// Only supports north and south
template<Direction D>
constexpr Bitboard fill(Bitboard b) {
    switch (D) {
        case NORTH:
            b |= (b << 8);
            b |= (b << 16);
            return b | (b << 32);
        case SOUTH:
            b |= (b >> 8);
            b |= (b >> 16);
            return b | (b >> 32);
        default: return b;
    }
}

template<Color color>
inline constexpr Bitboard doubled_pawns(Position& board) {
    Bitboard us_pawns = board.bitboard_of(color, PAWN);
    return us_pawns & shift<relative_dir<color>(SOUTH)>(fill<relative_dir<color>(SOUTH)>(us_pawns));
}

template<Color c>
inline constexpr Score evaluate_doubled_pawns(Position& board) {
    return DOUBLED_PAWN_PENALTY * pop_count(doubled_pawns<c>(board));
}