! This program reads two data files and writes out two new files
! containing the data from the times where both files have data.
! It assumes that the files are in date/time order
! Example:
! commonhours -f690070.org -g724916.org -p690070.com -q724916.com

      integer year(2), mo(2), day(2), hour(2), univtime(2), idx
      real dir(2), u(2)
      character*50 argv, name1, name2, name1out, name2out

      ! function definition
      integer julday

      if (iargc() .gt. 0) then
	do i = 1, iargc()
          call getarg(i,argv) 

          !command line help prompt
          if ((argv(2:2).eq.'?').or.(argv(2:2).eq.'h')) then
            write(*,*) 'Valid command line options:'
            write(*,*) '-? or -h Display this help screen'
            write(*,*) '-f<input_filename>'
            write(*,*) '-g<input_filename>'
            write(*,*) '-p<output_filename>'
            write(*,*) '-q<output_filename>'
            stop

          else if (argv(2:2) == 'f') then !First input file name
            read(argv(3:),*) name1

          else if (argv(2:2) == 'g') then !Second input file name
            read(argv(3:),*) name2

          else if (argv(2:2) == 'p') then !First output file name
            read(argv(3:),*) name1out

          else if (argv(2:2) == 'q') then !Second output file name
            read(argv(3:),*) name2out

          end if
        end do
      endif

      open (1, file = name1, status = "old")
      open (2, file = name2, status = "old")

      open (10, file = name1out)
      open (20, file = name2out)

      ! read from both files
      read (1,*,end=300) year(1), mo(1), day(1), hour(1), dir(1), u(1)
      read (2,*,end=300) year(2), mo(2), day(2), hour(2), dir(2), u(2)
      ! calculate a single value for date and time
      univtime(1) = julday(day(1), mo(1), year(1))*24 + hour(1)
      univtime(2) = julday(day(2), mo(2), year(2))*24 + hour(2)

      do while (2 > 1) !only leave this loop when eof.

        ! index forward
        do while ( univtime(1) .lt. univtime(2) )
            ! read first file until greater than or equal
            read (1,*,end=300) year(1), mo(1), day(1), hour(1), dir(1), u(1)
            univtime(1) = julday(day(1), mo(1), year(1))*24 + hour(1)
        end do
        do while ( univtime(2) .lt. univtime(1) )
            ! read second file until greater than or equal
            read (2,*,end=300) year(2), mo(2), day(2), hour(2), dir(2), u(2)
            univtime(2) = julday(day(2), mo(2), year(2))*24 + hour(2)
        end do

        !write data when time is equal
        if( univtime(2) .eq. univtime(1) ) then
          write(10,40) year(1), mo(1), day(1), hour(1), dir(1), u(1)
          write(20,40) year(2), mo(2), day(2), hour(2), dir(2), u(2)
40        format(x, 4i4, 2f6.1)
          ! read from both files
          read (1,*,end=300) year(1), mo(1), day(1), hour(1), dir(1), u(1)
          read (2,*,end=300) year(2), mo(2), day(2), hour(2), dir(2), u(2)
          ! calculate a single value for date and time
          univtime(1) = julday(day(1), mo(1), year(1))*24 + hour(1)
          univtime(2) = julday(day(2), mo(2), year(2))*24 + hour(2)
        end if

      end do
300   continue

      end