src/svgtiny_css.c: implement node_is_lang() select handler

[libsvgtiny.git] / src / svgtiny_css.c

#include <libcss/libcss.h>
#include <strings.h> /* strncasecmp */

#include "svgtiny.h"
#include "svgtiny_internal.h"

static css_error node_name(void *pw, void *node, css_qname *qname);
static css_error node_classes(void *pw, void *node,
                lwc_string ***classes, uint32_t *n_classes);
static css_error node_id(void *pw, void *node, lwc_string **id);
static css_error named_parent_node(void *pw, void *node,
                const css_qname *qname, void **parent);
static css_error named_sibling_node(void *pw, void *node,
                const css_qname *qname, void **sibling);
static css_error named_generic_sibling_node(void *pw, void *node,
                const css_qname *qname, void **sibling);
static css_error parent_node(void *pw, void *node, void **parent);
static css_error sibling_node(void *pw, void *node, void **sibling);
static css_error node_has_name(void *pw, void *node,
                const css_qname *qname, bool *match);
static css_error node_has_class(void *pw, void *node,
                lwc_string *name, bool *match);
static css_error node_has_id(void *pw, void *node,
                lwc_string *name, bool *match);
static css_error node_has_attribute(void *pw, void *node,
                const css_qname *qname, bool *match);
static css_error node_has_attribute_equal(void *pw, void *node,
                const css_qname *qname, lwc_string *value,
                bool *match);
static css_error node_has_attribute_dashmatch(void *pw, void *node,
                const css_qname *qname, lwc_string *value,
                bool *match);
static css_error node_has_attribute_includes(void *pw, void *node,
                const css_qname *qname, lwc_string *word,
                bool *match);
static css_error node_has_attribute_prefix(void *pw, void *node,
                const css_qname *qname, lwc_string *prefix,
                bool *match);
static css_error node_has_attribute_suffix(void *pw, void *node,
                const css_qname *qname, lwc_string *suffix,
                bool *match);
static css_error node_has_attribute_substring(void *pw, void *node,
                const css_qname *qname, lwc_string *substring,
                bool *match);
static css_error node_is_root(void *pw, void *node, bool *match);
static css_error node_count_siblings(void *pw, void *node,
                bool same_name, bool after, int32_t *count);
static css_error node_is_empty(void *pw, void *node, bool *is_empty);
static css_error node_is_link(void *pw, void *node, bool *is_link);
static css_error node_is_hover(void *pw, void *node, bool *is_hover);
static css_error node_is_active(void *pw, void *node, bool *is_active);
static css_error node_is_focus(void *pw, void *node, bool *is_focus);
static css_error node_is_enabled(void *pw, void *node, bool *is_enabled);
static css_error node_is_disabled(void *pw, void *node, bool *is_disabled);
static css_error node_is_checked(void *pw, void *node, bool *is_checked);
static css_error node_is_target(void *pw, void *node, bool *is_target);
static css_error node_is_lang(void *pw, void *node,
                lwc_string *lang, bool *is_lang);


/**
 * Resolve a relative URL to an absolute one by doing nothing. This is
 * the simplest possible implementation of a URL resolver, needed for
 * parsing CSS.
 */
css_error svgtiny_resolve_url(void *pw,
                const char *base, lwc_string *rel, lwc_string **abs)
{
        UNUSED(pw);
        UNUSED(base);

        /* Copy the relative URL to the absolute one (the return
           value) */
        *abs = lwc_string_ref(rel);
        return CSS_OK;
}

/**
 * Create a stylesheet with the default set of params.
 *
 * \param sheet A stylesheet pointer, passed in by reference, that
 * we use to store the newly-created stylesheet.
 * \param inline_style True if this stylesheet represents an inline
 * style, and false otherwise.
 *
 * \return The return value from css_stylesheet_create() is returned.
 */
css_error svgtiny_create_stylesheet(css_stylesheet **sheet,
                bool inline_style)
{
        css_stylesheet_params params;

        params.params_version = CSS_STYLESHEET_PARAMS_VERSION_1;
        params.level = CSS_LEVEL_DEFAULT;
        params.charset = NULL;
        params.url = "";
        params.title = NULL;
        params.allow_quirks = false;
        params.inline_style = inline_style;
        params.resolve = svgtiny_resolve_url;
        params.resolve_pw = NULL;
        params.import = NULL;
        params.import_pw = NULL;
        params.color = NULL;
        params.color_pw = NULL;
        params.font = NULL;
        params.font_pw = NULL;

        return css_stylesheet_create(&params, sheet);
}


/**************************/
/* libcss select handlers */
/**************************/
/*
 * From here on we implement the "select handler "API defined in
 * libcss's include/libcss/select.h and discussed briefly in its
 * docs/API document.
 */


/**
 * Retrieve the given node's name
 *
 * \param pw     Pointer to the current SVG parser state
 * \param node   Libdom SVG node
 * \param qname  Address at which to store the node name
 *
 * \return CSS_OK on success, or CSS_NOMEM if anything goes wrong
 */
css_error node_name(void *pw, void *node, css_qname *qname)
{
        dom_string *name;
        dom_exception err;
        struct svgtiny_parse_state *state;

        err = dom_node_get_node_name((dom_node *)node, &name);
        if (err != DOM_NO_ERR) {
                return CSS_NOMEM;
        }

        state = (struct svgtiny_parse_state *)pw;
        qname->ns = lwc_string_ref(state->interned_svg_xmlns);

        err = dom_string_intern(name, &qname->name);
        if (err != DOM_NO_ERR) {
                dom_string_unref(name);
                return CSS_NOMEM;
        }

        dom_string_unref(name);

        return CSS_OK;
}


/**
 * Retrieve the given node's classes
 *
 * \param pw         Pointer to the current SVG parser state
 * \param node       Libdom SVG node
 * \param classes    Address at which to store the class name array
 * \param n_classes  Address at which to store the length of the class
 *                   name array
 *
 * \return CSS_OK on success, or CSS_NOMEM if anything goes wrong
 *
 * \note CSS_NOMEM is not possible in practice as of libdom-0.4.1,
 *       because the underlying libdom function never fails
 */
css_error node_classes(void *pw, void *node,
                lwc_string ***classes, uint32_t *n_classes)
{
        UNUSED(pw);
        dom_exception err;

        err = dom_element_get_classes((dom_node *)node, classes, n_classes);

