhattan(sq2);
+            assert_eq!(
+                res, expected,
+                "failed {sq_str1} and {sq_str2}: got manhattan {}, expected {}",
+                res, expected
+            );
+        }
+    }
 }