/*  Part of SWI-Prolog

    Author:        Jan Wielemaker & Richard O'Keefe
    E-mail:        J.Wielemaker@vu.nl
    WWW:           http://www.swi-prolog.org
    Copyright (c)  2004-2016, University of Amsterdam
                              VU University Amsterdam
    All rights reserved.

    Redistribution and use in source and binary forms, with or without
    modification, are permitted provided that the following conditions
    are met:

    1. Redistributions of source code must retain the above copyright
       notice, this list of conditions and the following disclaimer.

    2. Redistributions in binary form must reproduce the above copyright
       notice, this list of conditions and the following disclaimer in
       the documentation and/or other materials provided with the
       distribution.

    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
    FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
    COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
    INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
    BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
    LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
    CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
    LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
    ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
    POSSIBILITY OF SUCH DAMAGE.
*/

:- module(sgml_write,
          [ html_write/2,               %          +Data, +Options
            html_write/3,               % +Stream, +Data, +Options
            sgml_write/2,               %          +Data, +Options
            sgml_write/3,               % +Stream, +Data, +Options
            xml_write/2,                %          +Data, +Options
            xml_write/3                 % +Stream, +Data, +Options
          ]).
:- use_module(library(lists)).
:- use_module(library(sgml)).
:- use_module(library(assoc)).
:- use_module(library(option)).
:- use_module(library(error)).

:- predicate_options(xml_write/2, 2, [pass_to(xml_write/3, 3)]).
:- predicate_options(xml_write/3, 3,
                     [ dtd(any),
                       doctype(atom),
                       public(atom),
                       system(atom),
                       header(boolean),
                       nsmap(list),
                       indent(nonneg),
                       layout(boolean),
                       net(boolean),
                       cleanns(boolean)
                     ]).

:- multifile
    xmlns/2.                        % NS, URI

/** <module> XML/SGML writer module

This library provides the inverse functionality   of  the sgml.pl parser
library, writing XML, SGML and HTML documents from the parsed output. It
is intended to allow rewriting in a  different dialect or encoding or to
perform document transformation in Prolog on the parsed representation.

The current implementation is  particularly   keen  on getting character
encoding and the use of character  entities   right.  Some work has been
done providing layout, but space handling in   XML  and SGML make this a
very hazardous area.

The Prolog-based low-level character and  escape   handling  is the real
bottleneck in this library and will probably be   moved  to C in a later
stage.

@see    library(http/html_write) provides a high-level library for
        emitting HTML and XHTML.
*/

%!  xml_write(+Data, +Options) is det.
%!  sgml_write(+Data, +Options) is det.
%!  html_write(+Data, +Options) is det.
%!  xml_write(+Stream, +Data, +Options) is det.
%!  sgml_write(+Stream, +Data, +Options) is det.
%!  html_write(+Stream, +Data, +Options) is det.
%
%   Write a term as created by the SGML/XML parser to a stream in
%   SGML or XML format.  Options:
%
%           * cleanns(Bool)
%           If `true` (default), remove duplicate `xmlns`
%           attributes.
%           * dtd(DTD)
%           The DTD.  This is needed for SGML documents that contain
%           elements with content model EMPTY.  Characters which may
%           not be written directly in the Stream's encoding will be
%           written using character data entities from the DTD if at
%           all possible, otherwise as numeric character references.
%           Note that the DTD will NOT be written out at all; as yet
%           there is no way to write out an internal subset,  though
%           it would not be hard to add one.
%
%           * doctype(DocType)
%           Document type for the SGML document type declaration.
%           If omitted it is taken from the root element.  There is
%           never any point in having this be disagree with the
%           root element.  A <!DOCTYPE> declaration will be written
%           if and only if at least one of doctype(_), public(_), or
%           system(_) is provided in Options.
%
%           * public(PubId)
%           The public identifier to be written in the <!DOCTYPE> line.
%
%           * system(SysId)
%           The system identifier to be written in the <!DOCTYPE> line.
%
%           * header(Bool)
%           If Bool is 'false', do not emit the <xml ...> header
%           line.  (xml_write/3 only)
%
%           * nsmap(Map:list(Id=URI))
%           When emitting embedded XML, assume these namespaces
%           are already defined from the environment.  (xml_write/3
%           only).
%
%           * indent(Indent)
%           Indentation of the document (for embedding)
%
%           * layout(Bool)
%           Emit/do not emit layout characters to make output
%           readable.
%
%           * net(Bool)
%           Use/do not use Null End Tags.
%           For XML, this applies only to empty elements, so you get
%
%           ==
%               <foo/>      (default, net(true))
%               <foo></foo> (net(false))
%           ==
%
%           For SGML, this applies to empty elements, so you get
%
%           ==
%               <foo>       (if foo is declared to be EMPTY in the DTD)
%               <foo></foo> (default, net(false))
%               <foo//      (net(true))
%           ==
%
%           and also to elements with character content not containing /
%
%           ==
%               <b>xxx</b>  (default, net(false))
%               <b/xxx/     (net(true)).
%           ==
%
%   Note that if the stream is UTF-8, the system will write special
%   characters as UTF-8 sequences, while if it is ISO Latin-1 it
%   will use (character) entities if there is a DTD that provides
%   them, otherwise it will use numeric character references.