        /* The implementation does not do it, but the documentation
           for dom_element_get_classes() says that a DOM_NO_MEM_ERR is
           possible here, so we handle it to be on the safe side. */
        if (err != DOM_NO_ERR) {
                return CSS_NOMEM;
        }

        return CSS_OK;
}


/**
 * Retrieve the given node's id
 *
 * \param pw    Pointer to the current SVG parser state
 * \param node  Libdom SVG node
 * \param id    Address at which to store the id
 *
 * \return      CSS_OK on success, or CSS_NOMEM if anything goes wrong
 */
css_error node_id(void *pw, void *node, lwc_string **id)
{
        dom_string *attr;
        dom_exception err;
        struct svgtiny_parse_state *state;

        /* Begin with the assumption that this node has no id */
        *id = NULL;

        state = (struct svgtiny_parse_state *)pw;
        err = dom_element_get_attribute((dom_node *)node,
                                        state->interned_id, &attr);
        if (err != DOM_NO_ERR) {
                return CSS_NOMEM;
        }
        else if (attr == NULL) {
                /* The node has no id attribute and our return value
                   is already set to NULL so we're done */
                return CSS_OK;
        }

        /* If we found an id attribute (a dom_string), intern it into
           an lwc_string that we can return, and then cleanup the
           dom_string. */
        err = dom_string_intern(attr, id);
        if (err != DOM_NO_ERR) {
                dom_string_unref(attr);
                return CSS_NOMEM;
        }
        dom_string_unref(attr);
        return CSS_OK;
}



/**
 * Find the first parent of the given element having the given name
 *
 * \param pw      Pointer to the current SVG parser state
 * \param node    Libdom SVG node
 * \param qname   Name of the parent node to search for
 * \param parent  Address at which to store the parent node pointer
 *
 * \return Always returns CSS_OK
 *
 * \post If a suitable element is found, a pointer to it will be
 *       stored at the address pointed to by \a parent; otherwise,
 *       NULL will be stored at the address pointed to by \a parent
 */
css_error named_parent_node(void *pw, void *node,
                const css_qname *qname, void **parent)
{
        UNUSED(pw);
        /* dom_element_named_parent_node() was invented to implement
         * this select handler so there isn't much for us to do except
         * call it. It's OK if node isn't an element, libdom checks
         * for it. */
        dom_element_named_parent_node((dom_element *)node,
                                qname->name,
                                (struct dom_element **)parent);

        /* Implementation detail: dom_element_named_parent_node()
         * increments the reference count of the parent element before
         * returning it to us. According to docs/RefCnt in the libdom
         * repository, this will prevent the parent element from being
         * destroyed if it is pruned from the DOM. That sounds good,
         * since we don't want to be using a pointer to an object that
         * has been destroyed... but we also have no way of later
         * decrementing the reference count ourselves, and don't want
         * to make the returned node eternal. Decrementing the
         * reference counter now allows it to be destroyed when the
         * DOM no longer needs it, and so long as no other parts of
         * libsvgtiny are messing with the DOM during parsing, that
         * shouldn't (ha ha) cause any problems. */
        dom_node_unref(*parent);

        return CSS_OK;
}


/**
 * Find the "next-sibling" of the given element having the given name
 *
 * This search corresponds to the "+ foo" combinator in CSS and will
 * find only "foo" element nodes that immediately precede the given
 * node under the same parent in the DOM. In CSS the tree is viewed
 * top-down and in libdom it is viewed from the bottom-up; as a result
 * "next" and "previous" are sometimes backwards. This is case-sensitive.
 *
 * \param pw       Pointer to the current SVG parser state
 * \param node     Libdom SVG node
 * \param qname    Name of the sibling node to search for
 * \param sibling  Address at which to store the sibling node pointer
 *
 * \return Always returns CSS_OK
 *
 * \post If a suitable element is found, a pointer to it will be
 *       stored at the address pointed to by \a sibling; otherwise,
 *       NULL will be stored at the address pointed to by \a sibling
 */
css_error named_sibling_node(void *pw, void *node,
                const css_qname *qname, void **sibling)
{
        UNUSED(pw);
        dom_node *n = node; /* the current node */
        dom_node *prev; /* the previous node */
        dom_exception err;
        dom_node_type type;
        dom_string *name;

        *sibling = NULL; /* default to nothing found */

        /* Begin the search; the first iteration we do outside of the
         * loop. Implementation detil: dom_node_get_previous_sibling()
         * increments the reference counter on the returned node. A
         * comment within named_parent_node() explains why we
         * decrement it ASAP. */
        err = dom_node_get_previous_sibling(n, &n);
        if (err != DOM_NO_ERR) {
                return CSS_OK;
        }

        while (n != NULL) {
                /* We're looking for the first ELEMENT sibling */
                err = dom_node_get_node_type(n, &type);
                if (err != DOM_NO_ERR) {
                        dom_node_unref(n);
                        return CSS_OK;
                }

                if (type == DOM_ELEMENT_NODE) {
                        /* We found an element node, does it have the
                         * right name? */
                        err = dom_node_get_node_name(n, &name);
                        if (err != DOM_NO_ERR) {
                                dom_node_unref(n);
                                return CSS_OK;
                        }

                        if (dom_string_lwc_isequal(name,
                                        qname->name)) {
                                /* The name is right, return it */
                                *sibling = n;
                        }

                        /* There's only one next-sibling element node
                         * and we've already found it, so if its name
                         * wasn't right, we return the default value
                         * of NULL below */
                        dom_string_unref(name);
                        dom_node_unref(n);
                        return CSS_OK;
                }

