; NAME:
;     reorder
;
; PURPOSE:
;     To reorder a list of FITS images to be in A, B order.
;
; CATEGORY:
;     Data Input
;
; CALLING SEQUENCE:
;     result = reorder(files,CANCEL=cancel)
;
; INPUTS:
;     files - A string array of (fullpath) file names
; 
; OUTUTS:
;    Returns a string array of file names in A, B order
;
; KEYWORD PARAMETERS:    
;    FILES  - An array of the file names in A,B order
;    CANCEL - Set on return if there is a problem
;
; PROCEDURES CALLED:
;    Requires the Astronomy User's Library
;
;  PROCEDURE: 
;    Extracts the BEAM keyword of pairs of images and reorders them 
;    to be in A, B order.  Useful since the dither pattern of the IRTF
;    is ABBAABBA.
;
;REVISION HISTORY:
;    2001-01-29 - Written by M. Cushing, Institute for Astronomy, UH
;
function reorder,fullpaths,FILES=files,CANCEL=cancel

cancel  = 0

;  Check parameters

if n_params() lt 1 then begin
    
    cancel = 1
    print, 'Syntax - result = reorder(files,FILES=files,CANCEL=cancel)'
    return, -1

endif

zparcheck, 'reorder',fullpaths, 1,7,[0,1], 'Full Paths' 

nfiles = n_elements(fullpaths)

if nfiles mod 2 ne 0 then begin
    
    mess = 'Need an even number of images for this program.'
    print, mess
    cancel = 1
    return,-1
    
endif
files =findgen(nfiles)

for i = 0, (nfiles-1)/2 do begin

    Ahdr= headfits(fullpaths[i*2])
    beam = fxpar(Ahdr,'BEAM')
    if strcompress(beam,/re) eq 'B' then begin

        tmp              = fullpaths[i*2] 
        fullpaths[i*2]   = fullpaths[i*2+1]
        fullpaths[i*2+1] = tmp

        tmp              = files[i*2]
        files[i*2]       = files[i*2+1]
        files[i*2+1]     = tmp
                           

    endif 

endfor

return, fullpaths

end