feat: add piece-square tables

1 files changed, 45 insertions, 26 deletions

diff --git a/src/grossmeister/evaluation.rs b/src/grossmeister/evaluation.rs
index d55d9c6..b7060ba 100644
--- a/src/grossmeister/evaluation.rs
+++ b/src/grossmeister/evaluation.rs
@@ -16,15 +16,15 @@ const PAWN_BONUS: [f32; 64] = [
      0.00,  0.00,  0.00,  0.00,  0.00,  0.00,  0.00,  0.00
 ];
 
-const PAWN_PASSER_BONUS: [f32; 64] = [
+const PAWN_BONUS_ENDGAME: [f32; 64] = [
   //  A      B      C      D      E      F      G      H
      0.00,  0.00,  0.00,  0.00,  0.00,  0.00,  0.00,  0.00,
      2.00,  2.00,  2.00,  2.00,  2.00,  2.00,  2.00,  2.00,
-     1.50,  1.50,  1.50,  1.50,  1.50,  1.50,  1.50,  1.50,
      1.00,  1.00,  1.00,  1.00,  1.00,  1.00,  1.00,  1.00,
      0.70,  0.70,  0.70,  0.70,  0.70,  0.70,  0.70,  0.70,
      0.50,  0.50,  0.50,  0.50,  0.50,  0.50,  0.50,  0.50,
-     0.20,  0.20,  0.20,  0.20,  0.20,  0.20,  0.20,  0.20,
+     0.30,  0.30,  0.30,  0.30,  0.30,  0.30,  0.30,  0.30,
+     0.10,  0.10,  0.10,  0.10,  0.10,  0.10,  0.10,  0.10,
      0.00,  0.00,  0.00,  0.00,  0.00,  0.00,  0.00,  0.00,
 ];
 
@@ -162,27 +162,6 @@ impl Grossmeister {
         (behind_pawns & (king | king.west_one() | king.east_one())).pop_count() as f32
     }
 
-    pub fn evaluate_endgame(&self) -> f32 {
-        let color = self.board.color();
-        let opponent_color = color.flip();
-
-        let mobility_advantage = self.mobility(color) - self.mobility(opponent_color);
-        let pawn_shield_advantage = self.pawn_shield(color) - self.pawn_shield(opponent_color);
-
-        mobility_advantage * 0.03 + pawn_shield_advantage * 0.20
-    }
-
-
-    pub fn evaluate_middlegame(&self) -> f32 {
-        let color = self.board.color();
-        let opponent_color = color.flip();
-
-        let mobility_advantage = self.mobility(color) - self.mobility(opponent_color);
-        let pawn_shield_advantage = self.pawn_shield(color) - self.pawn_shield(opponent_color);
-
-        mobility_advantage * 0.05 + pawn_shield_advantage * 0.15
-    }
-
     // Returns a value in [0, 240] representing how
     // much material is left in the game
     // Note: not related to actual material counting
@@ -209,6 +188,37 @@ impl Grossmeister {
         })
     }
 
+    /// Compute total bonus for well-positioned pieces
+    /// according to Piece-Square Tables
+    pub fn pst_bonus(&self, color: Color, is_endgame: bool) -> f32 {
+        self.board.pieces_by_color(color).iter().enumerate().fold(0., |total, (piece_index, bitboard)| {
+            let pst = match Piece::from(piece_index).without_color() {
+                Piece::Pawn => if is_endgame {
+                    PAWN_BONUS_ENDGAME
+                } else {
+                    PAWN_BONUS
+                }
+                Piece::Knight => KNIGHT_BONUS,
+                Piece::Bishop => BISHOP_BONUS,
+                Piece::Rook => ROOK_BONUS,
+                Piece::Queen => QUEEN_BONUS,
+                Piece::King => if is_endgame {
+                    KING_BONUS_ENGAME
+                } else {
+                    KING_BONUS
+                }
+                _ => panic!("Unreachable")
+            };
+            total + bitboard.serialize().iter().fold(0., |acc, &square| {
+                let pst_index = match color {
+                    Color::White => square.mirror(),
+                    Color::Black => square,
+                } as usize;
+                acc + pst[pst_index]
+            })
+        })
+    }
+
     /// Evaluate a position relative to the current player
     pub fn evaluate(&self) -> f32 {
         if self.is_dead_position() {
@@ -218,9 +228,18 @@ impl Grossmeister {
         let opponent_color = color.flip();
 
         let material_advantage = self.material(color) - self.material(opponent_color);
+        let mobility_advantage = self.mobility(color) - self.mobility(opponent_color);
+        let pawn_shield_advantage = self.pawn_shield(color) - self.pawn_shield(opponent_color);
+
+        let middlegame_eval =
+            mobility_advantage * 0.05 +
+            pawn_shield_advantage * 0.15 +
+            (self.pst_bonus(color, false) - self.pst_bonus(opponent_color, false));
 
-        let middlegame_eval = self.evaluate_middlegame();
-        let endgame_eval = self.evaluate_endgame();
+        let endgame_eval =
+            mobility_advantage * 0.03 +
+            pawn_shield_advantage * 0.20 +
+            (self.pst_bonus(color, true) - self.pst_bonus(opponent_color, true));
 
         let phase = self.phase();
         let tapered_eval = (middlegame_eval * phase as f32 + endgame_eval * (240 - phase) as f32) / 240.;