xml_write(Data, Options) :-
    current_output(Stream),
    xml_write(Stream, Data, Options).

xml_write(Stream0, Data, Options) :-
    fix_user_stream(Stream0, Stream),
    (   stream_property(Stream, encoding(text))
    ->  set_stream(Stream, encoding(utf8)),
        call_cleanup(xml_write(Stream, Data, Options),
                     set_stream(Stream, encoding(text)))
    ;   new_state(xml, State),
        init_state(Options, State),
        get_state(State, nsmap, NSMap),
        add_missing_namespaces(Data, NSMap, Data1),
        emit_xml_encoding(Stream, Options),
        emit_doctype(Options, Data, Stream),
        write_initial_indent(State, Stream),
        emit(Data1, Stream, State)
    ).


sgml_write(Data, Options) :-
    current_output(Stream),
    sgml_write(Stream, Data, Options).

sgml_write(Stream0, Data, Options) :-
    fix_user_stream(Stream0, Stream),
    (   stream_property(Stream, encoding(text))
    ->  set_stream(Stream, encoding(utf8)),
        call_cleanup(sgml_write(Stream, Data, Options),
                     set_stream(Stream, encoding(text)))
    ;   new_state(sgml, State),
        init_state(Options, State),
        write_initial_indent(State, Stream),
        emit_doctype(Options, Data, Stream),
        emit(Data, Stream, State)
    ).


html_write(Data, Options) :-
    current_output(Stream),
    html_write(Stream, Data, Options).

html_write(Stream, Data, Options) :-
    sgml_write(Stream, Data,
               [ dtd(html)
               | Options
               ]).

fix_user_stream(user, user_output) :- !.
fix_user_stream(Stream, Stream).


init_state([], _).
init_state([H|T], State) :-
    update_state(H, State),
    init_state(T, State).

update_state(dtd(DTD), State) :-
    !,
    (   atom(DTD)
    ->  dtd(DTD, DTDObj)
    ;   DTDObj = DTD
    ),
    set_state(State, dtd, DTDObj),
    dtd_character_entities(DTDObj, EntityMap),
    set_state(State, entity_map, EntityMap).
update_state(nsmap(Map), State) :-
    !,
    set_state(State, nsmap, Map).
update_state(cleanns(Bool), State) :-
    !,
    must_be(boolean, Bool),
    set_state(State, cleanns, Bool).
update_state(indent(Indent), State) :-
    !,
    must_be(integer, Indent),
    set_state(State, indent, Indent).
update_state(layout(Bool), State) :-
    !,
    must_be(boolean, Bool),
    set_state(State, layout, Bool).
update_state(doctype(_), _) :- !.
update_state(public(_),  _) :- !.
update_state(system(_),  _) :- !.
update_state(net(Bool), State) :-
    !,
    must_be(boolean, Bool),
    set_state(State, net, Bool).
update_state(header(Bool), _) :-
    !,
    must_be(boolean, Bool).
update_state(Option, _) :-
    domain_error(xml_write_option, Option).

