Simplify futility equation

This is a functional simplification. The 178 constant for the futility equation
in master can be removed.

STC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 42626 W: 9508 L: 9428 D: 23690
http://tests.stockfishchess.org/tests/view/5d5d4e320ebc5925cf11254e

LTC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 26182 W: 4432 L: 4320 D: 17430
http://tests.stockfishchess.org/tests/view/5d5df70d0ebc5925cf112fee

Closes https://github.com/official-stockfish/Stockfish/pull/2278

Bench: 3985701

diff --git a/src/search.cpp b/src/search.cpp
index 7f421b9cd38401f4e5d4d94c9f1adff3649b348b..7a16ef8cfc08af9da7ab9e374d5baf338a5d8026 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -64,7 +64,7 @@ namespace {
   // Razor and futility margins
   constexpr int RazorMargin = 661;
   Value futility_margin(Depth d, bool improving) {
   // Razor and futility margins
   constexpr int RazorMargin = 661;
   Value futility_margin(Depth d, bool improving) {

-    return Value(198 * (d / ONE_PLY) - 178 * improving);
+    return Value(198 * (d / ONE_PLY - improving));

   }
 
   // Reductions lookup table, initialized at startup
   }
 
   // Reductions lookup table, initialized at startup