/*  Part of SWI-Prolog

    Author:        Tom Schrijvers, Markus Triska and Jan Wielemaker
    E-mail:        Tom.Schrijvers@cs.kuleuven.ac.be
    WWW:           http://www.swi-prolog.org
    Copyright (c)  2004-2023, K.U.Leuven
                              SWI-Prolog Solutions b.v.
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       notice, this list of conditions and the following disclaimer.

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*/

:- module(dif,
          [ dif/2                               % +Term1, +Term2
          ]).
:- autoload(library(lists),[append/3, reverse/2, member/2]).


:- set_prolog_flag(generate_debug_info, false).

/** <module> The dif/2 constraint
*/

%!  dif(+Term1, +Term2) is semidet.
%
%   Constraint that expresses that  Term1   and  Term2  never become
%   identical (==/2). Fails if `Term1 ==   Term2`. Succeeds if Term1
%   can  never  become  identical  to  Term2.  In  other  cases  the
%   predicate succeeds after attaching constraints   to the relevant
%   parts of Term1 and Term2 that prevent   the  two terms to become
%   identical.

dif(X,Y) :-
    ?=(X,Y),
    !,
    X \== Y.
dif(X,Y) :-
    dif_c_c(X,Y,_).

/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
The constraint is helt in  an   attribute  `dif`. A constrained variable
holds a term  vardif(L1,L2)  where  `L1`   is  a  list  OrNode-Value for
constraints on this variable  and  `L2`   is  the  constraint list other
variables have on me.

The `OrNode` is a term node(Pairs), where `Pairs` is a of list Var=Value
terms representing the pending unifications. The  original dif/2 call is
represented by a single OrNode.

If a unification related to an  OrNode   fails  the terms are definitely
unequal and thus we can kill all   pending constraints and succeed. If a
unequal related to an OrNode succeeds we remove it from the node. If the
node becomes empty the terms are equal and we must fail.

The following invariants must hold

  - Any variable involved in a dif/2 constraint has an attribute
    vardif(L1,L2), Where each element of both lists is a term
    OrNode-Value, L1 represents the values this variable may __not__
    become equal to and L2 represents this variable involved in other
    constraints.  I.e, L2 is only used if a dif/2 requires two variables
    to be different.
  - An OrNode has an attribute node(Pairs), where Pairs contains the
    possible unifications.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

dif_unifiable(X, Y, Us) :-
    (    current_prolog_flag(occurs_check, error)
    ->   catch(unifiable(X,Y,Us), error(occurs_check(_,_),_), false)
    ;    unifiable(X, Y, Us)
    ).

%!  dif_c_c(+X,+Y,!OrNode)
%
%   Enforce dif(X,Y) that is related to the given OrNode. If X and Y are
%   equal we reduce the OrNode.  If  they   cannot  unify  we  are done.
%   Otherwise we extend the OrNode with  new pairs and create/extend the
%   vardif/2 terms for the left hand side of  the unifier as well as the
%   right hand if this is a variable.

dif_c_c(X,Y,OrNode) :-
    (   dif_unifiable(X, Y, Unifier)
    ->  (   Unifier == []
        ->  or_one_fail(OrNode)
        ;   dif_c_c_l(Unifier,OrNode, U),
            subunifier(U, OrNode)
        )
    ;   or_succeed(OrNode)
    ).

subunifier([], _).
subunifier([X=Y|T], OrNode) :-
    dif_c_c(X, Y, OrNode),
    subunifier(T, OrNode).


%!  dif_c_c_l(+Unifier, +OrNode)
%
%   Extend OrNode with new elements from the   unifier.  Note that it is
%   possible that a unification against the   same variable appears as a
%   result of how unifiable acts on  sharing subterms. This is prevented
%   by simplify_ornode/3.
%
%   @see test 14 in src/Tests/attvar/test_dif.pl.

dif_c_c_l(Unifier, OrNode, U) :-
    extend_ornode(OrNode, List, Tail),
    dif_c_c_l_aux(Unifier, OrNode, List0, Tail),
    (   simplify_ornode(List0, List, U)
    ->  true
    ;   List = List0,
        or_succeed(OrNode),
        U = []
    ).

extend_ornode(OrNode, List, Vars) :-
    (   get_attr(OrNode, dif, node(Vars))
    ->  true
    ;   Vars = []
    ),
    put_attr(OrNode,dif,node(List)).

dif_c_c_l_aux([],_,List,List).
dif_c_c_l_aux([X=Y|Unifier],OrNode,List,Tail) :-
    List = [X=Y|Rest],
    add_ornode(X,Y,OrNode),
    dif_c_c_l_aux(Unifier,OrNode,Rest,Tail).

