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
   45Gensym (_Generate Symbols_) is an  old   library  for  generating unique
   46symbols (atoms). Such symbols are generated from  a base atom which gets
   47a sequence number  appended.  Of  course   there  is  no  guarantee that
   48`catch22` is not an already defined atom and therefore one must be aware
   49these atoms are only unique in an isolated context.
   50
   51The SWI-Prolog gensym library is thread-safe.   The sequence numbers are
   52global over all threads and therefore   generated  atoms are unique over
   53all threads.
   54*/
   55
   56%!  gensym(+Base, -Unique)
   57%
   58%   Generate <Base>1, <Base>2, etc atoms   on  each subsequent call.
   59%   Note that there is nothing  that   prevents  other  parts of the
   60%   application to `invent'  the  same   identifier.  The  predicate
   61%   gensym/2 is thread-safe in the sense that two threads generating
   62%   identifiers from the same Base  will   never  generate  the same
   63%   identifier.
   64%
   65%   @see    uuid/1, term_hash/2, variant_sha1/2 may be used to
   66%           generate various unique or content-based identifiers
   67%           safely.
   68
   69gensym(Base, Atom) :-
   70    atom_concat('$gs_', Base, Key),
   71    flag(Key, Old, Old+1),
   72    record_gensym(Key, Old),
   73    New is Old+1,
   74    atom_concat(Base, New, Atom).
   75
   76record_gensym(Key, 0) :-
   77    !,
   78    recordz('$gensym', Key).
   79record_gensym(_, _).
   80
   81%!  reset_gensym
   82%
   83%   Reset gensym for all registered keys.   This  predicate is available
   84%   for compatibility only. New code is   strongly  advised to avoid the
   85%   use of reset_gensym or at least to reset  only the keys used by your
   86%   program to avoid unexpected side effects on other components.
   87
   88reset_gensym :-
   89    with_mutex('$gensym', do_reset_gensym).
   90
   91do_reset_gensym :-
   92    (   recorded('$gensym', Key, Ref),
   93        erase(Ref),
   94        set_flag(Key, 0),
   95        fail
   96    ;   true
   97    ).
   98
   99%!  reset_gensym(+Base)
  100%
  101%   Restart generation of identifiers from Base at <Base>1. Used to make
  102%   sure a program produces the  same   results  on subsequent runs. Use
  103%   with care.
  104
  105reset_gensym(Base) :-
  106    atom_concat('$gs_', Base, Key),
  107    set_flag(Key, 0).
  108
  109:- multifile sandbox:safe_primitive/1.  110
  111sandbox:safe_primitive(gensym:gensym(_,_))