json.pl

/*  Part of SWI-Prolog

    Author:        Jan Wielemaker
    E-mail:        J.Wielemaker@vu.nl
    WWW:           http://www.swi-prolog.org
    Copyright (C): 1985-2014, University of Amsterdam
			      VU University Amsterdam

    This program is free software; you can redistribute it and/or
    modify it under the terms of the GNU General Public License
    as published by the Free Software Foundation; either version 2
    of the License, or (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public
    License along with this library; if not, write to the Free Software
    Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA

    As a special exception, if you link this library with other files,
    compiled with a Free Software compiler, to produce an executable, this
    library does not by itself cause the resulting executable to be covered
    by the GNU General Public License. This exception does not however
    invalidate any other reasons why the executable file might be covered by
    the GNU General Public License.
*/

:- if(current_predicate(is_dict/1)).

:- module(json,
	  [ json_read/2,		% +Stream, -JSONTerm
	    json_read/3,		% +Stream, -JSONTerm, +Options
	    atom_json_term/3,		% ?Atom, ?JSONTerm, +Options
	    json_write/2,		% +Stream, +Term
	    json_write/3,		% +Stream, +Term, +Options
	    is_json_term/1,		% @Term
	    is_json_term/2,		% @Term, +Options
					% Version 7 dict support
	    json_read_dict/2,		% +Stream, -Dict
	    json_read_dict/3,		% +Stream, -Dict, +Options
	    json_write_dict/2,		% +Stream, +Dict
	    json_write_dict/3,		% +Stream, +Dict, +Options
	    atom_json_dict/3		% ?Atom, ?JSONDict, +Options
	  ]).

:- else.

:- module(json,
	  [ json_read/2,		% +Stream, -JSONTerm
	    json_read/3,		% +Stream, -JSONTerm, +Options
	    atom_json_term/3,		% ?Atom, ?JSONTerm, +Options
	    json_write/2,		% +Stream, +Term
	    json_write/3,		% +Stream, +Term, +Options
	    is_json_term/1,		% @Term
	    is_json_term/2
	  ]).

:- endif.

:- use_module(library(record)).
:- use_module(library(memfile)).
:- use_module(library(error)).
:- use_module(library(option)).

:- use_foreign_library(foreign(json)).

:- predicate_options(json_read/3, 3,
		     [ null(ground),
		       true(ground),
		       false(ground),
		       value_string_as(oneof([atom,string]))
		     ]).
:- predicate_options(json_write/3, 3,
		     [ indent(nonneg),
		       step(positive_integer),
		       tab(positive_integer),
		       width(nonneg),
		       null(ground),
		       true(ground),
		       false(ground),
		       serialize_unknown(boolean)
		     ]).
:- predicate_options(json_read_dict/3, 3,
		     [ tag(atom),
		       pass_to(json_read/3, 3)
		     ]).
:- predicate_options(json_write_dict/3, 3,
		     [ tag(atom),
		       pass_to(json_write/3, 3)
		     ]).
:- predicate_options(is_json_term/2, 2,
		     [ null(ground),
		       true(ground),
		       false(ground)
		     ]).
:- predicate_options(atom_json_term/3, 3,
		     [ as(oneof([atom,string,codes])),
		       pass_to(json_read/3, 3),
		       pass_to(json_write/3, 3)
		     ]).

/** <module> Reading and writing JSON serialization

This module supports reading and  writing   JSON  objects.  This library
supports two Prolog representations (the   _new_  representation is only
supported in SWI-Prolog version 7 and later):

  - The *classical* representation is provided by json_read/3 and
    json_write/3.  This represents a JSON object as json(NameValueList),
    a JSON string as an atom and the JSON constants =null=, =true= and
    =false= as @(null), @(true) and @false.

  - The *new* representation is provided by json_read_dict/3 and
    json_write_dict/3. This represents a JSON object as a dict, a JSON
    string as a Prolog string and the JSON constants using the Prolog
    atoms =null=, =true= and =false=.

@author Jan Wielemaker
@see	http_json.pl links JSON to the HTTP client and server modules.
@see	json_convert.pl converts JSON Prolog terms to more comfortable
terms.
*/

:- record json_options(null:ground = @(null),
		       true:ground = @(true),
		       false:ground = @(false),
		       value_string_as:oneof([atom,string]) = atom,
		       tag:atom = '').

default_json_dict_options(
    json_options(null, true, false, string, '')).