                /* Not an element node, so we move on the the previous
                 * previous sibling */
                err = dom_node_get_previous_sibling(n, &prev);
                if (err != DOM_NO_ERR) {
                        dom_node_unref(n);
                        return CSS_OK;
                }

                dom_node_unref(n);
                n = prev;
        }

        return CSS_OK;
}


/**
 * Find the first "subsequent-sibling" of the given element having the
 * given name
 *
 * This search corresponds to the "~ foo" combinator in CSS and will
 * find only "foo" element nodes that precede the given node (under
 * the same parent) in the DOM. In CSS the tree is viewed top-down and
 * in libdom it is viewed from the bottom-up; as a result "next" and
 * "previous" are sometimes backwards. This is case-sensitive.
 *
 * \param pw       Pointer to the current SVG parser state
 * \param node     Libdom SVG node
 * \param qname    Name of the sibling node to search for
 * \param sibling  Address at which to store the sibling node pointer
 *
 * \return Always returns CSS_OK
 *
 * \post If a suitable element is found, a pointer to it will be
 *       stored at the address pointed to by \a sibling; otherwise,
 *       NULL will be stored at the address pointed to by \a sibling
 */
css_error named_generic_sibling_node(void *pw, void *node,
                const css_qname *qname, void **sibling)
{
        UNUSED(pw);
        dom_node *n = node; /* the current node */
        dom_node *prev;     /* the previous node */
        dom_exception err;
        dom_node_type type;
        dom_string *name;


        *sibling = NULL; /* default to nothing found */

        /* Begin the search; the first iteration we do outside of the
         * loop. Implementation detil: dom_node_get_previous_sibling()
         * increments the reference counter on the returned node. A
         * comment within named_parent_node() explains why we
         * decrement it ASAP. */
        err = dom_node_get_previous_sibling(n, &n);
        if (err != DOM_NO_ERR) {
                return CSS_OK;
        }

        while (n != NULL) {
                err = dom_node_get_node_type(n, &type);
                if (err != DOM_NO_ERR) {
                        dom_node_unref(n);
                        return CSS_OK;
                }

                if (type == DOM_ELEMENT_NODE) {
                        /* We only want ELEMENT nodes */
                        err = dom_node_get_node_name(n, &name);
                        if (err != DOM_NO_ERR) {
                                dom_node_unref(n);
                                return CSS_OK;
                        }

                        if (dom_string_lwc_isequal(name,
                                        qname->name)) {
                                /* Found one. Save it and stop the search */
                                dom_string_unref(name);
                                dom_node_unref(n);
                                *sibling = n;
                                return CSS_OK;
                        }

                        dom_string_unref(name);
                }

                /* This sibling wasn't an element with the desired
                   name, so move on to the previous sibling */
                err = dom_node_get_previous_sibling(n, &prev);
                if (err != DOM_NO_ERR) {
                        dom_node_unref(n);
                        return CSS_OK;
                }

                dom_node_unref(n);
                n = prev;
        }

        return CSS_OK;
}


/**
 * Return a pointer to the given node's parent
 *
 * \param pw      Pointer to the current SVG parser state
 * \param node    Libdom SVG node
 * \param parent  Address at which to store the node's parent pointer
 *
 * \return Always returns CSS_OK
 */
css_error parent_node(void *pw, void *node, void **parent)
{
        UNUSED(pw);
        /* Libdom basically implements this for us */
        dom_element_parent_node(node, (struct dom_element **)parent);

        /* See the comment in named_parent_node() for why we decrement
         * this reference counter here. */
        dom_node_unref(*parent);

        return CSS_OK;
}


/**
 * Find the "next-sibling" of the given element
 *
 * This search corresponds "+ *" in CSS and will find the first
 * element node that immediately precedes the given node under the
 * same parent in the DOM. In CSS the tree is viewed top-down and in
 * libdom it is viewed from the bottom-up; as a result "next" and
 * "previous" are sometimes backwards.
 *
 * \param pw       Pointer to the current SVG parser state
 * \param node     Libdom SVG node
 * \param sibling  Address at which to store the sibling node pointer
 *
 * \return Always returns CSS_OK
 *
 * \post If a suitable element is found, a pointer to it will be
 *       stored at the address pointed to by \a sibling; otherwise,
 *       NULL will be stored at the address pointed to by \a sibling
 */
css_error sibling_node(void *pw, void *node, void **sibling)
{
        UNUSED(pw);
        dom_node *n = node; /* the current node */
        dom_node *prev;     /* the previous node */
        dom_exception err;
        dom_node_type type;

        *sibling = NULL; /* default to nothing found */

        /* Begin the search; the first iteration we do outside of the
         * loop. Implementation detil: dom_node_get_previous_sibling()
         * increments the reference counter on the returned node. A
         * comment within named_parent_node() explains why we
         * decrement it ASAP. */
        err = dom_node_get_previous_sibling(n, &n);
        if (err != DOM_NO_ERR) {
                return CSS_OK;
        }

        while (n != NULL) {
                err = dom_node_get_node_type(n, &type);
                if (err != DOM_NO_ERR) {
                        dom_node_unref(n);
                        return CSS_OK;
                }

                if (type == DOM_ELEMENT_NODE) {
                        /* We found a sibling node that is also an
                           element and that's all we wanted. */
                        *sibling = n;
                        dom_node_unref(n);
                        return CSS_OK;
                }

                /* This sibling node was not an element; move on to
                   the previous sibling */
                err = dom_node_get_previous_sibling(n, &prev);
                if (err != DOM_NO_ERR) {
                        dom_node_unref(n);
                        return CSS_OK;
                }

                dom_node_unref(n);
                n = prev;
        }

        return CSS_OK;
}


/**
 * Test the given node for the given name
 *
 * This will return true (via the "match" pointer) if the libdom node
 * has the given name or if that name is the universal selector;
 * otherwise it returns false. The comparison is case-sensitive. It
 * corresponds to a rule like "body { ... }" in CSS.
 *
 * \param pw       Pointer to the current SVG parser state
 * \param node     Libdom SVG node to test
 * \param qname    Name to check for
 * \param match    Pointer to the test result
 *
 * \return Always returns CSS_OK
 */
css_error node_has_name(void *pw, void *node,
                const css_qname *qname, bool *match)
{
        struct svgtiny_parse_state *state;
        dom_string *name;
        dom_exception err;

        /* Start by checking to see if qname is the universal selector */
        state = (struct svgtiny_parse_state *)pw;
        if (lwc_string_isequal(qname->name,
                        state->interned_universal, match) == lwc_error_ok) {
          if (*match) {
                /* It's the universal selector. In NetSurf, all node
                 * names match the universal selector, and nothing in
                 * the libcss documentation suggests another approach,
                 * so we follow NetSurf here. */
                return CSS_OK;
          }
        }

        err = dom_node_get_node_name((dom_node *)node, &name);
        if (err != DOM_NO_ERR) {
          return CSS_OK;
        }

