function tip,file

;returns a string vector which contains the data from a tip-tilt log file.
;the ttdata array contains the time and raw tip data
;ttdata(0,*)=time (hms), ttdata(1,*)=I3 (Y), ttdata(2,*)=I2 (Z) 
;if the file contains no data, the program returns ttdata=''

close,1
openr,1,file

;find out how many lines are in the file...
command='wc -l '+file
spawn,command,length
length=length(0)
length=strmid(length,0,strpos(length,' '))
length=long(length)

if length eq 0 then begin
 print,string(10b),string(7b),'This is a null file: ',file
 ttdata=''
 goto,exit
endif

;setup to collect data for both sensors (I2,I3)...
ttdata=strarr(3,length)	;ttdata contains the tip tilt data (see above)
dataflag=intarr(2)	;tracks whether data exists for both sensors, y & z
line=''			;line=string containg the data in the current line

n=0			;n tracks the number of data points
while not eof(1) do begin
 readf,1,line,format='(a)'
 start=strmid(line,0,2)		;start=first 2 characters of the line
 if start eq '$G' then begin	;gps data line
  gps:
  ttdata(0,n)=strmid(line,7,6)	;store gps time
  again:
  readf,1,line					;read the next line
  if eof(1) eq 1 then goto,finish		;quit at end of file
  start=strmid(line,0,2)			;first 2 characters
  if dataflag(0) and dataflag(1) eq 1 then begin	;when y & z data taken
   n=n+1						;increment n
   ttdata(0,n)=ttdata(0,n-1)				;set time eq. last gps time
   dataflag=[0,0]					;reset flags
  endif
  if start eq '$G' then goto,gps
  if start eq 'I2' then ttdata(2,n)=strmid(line,3,7)
  if start eq 'I2' then dataflag(1)=1
  if start eq 'I3' then ttdata(1,n)=strmid(line,3,7)
  if start eq 'I3' then dataflag(0)=1
  goto,again
 endif
endwhile
 
finish:
 
;was any data collected?...
if n eq 0 then begin
 Print,'No GPS data in: ,file
 ttdata=''
 goto,exit
endif 

;trim-off unused portion of data array...
temp=ttdata(*,0:n-1)
ttdata=fltarr(3,n)
ttdata=temp
 
exit:
return,ttdata
close,1
end