Call cycle detection before qsearch()

This has the property of raising alpha before calling qsearch(), thus
maybe giving some more cuts during qsearch(). The patch is equivalent
to the use of cycle detection inside qsearch() at depth 0, but is in
fact implemented by re-ordering code inside search(), which explains
the [0..4] bounds in the following tests.

STC (interrupted after 124250 games, with LLR=0.87):
http://tests.stockfishchess.org/tests/view/5b1500bd0ebc5902a8b420bf
LLR: 0.87 (-2.94,2.94) [0.00,4.00]
Total: 124250 W: 24973 L: 24470 D: 74807

LTC:
http://tests.stockfishchess.org/tests/view/5b1590eb0ebc5902a84dcd09
LLR: 2.96 (-2.94,2.94) [0.00,4.00]
Total: 74234 W: 11098 L: 10733 D: 52403

Closes https://github.com/official-stockfish/Stockfish/pull/1635

Bench: 4326784

diff --git a/src/search.cpp b/src/search.cpp
index 93651cc047b466724bb959f1102a463f8c2257f3..2185a3214fc59dd651c3e6639669cbcead83d7df 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -519,13 +519,25 @@ namespace {
   template <NodeType NT>
   Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
 
-    // Use quiescence search when needed
-    if (depth < ONE_PLY)
-        return qsearch<NT>(pos, ss, alpha, beta);
-
     constexpr bool PvNode = NT == PV;
     const bool rootNode = PvNode && ss->ply == 0;
 
+    // Check if we have an upcoming move which draws by repetition, or 
+    // if the opponent had an alternative move earlier to this position.
+    if (   pos.rule50_count() >= 3
+        && alpha < VALUE_DRAW
+        && !rootNode
+        && pos.has_game_cycle(ss->ply))
+    {
+        alpha = VALUE_DRAW;
+        if (alpha >= beta)
+            return alpha;
+    }
+
+    // Dive into quiescence search when the depth reaches zero
+    if (depth < ONE_PLY)
+        return qsearch<NT>(pos, ss, alpha, beta);
+
     assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
     assert(PvNode || (alpha == beta - 1));
     assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
@@ -578,17 +590,6 @@ namespace {
         beta = std::min(mate_in(ss->ply+1), beta);
         if (alpha >= beta)
             return alpha;
-
-        // Check if there exists a move which draws by repetition, or an alternative
-        // earlier move to this position.
-        if (   pos.rule50_count() >= 3
-            && alpha < VALUE_DRAW
-            && pos.has_game_cycle(ss->ply))
-        {
-            alpha = VALUE_DRAW;
-            if (alpha >= beta)
-                return alpha;
-        }
     }
 
     assert(0 <= ss->ply && ss->ply < MAX_PLY);