Simplify signature of remove_piece()

This is a non-functional simplification. Instead of passing the piece type
for remove_piece, we can rely on the board. The only exception is en-passant
which must be explicitly set because the destination square for the capture
is not the same as the piece to remove.

Verified also in the Chess960 castling case by running a couple of perft, see
the pull request discussion: #2460

STC
LLR: 2.94 (-2.94,2.94) [-3.00,1.00]
Total: 18624 W: 4147 L: 4070 D: 10407
Ptnml(0-2): 223, 1933, 4945, 1938, 260
http://tests.stockfishchess.org/tests/view/5dfeaa93e70446e17e451163

No functional change

src/position.cpp

@@ -743,8 +743,6 @@ void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
               assert(relative_rank(us, to) == RANK_6);
               assert(piece_on(to) == NO_PIECE);
               assert(piece_on(capsq) == make_piece(them, PAWN));
-
-              board[capsq] = NO_PIECE; // Not done by remove_piece()
           }
 
           st->pawnKey ^= Zobrist::psq[captured][capsq];
@@ -753,7 +751,10 @@ void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
           st->nonPawnMaterial[them] -= PieceValue[MG][captured];
 
       // Update board and piece lists
-      remove_piece(captured, capsq);
+      remove_piece(capsq);
+
+      if (type_of(m) == ENPASSANT)
+          board[capsq] = NO_PIECE;
 
       // Update material hash key and prefetch access to materialTable
       k ^= Zobrist::psq[captured][capsq];
@@ -784,7 +785,7 @@ void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
 
   // Move the piece. The tricky Chess960 castling is handled earlier
   if (type_of(m) != CASTLING)
-      move_piece(pc, from, to);
+      move_piece(from, to);
 
   // If the moving piece is a pawn do some special extra work
   if (type_of(pc) == PAWN)
@@ -804,7 +805,7 @@ void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
           assert(relative_rank(us, to) == RANK_8);
           assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
 
-          remove_piece(pc, to);
+          remove_piece(to);
           put_piece(promotion, to);
 
           // Update hash keys
@@ -884,7 +885,7 @@ void Position::undo_move(Move m) {
       assert(type_of(pc) == promotion_type(m));
       assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
 
-      remove_piece(pc, to);
+      remove_piece(to);
       pc = make_piece(us, PAWN);
       put_piece(pc, to);
   }
@@ -896,7 +897,7 @@ void Position::undo_move(Move m) {
   }
   else
   {
-      move_piece(pc, to, from); // Put the piece back at the source square
+      move_piece(to, from); // Put the piece back at the source square
 
       if (st->capturedPiece)
       {
@@ -936,9 +937,9 @@ void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Squ
   to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
 
   // Remove both pieces first since squares could overlap in Chess960
-  remove_piece(make_piece(us, KING), Do ? from : to);
-  remove_piece(make_piece(us, ROOK), Do ? rfrom : rto);
-  board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do it for us
+  remove_piece(Do ? from : to);
+  remove_piece(Do ? rfrom : rto);
+  board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do this for us
   put_piece(make_piece(us, KING), Do ? to : from);
   put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
 }

src/position.h

@@ -174,8 +174,8 @@ class Position {
 
   // Other helpers
   void put_piece(Piece pc, Square s);
-  void remove_piece(Piece pc, Square s);
-  void move_piece(Piece pc, Square from, Square to);
+  void remove_piece(Square s);
+  void move_piece(Square from, Square to);
   template<bool Do>
   void do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto);
 
@@ -412,12 +412,13 @@ inline void Position::put_piece(Piece pc, Square s) {
   psq += PSQT::psq[pc][s];
 }
 
-inline void Position::remove_piece(Piece pc, Square s) {
+inline void Position::remove_piece(Square s) {
 
   // WARNING: This is not a reversible operation. If we remove a piece in
   // do_move() and then replace it in undo_move() we will put it at the end of
   // the list and not in its original place, it means index[] and pieceList[]
   // are not invariant to a do_move() + undo_move() sequence.
+  Piece pc = board[s];
   byTypeBB[ALL_PIECES] ^= s;
   byTypeBB[type_of(pc)] ^= s;
   byColorBB[color_of(pc)] ^= s;
@@ -430,10 +431,11 @@ inline void Position::remove_piece(Piece pc, Square s) {
   psq -= PSQT::psq[pc][s];
 }
 
-inline void Position::move_piece(Piece pc, Square from, Square to) {
+inline void Position::move_piece(Square from, Square to) {
 
   // index[from] is not updated and becomes stale. This works as long as index[]
   // is accessed just by known occupied squares.
+  Piece pc = board[from];
   Bitboard fromTo = from | to;
   byTypeBB[ALL_PIECES] ^= fromTo;
   byTypeBB[type_of(pc)] ^= fromTo;