union_integer_int64 Function

    pure function union_integer_int64 (A, B) result(D)
        integer (int64), intent(in) :: A(:), B(:)
        integer (int64), allocatable :: D(:)
        character(*), parameter :: this_routine = 'union_complement'

        integer (int64), allocatable :: tmp(:)
        integer :: i, j, l

#ifdef DEBUG_
    block
        use util_mod, only: stop_all
        if (.not. (is_sorted(A))) then
            call stop_all (this_routine, "Assert fail: is_sorted(A)")
        end if
    end block
#endif
#ifdef DEBUG_
    block
        use util_mod, only: stop_all
        if (.not. (is_sorted(B))) then
            call stop_all (this_routine, "Assert fail: is_sorted(B)")
        end if
    end block
#endif

        allocate(tmp(size(A) + size(B)))

        i = 1; j = 1; l = 1
        do while (l <= size(tmp))
            if (i > size(A) .and. j > size(B)) then
                exit
            else if (i > size(A)) then
                tmp(l) = B(j)
                j = j + 1
                l = l + 1
            else if (j > size(B)) then
                tmp(l) = A(i)
                i = i + 1
                l = l + 1
            else if (A(i) == B(j)) then
                tmp(l) = A(i)
                i = i + 1
                j = j + 1
                l = l + 1
            else if (A(i) < B(j)) then
                tmp(l) = A(i)
                i = i + 1
                l = l + 1
            else if (A(i) > B(j)) then
                tmp(l) = B(j)
                j = j + 1
                l = l + 1
            end if
        end do

        associate(final_size => l - 1)
            D = tmp(:final_size)
        end associate
#ifdef DEBUG_
    block
        use util_mod, only: stop_all
        if (.not. (is_sorted(D))) then
            call stop_all (this_routine, "Assert fail: is_sorted(D)")
        end if
    end block
#endif
    end function union_integer_int64