; This function is for use with multi-processing (i.e. splitting an job
; over multiple CPUs). It is meant to be used in conjunction with
; mp_get_entries_to_process.
; 
; The function takes as input a filename, and the total number of processes
; you will be running. For example, if you will have 2 processes on 6 machines,
; set total_procs=12. Set filename to something unique, so you won't overwrite
; it yourself when running other tests, and so no one else will overwrite it 
; during your test.
;
; The function returns the process number (1..total_procs) for the process which
; called it.
;
; Example of how to use in your code:
;
;       ; Call this early on, so you can start your processes right after one another.
;       total_procs = 12
;       my_procnum = mp_get_procnum('.my_mp_file', total_procs)
;       ...
;       ...
;       obs = .....   ; An array of observations to process.
;
;       mp_get_entries_to_process(my_procnum, total_procs, n_elements(obs), start_index, end_index)
;
;       ; Cut down the array to what this process will work on.
;       obs = obs[start_index:end_index]
; 


function mp_get_procnum, mp_filename, total_procs

    ; Open the file which has the process number stamp.
    OPENR, 1, mp_filename, error=error

    ; Handle case when file does not (yet) exist.
    if (error ne 0) then begin
        ; File probably doesn't exist yet. This is procnum 1.
        CLOSE, 1

        this_procnum = 1

        ; Write this process number to the file.
        OPENW, 1, mp_filename
            printf, 1, this_procnum
        CLOSE, 1
        GOTO, OUT
    endif 

    ; Read the old proc num (last process stamped this here).
    prev_procnum = INT(0)
    readf, 1, prev_procnum
    CLOSE, 1

    ; Check for an old file.
    if (prev_procnum eq total_procs) then begin
        ; Start fresh.
        this_procnum = 1

        ; Write this process number to the file.
        OPENW, 1, mp_filename
            printf, 1, this_procnum
        CLOSE, 1

        GOTO, OUT
    endif

    ; Increase the procnum by 1.
    this_procnum = prev_procnum + 1

    ; Stamp new proc num to the file.
    OPENW, 1, mp_filename
        printf, 1, this_procnum
    CLOSE, 1

OUT:
    ; Print out the procnum to help the user avoid mistakes.
    print, 'I am process', strcompress(string(this_procnum)), '.'
    wait, 1
    return, this_procnum
end