From: Marco Costalba <mcostalba@gmail.com>
Date: Wed, 2 Jun 2010 10:47:53 +0000 (+0100)
Subject: Extend intermediate LMR to root search
X-Git-Url: https://git.sesse.net/?p=stockfish;a=commitdiff_plain;h=9337c6da46dae7c1664f36a4b27e1fedef910b9a

Extend intermediate LMR to root search

Almost no change, but it is in sync with what we do in search
and in any case the ELO difference is very small (because the
events when the intermediate research triggers are very rare),
too small to be measured, we just verify we don't have any
unexpected regressions.

After 802 games at 1+0 full QUAD
Mod vs Orig +114 =581 -107 +3 ELO

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
---

diff --git a/src/search.cpp b/src/search.cpp
index 6715d680..91b7a867 100644
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -892,17 +892,31 @@ namespace {
                         ss->reduction = reduction<PV>(depth, i - MultiPV + 2);
                         if (ss->reduction)
                         {
+                            assert(newDepth-ss->reduction >= OnePly);
+
                             // Reduced depth non-pv search using alpha as upperbound
                             value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth-ss->reduction);
                             doFullDepthSearch = (value > alpha);
                         }
+
+                        // The move failed high, but if reduction is very big we could
+                        // face a false positive, retry with a less aggressive reduction,
+                        // if the move fails high again then go with full depth search.
+                        if (doFullDepthSearch && ss->reduction > 2 * OnePly)
+                        {
+                            assert(newDepth - OnePly >= OnePly);
+
+                            ss->reduction = OnePly;
+                            value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth-ss->reduction);
+                            doFullDepthSearch = (value > alpha);
+                        }
+                        ss->reduction = Depth(0); // Restore original reduction
                     }
 
                     // Step 15. Full depth search
                     if (doFullDepthSearch)
                     {
                         // Full depth non-pv search using alpha as upperbound
-                        ss->reduction = Depth(0);
                         value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
 
                         // If we are above alpha then research at same depth but as PV