		 /*******************************
		 *	 MAP TO/FROM TEXT	*
		 *******************************/

%%	atom_json_term(+Atom, -JSONTerm, +Options) is det.
%%	atom_json_term(-Text, +JSONTerm, +Options) is det.
%
%	Convert between textual  representation  and   a  JSON  term. In
%	_write_ mode, the option
%
%	    * as(Type)
%	    defines the output type, which is one of =atom=,
%	    =string= or =codes=.

atom_json_term(Atom, Term, Options) :-
	ground(Atom), !,
	setup_call_cleanup(
	    ( atom_to_memory_file(Atom, MF),
	      open_memory_file(MF, read, In, [free_on_close(true)])
	    ),
	    json_read(In, Term, Options),
	    close(In)).
atom_json_term(Result, Term, Options) :-
	select_option(as(Type), Options, Options1),
	(   type_term(Type, Result, Out)
	->  true
	;   must_be(oneof([atom,string,codes]), Type)
	),
	with_output_to(Out,
		       json_write(current_output, Term, Options1)).

type_term(atom,   Result, atom(Result)).
type_term(string, Result, string(Result)).
type_term(codes,  Result, codes(Result)).


		 /*******************************
		 *	     READING		*
		 *******************************/

%%	json_read(+Stream, -Term) is det.
%%	json_read(+Stream, -Term, +Options) is det.
%
%	Read next JSON value from Stream into a Prolog term. The
%	canonical representation for Term is:
%
%	  * A JSON object is mapped to a term json(NameValueList), where
%	    NameValueList is a list of Name=Value. Name is an atom
%	    created from the JSON string.
%
%	  * A JSON array is mapped to a Prolog list of JSON values.
%
%	  * A JSON string is mapped to a Prolog atom
%
%	  * A JSON number is mapped to a Prolog number
%
%	  * The JSON constants =true= and =false= are mapped -like JPL-
%	    to @(true) and @(false).
%
%	  * The JSON constant =null= is mapped to the Prolog term
%	    @(null)
%
%	Here is a complete example in  JSON and its corresponding Prolog
%	term.
%
%	  ==
%	  { "name":"Demo term",
%	    "created": {
%	      "day":null,
%	      "month":"December",
%	      "year":2007
%	    },
%	    "confirmed":true,
%	    "members":[1,2,3]
%	  }
%	  ==
%
%	  ==
%	  json([ name='Demo term',
%	         created=json([day= @null, month='December', year=2007]),
%	         confirmed= @true,
%	         members=[1, 2, 3]
%	       ])
%	  ==
%
%	The following options are processed:
%
%		* null(NullTerm)
%		Term used to represent JSON =null=.  Default @(null)
%		* true(TrueTerm)
%		Term used to represent JSON =true=.  Default @(true)
%		* false(FalsTerm)
%		Term used to represent JSON =false=.  Default @(false)
%		* value_string_as(Type)
%		Prolog type used for strings used as value.  Default
%		is =atom=.  The alternative is =string=, producing a
%		packed string object.  Please note that =codes= or
%		=chars= would produce ambiguous output and is therefore
%		not supported.
%
%	If json_read/3 encounters end-of-file before any real data it
%	binds Term to the term @(end_of_file).
%
%	@see	json_read_dict/3 to read a JSON term using the version 7
%		extended data types.

json_read(Stream, Term) :-
	default_json_options(Options),
	(   json_value(Stream, Term, _, Options)
	->  true
	;   syntax_error(illegal_json, Stream)
	).
json_read(Stream, Term, Options) :-
	make_json_options(Options, OptionTerm, _RestOptions),
	(   json_value(Stream, Term, _, OptionTerm)
	->  true
	;   syntax_error(illegal_json, Stream)
	).

json_value(Stream, Term, Next, Options) :-
	get_code(Stream, C0),
	ws(C0, Stream, C1),
	(   C1 == -1
	->  Term = @(end_of_file),
	    Next = -1
	;   json_term(C1, Stream, Term, Next, Options)
	).