%       emit_xml_encoding(+Stream, +Options)
%
%       Emit the XML fileheader with   encoding information. Setting the
%       right encoding on the output stream  must be done before calling
%       xml_write/3.

emit_xml_encoding(Out, Options) :-
    option(header(Hdr), Options, true),
    Hdr == true,
    !,
    stream_property(Out, encoding(Encoding)),
    (   (   Encoding == utf8
        ;   Encoding == wchar_t
        )
    ->  format(Out, '<?xml version="1.0" encoding="UTF-8"?>~n~n', [])
    ;   Encoding == iso_latin_1
    ->  format(Out, '<?xml version="1.0" encoding="ISO-8859-1"?>~n~n', [])
    ;   domain_error(xml_encoding, Encoding)
    ).
emit_xml_encoding(_, _).


%!  emit_doctype(+Options, +Data, +Stream)
%
%   Emit the document-type declaration.
%   There is a problem with the first clause if we are emitting SGML:
%   the SGML DTDs for HTML up to HTML 4 *do not allow* any 'version'
%   attribute; so the only time this is useful is when it is illegal!

emit_doctype(_Options, Data, Out) :-
    (   Data = [_|_], memberchk(element(html,Att,_), Data)
    ;   Data = element(html,Att,_)
    ),
    memberchk(version=Version, Att),
    !,
    format(Out, '<!DOCTYPE HTML PUBLIC "~w">~n~n', [Version]).
emit_doctype(Options, Data, Out) :-
    (   memberchk(public(PubId), Options) -> true
    ;   PubId = (-)
    ),
    (   memberchk(system(SysId), Options) -> true
    ;   SysId = (-)
    ),
    \+ (PubId == (-),
        SysId == (-),
        \+ memberchk(doctype(_), Options)
    ),
    (   Data = element(DocType,_,_)
    ;   Data = [_|_], memberchk(element(DocType,_,_), Data)
    ;   memberchk(doctype(DocType), Options)
    ),
    !,
    write_doctype(Out, DocType, PubId, SysId).
emit_doctype(_, _, _).

write_doctype(Out, DocType, -, -) :-
    !,
    format(Out, '<!DOCTYPE ~w []>~n~n', [DocType]).
write_doctype(Out, DocType, -, SysId) :-
    !,
    format(Out, '<!DOCTYPE ~w SYSTEM "~w">~n~n', [DocType,SysId]).
write_doctype(Out, DocType, PubId, -) :-
    !,
    format(Out, '<!DOCTYPE ~w PUBLIC "~w">~n~n', [DocType,PubId]).
write_doctype(Out, DocType, PubId, SysId) :-
    format(Out, '<!DOCTYPE ~w PUBLIC "~w" "~w">~n~n', [DocType,PubId,SysId]).


%!  emit(+Element, +Out, +State, +Options)
%
%   Emit a single element

emit(Var, _, _) :-
    var(Var),
    !,
    instantiation_error(Var).
emit([], _, _) :- !.
emit([H|T], Out, State) :-
    !,
    emit(H, Out, State),
    emit(T, Out, State).
emit(CDATA, Out, State) :-
    atomic(CDATA),
    !,
    sgml_write_content(Out, CDATA, State).
emit(Element, Out, State) :-
    \+ \+ emit_element(Element, Out, State).

emit_element(pi(PI), Out, State) :-
    !,
    get_state(State, entity_map, EntityMap),
    write(Out, <?),
    write_quoted(Out, PI, '', EntityMap),
    (   get_state(State, dialect, xml) ->
        write(Out, ?>)
    ;   write(Out, >)
    ).
emit_element(element(Name, Attributes, Content), Out, State) :-
    !,
    must_be(list, Attributes),
    must_be(list, Content),
    (   get_state(State, dialect, xml)
    ->  update_nsmap(Attributes, CleanAttrs, State),
        (   get_state(State, cleanns, true)
        ->  WriteAttrs = CleanAttrs
        ;   WriteAttrs = Attributes
        )
    ;   WriteAttrs = Attributes
    ),
    att_length(WriteAttrs, State, Alen),
    (   Alen > 60,
        get_state(State, layout, true)
    ->  Sep = nl,
        AttIndent = 4
    ;   Sep = sp,
        AttIndent = 0
    ),
    put_char(Out, '<'),
    emit_name(Name, Out, State),
    (   AttIndent > 0
    ->  \+ \+ ( inc_indent(State, AttIndent),
                attributes(WriteAttrs, Sep, Out, State)
              )
    ;   attributes(WriteAttrs, Sep, Out, State)
    ),
    content(Content, Out, Name, State).
emit_element(E, _, _) :-
    type_error(xml_dom, E).

