From cc992e5e4a7110b21f85168bdedad7978edad140 Mon Sep 17 00:00:00 2001
From: FauziAkram <fauzi.dabat@hotmail.com>
Date: Mon, 17 Jun 2024 00:03:15 +0300
Subject: [PATCH] Internal iterative reductions: decrease depth more

For PV nodes without a ttMove, we decrease depth.
But in this patch, additionally, if the current position is found in the TT, and the stored depth in the TT is greater than or equal to
the current search depth, we decrease the search depth even further.

Passed STC:
LLR: 2.96 (-2.94,2.94) <0.00,2.00>
Total: 84384 W: 22154 L: 21761 D: 40469
Ptnml(0-2): 292, 9972, 21315, 10277, 336
https://tests.stockfishchess.org/tests/view/666b0a4d602682471b064db6

Passed LTC:
LLR: 2.95 (-2.94,2.94) <0.50,2.50>
Total: 92106 W: 23471 L: 23032 D: 45603
Ptnml(0-2): 79, 10155, 25154, 10578, 87
https://tests.stockfishchess.org/tests/view/666c423d602682471b064e56

closes https://github.com/official-stockfish/Stockfish/pull/5397

bench: 1038234
---
 src/search.cpp | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/src/search.cpp b/src/search.cpp
index 562bdbf9c0d..e63595c1b73 100644
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -829,9 +829,12 @@ Value Search::Worker::search(
     }
 
     // Step 10. Internal iterative reductions (~9 Elo)
-    // For PV nodes without a ttMove, we decrease depth by 3.
+    // For PV nodes without a ttMove, we decrease depth.
+    // Additionally, if the current position is found in the TT
+    // and the stored depth in the TT is greater than or equal to
+    // current search depth, we decrease search depth even further.
     if (PvNode && !ttData.move)
-        depth -= 3;
+        depth -= 3 + (ss->ttHit && ttData.depth >= depth);
 
     // Use qsearch if depth <= 0.
     if (depth <= 0)