json_term(0'{, Stream, json(Pairs), Next, Options) :- !,
	ws(Stream, C),
	json_pairs(C, Stream, Pairs, Options),
	get_code(Stream, Next).
json_term(0'[, Stream, Array, Next, Options) :- !,
	ws(Stream, C),
	json_array(C, Stream, Array, Options),
	get_code(Stream, Next).
json_term(0'", Stream, String, Next, Options) :- !,
	get_code(Stream, C1),
	json_string_codes(C1, Stream, Codes),
	json_options_value_string_as(Options, Type),
	codes_to_type(Type, Codes, String),
	get_code(Stream, Next).
json_term(0'-, Stream, Number, Next, _Options) :- !,
	json_number_codes(Stream, Codes, Next),
	number_codes(Number, [0'-|Codes]).
json_term(D, Stream, Number, Next, _Options) :-
	between(0'0, 0'9, D), !,
	json_number_codes(Stream, Codes, Next),
	number_codes(Number, [D|Codes]).
json_term(C, Stream, Constant, Next, Options) :-
	get_code(Stream, C1),
	json_identifier_codes(C1, Stream, Codes, Next),
	atom_codes(ID, [C|Codes]),
	json_constant(ID, Constant, Options).

json_pairs(0'}, _, [], _) :- !.
json_pairs(C0, Stream, [Pair|Tail], Options) :-
	json_pair(C0, Stream, Pair, C, Options),
	ws(C, Stream, Next),
	(   Next == 0',
	->  ws(Stream, C2),
	    json_pairs(C2, Stream, Tail, Options)
	;   Next == 0'}
	->  Tail = []
	;   syntax_error(illegal_object, Stream)
	).

json_pair(C0, Stream, Name=Value, Next, Options) :-
	json_string_as_atom(C0, Stream, Name),
	ws(Stream, C),
	C == 0':,
	json_value(Stream, Value, Next, Options).


json_array(0'], _, [], _) :- !.
json_array(C0, Stream, [Value|Tail], Options) :-
	json_term(C0, Stream, Value, C, Options),
	ws(C, Stream, Next),
	(   Next == 0',
	->  ws(Stream, C1),
	    json_array(C1, Stream, Tail, Options)
	;   Next == 0']
	->  Tail = []
	;   syntax_error(illegal_array, Stream)
	).

codes_to_type(atom, Codes, Atom) :-
	atom_codes(Atom, Codes).
codes_to_type(string, Codes, Atom) :-
	string_codes(Atom, Codes).
codes_to_type(codes, Codes, Codes).

json_string_as_atom(0'", Stream, Atom) :-
	get_code(Stream, C1),
	json_string_codes(C1, Stream, Codes),
	atom_codes(Atom, Codes).

json_string_codes(0'", _, []) :- !.
json_string_codes(0'\\, Stream, [H|T]) :- !,
	get_code(Stream, C0),
	(   escape(C0, Stream, H)
	->  true
	;   syntax_error(illegal_string_escape, Stream)
	),
	get_code(Stream, C1),
	json_string_codes(C1, Stream, T).
json_string_codes(-1, Stream, _) :- !,
	syntax_error(eof_in_string, Stream).
json_string_codes(C, Stream, [C|T]) :-
	get_code(Stream, C1),
	json_string_codes(C1, Stream, T).

escape(0'", _, 0'") :- !.
escape(0'\\, _, 0'\\) :- !.
escape(0'/, _, 0'/) :- !.
escape(0'b, _, 0'\b) :- !.
escape(0'f, _, 0'\f) :- !.
escape(0'n, _, 0'\n) :- !.
escape(0'r, _, 0'\r) :- !.
escape(0't, _, 0'\t) :- !.
escape(0'u, Stream, C) :- !,
	get_code(Stream, C1),
	get_code(Stream, C2),
	get_code(Stream, C3),
	get_code(Stream, C4),
	code_type(C1, xdigit(D1)),
	code_type(C2, xdigit(D2)),
	code_type(C3, xdigit(D3)),
	code_type(C4, xdigit(D4)),
	C is D1<<12+D2<<8+D3<<4+D4.

json_number_codes(Stream, Codes, Next) :-
	get_code(Stream, C1),
	json_number_codes(C1, Stream, Codes, Next).

