Further improve chain pawn evaluation

Passed both short TC:
LLR: 2.96 (-2.94,2.94) [-1.50,4.50]
Total: 28299 W: 5854 L: 5667 D: 16778

And long TC:
LLR: 2.96 (-2.94,2.94) [0.00,6.00]
Total: 9738 W: 1797 L: 1644 D: 6297

bench: 9294116

src/pawns.cpp

@@ -52,10 +52,16 @@ namespace {
     S(33, 31), S(33, 31), S(29, 31), S(20, 28) }};
 
   // Pawn chain membership bonus by file
-  const Score ChainMember[FILE_NB] = {
-    S(11,-1), S(13,-1), S(13,-1), S(14,-1),
-    S(14,-1), S(13,-1), S(13,-1), S(11,-1)
-  };
+  const Score ChainMember[FILE_NB][RANK_NB] = {
+    { S(0, 0), S(14, 0), S(16, 4), S(18,  9), S(56, 56), S(104, 208), S(236, 472) }, 
+    { S(0, 0), S(16, 0), S(18, 5), S(20, 10), S(60, 60), S(108, 216), S(240, 480) }, 
+    { S(0, 0), S(16, 0), S(18, 5), S(20, 10), S(60, 60), S(108, 216), S(240, 480) }, 
+    { S(0, 0), S(17, 0), S(19, 6), S(22, 11), S(66, 66), S(118, 236), S(254, 508) }, 
+    { S(0, 0), S(17, 0), S(19, 6), S(22, 11), S(66, 66), S(118, 236), S(254, 508) }, 
+    { S(0, 0), S(16, 0), S(18, 5), S(20, 10), S(60, 60), S(108, 216), S(240, 480) }, 
+    { S(0, 0), S(16, 0), S(18, 5), S(20, 10), S(60, 60), S(108, 216), S(240, 480) }, 
+    { S(0, 0), S(14, 0), S(16, 4), S(18,  9), S(56, 56), S(104, 208), S(236, 472) }
+  }; 
 
   // Candidate passed pawn bonus by rank
   const Score CandidatePassed[RANK_NB] = {
@@ -92,6 +98,7 @@ namespace {
     Bitboard b;
     Square s;
     File f;
+    Rank r;
     bool passed, isolated, doubled, opposed, chain, backward, candidate;
     Score value = SCORE_ZERO;
     const Square* pl = pos.list<PAWN>(Us);
@@ -112,6 +119,7 @@ namespace {
         assert(pos.piece_on(s) == make_piece(Us, PAWN));
 
         f = file_of(s);
+        r = relative_rank(Us, s);
 
         // This file cannot be semi-open
         e->semiopenFiles[Us] &= ~(1 << f);
@@ -176,11 +184,11 @@ namespace {
             value -= Backward[opposed][f];
 
         if (chain)
-            value += ChainMember[f] + CandidatePassed[relative_rank(Us, s)] / 2;
+            value += opposed ? ChainMember[f][r] / 2 : ChainMember[f][r];
 
         if (candidate)
         {
-            value += CandidatePassed[relative_rank(Us, s)];
+            value += CandidatePassed[r];
 
             if (!doubled)
                 e->candidatePawns[Us] |= s;