%!  add_ornode(+X, +Y, +OrNode)
%
%   Extend the vardif constraints on X and Y with the OrNode.

add_ornode(X,Y,OrNode) :-
    add_ornode_var1(X,Y,OrNode),
    (   var(Y)
    ->  add_ornode_var2(X,Y,OrNode)
    ;   true
    ).

add_ornode_var1(X,Y,OrNode) :-
    (   get_attr(X,dif,Attr)
    ->  Attr = vardif(V1,V2),
        put_attr(X,dif,vardif([OrNode-Y|V1],V2))
    ;   put_attr(X,dif,vardif([OrNode-Y],[]))
    ).

add_ornode_var2(X,Y,OrNode) :-
    (   get_attr(Y,dif,Attr)
    ->  Attr = vardif(V1,V2),
        put_attr(Y,dif,vardif(V1,[OrNode-X|V2]))
    ;   put_attr(Y,dif,vardif([],[OrNode-X]))
    ).

%!  simplify_ornode(+OrNode) is semidet.
%
%   Simplify the possible unifications left on the original dif/2 terms.
%   There are two reasons for simplification. First   of all, due to the
%   way unifiable works we may end up with variables in the unifier that
%   do not refer to the original terms,   but  to variables in subterms,
%   e.g. `[V1 = f(a, V2), V2 = b]`.   As a result of subsequent unifying
%   variables, the unifier may end up   having  multiple entries for the
%   same variable, possibly having different values, e.g.,  `[X = a, X =
%   b]`.  As  these  can  never  be  satified  both  we  have  prove  of
%   inequality.
%
%   Finally, we remove elements from the list that have become equal. If
%   the OrNode is empty, the original terms   are equal and thus we must
%   fail.
%
%   @tbd The simplification is complicated. Another approach would be to
%   turn `[X1=Y1, X2=Y2, ...]` into  `[X1,X2,...]` and `[Y1,Y2,...]` and
%   call  unifiable/3  on  these  lists  to    either   end  up  with  a
%   contradiction (satisfied) or a  new  unifier.   This  is  a stronger
%   propagation. Seems not so easy though.  Both pending constraints and
%   reconnecting to the proper variables need attention.

simplify_ornode(OrNode) :-
    (   get_attr(OrNode, dif, node(Pairs0))
    ->  simplify_ornode(Pairs0, Pairs, U),
	(   Pairs == [],
	    U == []
	->  fail
        ;   put_attr(OrNode, dif, node(Pairs)),
            subunifier(U, OrNode)
	)
    ;   true
    ).

simplify_ornode(List0, List, U) :-
    sort(1, @=<, List0, Sorted),
    simplify_ornode_(Sorted, List, U).

simplify_ornode_([], List, U) =>
    List = [],
    U = [].
simplify_ornode_([V1=V2|T], List, U), V1 == V2 =>
    simplify_ornode_(T, List, U).
simplify_ornode_([V1=Val1,V2=Val2|T], List, U), var(V1), V1 == V2 =>
    (   ?=(Val1, Val2)
    ->  Val1 == Val2,
        simplify_ornode_([V1=Val2|T], List, U)
    ;   U = [Val1=Val2|UT],
        simplify_ornode_([V2=Val2|T], List, UT)
    ).
simplify_ornode_([H|T], List, U) =>
    List = [H|Rest],
    simplify_ornode_(T, Rest, U).


%!  attr_unify_hook(+VarDif, +Other)
%
%   If two dif/2 variables are unified  we   must  join the two vardif/2
%   terms. To do so, we filter the vardif terms for the ones involved in
%   this unification. Those that  are  represent   OrNodes  that  have a
%   unification satisfied. For the rest we  remove the unifications with
%   _self_, append them and use this as new vardif term.
%
%   On unification with a value, we recursively call dif_c_c/3 using the
%   existing OrNodes.

attr_unify_hook(vardif(V1,V2),Other) :-
    (   get_attr(Other, dif, vardif(OV1,OV2))
    ->  (   (   or_nodes_completed(V1)
	    ;   or_nodes_completed(V2)
	    )
	->  true
	;   reverse_lookups(V1, Other, OrNodes1, NV1),
	    or_one_fails(OrNodes1),
	    reverse_lookups(OV1, Other, OrNodes2, NOV1),
	    or_one_fails(OrNodes2),
	    remove_obsolete(V2, Other, NV2),
	    remove_obsolete(OV2, Other, NOV2),
	    append(NV1, NOV1, CV1),
	    append(NV2, NOV2, CV2),
	    (   CV1 == [], CV2 == []
	    ->  del_attr(Other, dif)
	    ;   put_attr(Other, dif, vardif(CV1,CV2))
	    )
        )
    ;   var(Other)			% unrelated variable
    ->  put_attr(Other, dif, vardif(V1,V2))
    ;   verify_compounds(V1, Other),    % concrete value
        verify_compounds(V2, Other)
    ).

