Add log product formula for LMR

src/search/mod.rs

@@ -18,7 +18,8 @@ pub const ASPIRATION_WINDOW_DEPTH_THRESHOLD: u8 = 4;
 pub const ASPIRATION_WINDOW_EXPANSION: i16 = 2;
 pub const LMR_DEPTH_THRESHOLD: u8 = 2;
 pub const LMR_MOVE_COUNT_THRESHOLD: u8 = 3;
-pub const LMR_REDUCTION: u8 = 2;
+pub const LMR_BASE: f32 = 1.0;
+pub const LMR_DIVISION: f32 = 2.0;
 pub const NMP_REDUCTION: u8 = 2;
 pub const NMP_DEPTH_THRESHOLD: u8 = 2;
 pub const RFP_DEPTH_LIMIT: u8 = 6;

src/search/negamax.rs

@@ -10,7 +10,7 @@ use super::{
     quiescence::quiescence,
     time::TimeInfo,
     transposition_table::{NodeType, TTEntry},
-    CHECK_EXTENSION, LMR_DEPTH_THRESHOLD, LMR_MOVE_COUNT_THRESHOLD, LMR_REDUCTION, MAX_HISTORY,
+    CHECK_EXTENSION, LMR_BASE, LMR_DEPTH_THRESHOLD, LMR_DIVISION, LMR_MOVE_COUNT_THRESHOLD,
     NMP_DEPTH_THRESHOLD, NMP_REDUCTION, RFP_DEPTH_LIMIT, RFP_MARGIN,
 };
 
@@ -123,22 +123,29 @@ pub fn negamax(
         let score = if legal_moves == 1 {
             -negamax(game, -beta, -alpha, depth - 1, plies + 1, time, nodes, true)?
         } else {
-            #[rustfmt::skip]
             let lmr_reduction = if !in_check
                 && !mv.is_capture()
                 && depth > LMR_DEPTH_THRESHOLD
-                && legal_moves > LMR_MOVE_COUNT_THRESHOLD { LMR_REDUCTION } else { 0 };
+                && legal_moves > LMR_MOVE_COUNT_THRESHOLD
+            {
+                let log_product = (depth as f32).ln() * (legal_moves as f32).ln();
+                let reduction = LMR_BASE + (log_product / LMR_DIVISION);
+                reduction.clamp(1.0, (depth - 1) as f32).floor() as u8
+            } else {
+                1
+            };
 
             let mut score = -negamax(
                 game,
                 -alpha - 1,
                 -alpha,
-                depth - 1 - lmr_reduction,
+                depth - lmr_reduction,
                 plies + 1,
                 time,
                 nodes,
                 true,
             )?;
+
             if score > alpha && score < beta {
                 score = -negamax(game, -beta, -alpha, depth - 1, plies + 1, time, nodes, true)?;
             }