New voting formula for threads

We now use a quadratic formula during the vote for threads
when deciding on which thread to pick a move from.

time control 5+0.05, with 8 threads:
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 20202 W: 4031 L: 3813 D: 12358
http://tests.stockfishchess.org/tests/view/5c16c8e60ebc5902ba1223e2

time control 20+0.2, with 8 threads:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 14330 W: 2290 L: 2115 D: 9925
http://tests.stockfishchess.org/tests/view/5c16efca0ebc5902ba122993

20000 games match at time control 5+0.05, with 31 threads:
ELO: 5.63 +-2.8 (95%) LOS: 100.0%
Total: 20000 W: 3539 L: 3215 D: 13246
http://tests.stockfishchess.org/tests/view/5c16f07a0ebc5902ba122a20

Closes https://github.com/official-stockfish/Stockfish/pull/1876

No functional change (in simple thread mode)

diff --git a/src/search.cpp b/src/search.cpp
index 5b6a048500b0842c1b59a97f7913532139ae54e0..02f40199938810008ac2618bdd94e0ed15353861 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -254,7 +254,7 @@ void MainThread::search() {
       && !Skill(Options["Skill Level"]).enabled()
       &&  rootMoves[0].pv[0] != MOVE_NONE)
   {
-      std::map<Move, int> votes;
+      std::map<Move, int64_t> votes;
       Value minScore = this->rootMoves[0].score;
 
       // Find out minimum score and reset votes for moves which can be voted
@@ -265,12 +265,13 @@ void MainThread::search() {
       }
 
       // Vote according to score and depth
+      auto square = [](int64_t x) { return x * x; };
       for (Thread* th : Threads)
-          votes[th->rootMoves[0].pv[0]] +=  int(th->rootMoves[0].score - minScore)
-                                          + int(th->completedDepth);
+          votes[th->rootMoves[0].pv[0]] += 200 + (square(th->rootMoves[0].score - minScore + 1)
+                                                  * int64_t(th->completedDepth));
 
       // Select best thread
-      int bestVote = votes[this->rootMoves[0].pv[0]];
+      int64_t bestVote = votes[this->rootMoves[0].pv[0]];
       for (Thread* th : Threads)
       {
           if (votes[th->rootMoves[0].pv[0]] > bestVote)