; docformat = 'rst'

;+
; Find the difference of two sets of indices. A set of indices is 
; represented by an array of non-negative integers where a scalar -1L 
; indicates the empty set.
; 
; :Returns:
;    lonarr or -1L
;
; :Params:
;    ind1 : in, required, type=lonarr or -1L
;       array of indices where -1L indicates an empty set of indices
;    ind2 : in, required, type=lonarr or -1L
;       array of indices where -1L indicates an empty set of indices
;-
function mg_setdifference, ind1, ind2  
  compile_opt strictarr
  
  min1 = min(ind1, max=max1)
  min2 = min(ind2, max=max2)

  ; empty set difference if ind1 is empty
  if (ind1[0] eq -1L) then return, -1L
  
  ; return ind1 if no intersection
  if ((min2 gt max1) || (max2 lt min1)) then return, ind1
  
  r = where((histogram(ind1, min=min1, max=max1) ne 0L) and $
            (histogram(ind2, min=min1, max=max1) eq 0), count)
  
  return, count eq 0L ? -1L : r + min1
end