;-------------------------------------------------------------
;+
; NAME:
;        GET_FREELUN (function)
;
; PURPOSE:
;        Return next available logical unit number. Unlike 
;        the internal GET_LUN procedure, this function is not
;        restricted to unit numbers above 100, and it will 
;        detect any blocked unit number.
;
; CATEGORY:
;        I/O tools
;
; CALLING SEQUENCE:
;        lun = GET_FREELUN([LUN])
;
; INPUTS:
;        none
;
; KEYWORD PARAMETERS:
;        none
;
; OUTPUTS:
;        The lowest available logical unit number. This number is 
;        also returned in the LUN parameter for later use.
;
; SUBROUTINES:
;
; REQUIREMENTS:
;
; NOTES:
;
; EXAMPLE:
;        openw,get_freelun(lun),filename
;
; MODIFICATION HISTORY:
;        mgs, 17 Sep 1998: VERSION 1.00
;
;-
; Copyright (C) 1998, Martin Schultz, Harvard University
; This software is provided as is without any warranty
; whatsoever. It may be freely used, copied or distributed
; for non-commercial purposes. This copyright notice must be
; kept with any copy of this software. If this software shall
; be used commercially or sold as part of a larger package,
; please contact the author to arrange payment.
; The copyright is granted if this program becomes part of the 
; IDL distribution.
; Bugs and comments should be directed to mgs@io.harvard.edu
; with subject "IDL routine get_freelun"
;-------------------------------------------------------------


function get_freelun,lun
 
    help,/files,output=list

    lun = 1
 
    ; at least one file open 
    ; find lowest available unit number
    if (n_elements(list) gt 1) then begin
 
        ; maximum allowed number of open files exceeded?
        if (n_elements(list) gt  99) then $
            message,'Cannot handle any more open files'
 
        ; extract numbers and compare to expectation
        for i=1,n_elements(list)-1 do begin
            usedlun = fix(strmid(list[i],0,3))
            if (usedlun gt i) then begin
               lun = i
               return,lun   ; this one's free
            endif
        endfor
        ; next free unit is greater than all used ones
        lun = i
        return,lun
 
    endif else $    ; no file opened
       return,lun
 
 
end