        /* Unlike with HTML, SVG element names are case-sensitive */
        *match = dom_string_lwc_isequal(name, qname->name);
        dom_string_unref(name);

        return CSS_OK;
}


/**
 * Test the given node for the given class
 *
 * This will return true (via the "match" pointer) if the libdom node
 * has the given class. The comparison is case-sensitive. It
 * corresponds to node.class in CSS.
 *
 * \param pw       Pointer to the current SVG parser state
 * \param node     Libdom SVG node to test
 * \param name     Class name to check for
 * \param match    Pointer to the test result
 *
 * \return Always returns CSS_OK
 */
css_error node_has_class(void *pw, void *node,
                lwc_string *name, bool *match)
{
        UNUSED(pw);
        /* libdom implements this for us and apparently it cannot fail */
        dom_element_has_class((dom_node *)node, name, match);
        return CSS_OK;
}


/**
 * Test the given node for the given id
 *
 * This will return true (via the "match" pointer) if the libdom node
 * has the given id. The comparison is case-sensitive. It corresponds
 * to node#id in CSS.
 *
 * \param pw       Pointer to the current SVG parser state
 * \param node     Libdom SVG node to test
 * \param name     Id to check for
 * \param match    Pointer to the test result
 *
 * \return Always returns CSS_OK
 */
css_error node_has_id(void *pw, void *node,
                lwc_string *name, bool *match)
{
        dom_string *attr;
        dom_exception err;
        struct svgtiny_parse_state *state;

        attr = NULL;    /* a priori the "id" attribute may not exist */
        *match = false; /* default to no match */

        state = (struct svgtiny_parse_state *)pw;
        err = dom_element_get_attribute((dom_node *)node,
                                state->interned_id, &attr);
        if (err != DOM_NO_ERR || attr == NULL) {
                return CSS_OK;
        }

        *match = dom_string_lwc_isequal(attr, name);
        dom_string_unref(attr);

        return CSS_OK;
}


/**
 * Test the given node for the given attribute
 *
 * This will return true (via the "match" pointer) if the libdom node
 * has an attribute with the given name. The comparison is
 * case-sensitive. It corresponds to node[attr] in CSS.
 *
 * \param pw       Pointer to the current SVG parser state
 * \param node     Libdom SVG node to test
 * \param qname    Attribute name to check for
 * \param match    Pointer to the test result
 *
 * \return Returns CSS_OK if successful and CSS_NOMEM if anything
 *         goes wrong
 */
css_error node_has_attribute(void *pw, void *node,
                const css_qname *qname, bool *match)
{
        UNUSED(pw);
        dom_string *name;
        dom_exception err;

        /* intern the attribute name as a dom_string so we can
         * delegate to dom_element_has_attribute() */
        err = dom_string_create_interned(
                (const uint8_t *) lwc_string_data(qname->name),
                lwc_string_length(qname->name),
                &name);
        if (err != DOM_NO_ERR) {
                return CSS_NOMEM;
        }

        err = dom_element_has_attribute((dom_node *)node, name, match);
        if (err != DOM_NO_ERR) {
                dom_string_unref(name);
                return CSS_OK;
        }

        dom_string_unref(name);
        return CSS_OK;
}


/**
 * Test the given node for an attribute with a specific value
 *
 * This will return true (via the "match" pointer) if the libdom node
 * has an attribute with the given name and value. The comparison is
 * case-sensitive. It corresponds to node[attr=value] in CSS.
 *
 * \param pw       Pointer to the current SVG parser state
 * \param node     Libdom SVG node to test
 * \param qname    Attribute name to check for
 * \param value    Attribute value to check for
 * \param match    Pointer to the test result
 *
 * \return Returns CSS_OK if successful and CSS_NOMEM if we cannot
 *         intern the attribute name (which usually indicates memory
 *         exhaustion)
 */
css_error node_has_attribute_equal(void *pw, void *node,
                const css_qname *qname, lwc_string *value,
                bool *match)
{
        /* Implementation note: NetSurf always returns "no match" when
         * the value is empty (length zero). We allow it, because why
         * not? */

        UNUSED(pw);
        dom_string *name;
        dom_string *attr_val;
        dom_exception err;

        /* Intern the attribute name as a dom_string so we can
         * use dom_element_get_attribute() */
        err = dom_string_create_interned(
                (const uint8_t *) lwc_string_data(qname->name),
                lwc_string_length(qname->name),
                &name);
        if (err != DOM_NO_ERR) {
                return CSS_NOMEM;
        }

        err = dom_element_get_attribute((dom_node *)node, name, &attr_val);
        if ((err != DOM_NO_ERR) || (attr_val == NULL)) {
                /* There was an error getting the attribute's value or
                 * the attribute doesn't exist. So, no match? */
                dom_string_unref(name);
                *match = false;
                return CSS_OK;
        }

        /* Otherwise, we have the attribute value from the given node
         * and all we need to do is compare. */
        dom_string_unref(name);
        *match = dom_string_lwc_isequal(attr_val, value);
        dom_string_unref(attr_val);

        return CSS_OK;
}


/**
 * Test the given node for an attribute with a specific value,
 * possibly followed by a single hyphen
 *
 * This will return true (via the "match" pointer) if the libdom node
 * has an attribute with the given name and value or with the given
 * name and a value that is followed by exactly one hyphen. The
 * comparison is case-sensitive. This corresponds to [attr|=value]
 * in CSS.
 *
 * \param pw       Pointer to the current SVG parser state
 * \param node     Libdom SVG node to test
 * \param qname    Attribute name to check for
 * \param value    Attribute value to check for
 * \param match    Pointer to the test result
 *
 * \return Returns CSS_OK if successful and CSS_NOMEM if we cannot
 *         intern the attribute name (which usually indicates memory
 *         exhaustion)
 */
css_error node_has_attribute_dashmatch(void *pw, void *node,
                const css_qname *qname, lwc_string *value,
                bool *match)
{
        /* Implementation note: NetSurf always returns "no match" when
         * the value is empty (length zero). We allow it, because why
         * not? */

        UNUSED(pw);
        dom_string *name;
        dom_string *attr_val;
        dom_exception err;

        const char *vdata;   /* to hold the data underlying "value" */
        size_t vdata_len;
        const char *avdata;  /* to hold the found attribute value data */
        size_t avdata_len;

