df_shuffle Subroutine

    subroutine df_shuffle(df, seed)
        type(data_frame), intent(inout) :: df
        integer, intent(in), optional :: seed

        integer, dimension(:), allocatable :: indices, seed_array
        integer :: i, j, temp, seed_size
        real :: rand_val

        if (df % nrows() < 2) return

        ! Initialize random seed if provided
        if (present(seed)) then
            call random_seed(size=seed_size)
            allocate (seed_array(seed_size))
            seed_array = seed
            call random_seed(put=seed_array)
            deallocate (seed_array)
        end if

        ! Create array of indices
        allocate (indices(df % nrows()))
        do i = 1, df % nrows()
            indices(i) = i
        end do

        ! Fisher-Yates shuffle
        do i = df % nrows(), 2, -1
            call random_number(rand_val)
            j = int(rand_val * i) + 1
            temp = indices(i)
            indices(i) = indices(j)
            indices(j) = temp
        end do

        ! Reorder all columns according to shuffled indices
        call reorder_all_columns(df, indices)

        deallocate (indices)
    end subroutine df_shuffle