get_prefix_literal_internal Subroutine

   pure recursive subroutine get_prefix_literal_internal(tree, idx, prefix, res)
      use :: forgex_parameters_m
      implicit none
      type(tree_node_t), intent(in) :: tree(:)
      integer(int32), intent(in) :: idx
      character(:), allocatable, intent(inout) :: prefix
      logical, intent(inout) :: res

      logical :: res_left, res_right, unused
      type(tree_node_t) :: node
      character(:), allocatable :: candidate1, candidate2
      integer :: j, n
      
      if (idx < 1) return
      node = tree(idx)
      res_left = .false.
      res_right = .false.
      candidate1 = ''
      candidate2 = ''

      select case (node%op)
      case (op_concat)
         call get_prefix_literal_internal(tree, node%left_i, candidate1, res_left)

         if (res_left) then
            call get_prefix_literal_internal(tree, node%right_i, candidate2, res_right)
         end if

         prefix = prefix//candidate1//candidate2

         res = res_left .and. res_right

      case(op_union)
         call get_prefix_literal_internal(tree, node%left_i, candidate1, unused)
         call get_prefix_literal_internal(tree, node%right_i, candidate2, unused)
         prefix = extract_same_part_prefix(candidate1, candidate2)
         res = .false.
      case(op_repeat)
         n = node%min_repeat
         do j = 1, n
            call get_prefix_literal_internal(tree, node%left_i, prefix, res_left)
         end do
         res = res_left
      case (op_char)
         if (is_literal_tree_node(node)) then
            if (node%c(1)%min == node%c(1)%max) then
               prefix = prefix//adjustl_multi_byte(char_utf8(node%c(1)%min))
               res = .true.
               return
            end if
         end if
         res = .false.
      case default
         res = .false.
      end select
   end subroutine get_prefix_literal_internal