        /* Intern the attribute name as a dom_string so we can
         * use dom_element_get_attribute() */
        err = dom_string_create_interned(
                (const uint8_t *) lwc_string_data(qname->name),
                lwc_string_length(qname->name),
                &name);
        if (err != DOM_NO_ERR) {
                return CSS_NOMEM;
        }

        err = dom_element_get_attribute((dom_node *)node, name, &attr_val);
        if ((err != DOM_NO_ERR) || (attr_val == NULL)) {
                /* There was an error getting the attribute's value or
                 * the attribute doesn't exist. So, no match? */
                dom_string_unref(name);
                *match = false;
                return CSS_OK;
        }

        /* Otherwise, we have the attribute value from the given node
         * and all we need to do is compare. */
        dom_string_unref(name);
        *match = dom_string_lwc_isequal(attr_val, value);
        if (*match) {
                /* Exact match, we're done */
                dom_string_unref(attr_val);
                return CSS_OK;
        }

        /* No exact match, try it with a hyphen on the end */
        vdata = lwc_string_data(value);     /* needle */
        vdata_len = lwc_string_length(value);
        avdata = dom_string_data(attr_val); /* haystack */
        avdata_len = dom_string_byte_length(attr_val);
        dom_string_unref(attr_val);

        if (avdata_len > vdata_len && avdata[vdata_len] == '-') {
                if (strncasecmp(avdata, vdata, vdata_len) == 0) {
                        /* If there's a hyphen in the right position,
                         * it suffices to compare the strings only up
                         * to the hyphen */
                        *match = true;
                }
        }

        return CSS_OK;
}


/**
 * Test the given node for an attribute whose value is a
 * space-separated list of words, one of which is the given word
 *
 * This will return true (via the "match" pointer) if the libdom node
 * has an attribute with the given name and whose value when
 * considered as a space-separated list of words contains the given
 * word. The comparison is case-sensitive. This corresponds to
 * [attr~=value] in CSS.
 *
 * \param pw       Pointer to the current SVG parser state
 * \param node     Libdom SVG node to test
 * \param qname    Attribute name to check for
 * \param word     Value word to check for
 * \param match    Pointer to the test result
 *
 * \return Returns CSS_OK if successful and CSS_NOMEM if we cannot
 *         intern the attribute name (which usually indicates memory
 *         exhaustion)
 */
css_error node_has_attribute_includes(void *pw, void *node,
                const css_qname *qname, lwc_string *word,
                bool *match)
{
        UNUSED(pw);

        dom_string *name;
        dom_string *attr_val;
        dom_exception err;
        size_t wordlen; /* length of "word" */

        /* pointers used to parse a space-separated list of words */
        const char *p;
        const char *start;
        const char *end;

        *match = false; /* default to no match */

        wordlen = lwc_string_length(word);
        if (wordlen == 0) {
                /* In this case, the spec says that "if 'val' is the
                 * empty string, it will never represent anything." */
                return CSS_OK;
        }

        /* Intern the attribute name as a dom_string so we can
         * use dom_element_get_attribute() */
        err = dom_string_create_interned(
                (const uint8_t *) lwc_string_data(qname->name),
                lwc_string_length(qname->name),
                &name);
        if (err != DOM_NO_ERR) {
                return CSS_NOMEM;
        }

        err = dom_element_get_attribute((dom_node *)node, name, &attr_val);
        if ((err != DOM_NO_ERR) || (attr_val == NULL)) {
                /* There was an error getting the attribute's value or
                 * the attribute doesn't exist. So, no match? */
                dom_string_unref(name);
                return CSS_OK;
        }

        /* Parse the list comparing each word against "word" */
        start = dom_string_data(attr_val);
        end = start + dom_string_byte_length(attr_val);
        dom_string_unref(attr_val);

        for (p = start; p <= end; p++) {
                /* Move forward until we find the end of the first word */
                if (*p == ' ' || *p == '\0') {
                        /* If the length of that word is the length of the
                         * word we're looking for, do the comparison. */
                        if ((size_t) (p - start) == wordlen &&
                                strncasecmp(start,
                                        lwc_string_data(word),
                                        wordlen) == 0) {
                                *match = true;
                                break;
                        }
                        /* No match? Set "start" to the beginning of
                         * the next word and loop. */
                        start = p + 1;
                }
        }

        return CSS_OK;
}


/**
 * Test the given node for an attribute whose value begins with the
 * given prefix
 *
 * This will return true (via the "match" pointer) if the libdom node
 * has an attribute with the given name and whose value begins with
 * the given prefix string. The comparison is case-sensitive. This
 * corresponds to [attr^=value] in CSS.
 *
 * \param pw       Pointer to the current SVG parser state
 * \param node     Libdom SVG node to test
 * \param qname    Attribute name to check for
 * \param prefix   Value prefix to check for
 * \param match    Pointer to the test result
 *
 * \return Returns CSS_OK if successful and CSS_NOMEM if we cannot
 *         intern the attribute name (which usually indicates memory
 *         exhaustion)
 */
css_error node_has_attribute_prefix(void *pw, void *node,
                const css_qname *qname, lwc_string *prefix,
                bool *match)
{
        UNUSED(pw);
        dom_string *name;
        dom_string *attr_val;
        dom_exception err;
        const char *avdata; /* attribute value data */
        size_t avdata_len; /* length of that attribute value data */
        size_t prefixlen; /* length of "prefix" */

        prefixlen = lwc_string_length(prefix);
        if (prefixlen == 0) {
                /* In this case, the spec says that "if 'val' is the
                 * empty string, it will never represent anything." */
                return CSS_OK;
        }

        /* Intern the attribute name as a dom_string so we can
         * use dom_element_get_attribute() */
        err = dom_string_create_interned(
                (const uint8_t *) lwc_string_data(qname->name),
                lwc_string_length(qname->name),
                &name);
        if (err != DOM_NO_ERR) {
                return CSS_NOMEM;
        }

        err = dom_element_get_attribute((dom_node *)node, name, &attr_val);
        if ((err != DOM_NO_ERR) || (attr_val == NULL)) {
                /* There was an error getting the attribute's value or
                 * the attribute doesn't exist. So, no match? */
                dom_string_unref(name);
                *match = false;
                return CSS_OK;
        }

        /* Otherwise, we have the attribute value from the given node,
         * and the first thing we want to do is check to see if the
         * whole thing matches the prefix. */
        dom_string_unref(name);
        *match = dom_string_lwc_isequal(attr_val, prefix);

