get_index_list_forward Subroutine

   pure subroutine get_index_list_forward(text, prefix, suffix, index_array)
      use, intrinsic :: iso_fortran_env, only: int32
      use :: forgex_parameters_m
      implicit none
      character(*), intent(in) :: text, prefix, suffix
      integer(int32), allocatable, intent(inout) :: index_array(:)
      integer(int32), allocatable :: tmp(:)

      integer :: offset, idx, len_pre, len_suf, i, siz, suf_idx

      !! If the length of `prefix` equals to zero, return immediately.
      len_pre = len(prefix)
      len_suf = len(suffix)
      if (len_pre == 0) then
         return
      end if

      ! Intialize
      if (allocated(index_array)) deallocate(index_array)
      allocate(index_array(LIT_OPTS_INDEX_UNIT), source=INVALID_CHAR_INDEX)
      siz = LIT_OPTS_INDEX_UNIT

      ! Get the first position with the `index` intrinsic function.
      idx = index(text, prefix)
      suf_idx = index(text, suffix, back=.true.)
      if (suf_idx == 0) suf_idx = INVALID_CHAR_INDEX

      if (idx <= 0) then
         return
      else if (suf_idx /= INVALID_CHAR_INDEX) then
         if (idx <= suf_idx) index_array(1) = idx
      else
         index_array(1) = idx
      end if

      ! Calculate the offset to specify a substring.
      offset = idx + len_pre -1
      
      i = 2      
      do while (offset < len(text))

         ! Get the position and store it in the `idx` variable. 
         idx = index(text(offset+1:), prefix)
         if (idx <= 0) exit
         index_array(i) = idx + offset
         i = i + 1

         ! Reallocate
         if (i > siz) then
            call move_alloc(index_array, tmp)
            allocate(index_array(2*siz), source=INVALID_CHAR_INDEX)
            index_array(1:siz) = tmp(1:siz)
            siz = siz*2
         end if

         ! Update the offset to specify the next substring.
         offset = offset + idx + len_pre -1
         if (suf_idx /= INVALID_CHAR_INDEX .and. offset > suf_idx) exit
      end do
   end subroutine get_index_list_forward