Use an affine formula to mix stats and eval

Follow-up for the previous patch: we use an affine formula to mix stats
and evaluation in search. The idea is to give a bonus if the previous
move of the opponent was historically bad, and a malus if the previous
move of the opponent was historically good.

More precisely, if x is the stat score of the previous move by the opponent,
we implement the following formulas to tweak the evaluation at an internal
node of the tree for our pruning decisions at this node:

if x = 0, use v' = eval(P)
if x > 0, use v' = eval(P) - 5 - x/1024
if x < 0, use v' = eval(P) + 5 - x/1024

For reference, the previous master had this simpler rule:

if x > 0, use v' = eval(P) - 10
if x <= 0, use v' = eval(P)

STC:
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 29322 W: 6359 L: 6088 D: 16875
http://tests.stockfishchess.org/tests/view/5b76a5980ebc5902bdba957f

LTC:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 30893 W: 5154 L: 4910 D: 20829
http://tests.stockfishchess.org/tests/view/5b76ca6d0ebc5902bdba9914

Closes https://github.com/official-stockfish/Stockfish/pull/1740

Bench: 4592766

diff --git a/src/search.cpp b/src/search.cpp
index d43a875f68168f4c58f64907daba4c88daf4df7d..3d9c718b9df304c77c37885af263adb9f9b95727 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -722,12 +722,14 @@ namespace {
     }
     else
     {
-        ss->staticEval = eval =
-        (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) - 10 * ((ss-1)->statScore > 0)
-                                         : -(ss-1)->staticEval + 2 * Eval::Tempo;
+        int p = (ss-1)->statScore;
+        int malus = p > 0 ? (p + 5000) / 1024 :
+                    p < 0 ? (p - 5000) / 1024 : 0;
 
-        tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
-                  ss->staticEval);
+        ss->staticEval = eval = (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) - malus
+                                                                 : -(ss-1)->staticEval + 2 * Eval::Tempo;
+
+        tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, ss->staticEval);
     }
 
     // Step 7. Razoring (~2 Elo)