Rank-based outposts

Introduce OutpostRank[RANK_NB] which contains a bonus according to
the rank of the outpost. We use it for the primary Outpost bonus.
The values are based on the trends of the SPSA tuning run with some
manual tweaks.

Passed STC:
LLR: 2.96 (-2.94,2.94) [-1.50,4.50]
Total: 27454 W: 6059 L: 5869 D: 15526
http://tests.stockfishchess.org/tests/view/5dcadba20ebc590256922f09

Passed LTC:
LLR: 2.94 (-2.94,2.94) [0.00,3.50]
Total: 57950 W: 9443 L: 9112 D: 39395
http://tests.stockfishchess.org/tests/view/5dcaea880ebc5902569230bc

Bench: 4778405

----------------------------

The inspiration for this patch came from Stefan Geschwentner's attempt
of modifying BishopPawns into a rank-based penalty. Michael Stembera
suggested that maybe the S(0, 0) ranks (3rd, 7th and also maybe 8th)
can still be tuned. This would expand our definition of Outpost and
OutpostRanks would be removed altogether. Special thanks to Mark Tenzer
for all the help and excellent suggestions.

diff --git a/src/evaluate.cpp b/src/evaluate.cpp
index cefd9db427f19fd43f1d51ee08d621bec0b4cd61..07bacf8dcc0f0e40551a99df3e9866b88d9f98ba 100644 (file)
--- a/src/evaluate.cpp
+++ b/src/evaluate.cpp
@@ -125,6 +125,11 @@ namespace {
   constexpr Score PassedRank[RANK_NB] = {
     S(0, 0), S(10, 28), S(17, 33), S(15, 41), S(62, 72), S(168, 177), S(276, 260)
   };
+  
+  // OutpostRank[Rank] contains a bonus according to the rank of the outpost
+  constexpr Score OutpostRank[RANK_NB] = {
+    S(0, 0), S(0, 0), S(0, 0), S(28, 18), S(30, 24), S(32, 19)
+  };
 
   // Assorted bonuses and penalties
   constexpr Score BishopPawns        = S(  3,  7);
@@ -292,7 +297,7 @@ namespace {
             // Bonus if piece is on an outpost square or can reach one
             bb = OutpostRanks & attackedBy[Us][PAWN] & ~pe->pawn_attacks_span(Them);
             if (s & bb)
-                score += Outpost * (Pt == KNIGHT ? 2 : 1);
+                score += OutpostRank[relative_rank(Us, s)] * (Pt == KNIGHT ? 2 : 1);
 
             else if (Pt == KNIGHT && bb & b & ~pos.pieces(Us))
                 score += Outpost;