module get_divisors_m

  implicit none

contains

  subroutine get_divisors(n, div)

    ! Find all the divisors of a given integer.

    integer, intent(in):: n
    integer, pointer:: div(:)

    ! Variables local to the procedure:

    integer i, i_max
    integer n_div ! number of divisors of "n"
    integer work(n) ! temporary array to hold divisors

    !------------------------

    if (n <= 0) then
       print *, "get_divisors: n = ", n
       stop 1
    end if
    
    if (n == 1) then
       work(1) = 1
       n_div = 1
    else
       ! n >= 2, there are at least two divisors: 1 and "n"
       work(1) = 1
       work(2) = n
       n_div = 2

       i_max = int(sqrt(real(n)))

       do i = 2, i_max - 1
          if (mod(n, i) == 0) then
             work(n_div+1) = i
             work(n_div+2) = n / i
             n_div = n_div + 2
          end if
       end do

       if (i_max >= 2 .and. mod(n, i_max) == 0) then
          work(n_div+1) = i_max
          n_div = n_div + 1
          if (i_max**2 /= n) then
             work(n_div+1) = n / i_max
             n_div = n_div + 1
          end if
       end if
    end if

    allocate(div(n_div))
    div = work(:n_div)

  end subroutine get_divisors

end module get_divisors_m