Simplify time manager in search()

Remove the F[] array which I find unhelpful and rename `improvingFactor` to
`fallingEval` since larger values indicate a falling eval and more time use.

I realise a test was not strictly necessary, but I ran STC [-3,1] just to
check there are no foolish errors before creating the pull request:

STC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 35804 W: 7753 L: 7659 D: 20392
http://tests.stockfishchess.org/tests/view/5bef3a0c0ebc595e0ae39c19

It was then suggested to clean the constants around `fallingEval`
to make it more clear this is a factor around ~1 that adjusts time
up or downwards depending on some conditions. We then ran a double
test with this simplification suggestion:

STC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 68435 W: 14936 L: 14906 D: 38593
http://tests.stockfishchess.org/tests/view/5c02c56b0ebc5902bcee0184

LTC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 37258 W: 6324 L: 6230 D: 24704
http://tests.stockfishchess.org/tests/view/5c030a520ebc5902bcee0a32

No functional change

diff --git a/src/search.cpp b/src/search.cpp
index e63d72e85e4d1e08602701e3c006ca17aae0718d..6a63fcaa338910a221d2770374faea8ead35f183 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -492,10 +492,8 @@ void Thread::search() {
           && !Threads.stop
           && !Threads.stopOnPonderhit)
           {
-              const int F[] = { failedLow,
-                                bestValue - mainThread->previousScore };
-
-              int improvingFactor = std::max(246, std::min(832, 306 + 119 * F[0] - 6 * F[1]));
+              double fallingEval = (306 + 119 * failedLow + 6 * (mainThread->previousScore - bestValue)) / 581.0;
+              fallingEval        = std::max(0.5, std::min(1.5, fallingEval));
 
               // If the bestMove is stable over several iterations, reduce time accordingly
               timeReduction = 1.0;
@@ -509,7 +507,7 @@ void Thread::search() {
 
               // Stop the search if we have only one legal move, or if available time elapsed
               if (   rootMoves.size() == 1
-                  || Time.elapsed() > Time.optimum() * bestMoveInstability * improvingFactor / 581)
+                  || Time.elapsed() > Time.optimum() * bestMoveInstability * fallingEval)
               {
                   // If we are allowed to ponder do not stop the search now but
                   // keep pondering until the GUI sends "ponderhit" or "stop".