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)  1985-2013, University of Amsterdam
    7                              VU University Amsterdam
    8    All rights reserved.
    9
   10    Redistribution and use in source and binary forms, with or without
   11    modification, are permitted provided that the following conditions
   12    are met:
   13
   14    1. Redistributions of source code must retain the above copyright
   15       notice, this list of conditions and the following disclaimer.
   16
   17    2. Redistributions in binary form must reproduce the above copyright
   18       notice, this list of conditions and the following disclaimer in
   19       the documentation and/or other materials provided with the
   20       distribution.
   21
   22    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   23    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   24    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
   25    FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
   26    COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
   27    INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
   28    BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
   29    LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
   30    CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   31    LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
   32    ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
   33    POSSIBILITY OF SUCH DAMAGE.
   34*/
   35
   36:- module(gensym,
   37        [ reset_gensym/0,
   38          reset_gensym/1,               % +Base
   39          gensym/2                      % +Base, -Symbol
   40        ]).   41:- set_prolog_flag(generate_debug_info, false).   42
   43/** <module> Generate unique symbols
   44
   45The predicate gensym/2 is a  traditional   predicate  to generate unique
   46symbols.  It should be used with care.
   47*/
   48
   49%!  gensym(+Base, -Unique)
   50%
   51%   Generate <Base>1, <Base>2, etc atoms   on  each subsequent call.
   52%   Note that there is nothing  that   prevents  other  parts of the
   53%   application to `invent'  the  same   identifier.  The  predicate
   54%   gensym/2 is thread-safe in the sense that two threads generating
   55%   identifiers from the same Base  will   never  generate  the same
   56%   identifier.
   57%
   58%   @see    uuid/1, term_hash/2, variant_sha1/2 may be used to
   59%           generate various unique or content-based identifiers
   60%           safely.
   61
   62gensym(Base, Atom) :-
   63    atom_concat('$gs_', Base, Key),
   64    flag(Key, Old, Old+1),
   65    record_gensym(Key, Old),
   66    New is Old+1,
   67    atom_concat(Base, New, Atom).
   68
   69record_gensym(Key, 0) :-
   70    !,
   71    recordz('$gensym', Key).
   72record_gensym(_, _).
   73
   74%!  reset_gensym
   75%
   76%   Reset all gensym counters.  Please beware this is dangerous: gensym
   77%   may be in use by other modules that do not expect their counter to
   78%   be reset!
   79
   80reset_gensym :-
   81    with_mutex('$gensym', do_reset_gensym).
   82
   83do_reset_gensym :-
   84    (   recorded('$gensym', Key, Ref),
   85        erase(Ref),
   86        set_flag(Key, 0),
   87        fail
   88    ;   true
   89    ).
   90
   91%!  reset_gensym(+Base)
   92%
   93%   Reset a specific gensym counter.  Please beware this still is
   94%   dangerous as other code may use gensym with the same atom!
   95
   96reset_gensym(Base) :-
   97    atom_concat('$gs_', Base, Key),
   98    set_flag(Key, 0).
   99
  100:- multifile sandbox:safe_primitive/1.  101
  102sandbox:safe_primitive(gensym:gensym(_,_))