json_number_codes(C1, Stream, [C1|Codes], Next) :-
	number_code(C1), !,
	get_code(Stream, C2),
	json_number_codes(C2, Stream, Codes, Next).
json_number_codes(C, _, [], C).

number_code(C) :-
	between(0'0, 0'9, C), !.
number_code(0'.).
number_code(0'-).
number_code(0'e).
number_code(0'E).

json_identifier_codes(C1, Stream, [C1|T], Next) :-
	between(0'a, 0'z, C1), !,
	get_code(Stream, C2),
	json_identifier_codes(C2, Stream, T, Next).
json_identifier_codes(C, _, [], C).


json_constant(true, Constant, Options) :- !,
	json_options_true(Options, Constant).
json_constant(false, Constant, Options) :- !,
	json_options_false(Options, Constant).
json_constant(null, Constant, Options) :- !,
	json_options_null(Options, Constant).

%%	ws(+Stream, -Next) is det.
%%	ws(+C0, +Stream, -Next)
%
%	Skip white space on the Stream, returning the first non-ws
%	character.  Also skips =|//|= ... comments.

ws(Stream, Next) :-
	get_code(Stream, C0),
	ws(C0, Stream, Next).

ws(C0, Stream, C) :-
	ws(C0), !,
	get_code(Stream, C1),
	ws(C1, Stream, C).
ws(0'/, Stream, C) :- !,
	get_code(Stream, Cmt1), !,
	expect(Cmt1, 0'/, Stream),
	skip(Stream, 0'\n),
	get_code(Stream, C0),
	ws(C0, Stream, C).
ws(C, _, C).

ws(0' ).
ws(0'\t).
ws(0'\n).
ws(0'\r).

expect(C, C, _) :- !.
expect(_, 0'/, Stream) :- !,
	syntax_error(illegal_comment, Stream).

syntax_error(Message, Stream) :-
	stream_error_context(Stream, Context),
	throw(error(syntax_error(json(Message)), Context)).

stream_error_context(Stream, stream(Stream, Line, LinePos, CharNo)) :-
	character_count(Stream, CharNo),
	line_position(Stream, LinePos),
	line_count(Stream, Line).


		 /*******************************
		 *	    JSON OUTPUT		*
		 *******************************/

%%	json_write_string(+Stream, +Text) is det.
%
%	Write a JSON string to  Stream.  Stream   must  be  opened  in a
%	Unicode capable encoding, typically UTF-8.

% foreign json_write_string/2.

%%	json_write_indent(+Stream, +Indent, +TabDistance) is det.
%
%	Newline and indent to  Indent.  A   Newline  is  only written if
%	line_position(Stream, Pos) is not 0. Then   it  writes Indent //
%	TabDistance tab characters and Indent mode TabDistance spaces.

% foreign json_write_indent/3.

%%	json_write(+Stream, +Term) is det.
%%	json_write(+Stream, +Term, +Options) is det.
%
%	Write a JSON term to Stream.  The   JSON  object  is of the same
%	format as produced by json_read/2, though we allow for some more
%	flexibility with regard to pairs in  objects. All of Name=Value,
%	Name-Value and Name(Value) produce the  same output.
%
%	Values can be of the form  #(Term),   which  causes `Term` to be
%	_stringified_ if it is not an atom or string. Stringification is
%	based on term_string/2.
%
%	The version 7 _dict_ type is supported as well. If the dicts has
%	a _tag_, a property "type":"tag" is   added  to the object. This
%	behaviour can be changed using the =tag= option (see below). For
%	example:
%
%	  ==
%	  ?- json_write(current_output, point{x:1,y:2}).
%	  {
%	    "type":"point",
%	    "x":1,
%	    "y":2
%	  }
%	  ==
%
%	In addition to the options recognised by json_read/3, we process
%	the following options are recognised:
%
%	    * width(+Width)
%	    Width in which we try to format the result.  Too long lines
%	    switch from _horizontal_ to _vertical_ layout for better
%	    readability. If performance is critical and human
%	    readability is not an issue use Width = 0, which causes a
%	    single-line output.
%
%	    * step(+Step)
%	    Indentation increnment for next level.  Default is 2.
%
%	    * tab(+TabDistance)
%	    Distance between tab-stops.  If equal to Step, layout
%	    is generated with one tab per level.
%
%	    * serialize_unknown(+Boolean)
%	    If =true= (default =false=), serialize unknown terms and
%	    print them as a JSON string.  The default raises a type
%	    error.  Note that this option only makes sense if you can
%	    guarantee that the passed value is not an otherwise valid
%	    Prolog reporesentation of a Prolog term.
%
%	If a string is  emitted,  the   sequence  =|</|=  is  emitted as
%	=|<\/|=. This is valid  JSON  syntax   which  ensures  that JSON
%	objects  can  be  safely  embedded  into  an  HTML  =|<script>|=
%	element.

