feat: calculate pawn structure penalty

1 files changed, 41 insertions, 1 deletions

diff --git a/src/board/engine.rs b/src/board/engine.rs
index 18fb731..2ff3aed 100644
--- a/src/board/engine.rs
+++ b/src/board/engine.rs
@@ -3,6 +3,8 @@ use crate::{bitboard::pop_count, board::*};
 
 use super::ttable::{NodeType, TranspositionTableItem};
 
+static A_FILE:     Bitboard = 0x0101010101010101;
+
 #[derive(Debug, Default, PartialEq)]
 pub struct PerftResult {
     leaf_nodes: u64,
@@ -116,6 +118,42 @@ impl Board {
         eval
     }
 
+    /// Returns sum of the doubled, blocked and isolated pawns
+    /// The greater result is, the worse is the pawn structure
+    pub fn pawn_structure_penalty(&self, color: Color) -> f32 {
+        let mut result = 0.0;
+
+        let pawns = match color {
+            Color::White => self.pieces[PieceType::Pawn as usize],
+            Color::Black => self.pieces[PieceType::PawnBlack as usize],
+        };
+
+        for file in 0..8 {
+            let file_mask = A_FILE << file;
+            let pawns_on_file = pop_count(pawns & file_mask) as f32;
+
+            // Doubled pawns (-1 because one pawn on a file is ok)
+            result += (pawns_on_file - 1.).max(0.0);
+
+            // Isolated pawns (no pawns on neighbor files)
+            if [
+                A_FILE << (file - 1).max(0), // File to the left (if any)
+                A_FILE << (file + 1).min(7), // File to the right (if any)
+            ].iter().all(|file| file & pawns == 0) {
+                result += pawns_on_file;
+            }
+        }
+
+        // Blocked pawns
+        let blocked_mask = match color {
+            Color::White => self.occupancy >> 8,
+            Color::Black => self.occupancy << 8,
+        };
+        result += pop_count(pawns & blocked_mask) as f32;
+
+        result
+    }
+
     /// Evaluate a position relative to the current player
     pub fn evaluate(&self, precomputed_mobility: Option<f32>) -> f32 {
         let opponent_mobility = self.mobility(self.color().flip());
@@ -127,7 +165,9 @@ impl Board {
 
         let material_advantage = self.material_advantage();
 
-        material_advantage + 0.1 * mobility_advantage
+        let pawn_structure_penalty = self.pawn_structure_penalty(self.color()) - self.pawn_structure_penalty(self.color().flip());
+
+        material_advantage + 0.1 * mobility_advantage - 0.5 * pawn_structure_penalty
     }
 
     /// Evaluate move for move ordering, prioritizing efficient captures