attributes([], _, _, _).
attributes([H|T], Sep, Out, State) :-
    (   Sep == nl
    ->  write_indent(State, Out)
    ;   put_char(Out, ' ')
    ),
    attribute(H, Out, State),
    attributes(T, Sep, Out, State).

attribute(Name=Value, Out, State) :-
    emit_name(Name, Out, State),
    put_char(Out, =),
    sgml_write_attribute(Out, Value, State).

att_length(Atts, State, Len) :-
    att_length(Atts, State, 0, Len).

att_length([], _, Len, Len).
att_length([A0|T], State, Len0, Len) :-
    alen(A0, State, AL),
    Len1 is Len0 + 1 + AL,
    att_length(T, State, Len1, Len).

alen(ns(NS, _URI):Name=Value, _State, Len) :-
    !,
    atom_length(Value, AL),
    vlen(Name, NL),
    atom_length(NS, NsL),
    Len is AL+NL+NsL+3.
alen(URI:Name=Value, State, Len) :-
    !,
    atom_length(Value, AL),
    vlen(Name, NL),
    get_state(State, nsmap, Nsmap),
    (   memberchk(NS=URI, Nsmap)
    ->  atom_length(NS, NsL)
    ;   atom_length(URI, NsL)
    ),
    Len is AL+NL+NsL+3.
alen(Name=Value, _, Len) :-
    atom_length(Name, NL),
    vlen(Value, AL),
    Len is AL+NL+3.

vlen(Value, Len) :-
    is_list(Value),
    !,
    vlen_list(Value, 0, Len).
vlen(Value, Len) :-
    atom_length(Value, Len).

vlen_list([], L, L).
vlen_list([H|T], L0, L) :-
    atom_length(H, HL),
    (   L0 == 0
    ->  L1 is L0 + HL
    ;   L1 is L0 + HL + 1
    ),
    vlen_list(T, L1, L).


emit_name(Name, Out, _) :-
    atom(Name),
    !,
    write(Out, Name).
emit_name(ns(NS,_URI):Name, Out, _State) :-
    !,
    (  NS == ''
    -> write(Out, Name)
    ;  format(Out, '~w:~w', [NS, Name])
    ).
emit_name(URI:Name, Out, State) :-
    get_state(State, nsmap, NSMap),
    memberchk(NS=URI, NSMap),
    !,
    (   NS == []
    ->  write(Out, Name)
    ;   format(Out, '~w:~w', [NS, Name])
    ).
emit_name(Term, Out, _) :-              % error?
    write(Out, Term).

%!  update_nsmap(+Attributes, -Attributes1, !State)
%
%   Modify the nsmap of State to reflect modifications due to xmlns
%   arguments.
%
%   @arg    Attributes1 is a copy of Attributes with all redundant
%           namespace attributes deleted.

update_nsmap(Attributes, Attributes1, State) :-
    get_state(State, nsmap, Map0),
    update_nsmap(Attributes, Attributes1, Map0, Map),
    set_state(State, nsmap, Map).

