/* Part of SWI-Prolog
Author: Jan Wielemaker
E-mail: J.Wielemaker@vu.nl
WWW: http://www.swi-prolog.org
Copyright (c) 2005-2020, VU University Amsterdam
CWI, Amsterdam
All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in
the documentation and/or other materials provided with the
distribution.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
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*/
:- module(nb_set,
[ empty_nb_set/1, % -EmptySet
add_nb_set/2, % +Key, !Set
add_nb_set/3, % +Key, !Set, ?New
gen_nb_set/2, % +Set, -Key
size_nb_set/2, % +Set, -Size
nb_set_to_list/2 % +Set, -List
]).
:- autoload(library(lists),[member/2,append/2]).
:- autoload(library(terms),[term_factorized/3]).
/** Non-backtrackable sets
This library provides a non-backtrackabe _set_ of terms that are
variants of each other. It is primarily intended to implement distinct/1
from library(solution_sequences). The set is implemented as a hash table
that is built using non-backtrackable primitives, notably nb_setarg/3.
The original version of this library used binary trees which provides
immediate ordering. As the trees were not balanced, performance could
get really poor. The complexity of balancing trees using
non-backtrackable primitives is too high.
@author Jan Wielemaker
*/
initial_size(32). % initial hash-table size
%! empty_nb_set(-Set)
%
% Create an empty non-backtrackable set.
empty_nb_set(nb_set(Buckets, 0)) :-
initial_size(Size),
'$filled_array'(Buckets, buckets, Size, []).
%! add_nb_set(+Key, !Set) is det.
%! add_nb_set(+Key, !Set, ?New) is semidet.
%! add_nb_set(+Key, !Set, ?New) is semidet.
%
% Insert Key into the set. If a variant (see =@=/2) of Key is
% already in the set, the set is unchanged and New is unified with
% `false`. Otherwise, New is unified with `true` and a _copy of_
% Key is added to the set.
%
% @tbd Computing the hash for cyclic terms is performed with
% the help of term_factorized/3, which performs rather
% poorly.
add_nb_set(Key, Set) :-
add_nb_set(Key, Set, _).
add_nb_set(Key, Set, New) :-
arg(1, Set, Buckets),
compound_name_arity(Buckets, _, BCount),
hash_key(Key, BCount, Hash),
arg(Hash, Buckets, Bucket),
( member(X, Bucket),
Key =@= X
-> New = false
; New = true,
duplicate_term(Key, Copy),
nb_linkarg(Hash, Buckets, [Copy|Bucket]),
arg(2, Set, Size0),
Size is Size0+1,
nb_setarg(2, Set, Size),
( Size > BCount
-> rehash(Set)
; true
)
).
%! hash_key(+Term, +BucketCount, -Key) is det.
%
% Compute a hash for Term. Note that variant_hash/2 currently does
% not handle cyclic terms, so use term_factorized/3 to get rid of
% the cycles. This means that this library is rather slow when
% cyclic terms are involved.
:- if(catch((A = f(A), variant_hash(A,_)), _, fail)).
hash_key(Term, BCount, Key) :-
variant_hash(Term, IntHash),
Key is (IntHash mod BCount)+1.
:- else.
hash_key(Term, BCount, Key) :-
acyclic_term(Key),
!,
variant_hash(Term, IntHash),
Key is (IntHash mod BCount)+1.
hash_key(Term, BCount, Key) :-
term_factorized(Term, Skeleton, Substiution),
variant_hash(Skeleton+Substiution, IntHash),
Key is (IntHash mod BCount)+1.
:- endif.
rehash(Set) :-
arg(1, Set, Buckets0),
compound_name_arity(Buckets0, Name, Arity0),
Arity is Arity0*2,
'$filled_array'(Buckets, Name, Arity, []),
nb_setarg(1, Set, Buckets),
nb_setarg(2, Set, 0),
( arg(_, Buckets0, Chain),
member(Key, Chain),
add_nb_set(Key, Set, _),
fail
; true
).
%! nb_set_to_list(+Set, -List)
%
% Get the elements of a an nb_set. List is sorted to the standard
% order of terms.
nb_set_to_list(nb_set(Buckets, _Size), OrdSet) :-
compound_name_arguments(Buckets, _, Args),
append(Args, List),
sort(List, OrdSet).
%! gen_nb_set(+Set, -Key)
%
% Enumerate the members of a set in the standard order of terms.
gen_nb_set(Set, Key) :-
nb_set_to_list(Set, OrdSet),
member(Key, OrdSet).
%! size_nb_set(+Set, -Size)
%
% Unify Size with the number of elements in the set
size_nb_set(nb_set(_, Size), Size).