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    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)  2009-2019, University of Amsterdam
    7                              VU University Amsterdam
    8                              CWI, Amsterdam
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   35*/
   36
   37:- module(xpath,
   38          [ xpath/3,                    % +DOM, +Spec, -Value
   39            xpath_chk/3,                % +DOM, +Spec, -Value
   40
   41            op(400, fx, //),
   42            op(400, fx, /),
   43            op(200, fy, @)
   44          ]).   45:- use_module(library(record),[record/1, op(_,_,record)]).   46
   47:- autoload(library(debug),[assertion/1]).   48:- autoload(library(error),[instantiation_error/1,must_be/2]).   49:- autoload(library(lists),[member/2]).   50:- autoload(library(sgml),[xsd_number_string/2]).

Select nodes in an XML DOM

The library xpath.pl provides predicates to select nodes from an XML DOM tree as produced by library(sgml) based on descriptions inspired by the XPath language.

The predicate xpath/3 selects a sub-structure of the DOM non-deterministically based on an XPath-like specification. Not all selectors of XPath are implemented, but the ability to mix xpath/3 calls with arbitrary Prolog code provides a powerful tool for extracting information from XML parse-trees.

See also
- http://www.w3.org/TR/xpath */
   67:- record
   68    element(name, attributes, content).
 xpath_chk(+DOM, +Spec, ?Content) is semidet
Semi-deterministic version of xpath/3.
   74xpath_chk(DOM, Spec, Content) :-
   75    xpath(DOM, Spec, Content),
   76    !.
 xpath(+DOM, +Spec, ?Content) is nondet
Match an element in a DOM structure. The syntax is inspired by XPath, using () rather than [] to select inside an element. First we can construct paths using / and //:
//Term
Select any node in the DOM matching term.
/Term
Match the root against Term.
Term
Select the immediate children of the root matching Term.

The Terms above are of type callable. The functor specifies the element name. The element name '*' refers to any element. The name self refers to the top-element itself and is often used for processing matches of an earlier xpath/3 query. A term NS:Term refers to an XML name in the namespace NS. Optional arguments specify additional constraints and functions. The arguments are processed from left to right. Defined conditional argument values are:

index(?Index)
True if the element is the Index-th child of its parent, where 1 denotes the first child. Index can be one of:
Var
Var is unified with the index of the matched element.
last
True for the last element.
last - IntExpr
True for the last-minus-nth element. For example, last-1 is the element directly preceding the last one.
IntExpr
True for the element whose index equals IntExpr.
Integer
The N-th element with the given name, with 1 denoting the first element. Same as index(Integer).
last
The last element with the given name. Same as index(last).
last - IntExpr
The IntExpr-th element before the last. Same as index(last-IntExpr).

Defined function argument values are:

self
Evaluate to the entire element
content
Evaluate to the content of the element (a list)
text
Evaluates to all text from the sub-tree as an atom
text(As)
Evaluates to all text from the sub-tree according to As, which is either atom or string.
normalize_space
As text, but uses normalize_space/2 to normalise white-space in the output
number
Extract an integer or float from the value. Ignores leading and trailing white-space
@Attribute
Evaluates to the value of the given attribute. Attribute can be a compound term. In this case the functor name denotes the element and arguments perform transformations on the attribute value. Defined transformations are:
number
Translate the value into a number using xsd_number_string/2 from library(sgml).
integer
As number, but subsequently transform the value into an integer using the round/1 function.
float
As number, but subsequently transform the value into a float using the float/1 function.
string
Translate the value into a Prolog string.
lower
Translate the value to lower case, preserving the type.
upper
Translate the value to upper case, preserving the type.

In addition, the argument-list can be conditions:

Left = Right
Succeeds if the left-hand unifies with the right-hand. If the left-hand side is a function, this is evaluated. The right-hand side is never evaluated, and thus the condition content = content defines that the content of the element is the atom content. The functions lower_case and upper_case can be applied to Right (see example below).
contains(Haystack, Needle)
Succeeds if Needle is a sub-string of Haystack.
XPath
Succeeds if XPath matches in the currently selected sub-DOM. For example, the following expression finds an h3 element inside a div element, where the div element itself contains an h2 child with a strong child.
//div(h2/strong)/h3

This is equivalent to the conjunction of XPath goals below.

