;+
;
; @file_comments
; find the closest point of (P0) within a list of np1 points
; P1 which can be on a sphere
;
; @categories
; Maps
;
; @param p0lon {in}{required} {type=scalar}
; longitudes of point P0.
;
; @param p0lat {in}{required} {type=scalar}
; latitudes of point P0.
;
; @param neighlon {in}{optional}
;
; @param neighlat {in}{optional}
;
; @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 closest 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
;
; @version
; $Id$
;
;-
FUNCTION neighbor, p0lon, p0lat, neighlon, neighlat $
                 , SPHERE=sphere, DISTANCE=distance, RADIANS=radians
;
  compile_opt idl2, strictarrsubs
;
; some checks
  IF  n_elements(p0lon) NE 1 THEN ras = report('Sorry p0lon must be a scalar')
  p0lon = p0lon[0]
  IF  n_elements(p0lat) NE 1 THEN ras = report('Sorry p0lat must be a scalar')
  p0lat = p0lat[0]
  nneig = n_elements(neighlon)
  IF  n_elements(neighlat) NE nneig  THEN $
    ras = report('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