:- record json_write_state(indent:nonneg = 0,
			   step:positive_integer = 2,
			   tab:positive_integer = 8,
			   width:nonneg = 72,
			   serialize_unknown:boolean = false
			  ).

json_write(Stream, Term) :-
	json_write(Stream, Term, []).
json_write(Stream, Term, Options) :-
	make_json_write_state(Options, State, Options1),
	make_json_options(Options1, OptionTerm, _RestOptions),
	json_write_term(Term, Stream, State, OptionTerm).

json_write_term(Var, _, _, _) :-
	var(Var), !,
	instantiation_error(Var).
json_write_term(json(Pairs), Stream, State, Options) :- !,
	json_write_object(Pairs, Stream, State, Options).
:- if(current_predicate(is_dict/1)).
json_write_term(Dict, Stream, State, Options) :-
	is_dict(Dict), !,
	dict_pairs(Dict, Tag, Pairs0),
	(   nonvar(Tag),
	    json_options_tag(Options, Name),
	    Name \== ''
	->  Pairs = [Name-Tag|Pairs0]
	;   Pairs = Pairs0
	),
	json_write_object(Pairs, Stream, State, Options).
:- endif.
json_write_term(List, Stream, State, Options) :-
	is_list(List), !,
	space_if_not_at_left_margin(Stream, State),
	write(Stream, '['),
	(   json_write_state_width(State, Width),
	    (   Width == 0
	    ->  true
	    ;   json_write_state_indent(State, Indent),
		json_print_length(List, Options, Width, Indent, _)
	    )
	->  set_width_of_json_write_state(0, State, State2),
	    write_array_hor(List, Stream, State2, Options),
	    write(Stream, ']')
	;   step_indent(State, State2),
	    write_array_ver(List, Stream, State2, Options),
	    indent(Stream, State),
	    write(Stream, ']')
	).
json_write_term(Number, Stream, _State, _Options) :-
	number(Number), !,
	write(Stream, Number).
json_write_term(True, Stream, _State, Options) :-
	json_options_true(Options, True), !,
	write(Stream, true).
json_write_term(False, Stream, _State, Options) :-
	json_options_false(Options, False), !,
	write(Stream, false).
json_write_term(Null, Stream, _State, Options) :-
	json_options_null(Options, Null), !,
	write(Stream, null).
json_write_term(#(Text), Stream, _State, _Options) :- !,
	(   (   atom(Text)
	    ;	string(Text)
	    )
	->  json_write_string(Stream, Text)
	;   term_string(Text, String),
	    json_write_string(Stream, String)
	).
json_write_term(String, Stream, _State, _Options) :-
	atom(String), !,
	json_write_string(Stream, String).
json_write_term(String, Stream, _State, _Options) :-
	string(String), !,
	json_write_string(Stream, String).
json_write_term(AnyTerm, Stream, State, _Options) :-
	(   json_write_state_serialize_unknown(State, true)
	->  term_string(AnyTerm, String),
	    json_write_string(Stream, String)
	;   type_error(json_term, AnyTerm)
	).

json_write_object(Pairs, Stream, State, Options) :-
	space_if_not_at_left_margin(Stream, State),
	write(Stream, '{'),
	(   json_write_state_width(State, Width),
	    (   Width == 0
	    ->  true
	    ;   json_write_state_indent(State, Indent),
		json_print_length(json(Pairs), Options, Width, Indent, _)
	    )
	->  set_width_of_json_write_state(0, State, State2),
	    write_pairs_hor(Pairs, Stream, State2, Options),
	    write(Stream, '}')
	;   step_indent(State, State2),
	    write_pairs_ver(Pairs, Stream, State2, Options),
	    indent(Stream, State),
	    write(Stream, '}')
	).