   ...,
   xpath(DOM, //(div), Div),
   xpath(Div, h2/strong, _),
   xpath(Div, h3, Result)

Examples:

Match each table-row in DOM:

xpath(DOM, //tr, TR)

Match the last cell of each tablerow in DOM. This example illustrates that a result can be the input of subsequent xpath/3 queries. Using multiple queries on the intermediate TR term guarantee that all results come from the same table-row:

xpath(DOM, //tr, TR),
xpath(TR,  /td(last), TD)

Match each href attribute in an <a> element

xpath(DOM, //a(@href), HREF)

Suppose we have a table containing rows where each first column is the name of a product with a link to details and the second is the price (a number). The following predicate matches the name, URL and price:

product(DOM, Name, URL, Price) :-
    xpath(DOM, //tr, TR),
    xpath(TR, td(1), C1),
    xpath(C1, /self(normalize_space), Name),
    xpath(C1, a(@href), URL),
    xpath(TR, td(2, number), Price).

Suppose we want to select books with genre="thriller" from a tree containing elements <book genre=...>

thriller(DOM, Book) :-
    xpath(DOM, //book(@genre=thiller), Book).

Match the elements <table align="center"> and <table align="CENTER">:

    //table(@align(lower) = center)

Get the width and height of a div element as a number, and the div node itself:

    xpath(DOM, //div(@width(number)=W, @height(number)=H), Div)

Note that div is an infix operator, so parentheses must be used in cases like the following:

