position: add captured piece

src/piece.c

@@ -65,10 +65,6 @@ char *piece_to_low(piece_t p)
 {
     return piece_details[p].low;
 }
-//char *piece_to_sym(piece_t p)
-//{
-//    return piece_details[PIECE(p)].sym;
-//}
 
 char *piece_to_sym(piece_t p)
 {

src/position.h

@@ -41,6 +41,7 @@ typedef struct __pos_s {
                         castle_rights_t castle;
                         u16 clock_50;
                         u16 plycount;             /* plies so far, start from 1 */
+                        piece_t captured;         /* only for move_undo */
         );
 
     piece_t board[BOARDSIZE];
@@ -130,7 +131,7 @@ static __always_inline bitboard_t pos_between_occ(const pos_t *pos,
 static __always_inline int pos_between_count(const pos_t *pos,
                                              const square_t sq1, const square_t sq2)
 {
-    return bb_between_excl[sq1][sq2] & pos_occ(pos);
+    return popcount64(pos_between_occ(pos, sq1, sq2));
 }
 
 /**