write_pairs_hor([], _, _, _).
write_pairs_hor([H|T], Stream, State, Options) :-
	json_pair(H, Name, Value),
	json_write_string(Stream, Name),
	write(Stream, ':'),
	json_write_term(Value, Stream, State, Options),
	(   T == []
	->  true
	;   write(Stream, ', '),
	    write_pairs_hor(T, Stream, State, Options)
	).

write_pairs_ver([], _, _, _).
write_pairs_ver([H|T], Stream, State, Options) :-
	indent(Stream, State),
	json_pair(H, Name, Value),
	json_write_string(Stream, Name),
	write(Stream, ':'),
	json_write_term(Value, Stream, State, Options),
	(   T == []
	->  true
	;   write(Stream, ','),
	    write_pairs_ver(T, Stream, State, Options)
	).


json_pair(Var, _, _) :-
	var(Var), !,
	instantiation_error(Var).
json_pair(Name=Value, Name, Value) :- !.
json_pair(Name-Value, Name, Value) :- !.
json_pair(NameValue, Name, Value) :-
	compound(NameValue),
	NameValue =.. [Name, Value], !.
json_pair(Pair, _, _) :-
	type_error(json_pair, Pair).


write_array_hor([], _, _, _).
write_array_hor([H|T], Stream, State, Options) :-
	json_write_term(H, Stream, State, Options),
	(   T == []
	->  write(Stream, ' ')
	;   write(Stream, ', '),
	    write_array_hor(T, Stream, State, Options)
	).

write_array_ver([], _, _, _).
write_array_ver([H|T], Stream, State, Options) :-
	indent(Stream, State),
	json_write_term(H, Stream, State, Options),
	(   T == []
	->  true
	;   write(Stream, ','),
	    write_array_ver(T, Stream, State, Options)
	).


indent(Stream, State) :-
	json_write_state_indent(State, Indent),
	json_write_state_tab(State, Tab),
	json_write_indent(Stream, Indent, Tab).

step_indent(State0, State) :-
	json_write_state_indent(State0, Indent),
	json_write_state_step(State0, Step),
	NewIndent is Indent+Step,
	set_indent_of_json_write_state(NewIndent, State0, State).

space_if_not_at_left_margin(Stream, State) :-
	line_position(Stream, LinePos),
	(   LinePos == 0
	;   json_write_state_indent(State, LinePos)
	), !.
space_if_not_at_left_margin(Stream, _) :-
	put_char(Stream, ' ').


%%	json_print_length(+Value, +Options, +Max, +Len0, +Len) is semidet.
%
%	True if Len-Len0 is the print-length of Value on a single line
%	and Len-Len0 =< Max.
%
%	@tbd	Escape sequences in strings are not considered.

json_print_length(json(Pairs), Options, Max, Len0, Len) :- !,
	Len1 is Len0 + 2,
	Len1 =< Max,
	pairs_print_length(Pairs, Options, Max, Len1, Len).
:- if(current_predicate(is_dict/1)).
json_print_length(Dict, Options, Max, Len0, Len) :-
	is_dict(Dict), !,
	dict_pairs(Dict, _Tag, Pairs),
	Len1 is Len0 + 2,
	Len1 =< Max,
	pairs_print_length(Pairs, Options, Max, Len1, Len).
:- endif.
json_print_length(Array, Options, Max, Len0, Len) :-
	is_list(Array), !,
	Len1 is Len0 + 2,
	Len1 =< Max,
	array_print_length(Array, Options, Max, Len1, Len).
json_print_length(Null, Options, Max, Len0, Len) :-
	json_options_null(Options, Null), !,
	Len is Len0 + 4,
	Len =< Max.
json_print_length(False, Options, Max, Len0, Len) :-
	json_options_false(Options, False), !,
	Len is Len0 + 5,
	Len =< Max.
json_print_length(True, Options, Max, Len0, Len) :-
	json_options_true(Options, True), !,
	Len is Len0 + 4,
	Len =< Max.
json_print_length(Number, _Options, Max, Len0, Len) :-
	number(Number), !,
	write_length(Number, AL, []),
	Len is Len0 + AL,
	Len =< Max.
json_print_length(@(Id), _Options, Max, Len0, Len) :-
	atom(Id), !,
	atom_length(Id, IdLen),
	Len is Len0+IdLen,
	Len =< Max.
json_print_length(String, _Options, Max, Len0, Len) :-
	string_len(String, Len0, Len), !,
	Len =< Max.
json_print_length(AnyTerm, _Options, Max, Len0, Len) :-
	write_length(AnyTerm, AL, []),		% will be serialized
	Len is Len0 + AL+2,
	Len =< Max.

