Sort PV moves always in two steps

This should fix following issue:

Suppose the search with MultiPVIteration == 0 returns an exact score

move = Nxf4, score = 100

Now search with MultiPVIteration == 1 and get two scores

move = Qg8, score = 150
move = Ra1, score = 180

If we now reorder all the moves in one step we end up with

pv[0] = Ra1, pv[1] = Qg8

Instead reordering as the current patch we end up in:

pv[0] = Ra1, pv[1] = Nxf4

preserving the first searched move.

No functional change in single PV.

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

diff --git a/src/search.cpp b/src/search.cpp
index b271e0e20f151e2997f06109ffea50b5da3caf9f..5921ae0fd77a093c95b59e3c0c8f3a1b2d02bc04 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -570,12 +570,13 @@ namespace {
                 // because all the values but the first are usually set to
                 // -VALUE_INFINITE and we want to keep the same order for all
                 // the moves but the new PV that goes to head.
+                sort<RootMove>(Rml.begin() + MultiPVIteration, Rml.end());
+
+                // In case we have found an exact score reorder the PV moves
+                // before leaving the fail high/low loop, otherwise leave the
+                // last PV move in its position so to be searched again.
                 if (value > alpha && value < beta)
-                    sort<RootMove>(Rml.begin(), Rml.end());
-                else
-                    // In MultiPV mode, sort only the tail of the list
-                    // until all fail-highs and fail-lows have been resolved
-                    sort<RootMove>(Rml.begin() + MultiPVIteration, Rml.end());
+                    sort<RootMove>(Rml.begin(), Rml.begin() + MultiPVIteration);
 
                 // Write PV back to transposition table in case the relevant entries
                 // have been overwritten during the search.