        /* If not, check to see if an, uh, prefix matches the
         * prefix */
        if (*match == false) {
                avdata = dom_string_data(attr_val);
                avdata_len = dom_string_byte_length(attr_val);
                if ((avdata_len >= prefixlen) &&
                                (strncasecmp(avdata,
                                        lwc_string_data(prefix),
                                        prefixlen) == 0)) {
                        /* Use strncasecmp to compare only the first
                         * "n" characters, where "n" is the length of
                         * the prefix. */
                        *match = true;
                }
        }

        dom_string_unref(attr_val);

        return CSS_OK;
}


/**
 * Test the given node for an attribute whose value end with the
 * given suffix
 *
 * This will return true (via the "match" pointer) if the libdom node
 * has an attribute with the given name and whose value ends with
 * the given suffix string. The comparison is case-sensitive. This
 * corresponds to [attr$=value] in CSS.
 *
 * \param pw       Pointer to the current SVG parser state
 * \param node     Libdom SVG node to test
 * \param qname    Attribute name to check for
 * \param suffix   Value suffix to check for
 * \param match    Pointer to the test result
 *
 * \return Returns CSS_OK if successful and CSS_NOMEM if we cannot
 *         intern the attribute name (which usually indicates memory
 *         exhaustion)
 */
css_error node_has_attribute_suffix(void *pw, void *node,
                const css_qname *qname, lwc_string *suffix,
                bool *match)
{
        UNUSED(pw);
        dom_string *name;
        dom_string *attr_val;
        dom_exception err;
        const char *avdata; /* attribute value data */
        size_t avdata_len; /* length of that attribute value data */
        size_t suffixlen; /* length of "suffix" */

        /* convenience pointer we'll use when matching the suffix */
        const char *suffix_start;

        suffixlen = lwc_string_length(suffix);
        if (suffixlen == 0) {
                /* In this case, the spec says that "if 'val' is the
                 * empty string, it will never represent anything." */
                return CSS_OK;
        }

        /* Intern the attribute name as a dom_string so we can
         * use dom_element_get_attribute() */
        err = dom_string_create_interned(
                (const uint8_t *) lwc_string_data(qname->name),
                lwc_string_length(qname->name),
                &name);
        if (err != DOM_NO_ERR) {
                return CSS_NOMEM;
        }

        err = dom_element_get_attribute((dom_node *)node, name, &attr_val);
        if ((err != DOM_NO_ERR) || (attr_val == NULL)) {
                /* There was an error getting the attribute's value or
                 * the attribute doesn't exist. So, no match? */
                dom_string_unref(name);
                *match = false;
                return CSS_OK;
        }

        /* Otherwise, we have the attribute value from the given node,
         * and the first thing we want to do is check to see if the
         * whole thing matches the suffix. */
        dom_string_unref(name);
        *match = dom_string_lwc_isequal(attr_val, suffix);

        /* If not, check to see if an, uh, suffix matches the
         * suffix */
        if (*match == false) {
                avdata = dom_string_data(attr_val);
                avdata_len = dom_string_byte_length(attr_val);

                suffix_start = (char *)(avdata + avdata_len - suffixlen);

                if ((avdata_len >= suffixlen) &&
                                (strncasecmp(suffix_start,
                                        lwc_string_data(suffix),
                                        suffixlen) == 0)) {
                        /* Use strncasecmp to compare only the last
                         * "n" characters, where "n" is the length of
                         * the suffix. */
                        *match = true;
                }
        }

        dom_string_unref(attr_val);

        return CSS_OK;
}


/**
 * Implement node_has_attribute_substring() with optional case-
 * insensitivity. This corresponds to [attr*=value i] in CSS and is
 * not supported by libcss yet, but it allows us to factor out some
 * common code.
 */
static css_error _node_has_attribute_substring(void *pw, void *node,
                const css_qname *qname, lwc_string *substring,
                bool *match, bool insensitive)
{
        UNUSED(pw);
        dom_string *name;
        dom_string *attr_val;
        dom_exception err;
        size_t attr_len;  /* length of attr_val */
        size_t substrlen; /* length of "substring" */

        /* Convenience pointers we use when comparing substrings */
        const char *p;
        const char *p_max;

        substrlen = lwc_string_length(substring);
        if (substrlen == 0) {
                /* In this case, the spec says that "if 'val' is the
                 * empty string, it will never represent anything." */
                return CSS_OK;
        }

        /* Intern the attribute name as a dom_string so we can
         * use dom_element_get_attribute() */
        err = dom_string_create_interned(
                (const uint8_t *) lwc_string_data(qname->name),
                lwc_string_length(qname->name),
                &name);
        if (err != DOM_NO_ERR) {
                return CSS_NOMEM;
        }

        err = dom_element_get_attribute((dom_node *)node, name, &attr_val);
        if ((err != DOM_NO_ERR) || (attr_val == NULL)) {
                /* There was an error getting the attribute's value or
                 * the attribute doesn't exist. So, no match? */
                dom_string_unref(name);
                *match = false;
                return CSS_OK;
        }

        /* Otherwise, we have the attribute value from the given node,
         * and the first thing we want to do is check to see if the
         * whole thing matches the substring. */
        dom_string_unref(name);

        if (insensitive) {
                *match = dom_string_caseless_lwc_isequal(attr_val, substring);
        }
        else {
                *match = dom_string_lwc_isequal(attr_val, substring);
        }

        /* If not, check to see if an, uh, substring matches the
         * substring */
        if (*match == false) {
                p = dom_string_data(attr_val);

                /* Check every long-enough suffix for a prefix match */
                attr_len = dom_string_byte_length(attr_val);
                if (attr_len >= substrlen) {
                        p_max = p + attr_len - substrlen;
                        while (p <= p_max) {
                                if (strncasecmp(p,
                                                lwc_string_data(substring),
                                                substrlen) == 0) {
                                        *match = true;
                                        break;
                                }
                                p++;
                        }
                }
        }

        dom_string_unref(attr_val);

        return CSS_OK;
}

/**
 * Test the given node for an attribute whose value contains the
 * given substring
 *
 * This will return true (via the "match" pointer) if the libdom node
 * has an attribute with the given name and whose value contains the
 * given substring. The comparison is case-sensitive. This corresponds
 * to [attr*=value] in CSS.
 *
 * \param pw          Pointer to the current SVG parser state
 * \param node        Libdom SVG node to test
 * \param qname       Attribute name to check for
 * \param substring   Value substring to check for
 * \param match       Pointer to the test result
 *
 * \return Returns CSS_OK if successful and CSS_NOMEM if we cannot
 *         intern the attribute name (which usually indicates memory
 *         exhaustion)
 */
css_error node_has_attribute_substring(void *pw, void *node,
                const css_qname *qname, lwc_string *substring,
                bool *match)
{
        return _node_has_attribute_substring(pw, node, qname, substring,
                        match, false);
}


