;+
;
; @file_comments
;find the closetest point of (P0) within a list of np1 points
;P1 Which can be on a sphere 
;
; @categories Maps
;
; @examples 
; IDL> Result = neighbor(lon0, lat0, lon1, lat1)
;
;@param p0lon {in}{required}  scalar. longitudes of point P0. 
;@param p0lat  {in}{required}  scalar. latitudes of point P0. 
;
; @keyword   RADIANS if set, inputs and angular outputs are in radians, otherwise
;degrees.
; @keyword   DISTANCE dis, to get back the distances between P0 and the np1
;   points P1 in the variable dis.
; @keyword   /SPHERE to activate if points are located on a sphere.
;
; @returns
;       index giving the P1[index] point that is the closetest point
;       of (P0)
;
; @examples
;       IDL> print, neighbor(-105.15,40.02,[-0.07,100,50],[51.30,20,0], $
;            distance=dis)
;                  0
;       IDL> print, dis
;             105.684      206.125      160.228
;
; @history
; Sebastien Masson (smasson\@lodyc.jussieu.fr)
;                  October 2003
;-
FUNCTION neighbor, p0lon, p0lat, neighlon, neighlat, sphere = sphere, distance = distance, radians = radians
;
; somme checks
  IF  n_elements(p0lon) NE 1 THEN MESSAGE, 'Sorry p0lon must be a scalar'
  p0lon = p0lon[0]
  IF  n_elements(p0lat) NE 1 THEN MESSAGE, 'Sorry p0lat must be a scalar'
  p0lat = p0lat[0]
  nneig = n_elements(neighlon) 
  IF  n_elements(neighlat) NE nneig  THEN $
    MESSAGE, 'neighlon and neighlat must have the same number of elements'
; distance between P0 and the others points
  IF keyword_set(sphere) THEN BEGIN
    IF sphere NE 1 THEN radius = sphere
    distance = Map_nPoints(p0lon, p0lat, neighlon, neighlat $
                       , radius = radius, radians = radians)
  ENDIF ELSE BEGIN 
    distance = (neighlon-p0lon)^2+(neighlat-p0lat)^2
    IF arg_present(distance) THEN distance = sqrt(distance)
  ENDELSE
  RETURN, where(distance EQ min(distance))
END