;+
; NAME:
;  nthline_lookup
;
; PURPOSE: (one line)
;  make a lookup table for use by nthline
;
; DESCRIPTION:
;  make a lookup table for use by nthline
;
; CATEGORY:
;  File IO
;  
; CALLING SEQUENCE:
;  nthline_lookup, filename, lookup=lookup, recalc=recalc
;
; INPUTS:
;  filename - file for which to make the lookup
;
; OPTIONAL INPUT PARAMETERS:
;  
; KEYWORD INPUT PARAMETERS:
;
;  lookup - name of lookup table (otherwise filename.lookup)
;  recacl - if set, recalculate and save lookup table, even if file exists
;
; KEYWORD OUTPUT PARAMETERS:
;
; OUTPUTS:
;  
; COMMON BLOCKS:
;  None
;
; SIDE EFFECTS:
;  Creates lookup table, a file of 2 x n longs
;  startbyte  nbyte
;
; PROCEDURES:
;
; RESTRICTIONS:
;
; EXAMPLE:
; 
; MODIFICATION HISTORY:
;  Written   2006 Apr 23 Leslie Young SwRI
;-
pro nthline_lookup, filename, lookup=lookup, recalc=recalc

    if not keyword_set(lookup) then lookup=filename+'.lookup'

    tmp = ' ' 
    if not isfile(lookup) or keyword_set(recalc) then begin
        openr, ilun, filename, /get_lun
        openw, olun, lookup, /get_lun
        startbyte = 0L
        repeat begin
            readf,ilun,tmp
            reclen = strlen(tmp)
            writeu, olun, startbyte, reclen
            startbyte = startbyte + reclen + 1  ; add 1 for EOL
        endrep until eof(ilun)
        close, ilun & free_lun,ilun
        close, olun & free_lun,olun
    endif
end