/**
 * Test whether or not the given node is the document's root element
 * This corresponds to the CSS :root pseudo-selector.
 *
 * \param pw          Pointer to the current SVG parser state
 * \param node        Libdom SVG node to test
 * \param match       Pointer to the test result
 *
 * \return CSS_OK on success, or CSS_NOMEM if anything goes wrong
 */
css_error node_is_root(void *pw, void *node, bool *match)
{
        UNUSED(pw);
        dom_node *parent;
        dom_node_type type;
        dom_exception err;

        err = dom_node_get_parent_node((dom_node *)node, &parent);
        if (err != DOM_NO_ERR) {
                return CSS_NOMEM;
        }

        /* It's the root element if it doesn't have a parent element */
        if (parent != NULL) {
                err = dom_node_get_node_type(parent, &type);
                dom_node_unref(parent);
                if (err != DOM_NO_ERR) {
                        return CSS_NOMEM;
                }
                if (type != DOM_DOCUMENT_NODE) {
                        /* DOM_DOCUMENT_NODE is the only allowable
                         * type of parent node for the root element */
                        *match = false;
                        return CSS_OK;
                }
        }

        *match = true;
        return CSS_OK;
}


/**
 * Used internally in node_count_siblings() to "count" the given
 * sibling node. It factors out the node type and name checks.
 */
static int node_count_siblings_check(dom_node *dnode,
                bool check_name,
                dom_string *name)
{
        int ret;
        dom_node_type type;
        dom_exception exc;
        dom_string *dnode_name;

        /* We flip this to 1 if/when we count this node */
        ret = 0;

        if (dnode == NULL) {
                return ret;
        }

        exc = dom_node_get_node_type(dnode, &type);
        if ((exc != DOM_NO_ERR) || (type != DOM_ELEMENT_NODE)) {
                /* We only count element siblings */
                return ret;
        }

        /* ... with the right name */
        if (check_name) {
                dnode_name = NULL;
                exc = dom_node_get_node_name(dnode, &dnode_name);

                if ((exc == DOM_NO_ERR) && (dnode_name != NULL)) {
                        if (dom_string_isequal(name,
                                        dnode_name)) {
                                        ret = 1;
                        }
                        dom_string_unref(dnode_name);
                }
        }
        else {
                ret = 1;
        }

        return ret;
}

/**
 * Count the given node's sibling elements
 *
 * This counts the given node's sibling elements in one direction,
 * either forwards or backwards, in the DOM. Keep in mind that the
 * libdom tree is upside-down compared to the CSS one; so "next" and
 * "previous" are actually reversed; the default is to count preceding
 * libdom siblings which correspond to subsequent CSS siblings.
 *
 * This operation is central to the CSS :first-child, :nth-child, and
 * :last-child (et cetera) pseudo-selectors.
 *
 * If same_name is true, then only nodes having the same
 * (case-sensitive) name as the given node are counted.
 *
 * \param pw          Pointer to the current SVG parser state
 * \param node        Libdom SVG node whose siblings we're counting
 * \param same_name   Whether or not to count only siblings having
 *                    the same name as the given node
 * \param after       Count subsequent siblings rather than precedent
 *                    ones (the default)
 * \param count       Pointer to the return value, the number of sibling
 *                    elements
 *
 * \return CSS_OK on success, or CSS_NOMEM if anything goes wrong
 */
css_error node_count_siblings(void *pw, void *node,
                bool same_name, bool after, int32_t *count)
{
        UNUSED(pw);
        dom_exception exc;
        dom_node *dnode; /* node, but with the right type */
        dom_string *dnode_name;
        dom_node *next; /* "next" sibling (depends on direction) */

        /* Pointer to the "next sibling" function */
        dom_exception (*next_func)(dom_node *, dom_node **);

        *count = 0;

        dnode_name = NULL;
        dnode = (dom_node *)node;
        if (same_name) {
                exc = dom_node_get_node_name(dnode, &dnode_name);
                if ((exc != DOM_NO_ERR) || (dnode_name == NULL)) {
                        return CSS_NOMEM;
                }
        }

        /* Increment the reference counter for dnode for as long as
         * we retain a reference to it. */
        dnode = dom_node_ref(dnode);

        next_func = dom_node_get_previous_sibling;
        if (after) {
                next_func = dom_node_get_next_sibling;
        }

        do {
                exc = next_func(dnode, &next);
                if (exc != DOM_NO_ERR) {
                        break;
                }

                /* If next_func worked, we're about to swap "next"
                 * with "dnode" meaning that we will no longer retain
                 * a reference to the current dnode. */
                dom_node_unref(dnode);
                dnode = next;

                *count += node_count_siblings_check(dnode,
                                        same_name,
                                        dnode_name);
        } while (dnode != NULL);

        if (dnode_name != NULL) {
                dom_string_unref(dnode_name);
        }

        return CSS_OK;
}


/**
 * Determine whether or not the given element is empty
 *
 * An element is "nonempty" if it has a child that is either an
 * element node or a text node.
 *
 * \param pw          Pointer to the current SVG parser state
 * \param node        Libdom SVG node to check for emptiness
 * \param is_empty    Pointer to the return value
 *
 * \return CSS_OK on success, or CSS_NOMEM if anything goes wrong
 */
css_error node_is_empty(void *pw, void *node, bool *is_empty)
{
        UNUSED(pw);
        dom_node *child; /* current child node pointer */
        dom_node *next;  /* next child node pointer */
        dom_node_type type; /* what type of node is "child" */
        dom_exception err;

        /* Assume that it's empty by default */
        *is_empty = true;

