(* Copyright (C) 2000, HELM Team.
 * 
 * This file is part of HELM, an Hypertextual, Electronic
 * Library of Mathematics, developed at the Computer Science
 * Department, University of Bologna, Italy.
 * 
 * HELM is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 * 
 * HELM is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with HELM; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston,
 * MA  02111-1307, USA.
 * 
 * For details, see the HELM World-Wide-Web page,
 * http://cs.unibo.it/helm/.
 *)

(*
 * implementazione del comando UNION
 *)


(* Merges two attribute group lists preserves order and gets rid of duplicates*)
let rec merge l1 l2 =
  match (l1,l2) with
     [],l
   | l,[] -> l
   | g1::tl1,g2::_ when g1 < g2 -> g1::(merge tl1 l2)
   | g1::_,g2::tl2 when g2 < g1 -> g2::(merge l1 tl2)
   | g1::tl1,g2::tl2 -> g1::(merge tl1 tl2)
;;       

(* preserves order and gets rid of duplicates *)
let rec union_ex rs1 rs2 =
  match (rs1, rs2) with
     [],l
   | l,[] -> l
   | (uri1,l1)::tl1,(uri2,_)::_ when uri1 < uri2 -> (uri1,l1)::(union_ex tl1 rs2)
   | (uri1,_)::_,(uri2,l2)::tl2 when uri2 < uri1 -> (uri2,l2)::(union_ex rs1 tl2)
   | (uri1,l1)::tl1,(uri2,l2)::tl2 -> if l1 = l2 then (uri1,l1)::(union_ex tl1 tl2)
				      else (uri1,merge l1 l2)::(union_ex tl1 tl2)	
;;