%!  or_nodes_completed(+List) is semidet.
%
%   Unification  may  have  made  some  of  the  or  nodes  internally
%   inconsistent.  This  code checks  for that and  makes the  or node
%   succeed if this is the case.

or_nodes_completed(List) :-
    member(Or-_Value, List),
    get_attr(Or, dif, node(Unifiers0)),
    copy_term_nat(Unifiers0, Unifiers),
    \+ unify_list(Unifiers),
    !,
    or_succeed(Or).

unify_list([]).
unify_list([A=A|T]) :-
    unify_list(T).


remove_obsolete([], _, []).
remove_obsolete([N-Y|T], X, L) :-
    (   Y==X
    ->  remove_obsolete(T, X, L)
    ;   L=[N-Y|RT],
        remove_obsolete(T, X, RT)
    ).

reverse_lookups([],_,[],[]).
reverse_lookups([N-X|NXs],Value,Nodes,Rest) :-
    (   X == Value
    ->  Nodes = [N|RNodes],
        Rest = RRest
    ;   Nodes = RNodes,
        Rest = [N-X|RRest]
    ),
    reverse_lookups(NXs,Value,RNodes,RRest).

%!  verify_compounds(+Nodes, +Value)
%
%   Unification to a concrete Value (no variable)

verify_compounds([],_).
verify_compounds([OrNode-Y|Rest],X) :-
    (   var(Y)
    ->  true
    ;   OrNode == (-)
    ->  true
    ;   dif_c_c(X,Y,OrNode)
    ),
    verify_compounds(Rest,X).

%!  or_succeed(+OrNode) is det.
%
%   The dif/2 constraint related  to  OrNode   is  complete,  i.e., some
%   (sub)terms can definitely not become equal.   Next,  we can clean up
%   the constraints. We do so by setting   the  OrNode to `-` and remove
%   this _dead_ OrNode from every vardif/2 attribute we can find.

or_succeed(OrNode) :-
    (   get_attr(OrNode,dif,Attr)
    ->  Attr = node(Pairs),
        del_attr(OrNode,dif),
        OrNode = (-),
        del_or_dif(Pairs)
    ;   true
    ).

del_or_dif([]).
del_or_dif([X=Y|Xs]) :-
    cleanup_dead_nodes(X),
    cleanup_dead_nodes(Y),              % JW: what about embedded variables?
    del_or_dif(Xs).

cleanup_dead_nodes(X) :-
    (   get_attr(X,dif,Attr)
    ->  Attr = vardif(V1,V2),
        filter_dead_ors(V1,NV1),
        filter_dead_ors(V2,NV2),
        (   NV1 == [], NV2 == []
        ->  del_attr(X,dif)
        ;   put_attr(X,dif,vardif(NV1,NV2))
        )
    ;   true
    ).

filter_dead_ors([],[]).
filter_dead_ors([Or-Y|Rest],List) :-
    (   var(Or)
    ->  List = [Or-Y|NRest]
    ;   List = NRest
    ),
    filter_dead_ors(Rest,NRest).


%!  or_one_fail(+OrNode) is semidet.
%
%   Some unification related to OrNode succeeded.

or_one_fail(OrNode) :-
    simplify_ornode(OrNode).

or_one_fails([]).
or_one_fails([N|Ns]) :-
    or_one_fail(N),
    or_one_fails(Ns).


/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
   The attribute of a variable X is vardif/2. The first argument is a
   list of pairs. The first component of each pair is an OrNode. The
   attribute of each OrNode is node/2. The second argument of node/2
   is a list of equations A = B. If the LHS of the first equation is
   X, then return a goal, otherwise don't because someone else will.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

attribute_goals(Var) -->
    (   { get_attr(Var, dif, vardif(Ors,_)) }
    ->  or_nodes(Ors, Var)
    ;   or_node(Var)
    ).

or_node(O) -->
    (   { get_attr(O, dif, node(Pairs)) }
    ->  { eqs_lefts_rights(Pairs, As, Bs) },
        mydif(As, Bs),
        { del_attr(O, dif) }
    ;   []
    ).

or_nodes([], _)       --> [].
or_nodes([O-_|Os], X) -->
    (   { get_attr(O, dif, node(Eqs)) }
    ->  (   { Eqs = [LHS=_|_], LHS == X }
        ->  { eqs_lefts_rights(Eqs, As, Bs) },
            mydif(As, Bs),
            { del_attr(O, dif) }
        ;   []
        )
    ;   [] % or-node already removed
    ),
    or_nodes(Os, X).

mydif([X], [Y]) --> !, dif_if_necessary(X, Y).
mydif(Xs0, Ys0) -->
    { reverse(Xs0, Xs), reverse(Ys0, Ys), % follow original order
      X =.. [f|Xs], Y =.. [f|Ys]
    },
    dif_if_necessary(X, Y).

dif_if_necessary(X, Y) -->
    (   { dif_unifiable(X, Y, _) }
    ->  [dif(X,Y)]
    ;   []
    ).

eqs_lefts_rights([], [], []).
eqs_lefts_rights([A=B|ABs], [A|As], [B|Bs]) :-
    eqs_lefts_rights(ABs, As, Bs).
