enqueue Subroutine

   pure subroutine enqueue(pq, seg)
      implicit none
      class(priority_queue_t), intent(inout) :: pq
      type(segment_t),         intent(in) :: seg

      type(segment_t)              :: t
      type(segment_t), allocatable :: tmp(:)
      integer(int32)               :: n, i

      if (.not.allocated(pq%heap)) allocate(pq%heap(1))

      !  Managing the size of array in the queue.
      !! @note This implementation shall be rewritten using the `move_alloc` statement.
      n = pq%number
      if (n == size(pq%heap)) then
         allocate(tmp(n))
         tmp(:) = pq%heap(:)
         deallocate(pq%heap)
         allocate(pq%heap(n*2))
         pq%heap(1:n) = tmp(1:n)
      end if

      pq%number = pq%number + 1
      pq%heap(pq%number) = seg

      ! Implementing a queue using arrays.
      ! The following loop ensures that the data structure is a heap:
      n = pq%number
      do while (n > 1)
         i = n/2
         if (pq%heap(n)%min < pq%heap(i)%min &
               .or. (pq%heap(n)%min == pq%heap(i)%min .and. pq%heap(n)%max < pq%heap(i)%max)) then
            t = pq%heap(n)
            pq%heap(n) = pq%heap(i)
            pq%heap(i) = t
         end if
         n = i
      end do
   end subroutine enqueue