pairs_print_length([], _, _, Len, Len).
pairs_print_length([H|T], Options, Max, Len0, Len) :-
	pair_len(H, Options, Max, Len0, Len1),
	(   T == []
	->  Len = Len1
	;   Len2 is Len1 + 2,
	    Len2 =< Max,
	    pairs_print_length(T, Options, Max, Len2, Len)
	).

pair_len(Pair, Options, Max, Len0, Len) :-
	pair_nv(Pair, Name, Value),
	string_len(Name, Len0, Len1),
	Len2 is Len1+2,
	Len2 =< Max,
	json_print_length(Value, Options, Max, Len2, Len).

pair_nv(Name=Value, Name, Value).
pair_nv(Name-Value, Name, Value).

array_print_length([], _, _, Len, Len).
array_print_length([H|T], Options, Max, Len0, Len) :-
	json_print_length(H, Options, Max, Len0, Len1),
	(   T == []
	->  Len = Len1
	;   Len2 is Len1+2,
	    Len2 =< Max,
	    array_print_length(T, Options, Max, Len2, Len)
	).

string_len(String, Len0, Len) :-
	atom(String), !,
	atom_length(String, AL),
	Len is Len0 + AL + 2.
string_len(String, Len0, Len) :-
	string(String), !,
	string_length(String, AL),
	Len is Len0 + AL + 2.


		 /*******************************
		 *	       TEST		*
		 *******************************/

%%	is_json_term(@Term) is semidet.
%%	is_json_term(@Term, +Options) is semidet.
%
%	True if Term is  a  json  term.   Options  are  the  same as for
%	json_read/2, defining the Prolog  representation   for  the JSON
%	=true=, =false= and =null= constants.

is_json_term(Term) :-
	default_json_options(Options),
	is_json_term2(Options, Term).

is_json_term(Term, Options) :-
	make_json_options(Options, OptionTerm, _RestOptions),
	is_json_term2(OptionTerm, Term).

is_json_term2(_, Var) :-
	var(Var), !, fail.
is_json_term2(Options, json(Pairs)) :- !,
	is_list(Pairs),
	maplist(is_json_pair(Options), Pairs).
is_json_term2(Options, List) :-
	is_list(List), !,
	maplist(is_json_term2(Options), List).
is_json_term2(_, Primitive) :-
	atomic(Primitive), !.		% atom, string or number
is_json_term2(Options, True) :-
	json_options_true(Options, True).
is_json_term2(Options, False) :-
	json_options_false(Options, False).
is_json_term2(Options, Null) :-
	json_options_null(Options, Null).

is_json_pair(_, Var) :-
	var(Var), !, fail.
is_json_pair(Options, Name=Value) :-
	atom(Name),
	is_json_term2(Options, Value).

:- if(current_predicate(is_dict/1)).

		 /*******************************
		 *	   DICT SUPPORT		*
		 *******************************/

%%	json_read_dict(+Stream, -Dict) is det.
%%	json_read_dict(+Stream, -Dict, +Options) is det.
%
%	Read  a  JSON  object,  returning  objects    as  a  dicts.  The
%	representation depends on the options, where the default is:
%
%	  * String values are mapped to Prolog strings
%	  * JSON =true=, =false= and =null= are represented using these
%	    Prolog atoms.
%	  * JSON objects are mapped to dicts.
%	  * By default, a =type= field in an object assigns a tag for
%	    the dict.
%
%	On  addition  to  the   options    processed   by   json_read/3,
%	json_read_dict/3 processes this additional option:
%
%	  * tag(+Name)
%	    When converting to/from a dict, map the indicated JSON
%	    attribute to the dict _tag_. No mapping is performed if Name
%	    is the empty atom ('', default). See json_read_dict/2 and
%	    json_write_dict/2.

