extract_same_part_prefix Function

   pure function extract_same_part_prefix (a, b) result(res)
      use :: forgex_utf8_m
      implicit none
      character(*), intent(in) :: a, b
      character(:), allocatable :: res

      character(:), allocatable :: buf
      integer :: i, ie, n
      res = ''
      buf = ''
      n = min(len(a), len(b))
      do i = 1, n
         if (a(i:i) == b(i:i)) then
            buf = buf//a(i:i)
         else
            exit
         end if
      end do

      ! Handling UTF8 fragment bytes
      n = len(buf)
      i = 1
      do while (i <= n)
         ie = idxutf8(buf, i)
         if (n < ie) exit

         if (is_valid_multiple_byte_character(buf(i:ie))) then
            res = res//adjustl_multi_byte(buf(i:ie))
         end if
         i = ie + 1
      end do

   end function extract_same_part_prefix