feat: prioritize equal captures over non-captures

2 files changed, 31 insertions, 15 deletions

diff --git a/src/board/engine.rs b/src/board/engine.rs
index 47890ad..05cb1e7 100644
--- a/src/board/engine.rs
+++ b/src/board/engine.rs
@@ -1,4 +1,4 @@
-use std::{time::{Instant, Duration}, f32::INFINITY};
+use std::{time::{Instant, Duration}, f32::INFINITY, cmp::Ordering};
 use crate::{bitboard::pop_count, board::*};
 
 use super::ttable::{NodeType, TranspositionTableItem};
@@ -173,7 +173,7 @@ impl Board {
     /// Evaluate move for move ordering, prioritizing efficient captures
     /// where low-value pieces capture high-value pieces
     fn eval_move(&self, m: Move) -> f32 {
-        if m.kind == MoveKind::Capture || m.kind == MoveKind::EnPassant {
+        if m.is_tactical() {
             let [source_eval, target_eval] = [m.source, m.target]
                 .map(|sq| self.piece_by_square(sq))
                 .map(|p| {
@@ -187,17 +187,29 @@ impl Board {
         0.0
     }
 
-    pub fn negamax_search(&mut self, mut alpha: f32, beta: f32, depth_left: u8, deadline: Instant) -> (f32, Vec<Move>) {
-        let mut principal_variation = Vec::new();
-        let color = self.color();
-
-        let mut moves = self.generate_pseudolegal_moves(color);
-
+    pub fn order_moves(&self, moves: &mut Vec<Move>) {
         moves.sort_unstable_by(|a, b| {
             let a_eval = self.eval_move(*a);
             let b_eval = self.eval_move(*b);
+            if a_eval == 0.0 && b_eval == 0.0 {
+                // Prioritize equal captures over non-captures
+                if a.is_tactical() && !b.is_tactical() {
+                    return Ordering::Less
+                }
+                if b.is_tactical() && !a.is_tactical() {
+                    return Ordering::Greater
+                }
+            }
             a_eval.total_cmp(&b_eval).reverse()
         });
+    }
+
+    pub fn negamax_search(&mut self, mut alpha: f32, beta: f32, depth_left: u8, deadline: Instant) -> (f32, Vec<Move>) {
+        let mut principal_variation = Vec::new();
+        let color = self.color();
+
+        let mut moves = self.generate_pseudolegal_moves(color);
+        self.order_moves(&mut moves);
 
         match self.transposition_table[(self.hash % TTABLE_SIZE) as usize] {
             Some(item) => {
@@ -273,12 +285,7 @@ impl Board {
     pub fn quiscence(&mut self, mut alpha: f32, beta: f32) -> f32 {
         let color = self.color();
         let mut moves = self.generate_pseudolegal_moves(color);
-
-        moves.sort_unstable_by(|a, b| {
-            let a_eval = self.eval_move(*a);
-            let b_eval = self.eval_move(*b);
-            a_eval.total_cmp(&b_eval).reverse()
-        });
+        self.order_moves(&mut moves);
 
         match self.transposition_table[(self.hash % TTABLE_SIZE) as usize] {
             Some(item) => {
@@ -298,7 +305,7 @@ impl Board {
             alpha = stand_pat;
         }
 
-        let tactical_moves = moves.iter().filter(|m| m.kind == MoveKind::Capture || m.kind == MoveKind::EnPassant);
+        let tactical_moves = moves.iter().filter(|m| m.is_tactical());
 
         for mov in tactical_moves {
             let ep_target_before = self.ep_target.clone();
diff --git a/src/moves.rs b/src/moves.rs
index 87b7c4b..34f8370 100644
--- a/src/moves.rs
+++ b/src/moves.rs
@@ -22,6 +22,15 @@ impl Move {
         let bb = self.source.to_bitboard() | self.target.to_bitboard();
         print(bb, format!("{:?}", self).as_str());
     }
+
+    /// Tactical move is a move that changes material score
+    pub fn is_tactical(&self) -> bool {
+        match self.kind {
+            MoveKind::Capture => true,
+            MoveKind::EnPassant => true,
+            _ => false,
+        }
+    }
 }