From 00e34a758f2ca170986550a1f8f25dfe691ca511 Mon Sep 17 00:00:00 2001
From: OfekShochat <o@shoch.at>
Date: Thu, 23 Sep 2021 23:16:17 +0300
Subject: [PATCH] Range reductions

adding reductions for when the delta between the static eval and the child's eval is consistently low.

passed STC
https://tests.stockfishchess.org/html/live_elo.html?614d7b3c7bdc23e77ceb8a5d
LLR: 2.95 (-2.94,2.94) <-0.50,2.50>
Total: 88872 W: 22672 L: 22366 D: 43834
Ptnml(0-2): 343, 10150, 23117, 10510, 316

passed LTC
https://tests.stockfishchess.org/html/live_elo.html?614daf3e7bdc23e77ceb8a82
LLR: 2.93 (-2.94,2.94) <0.50,3.50>
Total: 24368 W: 6153 L: 5928 D: 12287
Ptnml(0-2): 13, 2503, 6937, 2708, 23

closes https://github.com/official-stockfish/Stockfish/pull/3717

Bench: 5443950
---
 AUTHORS        |  1 +
 src/search.cpp | 13 +++++++++----
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/AUTHORS b/AUTHORS
index 5b5bbf22b67..be35127a706 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -143,6 +143,7 @@ Nikolay Kostov (NikolayIT)
 Nguyen Pham (nguyenpham)
 Norman Schmidt (FireFather)
 notruck
+Ofek Shochat (OfekShochat, ghostway)
 Ondrej Mosnáček (WOnder93)
 Oskar Werkelin Ahlin
 Pablo Vazquez
diff --git a/src/search.cpp b/src/search.cpp
index 9b56bd2b083..ffeb511095d 100644
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -69,9 +69,9 @@ namespace {
   // Reductions lookup table, initialized at startup
   int Reductions[MAX_MOVES]; // [depth or moveNumber]
 
-  Depth reduction(bool i, Depth d, int mn) {
+  Depth reduction(bool i, Depth d, int mn, bool rangeReduction) {
     int r = Reductions[d] * Reductions[mn];
-    return (r + 534) / 1024 + (!i && r > 904);
+    return (r + 534) / 1024 + (!i && r > 904) + rangeReduction;
   }
 
   constexpr int futility_move_count(bool improving, Depth depth) {
@@ -954,6 +954,7 @@ namespace {
 moves_loop: // When in check, search starts here
 
     ttCapture = ttMove && pos.capture_or_promotion(ttMove);
+    int rangeReduction = 0;
 
     // Step 11. A small Probcut idea, when we are in check
     probCutBeta = beta + 409;
@@ -1041,7 +1042,7 @@ namespace {
           moveCountPruning = moveCount >= futility_move_count(improving, depth);
 
           // Reduced depth of the next LMR search
-          int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), 0);
+          int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount, rangeReduction > 2), 0);
 
           if (   captureOrPromotion
               || givesCheck)
@@ -1176,7 +1177,7 @@ namespace {
               || !ss->ttPv)
           && (!PvNode || ss->ply > 1 || thisThread->id() % 4 != 3))
       {
-          Depth r = reduction(improving, depth, moveCount);
+          Depth r = reduction(improving, depth, moveCount, rangeReduction > 2);
 
           if (PvNode)
               r--;
@@ -1236,6 +1237,10 @@ namespace {
 
           value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
 
+          // Range reductions (~3 Elo)
+          if (ss->staticEval - value < 30 && depth > 7)
+              rangeReduction++;
+
           // If the son is reduced and fails high it will be re-searched at full depth
           doFullDepthSearch = value > alpha && d < newDepth;
           didLMR = true;