update_nsmap([], [], Map, Map).
update_nsmap([xmlns:NS=URI|T], Attrs, Map0, Map) :-
    !,
    (   memberchk(NS=URI, Map0)
    ->  update_nsmap(T, Attrs, Map0, Map)
    ;   set_nsmap(NS, URI, Map0, Map1),
        Attrs = [xmlns:NS=URI|Attrs1],
        update_nsmap(T, Attrs1, Map1, Map)
    ).
update_nsmap([xmlns=URI|T], Attrs, Map0, Map) :-
    !,
    (   memberchk([]=URI, Map0)
    ->  update_nsmap(T, Attrs, Map0, Map)
    ;   set_nsmap([], URI, Map0, Map1),
        Attrs = [xmlns=URI|Attrs1],
        update_nsmap(T, Attrs1, Map1, Map)
    ).
update_nsmap([H|T0], [H|T], Map0, Map) :-
    !,
    update_nsmap(T0, T, Map0, Map).

set_nsmap(NS, URI, Map0, Map) :-
    select(NS=_, Map0, Map1),
    !,
    Map = [NS=URI|Map1].
set_nsmap(NS, URI, Map, [NS=URI|Map]).


%!  content(+Content, +Out, +Element, +State, +Options)
%
%   Emit the content part of a structure  as well as the termination
%   for the content. For empty content   we have three versions: XML
%   style '/>', SGML declared EMPTY element (nothing) or normal SGML
%   element (we must close with the same element name).

content([], Out, Element, State) :-    % empty element
    !,
    (   get_state(State, net, true)
    ->  (   get_state(State, dialect, xml) ->
            write(Out, />)
        ;   empty_element(State, Element) ->
            write(Out, >)
        ;   write(Out, //)
        )
    ;/* get_state(State, net, false) */
        write(Out, >),
        (   get_state(State, dialect, sgml),
            empty_element(State, Element)
        ->  true
        ;   emit_close(Element, Out, State)
        )
    ).
content([CDATA], Out, Element, State) :-
    atomic(CDATA),
    !,
    (   get_state(State, dialect, sgml),
        get_state(State, net, true),
        \+ sub_atom(CDATA, _, _, _, /),
        write_length(CDATA, Len, []),
        Len < 20
    ->  write(Out, /),
        sgml_write_content(Out, CDATA, State),
        write(Out, /)
    ;   verbatim_element(Element, State)
    ->  write(Out, >),
        write(Out, CDATA),
        emit_close(Element, Out, State)
    ;/* XML or not NET */
            write(Out, >),
        sgml_write_content(Out, CDATA, State),
        emit_close(Element, Out, State)
    ).
content(Content, Out, Element, State) :-
    get_state(State, layout, true),
    /* If xml:space='preserve' is present, */
        /* we MUST NOT tamper with white space at all. */
        \+ (Element = element(_,Atts,_),
        memberchk('xml:space'=preserve, Atts)
    ),
    element_content(Content, Elements),
    !,
    format(Out, >, []),
    \+ \+ (
        inc_indent(State),
        write_element_content(Elements, Out, State)
    ),
    write_indent(State, Out),
    emit_close(Element, Out, State).
content(Content, Out, Element, State) :-
    format(Out, >, []),
    write_mixed_content(Content, Out, Element, State),
    emit_close(Element, Out, State).

verbatim_element(Element, State) :-
    verbatim_element(Element),
    get_state(State, dtd, DTD),
    DTD \== (-),
    dtd_property(DTD, doctype(html)).

verbatim_element(script).
verbatim_element(style).

emit_close(Element, Out, State) :-
    write(Out, '</'),
    emit_name(Element, Out, State),
    write(Out, '>').


write_mixed_content([], _, _, _).
write_mixed_content([H|T], Out, Element, State) :-
    write_mixed_content_element(H, Out, State),
    write_mixed_content(T, Out, Element, State).

write_mixed_content_element(H, Out, State) :-
    (   atom(H)
    ->  sgml_write_content(Out, H, State)
    ;   string(H)
    ->  sgml_write_content(Out, H, State)
    ;   functor(H, element, 3)
    ->  emit(H, Out, State)
    ;   functor(H, pi, 1)
    ->  emit(H, Out, State)
    ;   var(H)
    ->  instantiation_error(H)
    ;   H = sdata(Data)             % cannot be written without entity!
    ->  print_message(warning, sgml_write(sdata_as_cdata(Data))),
        sgml_write_content(Out, Data, State)
    ;   type_error(sgml_content, H)
    ).


