;-------------------------------------------------------------
;+
; NAME:
;        CHKSTRU  (function)
;
; PURPOSE:
;        check validity of a structure and test if necessary
;        fields are contained
;
; CATEGORY:
;        tools
;
; CALLING SEQUENCE:
;        res=CHKSTRU(STRUCTURE,FIELDS [,/VERBOSE]) 
;
; INPUTS:
;        STRUCTURE --> the structure to be tested. If STRUCTURE is
;             not of type structure, the function will return 0
;
;        FIELDS --> a string or string array with field names to 
;             be contained in STRUCTURE. CHKSTRU returns 1 (true)
;             only if all field names are contained in STRUCTURE.
;             The entries of FIELDS may be upper or lowercase.
;
; KEYWORD PARAMETERS:
;        INDEX --> a named variable that will contain the indices of
;             the required field names in the structure. They can then
;             be assessed through structure.(index(i)) . Index will
;             contain -1 for all fields entries that are not in the
;             structure.
;
;        /VERBOSE --> set this keyword to return an error message 
;             in case of an error.
;
; OUTPUTS:
;        CHKSTRU returns 1 if successful, otherwise 0.
;
; SUBROUTINES:
;
; REQUIREMENTS:
;
; NOTES:
;
; EXAMPLE:
;        test = { a:1, b:2, c:3 }
;        required = ['a','c']
;        if CHKSTRU(test,required) then print,'found a and c.'
;
; MODIFICATION HISTORY:
;        mgs, 02 Mar 1998: VERSION 1.00
;        mgs, 07 Apr 1998: - second parameter (FIELDS) now optional
;
;-
; Copyright (C) 1998, Martin Schultz, Harvard University
; This software is provided as is without any warranty
; whatsoever. It may be freely used, copied or distributed
; for non-commercial purposes. This copyright notice must be
; kept with any copy of this software. If this software shall
; be used commercially or sold as part of a larger package,
; please contact the author to arrange payment.
; Bugs and comments should be directed to mgs@io.harvard.edu
; with subject "IDL routine chkstru"
;-------------------------------------------------------------


function chkstru,structure,fields,index=index,verbose=verbose
 

     ; default index
     index = -1
 
     ; first check number of parameters (must be at least 1)
     if (n_params() lt 1) then begin
         if(keyword_set(verbose)) then $
             print,'CHKSTRU: ** invalid number of parameters ! **'
             return,0
         endif
 
 
     ; check if the user really passed a structure
 
     s = size(structure)
     if (s(1+s(0)) ne 8) then begin
         if(keyword_set(verbose)) then $
             print,'CHKSTRU: ** No structure passed ! **'
         return,0
     endif
 
     ; only one parameter: then we are finished
     if (n_params() eq 1) then return,1


 
     ; see if required field names are contained in the structure
     ; and return indices of these fields
 
     names = tag_names(structure)
     index = intarr(n_elements(fields)) - 1   ; default index to 'not found'

     for i=0,n_elements(fields)-1 do begin
         ind = where(names eq strupcase(fields(i)))
         if (ind(0) lt 0) then begin
             if(keyword_set(verbose)) then $
                print,'CHKSTRU: ** Cannot find field '+fields(i)+' ! **'  
         endif else index(i) = ind(0)
     endfor
 
 
     ; check minimum value of index field: -1 indicates error
     return,(min(index) ge 0)
 
end