;+
;
; @file_comments
; find the intersection between 2 matrices of whole numbers
;
; see also <pro>different</pro> and <pro>union</pro>
;
; @categories
; Calculation
;
; @param a {in}{required} 
; arrays of positive integers, which need not to be
; sorted. Duplicate elements are ignored, as they have no effect on the
; result
;
; @param b {in}{required} 
; see a
;
; @returns
; an array containing the set of values in both a and b.
;
; The empty set is denoted by an array with the first element equal to
; -1.
;
; @restrictions 
; These functions will not be efficient on sparse sets with wide
; ranges, as they trade memory for efficiency. 
; The <proidl>HISTOGRAM</proidl> function
; is used, which creates arrays of size equal to the range of the
; resulting set.
;
; @examples
;
;   IDL> a = [2,4,6,8]
;   IDL> b = [6,1,3,2]
;   IDL> res=inter(a,b)
;              2           6
; Right because 2 and 6 are in a and B.
;
; @history
; <a href="http://www.dfanning.com/tips/set_operations.html"/>
;
; @version
; $Id$
;
;-
FUNCTION inter, a, b
;
  compile_opt idl2, strictarrsubs
;
   case 1 of
      n_elements(a) EQ 0:return,  -1
      n_elements(b) EQ 0:return,  -1
      n_elements(a) EQ 1 AND n_elements(b) NE 1: $
       if (where(b EQ a[0]))[0] EQ -1 then return, -1 ELSE return,  a[0]
      n_elements(b) EQ 1 AND n_elements(a) NE 1: $
       if (where(a EQ b[0]))[0] EQ -1 then return, -1 ELSE return,  b[0]
      n_elements(a) EQ 1 AND n_elements(b) EQ 1: $
       if (where(a[0] EQ b[0]))[0] EQ -1 then return, -1 ELSE return,  a[0]
      ELSE:
   ENDCASE
;
minab = Min(a, Max=maxa) > Min(b, Max=maxb) ;Only need intersection of ranges
maxab = maxa < maxb

   ; If either set is empty, or their ranges don't intersect: result = NULL.

IF maxab LT minab OR maxab LT 0 THEN RETURN, -1
r = Where(Histogram(a, Min=minab, Max=maxab) $
          *Histogram(b, Min=minab, Max=maxab), count)

IF count EQ 0 THEN RETURN, -1 ELSE RETURN, r + minab
END