From f590767b91f6e7b95ee5797b3cf32c143e50d3c2 Mon Sep 17 00:00:00 2001
From: breatn <andante_chantry.6f@icloud.com>
Date: Tue, 22 Apr 2025 18:54:04 +0100
Subject: [PATCH] Adaptive beta cut

passed STC:
LLR: 2.94 (-2.94,2.94) <0.00,2.00>
Total: 197088 W: 51084 L: 50533 D: 95471
Ptnml(0-2): 547, 23201, 50577, 23592, 627
https://tests.stockfishchess.org/tests/view/680604d798cd372e3aea58fe

passed LTC
LLR: 2.96 (-2.94,2.94) <0.50,2.50>
Total: 127950 W: 32719 L: 32214 D: 63017
Ptnml(0-2): 69, 13825, 35673, 14348, 60
https://tests.stockfishchess.org/tests/view/6805eae498cd372e3aea588a

closes https://github.com/official-stockfish/Stockfish/pull/6012

bench 1579003
---
 AUTHORS        |  1 +
 src/search.cpp | 14 +++++++++++++-
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/AUTHORS b/AUTHORS
index 54c5f8a2..f1675860 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -34,6 +34,7 @@ Artem Solopiy (EntityFX)
 Auguste Pop
 Balazs Szilagyi
 Balint Pfliegel
+Baptiste Rech (breatn)
 Ben Chaney (Chaneybenjamini)
 Ben Koshy (BKSpurgeon)
 Bill Henry (VoyagerOne)
diff --git a/src/search.cpp b/src/search.cpp
index 85e4d007..d0841881 100644
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -103,6 +103,18 @@ Value to_corrected_static_eval(const Value v, const int cv) {
     return std::clamp(v + cv / 131072, VALUE_TB_LOSS_IN_MAX_PLY + 1, VALUE_TB_WIN_IN_MAX_PLY - 1);
 }
 
+int adaptive_probcut_margin(Depth depth) {
+    // Base margin
+    constexpr int base = 180;
+
+    // Approximate log2(depth) using a fast lookup table
+    static constexpr int logTable[32] = {0, 0, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3,
+                                         4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4};
+
+    int logDepth = logTable[std::min(depth, 31)];
+    return base + logDepth * 60 + std::min(10, (depth - 16) * 2);
+};
+
 void update_correction_history(const Position& pos,
                                Stack* const    ss,
                                Search::Worker& workerThread,
@@ -972,7 +984,7 @@ Value Search::Worker::search(
 moves_loop:  // When in check, search starts here
 
     // Step 12. A small Probcut idea
-    probCutBeta = beta + 415;
+    probCutBeta = beta + adaptive_probcut_margin(depth);
     if ((ttData.bound & BOUND_LOWER) && ttData.depth >= depth - 4 && ttData.value >= probCutBeta
         && !is_decisive(beta) && is_valid(ttData.value) && !is_decisive(ttData.value))
         return probCutBeta;