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(_,_)).