From: cj5716 <125858804+cj5716@users.noreply.github.com>
Date: Mon, 26 Jun 2023 11:40:22 +0000 (+0800)
Subject: Negative extension on cutNodes based on depth
X-Git-Url: https://git.sesse.net/?p=stockfish;a=commitdiff_plain;h=e551964ef63e4e4af4bb6132538b98fad4a51afe

Negative extension on cutNodes based on depth

This patch was inspired by candirufish original attempt at negative extensions
here that failed yellow: https://tests.stockfishchess.org/tests/view/6486529065ffe077ca124f32

I tested some variations of the idea and tuned a depth condition for
a modified version of it here https://tests.stockfishchess.org/tests/view/648db80a91c58631ce31fe00
after noticing abnormal scaling (ie many passed STC but not LTC)
After some small tweaks I got the final version here

Passed STC:
LLR: 2.98 (-2.94,2.94) <0.00,2.00>
Total: 122208 W: 32776 L: 32350 D: 57082
Ptnml(0-2): 310, 13250, 33553, 13686, 305
https://tests.stockfishchess.org/tests/view/64997934dc7002ce609d01e3

Passed LTC:
LLR: 2.94 (-2.94,2.94) <0.50,2.50>
Total: 145092 W: 39617 L: 39115 D: 66360
Ptnml(0-2): 54, 13691, 44552, 14197, 52
https://tests.stockfishchess.org/tests/view/649a1c5ddc7002ce609d0bff

closes https://github.com/official-stockfish/Stockfish/pull/4644

Bench: 2637784
---

diff --git a/src/search.cpp b/src/search.cpp
index 740ad71e..fbc1755b 100644
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1097,6 +1097,10 @@ moves_loop: // When in check, search starts here
               else if (ttValue >= beta)
                   extension = -2 - !PvNode;
 
+              // If we are on a cutNode, reduce it based on depth (negative extension) (~1 Elo)
+              else if (cutNode)
+                  extension = depth > 8 && depth < 17 ? -3 : -1;
+
               // If the eval of ttMove is less than value, we reduce it (negative extension) (~1 Elo)
               else if (ttValue <= value)
                   extension = -1;