element_content([], []).
element_content([element(Name,Atts,C)|T0], [element(Name,Atts,C)|T]) :-
    !,
    element_content(T0, T).
element_content([Blank|T0], T) :-
    atom(Blank),
    atom_codes(Blank, Codes),
    all_blanks(Codes),
    element_content(T0, T).

all_blanks([]).
all_blanks([H|T]) :-
    code_type(H, space),
    all_blanks(T).

write_element_content([], _, _).
write_element_content([H|T], Out, State) :-
    write_indent(State, Out),
    emit(H, Out, State),
    write_element_content(T, Out, State).


                 /*******************************
                 *           NAMESPACES         *
                 *******************************/

%!  add_missing_namespaces(+DOM0, +NsMap, -DOM)
%
%   Add xmlns:NS=URI definitions to the toplevel element(s) to
%   deal with missing namespaces.

add_missing_namespaces([], _, []) :- !.
add_missing_namespaces([H0|T0], Def, [H|T]) :-
    !,
    add_missing_namespaces(H0, Def, H),
    add_missing_namespaces(T0, Def, T).
add_missing_namespaces(Elem0, Def, Elem) :-
    Elem0 = element(Name, Atts0, Content),
    !,
    missing_namespaces(Elem0, Def, Missing),
    (   Missing == []
    ->  Elem = Elem0
    ;   add_missing_ns(Missing, Atts0, Atts),
        Elem = element(Name, Atts, Content)
    ).
add_missing_namespaces(DOM, _, DOM).    % CDATA, etc.

add_missing_ns([], Atts, Atts).
add_missing_ns([H|T], Atts0, Atts) :-
    generate_ns(H, NS),
    add_missing_ns(T, [xmlns:NS=H|Atts0], Atts).

%!  generate_ns(+URI, -NS) is det.
%
%   Generate a namespace (NS) identifier for URI.

generate_ns(URI, NS) :-
    xmlns(NS, URI),
    !.
generate_ns(URI, NS) :-
    default_ns(URI, NS),
    !.
generate_ns(_, NS) :-
    gensym(xns, NS).

%!  xmlns(?NS, ?URI) is nondet.
%
%   Hook to define human readable  abbreviations for XML namespaces.
%   xml_write/3 tries these locations:
%
%     1. This hook
%     2. Defaults (see below)
%     3. rdf_db:ns/2 for RDF-DB integration
%
%   Default XML namespaces are:
%
%     | xsi    | http://www.w3.org/2001/XMLSchema-instance |
%     | xs     | http://www.w3.org/2001/XMLSchema          |
%     | xhtml  | http://www.w3.org/1999/xhtml              |
%     | soap11 | http://schemas.xmlsoap.org/soap/envelope/ |
%     | soap12 | http://www.w3.org/2003/05/soap-envelope   |
%
%   @see xml_write/2, rdf_register_ns/2.

:- multifile
    rdf_db:ns/2.

default_ns('http://www.w3.org/2001/XMLSchema-instance', xsi).
default_ns('http://www.w3.org/2001/XMLSchema',          xs).
default_ns('http://www.w3.org/1999/xhtml',              xhtml).
default_ns('http://schemas.xmlsoap.org/soap/envelope/', soap11).
default_ns('http://www.w3.org/2003/05/soap-envelope',   soap12).
default_ns(URI, NS) :-
    rdf_db:ns(NS, URI).

%!  missing_namespaces(+DOM, +NSMap, -Missing)
%
%   Return a list of URIs appearing in DOM that are not covered
%   by xmlns definitions.

missing_namespaces(DOM, Defined, Missing) :-
    missing_namespaces(DOM, Defined, [], Missing).

missing_namespaces([], _, L, L) :- !.
missing_namespaces([H|T], Def, L0, L) :-
    !,
    missing_namespaces(H, Def, L0, L1),
    missing_namespaces(T, Def, L1, L).
missing_namespaces(element(Name, Atts, Content), Def, L0, L) :-
    !,
    update_nsmap(Atts, _, Def, Def1),
    missing_ns(Name, Def1, L0, L1),
    missing_att_ns(Atts, Def1, L1, L2),
    missing_namespaces(Content, Def1, L2, L).
missing_namespaces(_, _, L, L).

