View source with formatted comments or as raw
    1/*  Part of SWI-Prolog
    2
    3    Author:        Jan Wielemaker
    4    E-mail:        J.Wielemaker@vu.nl
    5    WWW:           http://www.swi-prolog.org
    6    Copyright (c)  2005-2015, VU University Amsterdam
    7    All rights reserved.
    8
    9    Redistribution and use in source and binary forms, with or without
   10    modification, are permitted provided that the following conditions
   11    are met:
   12
   13    1. Redistributions of source code must retain the above copyright
   14       notice, this list of conditions and the following disclaimer.
   15
   16    2. Redistributions in binary form must reproduce the above copyright
   17       notice, this list of conditions and the following disclaimer in
   18       the documentation and/or other materials provided with the
   19       distribution.
   20
   21    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   22    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   23    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
   24    FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
   25    COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
   26    INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
   27    BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
   28    LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
   29    CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   30    LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
   31    ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
   32    POSSIBILITY OF SUCH DAMAGE.
   33*/
   34
   35:- module(nb_set,
   36          [ empty_nb_set/1,             % -EmptySet
   37            add_nb_set/2,               % +Key, !Set
   38            add_nb_set/3,               % +Key, !Set, ?New
   39            gen_nb_set/2,               % +Set, -Key
   40            size_nb_set/2,              % +Set, -Size
   41            nb_set_to_list/2            % +Set, -List
   42          ]).   43:- use_module(library(lists)).   44:- use_module(library(terms)).   45:- use_module(library(apply_macros), []).   46
   47/** <module> Non-backtrackable sets
   48
   49This library provides  a  non-backtrackabe  _set_   of  terms  that  are
   50variants of each other. It is primarily intended to implement distinct/1
   51from library(solution_sequences). The set is implemented as a hash table
   52that is built using non-backtrackable primitives, notably nb_setarg/3.
   53
   54The original version of this library   used  binary trees which provides
   55immediate ordering. As the trees were   not  balanced, performance could
   56get   really   poor.   The   complexity   of   balancing   trees   using
   57non-backtrackable primitives is too high.
   58
   59@author Jan Wielemaker
   60*/
   61
   62initial_size(32).                       % initial hash-table size
   63
   64%!  empty_nb_set(-Set)
   65%
   66%   Create an empty non-backtrackable set.
   67
   68empty_nb_set(nb_set(Buckets, 0)) :-
   69    initial_size(Size),
   70    '$filled_array'(Buckets, buckets, Size, []).
   71
   72%!  add_nb_set(+Key, !Set) is det.
   73%!  add_nb_set(+Key, !Set, ?New) is semidet.
   74%!  add_nb_set(+Key, !Set, ?New) is semidet.
   75%
   76%   Insert Key into the set. If  a   variant  (see  =@=/2) of Key is
   77%   already in the set, the set is unchanged and New is unified with
   78%   `false`. Otherwise, New is unified with   `true` and a _copy of_
   79%   Key is added to the set.
   80%
   81%   @tbd    Computing the hash for cyclic terms is performed with
   82%           the help of term_factorized/3, which performs rather
   83%           poorly.
   84
   85add_nb_set(Key, Set) :-
   86    add_nb_set(Key, Set, _).
   87add_nb_set(Key, Set, New) :-
   88    arg(1, Set, Buckets),
   89    compound_name_arity(Buckets, _, BCount),
   90    hash_key(Key, BCount, Hash),
   91    arg(Hash, Buckets, Bucket),
   92    (   member(X, Bucket),
   93        Key =@= X
   94    ->  New = false
   95    ;   New = true,
   96        duplicate_term(Key, Copy),
   97        nb_linkarg(Hash, Buckets, [Copy|Bucket]),
   98        arg(2, Set, Size0),
   99        Size is Size0+1,
  100        nb_setarg(2, Set, Size),
  101        (   Size > BCount
  102        ->  rehash(Set)
  103        ;   true
  104        )
  105    ).
  106
  107%!  hash_key(+Term, +BucketCount, -Key) is det.
  108%
  109%   Compute a hash for Term. Note that variant_hash/2 currently does
  110%   not handle cyclic terms, so use  term_factorized/3 to get rid of
  111%   the cycles. This means that  this   library  is rather slow when
  112%   cyclic terms are involved.
  113
  114:- if(catch((A = f(A), variant_hash(A,_)), error(type_error(_,_),_), fail)).  115hash_key(Term, BCount, Key) :-
  116    variant_hash(Term, IntHash),
  117    Key is (IntHash mod BCount)+1.
  118:- else.  119hash_key(Term, BCount, Key) :-
  120    acyclic_term(Key),
  121    !,
  122    variant_hash(Term, IntHash),
  123    Key is (IntHash mod BCount)+1.
  124hash_key(Term, BCount, Key) :-
  125    term_factorized(Term, Skeleton, Substiution),
  126    variant_hash(Skeleton+Substiution, IntHash),
  127    Key is (IntHash mod BCount)+1.
  128:- endif.  129
  130rehash(Set) :-
  131    arg(1, Set, Buckets0),
  132    compound_name_arity(Buckets0, Name, Arity0),
  133    Arity is Arity0*2,
  134    '$filled_array'(Buckets, Name, Arity, []),
  135    nb_setarg(1, Set, Buckets),
  136    nb_setarg(2, Set, 0),
  137    forall(( arg(_, Buckets0, Chain),
  138             member(Key, Chain)
  139           ),
  140           add_nb_set(Key, Set, _)).
  141
  142%!  nb_set_to_list(+Set, -List)
  143%
  144%   Get the elements of a an nb_set. List is sorted to the standard
  145%   order of terms.
  146
  147nb_set_to_list(nb_set(Buckets, _Size), OrdSet) :-
  148    compound_name_arguments(Buckets, _, Args),
  149    append(Args, List),
  150    sort(List, OrdSet).
  151
  152%!  gen_nb_set(+Set, -Key)
  153%
  154%   Enumerate the members of a set in the standard order of terms.
  155
  156gen_nb_set(Set, Key) :-
  157    nb_set_to_list(Set, OrdSet),
  158    member(Key, OrdSet).
  159
  160%!  size_nb_set(+Set, -Size)
  161%
  162%   Unify Size with the number of elements in the set
  163
  164size_nb_set(nb_set(_, Size), Size)