    xpath(DOM, //(div), Div)
  262xpath(DOM, Spec, Content) :-
  263    in_dom(Spec, DOM, Content).
  264
  265in_dom(//Spec, DOM, Value) :-
  266    !,
  267    element_spec(Spec, Name, Modifiers),
  268    sub_dom(I, Len, Name, E, DOM),
  269    modifiers(Modifiers, I, Len, E, Value).
  270in_dom(/Spec, E, Value) :-
  271    !,
  272    element_spec(Spec, Name, Modifiers),
  273    (   Name == self
  274    ->  true
  275    ;   element_name(E, Name)
  276    ),
  277    modifiers(Modifiers, 1, 1, E, Value).
  278in_dom(A/B, DOM, Value) :-
  279    !,
  280    in_dom(A, DOM, Value0),
  281    in_dom(B, Value0, Value).
  282in_dom(A//B, DOM, Value) :-
  283    !,
  284    in_dom(A, DOM, Value0),
  285    in_dom(//B, Value0, Value).
  286in_dom(Spec, element(_, _, Content), Value) :-
  287    element_spec(Spec, Name, Modifiers),
  288    count_named_elements(Content, Name, CLen),
  289    CLen > 0,
  290    nth_element(N, Name, E, Content),
  291    modifiers(Modifiers, N, CLen, E, Value).
  292
  293element_spec(Var, _, _) :-
  294    var(Var),
  295    !,
  296    instantiation_error(Var).
  297element_spec(NS:Term, NS:Name, Modifiers) :-
  298    !,
  299    callable_name_arguments(Term, Name0, Modifiers),
  300    star(Name0, Name).
  301element_spec(Term, Name, Modifiers) :-
  302    !,
  303    callable_name_arguments(Term, Name0, Modifiers),
  304    star(Name0, Name).
  305
  306callable_name_arguments(Atom, Name, Arguments) :-
  307    atom(Atom),
  308    !,
  309    Name = Atom, Arguments = [].
  310callable_name_arguments(Compound, Name, Arguments) :-
  311    compound_name_arguments(Compound, Name, Arguments).
  312
  313
  314star(*, _) :- !.
  315star(Name, Name).
 sub_dom(-Index, -Count, +Name, -Sub, +DOM) is nondet
Sub is a node in DOM with Name.
Arguments:
Count- is the total number of nodes in the content list Sub appears that have the same name.
Index- is the 1-based index of Sub of nodes with Name.
  327sub_dom(1, 1, Name, DOM, DOM) :-
  328    element_name(DOM, Name0),
  329    \+ Name \= Name0.
  330sub_dom(N, Len, Name, E, element(_,_,Content)) :-
  331    !,
  332    sub_dom_2(N, Len, Name, E, Content).
  333sub_dom(N, Len, Name, E, Content) :-
  334    is_list(Content),
  335    sub_dom_2(N, Len, Name, E, Content).
  336
  337sub_dom_2(N, Len, Name, Element, Content) :-
  338    (   count_named_elements(Content, Name, Len),
  339        nth_element(N, Name, Element, Content)
  340    ;   member(element(_,_,C2), Content),
  341        sub_dom_2(N, Len, Name, Element, C2)
  342    ).
 count_named_elements(+Content, +Name, -Count) is det
Count is the number of nodes with Name in Content.
  349count_named_elements(Content, Name, Count) :-
  350    count_named_elements(Content, Name, 0, Count).
  351
  352count_named_elements([], _, Count, Count).
  353count_named_elements([element(Name,_,_)|T], Name0, C0, C) :-
  354    \+ Name \= Name0,
  355    !,
  356    C1 is C0+1,
  357    count_named_elements(T, Name0, C1, C).
  358count_named_elements([_|T], Name, C0, C) :-
  359    count_named_elements(T, Name, C0, C).
 nth_element(?N, +Name, -Element, +Content:list) is nondet
True if Element is the N-th element with name in Content.
  366nth_element(N, Name, Element, Content) :-
  367    nth_element_(1, N, Name, Element, Content).
  368
  369nth_element_(I, N, Name, E, [H|T]) :-
  370    element_name(H, Name0),
  371    \+ Name \= Name0,
  372    !,
  373    (   N = I,
  374        E = H
  375    ;   I2 is I + 1,
  376        (   nonvar(N), I2 > N
  377        ->  !, fail
  378        ;   true
  379        ),
  380        nth_element_(I2, N, Name, E, T)
  381    ).
  382nth_element_(I, N, Name, E, [_|T]) :-
  383    nth_element_(I, N, Name, E, T).
 modifiers(+Modifiers, +I, +Clen, +DOM, -Value)
  390modifiers([], _, _, Value, Value).
  391modifiers([H|T], I, L, Value0, Value) :-
  392    modifier(H, I, L, Value0, Value1),
  393    modifiers(T, I, L, Value1, Value).
  394
  395modifier(M, _, _, _, _) :-
  396    var(M),
  397    !,
  398    instantiation_error(M).
  399modifier(Index, I, L, Value0, Value) :-
  400    implicit_index_modifier(Index),
  401    !,
  402    Value = Value0,
  403    index_modifier(Index, I, L).
  404modifier(index(Index), I, L, Value, Value) :-
  405    !,
  406    index_modifier(Index, I, L).
  407modifier(Function, _, _, In, Out) :-
  408    xpath_function(Function),
  409    !,
  410    xpath_function(Function, In, Out).
  411modifier(Function, _, _, In, Out) :-
  412    xpath_condition(Function, In),
  413    Out = In.
  414
  415implicit_index_modifier(I) :-
  416    integer(I),
  417    !.
  418implicit_index_modifier(last).
  419implicit_index_modifier(last-_Expr).
  420
  421index_modifier(Var, I, _L) :-
  422    var(Var),
  423    !,
  424    Var = I.
  425index_modifier(last, I, L) :-
  426    !,
  427    I =:= L.
  428index_modifier(last-Expr, I, L) :-
  429    !,
  430    I =:= L-Expr.
  431index_modifier(N, I, _) :-
  432    N =:= I.
  433
  434xpath_function(self, DOM, Value) :-                            % self
  435    !,
  436    Value = DOM.
  437xpath_function(content, Element, Value) :-                     % content
  438    !,
  439    element_content(Element, Value).
  440xpath_function(text, DOM, Text) :-                             % text
  441    !,
  442    text_of_dom(DOM, atom, Text).
  443xpath_function(text(As), DOM, Text) :-                         % text(As)
  444    !,
  445    text_of_dom(DOM, As, Text).
  446xpath_function(normalize_space, DOM, Text) :-                  % normalize_space
  447    !,
  448    text_of_dom(DOM, string, Text0),
  449    normalize_space(atom(Text), Text0).
  450xpath_function(number, DOM, Number) :-                         % number
  451    !,
  452    text_of_dom(DOM, string, Text0),
  453    normalize_space(string(Text), Text0),
  454    catch(xsd_number_string(Number, Text), _, fail).
  455xpath_function(@Name, element(_, Attrs, _), Value) :-          % @Name
  456    !,
  457    (   atom(Name)
  458    ->  memberchk(Name=Value, Attrs)
  459    ;   compound(Name)
  460    ->  compound_name_arguments(Name, AName, AOps),
  461        memberchk(AName=Value0, Attrs),
  462        translate_attribute(AOps, Value0, Value)
  463    ;   member(Name=Value, Attrs)
  464    ).
  465xpath_function(quote(Value), _, Value).                         % quote(Value)
  466
  467xpath_function(self).
  468xpath_function(content).
  469xpath_function(text).
  470xpath_function(text(_)).
  471xpath_function(normalize_space).
  472xpath_function(number).
  473xpath_function(@_).
  474xpath_function(quote(_)).
  475
  476translate_attribute([], Value, Value).
  477translate_attribute([H|T], Value0, Value) :-
  478    translate_attr(H, Value0, Value1),
  479    translate_attribute(T, Value1, Value).
  480
  481translate_attr(number, Value0, Value) :-
  482    xsd_number_string(Value, Value0).
  483translate_attr(integer, Value0, Value) :-
  484    xsd_number_string(Value1, Value0),
  485    Value is round(Value1).
  486translate_attr(float, Value0, Value) :-
  487    xsd_number_string(Value1, Value0),
  488    Value is float(Value1).
  489translate_attr(string, Value0, Value) :-
  490    atom_string(Value0, Value).
  491translate_attr(lower, Value0, Value) :-
  492    (   atom(Value0)
  493    ->  downcase_atom(Value0, Value)
  494    ;   string_lower(Value0, Value)
  495    ).
  496translate_attr(upper, Value0, Value) :-
  497    (   atom(Value0)
  498    ->  upcase_atom(Value0, Value)
  499    ;   string_upper(Value0, Value)
  500    ).
  501
  502xpath_condition(Left = Right, Value) :-                        % =
  503    !,
  504    var_or_function(Left, Value, LeftValue),
  505    process_equality(LeftValue, Right).
  506xpath_condition(contains(Haystack, Needle), Value) :-          % contains(Haystack, Needle)
  507    !,
  508    val_or_function(Haystack, Value, HaystackValue),
  509    val_or_function(Needle, Value, NeedleValue),
  510    atom(HaystackValue), atom(NeedleValue),
  511    (   sub_atom(HaystackValue, _, _, _, NeedleValue)
  512    ->  true
  513    ).
  514xpath_condition(Spec, Dom) :-
  515    in_dom(Spec, Dom, _).
 process_equality(+Left, +Right) is semidet
Provides (very) partial support for XSLT functions that can be applied according to the XPath 2 specification.