missing_att_ns([], _, M, M).
missing_att_ns([Name=_|T], Def, M0, M) :-
    missing_ns(Name, Def, M0, M1),
    missing_att_ns(T, Def, M1, M).

missing_ns(ns(NS, URI):_, Def, M0, M) :-
    !,
    (  memberchk(NS=URI, Def)
    -> M = M0
    ;  NS == ''
    -> M = M0
    ;  M = [URI|M0]
    ).
missing_ns(URI:_, Def, M0, M) :-
    !,
    (   (   memberchk(_=URI, Def)
        ;   memberchk(URI, M0)
        ;   URI = xml               % predefined ones
        ;   URI = xmlns
        )
    ->  M = M0
    ;   M = [URI|M0]
    ).
missing_ns(_, _, M, M).

                 /*******************************
                 *         QUOTED WRITE         *
                 *******************************/

sgml_write_attribute(Out, Values, State) :-
    is_list(Values),
    !,
    get_state(State, entity_map, EntityMap),
    put_char(Out, '"'),
    write_quoted_list(Values, Out, '"<&\r\n\t', EntityMap),
    put_char(Out, '"').
sgml_write_attribute(Out, Value, State) :-
    is_text(Value),
    !,
    get_state(State, entity_map, EntityMap),
    put_char(Out, '"'),
    write_quoted(Out, Value, '"<&\r\n\t', EntityMap),
    put_char(Out, '"').
sgml_write_attribute(Out, Value, _State) :-
    number(Value),
    !,
    format(Out, '"~w"', [Value]).
sgml_write_attribute(_, Value, _) :-
    type_error(sgml_attribute_value, Value).

write_quoted_list([], _, _, _).
write_quoted_list([H|T], Out, Escape, EntityMap) :-
    write_quoted(Out, H, Escape, EntityMap),
    (   T == []
    ->  true
    ;   put_char(Out, ' '),
        write_quoted_list(T, Out, Escape, EntityMap)
    ).


sgml_write_content(Out, Value, State) :-
    is_text(Value),
    !,
    get_state(State, entity_map, EntityMap),
    write_quoted(Out, Value, '<&>\r', EntityMap).
sgml_write_content(Out, Value, _) :-
    write(Out, Value).

is_text(Value) :- atom(Value), !.
is_text(Value) :- string(Value), !.

write_quoted(Out, Atom, Escape, EntityMap) :-
    atom(Atom),
    !,
    atom_codes(Atom, Codes),
    writeq(Codes, Out, Escape, EntityMap).
write_quoted(Out, String, Escape, EntityMap) :-
    string(String),
    !,
    string_codes(String, Codes),
    writeq(Codes, Out, Escape, EntityMap).
write_quoted(_, String, _, _) :-
    type_error(atom_or_string, String).


%!  writeq(+Text:codes, +Out:stream, +Escape:atom, +Escape:assoc) is det.

writeq([], _, _, _).
writeq([H|T], Out, Escape, EntityMap) :-
    (   char_code(HC, H),
        sub_atom(Escape, _, _, _, HC)
    ->  write_entity(H, Out, EntityMap)
    ;   H >= 256
    ->  (   stream_property(Out, encoding(Enc)),
            unicode_encoding(Enc)
        ->  put_code(Out, H)
        ;   write_entity(H, Out, EntityMap)
        )
    ;   put_code(Out, H)
    ),
    writeq(T, Out, Escape, EntityMap).

unicode_encoding(utf8).
unicode_encoding(wchar_t).
unicode_encoding(unicode_le).
unicode_encoding(unicode_be).

write_entity(Code, Out, EntityMap) :-
    (   get_assoc(Code, EntityMap, EntityName)
    ->  format(Out, '&~w;', [EntityName])
    ;   format(Out, '&#x~16R;', [Code])
    ).


