Retire unstoppable pawns evaluation

One hundred lines of code should be compensated by an
important ELO increase, otherwise are candidate for removal...

...and is not the case. We are well within error margin, so
remove the code even if we lose a couple of elo points, but
semplification is huge.

After 6494 games on russian cluster
Orig vs Mod 1145 - 1107 - 4242 (-2 ELO)

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/evaluate.cpp

@@ -147,9 +147,6 @@ namespace {
 
   #undef S
 
-  // Bonus for unstoppable passed pawns
-  const Value UnstoppablePawnValue = Value(0x500);
-
   // Rooks and queens on the 7th rank (modified by Joona Kiiski)
   const Score RookOn7thBonus  = make_score(47, 98);
   const Score QueenOn7thBonus = make_score(27, 54);
@@ -258,7 +255,6 @@ namespace {
   template<Color Us>
   void evaluate_passed_pawns(const Position& pos, EvalInfo& ei);
 
-  void evaluate_unstoppable_pawns(const Position& pos, EvalInfo& ei);
   void evaluate_trapped_bishop_a7h7(const Position& pos, Square s, Color us, EvalInfo& ei);
   void evaluate_trapped_bishop_a1h1(const Position& pos, Square s, Color us, EvalInfo& ei);
   inline Score apply_weight(Score v, Score weight);
@@ -337,10 +333,6 @@ Value do_evaluate(const Position& pos, EvalInfo& ei) {
   evaluate_passed_pawns<WHITE>(pos, ei);
   evaluate_passed_pawns<BLACK>(pos, ei);
 
-  // If one side has only a king, check whether exsists any unstoppable passed pawn
-  if (!pos.non_pawn_material(WHITE) || !pos.non_pawn_material(BLACK))
-      evaluate_unstoppable_pawns(pos, ei);
-
   Phase phase = ei.mi->game_phase();
 
   // Middle-game specific evaluation terms
@@ -873,98 +865,6 @@ namespace {
   }
 
 
-  // evaluate_unstoppable_pawns() evaluates the unstoppable passed pawns for both sides
-
-  void evaluate_unstoppable_pawns(const Position& pos, EvalInfo& ei) {
-
-    int movesToGo[2] = {0, 0};
-    Square pawnToGo[2] = {SQ_NONE, SQ_NONE};
-
-    for (Color c = WHITE; c <= BLACK; c++)
-    {
-        // Skip evaluation if other side has non-pawn pieces
-        if (pos.non_pawn_material(opposite_color(c)))
-            continue;
-
-        Bitboard b = ei.pi->passed_pawns() & pos.pieces_of_color(c);
-
-        while (b)
-        {
-            Square s = pop_1st_bit(&b);
-            Square queeningSquare = relative_square(c, make_square(square_file(s), RANK_8));
-            int d =  square_distance(s, queeningSquare)
-                   - int(relative_rank(c, s) == RANK_2) // Double pawn push
-                   - square_distance(pos.king_square(opposite_color(c)), queeningSquare)
-                   + int(c != pos.side_to_move());
-
-            // Do we protect the path to queening ?
-            bool pathDefended = (ei.attacked_by(c) & squares_in_front_of(c, s)) == squares_in_front_of(c, s);
-
-            if (d < 0 || pathDefended)
-            {
-                int mtg = RANK_8 - relative_rank(c, s) - int(relative_rank(c, s) == RANK_2);
-                int blockerCount = count_1s_max_15(squares_in_front_of(c, s) & pos.occupied_squares());
-                mtg += blockerCount;
-                d += blockerCount;
-                if ((d < 0 || pathDefended) && (!movesToGo[c] || movesToGo[c] > mtg))
-                {
-                    movesToGo[c] = mtg;
-                    pawnToGo[c] = s;
-                }
-            }
-        }
-    }
-
-    // Neither side has an unstoppable passed pawn?
-    if (!(movesToGo[WHITE] | movesToGo[BLACK]))
-        return;
-
-    // Does only one side have an unstoppable passed pawn?
-    if (!movesToGo[WHITE] || !movesToGo[BLACK])
-    {
-        Color winnerSide = movesToGo[WHITE] ? WHITE : BLACK;
-        ei.value += make_score(0, Sign[winnerSide] * (UnstoppablePawnValue - Value(0x40 * movesToGo[winnerSide])));
-    }
-    else
-    {   // Both sides have unstoppable pawns! Try to find out who queens
-        // first. We begin by transforming 'movesToGo' to the number of
-        // plies until the pawn queens for both sides.
-        movesToGo[WHITE] *= 2;
-        movesToGo[BLACK] *= 2;
-        movesToGo[pos.side_to_move()]--;
-
-        Color winnerSide = movesToGo[WHITE] < movesToGo[BLACK] ? WHITE : BLACK;
-        Color loserSide = opposite_color(winnerSide);
-
-        // If one side queens at least three plies before the other, that side wins
-        if (movesToGo[winnerSide] <= movesToGo[loserSide] - 3)
-            ei.value += Sign[winnerSide] * make_score(0, UnstoppablePawnValue - Value(0x40 * (movesToGo[winnerSide]/2)));
-
-        // If one side queens one ply before the other and checks the king or attacks
-        // the undefended opponent's queening square, that side wins. To avoid cases
-        // where the opponent's king could move somewhere before first pawn queens we
-        // consider only free paths to queen for both pawns.
-        else if (   !(squares_in_front_of(WHITE, pawnToGo[WHITE]) & pos.occupied_squares())
-                 && !(squares_in_front_of(BLACK, pawnToGo[BLACK]) & pos.occupied_squares()))
-        {
-            assert(movesToGo[loserSide] - movesToGo[winnerSide] == 1);
-
-            Square winnerQSq = relative_square(winnerSide, make_square(square_file(pawnToGo[winnerSide]), RANK_8));
-            Square loserQSq = relative_square(loserSide, make_square(square_file(pawnToGo[loserSide]), RANK_8));
-
-            Bitboard b = pos.occupied_squares();
-            clear_bit(&b, pawnToGo[winnerSide]);
-            clear_bit(&b, pawnToGo[loserSide]);
-            b = queen_attacks_bb(winnerQSq, b);
-
-            if (  (b & pos.pieces(KING, loserSide))
-                ||(bit_is_set(b, loserQSq) && !bit_is_set(ei.attacked_by(loserSide), loserQSq)))
-                ei.value += Sign[winnerSide] * make_score(0, UnstoppablePawnValue - Value(0x40 * (movesToGo[winnerSide]/2)));
-        }
-    }
-  }
-
-
   // evaluate_trapped_bishop_a7h7() determines whether a bishop on a7/h7
   // (a2/h2 for black) is trapped by enemy pawns, and assigns a penalty
   // if it is.