X-Git-Url: https://git.sesse.net/?p=stockfish;a=blobdiff_plain;f=src%2Ftt.cpp;h=151b71f58df6ac789a87ac200b543169a6fe5c6b;hp=5ebd10d2d6cdfa7657c3d967716e03f8b848961f;hb=8de29390f2d2bd31585b93ff46eae3051126f666;hpb=51330414c4a773a9c3d1dd562fdc8536344a67b8

diff --git a/src/tt.cpp b/src/tt.cpp
index 5ebd10d2..151b71f5 100644
--- a/src/tt.cpp
+++ b/src/tt.cpp
@@ -2,6 +2,7 @@
   Stockfish, a UCI chess playing engine derived from Glaurung 2.1
   Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
   Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
+  Copyright (C) 2015-2016 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
 
   Stockfish is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
@@ -66,9 +67,9 @@ void TranspositionTable::clear() {
 /// TranspositionTable::probe() looks up the current position in the transposition
 /// table. It returns true and a pointer to the TTEntry if the position is found.
 /// Otherwise, it returns false and a pointer to an empty or least valuable TTEntry
-/// to be replaced later. TTEntry t1 is considered more valuable than TTEntry t2 if
-/// both are from the current search and the depth of t1 is greater than the depth of t2,
-/// or if t1 is from a previous search but its depth is at least 8 ply deeper than t2.
+/// to be replaced later. The replace value of an entry is calculated as its depth
+/// minus 8 times its relative age. TTEntry t1 is considered more valuable than
+/// TTEntry t2 if its replace value is greater than that of t2.
 
 TTEntry* TranspositionTable::probe(const Key key, bool& found) const {
 
@@ -78,7 +79,7 @@ TTEntry* TranspositionTable::probe(const Key key, bool& found) const {
   for (int i = 0; i < ClusterSize; ++i)
       if (!tte[i].key16 || tte[i].key16 == key16)
       {
-          if (tte[i].key16)
+          if ((tte[i].genBound8 & 0xFC) != generation8 && tte[i].key16)
               tte[i].genBound8 = uint8_t(generation8 | tte[i].bound()); // Refresh
 
           return found = (bool)tte[i].key16, &tte[i];
@@ -87,8 +88,12 @@ TTEntry* TranspositionTable::probe(const Key key, bool& found) const {
   // Find an entry to be replaced according to the replacement strategy
   TTEntry* replace = tte;
   for (int i = 1; i < ClusterSize; ++i)
-      if (  ((  tte[i].genBound8 & 0xFC) == generation8) * 8 * ONE_PLY +   tte[i].depth8
-          < ((replace->genBound8 & 0xFC) == generation8) * 8 * ONE_PLY + replace->depth8)
+      // Due to our packed storage format for generation and its cyclic
+      // nature we add 259 (256 is the modulus plus 3 to keep the lowest
+      // two bound bits from affecting the result) to calculate the entry
+      // age correctly even after generation8 overflows into the next cycle.
+      if (  replace->depth8 - ((259 + generation8 - replace->genBound8) & 0xFC) * 2 * ONE_PLY
+          >   tte[i].depth8 - ((259 + generation8 -   tte[i].genBound8) & 0xFC) * 2 * ONE_PLY)
           replace = &tte[i];
 
   return found = false, replace;