Revert "Tweak reductions formula: 0.88 * depth + 0.12"

This patch reverts the recent commit called "Tweak reductions formula, etc."
The decisions for the revert decision were as follows:

1) The original commit called "Tweak reductions formula: 0.88 * depth + 0.12"
showed bad scaling at in a Very Long Time Control (VLTC) test:

VLTC (180+1.8):
LLR: -1.59 (-2.94,2.94) [0.00,5.00]
Total: 14968 W: 2247 L: 2257 D: 10464
http://tests.stockfishchess.org/tests/view/5b559ffa0ebc5902bdb84f36

2) So there was a suspicion that the original fast passing LTC test which lead
us to accept the patch may have been a statistical accident, so we organized
a match against the previous master at LTC to get an Elo estimate for the
patch:

LTC match:
ELO: -1.83 +-2.1 (95%) LOS: 4.3%
Total: 36018 W: 6018 L: 6208 D: 23792
http://tests.stockfishchess.org/tests/view/5b55f8110ebc5902bdb8526f

3) Based on these results, we ran a simplification test with [-3..1] bounds
for the revert at LTC:

LTC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 41501 W: 7107 L: 7020 D: 27374
http://tests.stockfishchess.org/tests/view/5b5738670ebc5902bdb86932

4) So we revert.

Bench: 4491691

diff --git a/src/search.cpp b/src/search.cpp
index 89636f23575398840559967029541bf94ad3d736..af7a5c13dd643f411e8bf1f94906e8269423f714 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -153,8 +153,7 @@ void Search::init() {
       for (int d = 1; d < 64; ++d)
           for (int mc = 1; mc < 64; ++mc)
           {
-              double slope = d > 2 ? 0.88 * d + 0.36 : d;
-              double r = log(slope) * log(mc) / 1.95;
+              double r = log(d) * log(mc) / 1.95;
 
               Reductions[NonPV][imp][d][mc] = int(std::round(r));
               Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);