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/* Part of SWI-Prolog
Author: Jan Wielemaker
E-mail: J.Wielemaker@cs.vu.nl
WWW: http://www.swi-prolog.org
Copyright (C): 2011, VU University Amsterdam
This program 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.
This program 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 this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
As a special exception, if you link this library with other files,
compiled with a Free Software compiler, to produce an executable, this
library does not by itself cause the resulting executable to be covered
by the GNU General Public License. This exception does not however
invalidate any other reasons why the executable file might be covered by
the GNU General Public License.
*/
:- module(varnumbers,
[ numbervars/1, % +Term
varnumbers/2, % +Term, -Copy
max_var_number/3, % +Term, +Start, -Max
varnumbers/3 % +Term, +No, -Copy
]).
:- use_module(library(error)).
/** <module> Utilities for numbered terms
This library provides the inverse functionality of the built-in
numbervars/3. Note that this library suffers from the known issues that
'$VAR'(X) is a normal Prolog term and, -unlike the built-in numbervars-,
the inverse predicates do _not_ process cyclic terms. The following
predicate is true for any acyclic term that contains no '$VAR'(X),
integer(X) terms and no constraint variables:
==
always_true(X) :-
copy_term(X, X2),
numbervars(X),
varnumbers(X, Copy),
Copy =@= X2.
==
@see numbervars/4, =@=/2 (variant/2).
@compat This library was introduced by Quintus and available in
many related implementations, although not with exactly the
same set of predicates.
*/
%% numbervars(+Term) is det.
%
% Number variables in Term using $VAR(N). Equivalent to
% numbervars(Term, 0, _).
%
% @see numbervars/3, numbervars/4
numbervars(Term) :-
numbervars(Term, 0, _).
%% varnumbers(+Term, -Copy) is det.
%
% Inverse of numbervars/1. Equivalent to varnumbers(Term, 0,
% Copy).
varnumbers(Term, Copy) :-
varnumbers(Term, 0, Copy).
%% varnumbers(+Term, +Start, -Copy) is det.
%
% Inverse of numbervars/3. True when Copy is a copy of Term with
% all variables numbered >= Start consistently replaced by fresh
% variables. Variables in Term are _shared_ with Copy rather than
% replaced by fresh variables.
%
% @error domain_error(acyclic_term, Term) if Term is cyclic.
% @compat Quintus, SICStus. Not in YAP version of this library
varnumbers(Term, Min, Copy) :-
must_be(acyclic, Term),
MaxStart is Min-1,
max_var_number(Term, MaxStart, Max),
NVars is Max-MaxStart,
( NVars == 0
-> Copy = Term
; roundup_next_power_two(NVars, Len),
functor(Vars, v, Len),
varnumbers(Term, MaxStart, Vars, Copy)
).
varnumbers(Var, _, _, Copy) :-
var(Var), !,
Copy = Var.
varnumbers(Var, _, _, Copy) :-
atomic(Var), !,
Copy = Var.
varnumbers('$VAR'(I), Min, Vars, Copy) :-
integer(I),
I > Min, !,
Index is I-Min,
arg(Index, Vars, Copy).
varnumbers(Term, Min, Vars, Copy) :-
functor(Term, Name, Arity),
functor(Copy, Name, Arity),
varnumbers_args(1, Arity, Term, Min, Vars, Copy).
varnumbers_args(I, Arity, Term, Min, Vars, Copy) :-
I =< Arity, !,
arg(I, Term, AT),
arg(I, Copy, CT),
varnumbers(AT, Min, Vars, CT),
I2 is I + 1,
varnumbers_args(I2, Arity, Term, Min, Vars, Copy).
varnumbers_args(_, _, _, _, _, _).
%% roundup_next_power_two(+Int, -NextPower) is det.
%
% NextPower is I**2, such that NextPower >= Int.
roundup_next_power_two(1, 1) :- !.
roundup_next_power_two(N, L) :-
L is 1<<(msb(N-1)+1).
%% max_var_number(+Term, +Start, -Max) is det.
%
% True when Max is the max of Start and the highest numbered
% $VAR(N) term.
%
% @author Vitor Santos Costa
% @compat YAP
max_var_number(V, Max, Max) :-
var(V), !.
max_var_number('$VAR'(I), Max0, Max) :-
integer(I), !,
Max is max(I,Max0).
max_var_number(S, Max0, Max) :-
functor(S, _, Ar),
max_var_numberl(Ar, S, Max0, Max).
max_var_numberl(0, _, Max, Max) :- !.
max_var_numberl(I, T, Max0, Max) :-
arg(I, T, Arg),
I2 is I-1,
max_var_number(Arg, Max0, Max1),
max_var_numberl(I2, T, Max1, Max).
|