For example the XPath expression in [1], and the equivalent Prolog expression in [2], would both match the HTML element in [3].

[1] //table[align=lower-case(center)]
[2] //table(@align=lower_case(center))
[3] <table align="CENTER">
  533process_equality(Left, Right) :-
  534    var(Right),
  535    !,
  536    Left = Right.
  537process_equality(Left, lower_case(Right)) :-
  538    !,
  539    downcase_atom(Left, Right).
  540process_equality(Left, upper_case(Right)) :-
  541    !,
  542    upcase_atom(Left, Right).
  543process_equality(Left, Right) :-
  544    Left = Right.
  545
  546
  547var_or_function(Arg, _, Arg) :-
  548    var(Arg),
  549    !.
  550var_or_function(Func, Value0, Value) :-
  551    xpath_function(Func),
  552    !,
  553    xpath_function(Func, Value0, Value).
  554var_or_function(Value, _, Value).
  555
  556val_or_function(Arg, _, Arg) :-
  557    var(Arg),
  558    !,
  559    instantiation_error(Arg).
  560val_or_function(Func, Value0, Value) :-                         % TBD
  561    xpath_function(Func, Value0, Value),
  562    !.
  563val_or_function(Value, _, Value).
 text_of_dom(+DOM, +As, -Text:atom) is det
Text is the joined textual content of DOM.
  570text_of_dom(DOM, As, Text) :-
  571    phrase(text_of(DOM), Tokens),
  572    (   As == atom
  573    ->  atomic_list_concat(Tokens, Text)
  574    ;   As == string
  575    ->  atomics_to_string(Tokens, Text)
  576    ;   must_be(oneof([atom,string]), As)
  577    ).
  578
  579text_of(element(_,_,Content)) -->
  580    text_of_list(Content).
  581text_of([]) -->
  582    [].
  583text_of([H|T]) -->
  584    text_of(H),
  585    text_of(T).
  586
  587
  588text_of_list([]) -->
  589    [].
  590text_of_list([H|T]) -->
  591    text_of_1(H),
  592    text_of_list(T).
  593
  594
  595text_of_1(element(_,_,Content)) -->
  596    !,
  597    text_of_list(Content).
  598text_of_1(Data) -->
  599    { assertion(atom_or_string(Data)) },
  600    [Data].
  601
  602atom_or_string(Data) :-
  603    (   atom(Data)
  604    ->  true
  605    ;   string(Data)
  606    )