                 /*******************************
                 *          INDENTATION         *
                 *******************************/

write_initial_indent(State, Out) :-
    (   get_state(State, indent, Indent),
        Indent > 0
    ->  emit_indent(Indent, Out)
    ;   true
    ).

write_indent(State, _) :-
    get_state(State, layout, false),
    !.
write_indent(State, Out) :-
    get_state(State, indent, Indent),
    emit_indent(Indent, Out).

emit_indent(Indent, Out) :-
    Tabs is Indent // 8,
    Spaces is Indent mod 8,
    format(Out, '~N', []),
    write_n(Tabs, '\t', Out),
    write_n(Spaces, ' ', Out).

write_n(N, Char, Out) :-
    (   N > 0
    ->  put_char(Out, Char),
        N2 is N - 1,
        write_n(N2, Char, Out)
    ;   true
    ).

inc_indent(State) :-
    inc_indent(State, 2).

inc_indent(State, Inc) :-
    state(indent, Arg),
    arg(Arg, State, I0),
    I is I0 + Inc,
    setarg(Arg, State, I).


                 /*******************************
                 *         DTD HANDLING         *
                 *******************************/

%!  empty_element(+State, +Element)
%
%   True if Element is declared  with   EMPTY  content in the (SGML)
%   DTD.

empty_element(State, Element) :-
    get_state(State, dtd, DTD),
    DTD \== (-),
    dtd_property(DTD, element(Element, _, empty)).

%!  dtd_character_entities(+DTD, -Map)
%
%   Return an assoc mapping character entities   to their name. Note
%   that the entity representation is a bit dubious. Entities should
%   allow for a wide-character version and avoid the &#..; trick.

dtd_character_entities(DTD, Map) :-
    empty_assoc(Empty),
    dtd_property(DTD, entities(Entities)),
    fill_entity_map(Entities, DTD, Empty, Map).

fill_entity_map([], _, Map, Map).
fill_entity_map([H|T], DTD, Map0, Map) :-
    (   dtd_property(DTD, entity(H, CharEntity)),
        atom(CharEntity),
        (   sub_atom(CharEntity, 0, _, _, '&#'),
            sub_atom(CharEntity, _, _, 0, ';')
        ->  sub_atom(CharEntity, 2, _, 1, Name),
            atom_number(Name, Code)
        ;   atom_length(CharEntity, 1),
            char_code(CharEntity, Code)
        )
    ->  put_assoc(Code, Map0, H, Map1),
        fill_entity_map(T, DTD, Map1, Map)
    ;   fill_entity_map(T, DTD, Map0, Map)
    ).



                 /*******************************
                 *            FIELDS            *
                 *******************************/

state(indent,     1).                   % current indentation
state(layout,     2).                   % use layout (true/false)
state(dtd,        3).                   % DTD for entity names
state(entity_map, 4).                   % compiled entity-map
state(dialect,    5).                   % xml/sgml
state(nsmap,      6).                   % defined namespaces
state(net,        7).                   % Should null end-tags be used?
state(cleanns,    8).                   % Remove duplicate xmlns declarations

new_state(Dialect,
    state(
        0,              % indent
        true,           % layout
        -,              % DTD
        EntityMap,      % entity_map
        Dialect,        % dialect
        [],             % NS=Full map
        Net,            % Null End-Tags?
        true            % Remove duplicate xmlns declarations
    )) :-
    (   Dialect == sgml
    ->  Net = false,
        empty_assoc(EntityMap)
    ;   Net = true,
        xml_entities(EntityMap)
    ).

get_state(State, Field, Value) :-
    state(Field, Arg),
    arg(Arg, State, Value).

set_state(State, Field, Value) :-
    state(Field, Arg),
    setarg(Arg, State, Value).

term_expansion(xml_entities(map),
               xml_entities(Map)) :-
    list_to_assoc([ 0'< - lt,
                    0'& - amp,
                    0'> - gt,
                    0'\' - apos,
                    0'\" - quot
                  ], Map).
xml_entities(map).

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

:- multifile
    prolog:message/3.

prolog:message(sgml_write(sdata_as_cdata(Data))) -->
    [ 'SGML-write: emitting SDATA as CDATA: "~p"'-[Data] ].
