(* 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 INTERSECT
 *)

(*
 * eccezione sollevata quando il join dei contesti
 * deve essere vuoto
 *)
exception Join_must_be_empty;;

(*
 * join fra due contesti
 *)
let xres_join_context h1 l1 h2 l2 =
 match (l1, l2) with
    ([], _) -> l2
 |  (_, []) -> l1
 |  (_,  _) ->
     let hh = h2 @ (List.find_all (function t -> not (List.mem t h2)) h1)
     and m1 = List.combine h1 l1
     and m2 = List.combine h2 l2
     in
      try 
       (List.map
        (fun elem ->
         let value1 = try (List.assoc elem m1) with Not_found -> List.assoc elem m2
         and value2 = try (List.assoc elem m2) with Not_found -> List.assoc elem m1
         in
          if value1 = value2 then value1 else raise Join_must_be_empty
        )
        hh
       ) with
        Join_must_be_empty -> []
;;

(*
 * implementazione del comando INTERSECT
 *)
let intersect_ex alist1 alist2 =
 let head1 = List.hd alist1
 and tail1 = List.tl alist1
 and head2 = List.hd alist2
 and tail2 = List.tl alist2 (* e fin qui ... *)
 in
  match (head1, head2) with
     ([], _) -> assert false (* gli header non devono mai essere vuoti *)
  |  (_, []) -> assert false (* devono contenere almeno [retVal] *)
  |  (_,  _) ->
      (match (tail1, tail2) with
          ([], _) -> [["retVal"]] (* se una delle due code e' vuota... *)
       |  (_, []) -> [["retVal"]] (* ... l'intersezione e' vuota *)
       |  (_,  _) ->
           [head2 @
            (List.find_all
	     (function t -> not (List.mem t head2))
	     head1
	    )
	   ] (* header del risultato finale *)
	   @
	   List.fold_left
            (fun par1 elem1 -> par1 @
             List.map
	      (fun elem2 ->
	       [(List.hd elem1)] @
	       (xres_join_context (List.tl head1) (List.tl elem1)
	                          (List.tl head2) (List.tl elem2))
	      )
              (List.find_all
               (fun elem2 -> (* trova tutti gli elementi della lista tail2 *)
		((List.hd elem1) = (List.hd elem2)) && (* che stanno in tail1 *)
		not ((xres_join_context (List.tl head1) (List.tl elem1)
		                        (List.tl head2) (List.tl elem2)) = [])
		(* e per i quali la xres_join_context non sia vuota *)
	       )
               tail2
	      )
             )
           []
           tail1 (* per ogni elemento di tail1 applica la List.fold_left *)
      ) (* match *)
;;