json_read_dict(Stream, Dict) :-
	json_read_dict(Stream, Dict, []).

json_read_dict(Stream, Dict, Options) :-
	make_json_dict_options(Options, OptionTerm, _RestOptions),
	json_value(Stream, Term, _, OptionTerm),
	term_to_dict(Term, Dict, OptionTerm).

term_to_dict(json(Pairs), Dict, Options) :- !,
	(   json_options_tag(Options, TagName),
	    Tag \== '',
	    select(TagName = Tag0, Pairs, NVPairs),
	    to_atom(Tag0, Tag)
	->  json_dict_pairs(NVPairs, DictPairs, Options)
	;   json_dict_pairs(Pairs, DictPairs, Options)
	),
	dict_create(Dict, Tag, DictPairs).
term_to_dict(Value0, Value, _Options) :-
	atomic(Value0), Value0 \== [], !,
	Value = Value0.
term_to_dict(List0, List, Options) :-
	assertion(is_list(List0)),
	terms_to_dicts(List0, List, Options).

json_dict_pairs([], [], _).
json_dict_pairs([Name=Value0|T0], [Name=Value|T], Options) :-
	term_to_dict(Value0, Value, Options),
	json_dict_pairs(T0, T, Options).

terms_to_dicts([], [], _).
terms_to_dicts([Value0|T0], [Value|T], Options) :-
	term_to_dict(Value0, Value, Options),
	terms_to_dicts(T0, T, Options).

to_atom(Tag, Atom) :-
	string(Tag), !,
	atom_string(Atom, Tag).
to_atom(Atom, Atom) :-
	atom(Atom).

%%	json_write_dict(+Stream, +Dict) is det.
%%	json_write_dict(+Stream, +Dict, +Options) is det.
%
%	Write a JSON term, represented using dicts.  This is the same as
%	json_write/3, but assuming the default   representation  of JSON
%	objects as dicts.

json_write_dict(Stream, Dict) :-
	json_write_dict(Stream, Dict, []).

json_write_dict(Stream, Dict, Options) :-
	make_json_write_state(Options, State, Options1),
	make_json_dict_options(Options1, OptionTerm, _RestOptions),
	json_write_term(Dict, Stream, State, OptionTerm).


make_json_dict_options(Options, Record, RestOptions) :-
	default_json_dict_options(Record0),
	set_json_options_fields(Options, Record0, Record, RestOptions).

%%	atom_json_dict(+Atom, -JSONDict, +Options) is det.
%%	atom_json_dict(-Text, +JSONDict, +Options) is det.
%
%	Convert  between  textual  representation  and    a   JSON  term
%	represented as a dict. Options are as for json_read/3.
%	In _write_ mode, the addtional option
%
%	    * as(Type)
%	    defines the output type, which is one of =atom=,
%	    =string= or =codes=.

atom_json_dict(Atom, Term, Options) :-
	ground(Atom), !,
	setup_call_cleanup(
	    ( atom_to_memory_file(Atom, MF),
	      open_memory_file(MF, read, In, [free_on_close(true)])
	    ),
	    json_read_dict(In, Term, Options),
	    close(In)).
atom_json_dict(Result, Term, Options) :-
	select_option(as(Type), Options, Options1, atom),
	(   type_term(Type, Result, Out)
	->  true
	;   must_be(oneof([atom,string,codes]), Type)
	),
	with_output_to(Out,
		       json_write_dict(current_output, Term, Options1)).

:- endif.

		 /*******************************
		 *	     MESSAGES		*
		 *******************************/

:- multifile
	prolog:error_message/3.

prolog:error_message(syntax_error(json(Id))) -->
	[ 'JSON syntax error: ' ],
	json_syntax_error(Id).

json_syntax_error(illegal_comment) -->
	[ 'Illegal comment' ].
json_syntax_error(illegal_string_escape) -->
	[ 'Illegal escape sequence in string' ].