        /* Get the given node's first child. Implementation detail:
         * this increments the reference counter on the child node. */
        err = dom_node_get_first_child((dom_node *)node, &child);
        if (err != DOM_NO_ERR) {
                return CSS_NOMEM;
        }

        /* And now loop through all children looking for a
         * text/element node. If we find one, the original
         * node is "nonempty" */
        while (child != NULL) {
                err = dom_node_get_node_type(child, &type);
                if (err != DOM_NO_ERR) {
                        dom_node_unref(child);
                        return CSS_NOMEM;
                }

                if (type == DOM_ELEMENT_NODE || type == DOM_TEXT_NODE) {
                        *is_empty = false;
                        dom_node_unref(child);
                        return CSS_OK;
                }

                err = dom_node_get_next_sibling(child, &next);
                if (err != DOM_NO_ERR) {
                        dom_node_unref(child);
                        return CSS_NOMEM;
                }

                /* If we're moving to the next node, we can release
                 * the reference to the current one */
                dom_node_unref(child);
                child = next;
        }

        return CSS_OK;
}


/**
 * Determine whether or not the given node is a link
 *
 * A node is a link if it is an element node whose name is "a" and if
 * it has an "href" attribute (case-sensitive). This selector
 * corresponds to node:link pseudo-class in CSS.
 *
 * This pseudo-class is a bit awkward because the two standards (HTML5
 * and CSS) disagree on what it means, and because libsvgtiny does not
 * have enough information to determine if a link has been "visited"
 * yet -- that's a UI property. CSS says that :link is for unvisited
 * links, which we can't determine. HTML5 says that each link must
 * be either a :link or :visited. Since we can't decide either way,
 * It seems less wrong to declare that all links are unvisited; i.e.
 * that they match :link.
 *
 * \param pw          Pointer to the current SVG parser state
 * \param node        Libdom SVG node to check
 * \param is_link     Pointer to the boolean return value
 *
 * \return CSS_OK on success, or CSS_NOMEM if anything goes wrong
 */
css_error node_is_link(void *pw, void *node, bool *is_link)
{
        dom_exception exc;
        dom_node *dnode; /* node, but with the right type */
        dom_string *dnode_name;
        bool has_href;
        struct svgtiny_parse_state* state;

        dnode = (dom_node *)node;
        dnode_name = NULL;
        has_href = false; /* assume no href attribute */
        *is_link = false; /* assume that it's not a link */

        exc = dom_node_get_node_name(dnode, &dnode_name);
        if ((exc != DOM_NO_ERR) || (dnode_name == NULL)) {
                return CSS_NOMEM;
        }

        state = (struct svgtiny_parse_state *)pw;
        if (dom_string_isequal(dnode_name, state->interned_a)) {
                exc = dom_element_has_attribute(node,
                                        state->interned_href,
                                        &has_href);
                if (exc == DOM_NO_ERR && has_href) {
                        *is_link = true;
                }
        }

        dom_string_unref(dnode_name);
        return CSS_OK;
}


/**
 * Check if the given node is being "hovered" over
 *
 * This check always fails because the SVG DOM does not have the
 * necessary information (it's a UI property).
 *
 * \param pw          Pointer to the current SVG parser state; unused
 * \param node        Libdom SVG node to check; unused
 * \param is_hover    Pointer to the boolean return value
 *
 * \return Always returns CSS_OK
 */
css_error node_is_hover(void *pw, void *node, bool *is_hover)
{
        UNUSED(pw);
        UNUSED(node);
        *is_hover = false;
        return CSS_OK;
}


/**
 * Check if the given node is "active"
 *
 * This check always fails because the SVG DOM does not have the
 * necessary information (it's a UI property).
 *
 * \param pw          Pointer to the current SVG parser state; unused
 * \param node        Libdom SVG node to check; unused
 * \param is_active   Pointer to the boolean return value
 *
 * \return Always returns CSS_OK
 */
css_error node_is_active(void *pw, void *node, bool *is_active)
{
        UNUSED(pw);
        UNUSED(node);
        *is_active = false;
        return CSS_OK;
}


/**
 * Check if the given node has the focus
 *
 * This check always fails because the SVG DOM does not have the
 * necessary information (it's a UI property).
 *
 * \param pw          Pointer to the current SVG parser state; unused
 * \param node        Libdom SVG node to check; unused
 * \param is_focus    Pointer to the boolean return value
 *
 * \return Always returns CSS_OK
 */
css_error node_is_focus(void *pw, void *node, bool *is_focus)
{
        UNUSED(pw);
        UNUSED(node);
        *is_focus = false;
        return CSS_OK;
}


/**
 * Check if the given node is enabled
 *
 * This check always fails because the SVG DOM does not have the
 * necessary information (it's a UI property).
 *
 * \param pw            Pointer to the current SVG parser state; unused
 * \param node          Libdom SVG node to check; unused
 * \param is_enabled    Pointer to the boolean return value
 *
 * \return Always returns CSS_OK
 */
css_error node_is_enabled(void *pw, void *node, bool *is_enabled)
{
        UNUSED(pw);
        UNUSED(node);
        *is_enabled = false;
        return CSS_OK;
}


/**
 * Check if the given node is disabled
 *
 * This check always fails because the SVG DOM does not have the
 * necessary information (it's a UI property). Beware, until they are
 * implemented, this is NOT the logical negation of node_is_enabled!
 *
 * \param pw            Pointer to the current SVG parser state; unused
 * \param node          Libdom SVG node to check; unused
 * \param is_disabled   Pointer to the boolean return value
 *
 * \return Always returns CSS_OK
 */
css_error node_is_disabled(void *pw, void *node, bool *is_disabled)
{
        UNUSED(pw);
        UNUSED(node);
        *is_disabled = false;
        return CSS_OK;
}


/**
 * Test whether or not the given node is "checked"
 *
 * This test always fails because the SVG DOM does not have the
 * necessary information (it's a UI property).
 *
 * \param pw            Pointer to the current SVG parser state; unused
 * \param node          Libdom SVG node to check; unused
 * \param is_checked    Pointer to the boolean return value
 *
 * \return Always returns CSS_OK
 */
css_error node_is_checked(void *pw, void *node, bool *is_checked)
{
        UNUSED(pw);
        UNUSED(node);
        *is_checked = false;
        return CSS_OK;
}


/**
 * Check if the given node is the "target" of the document URL
 *
 * This test always fails because the SVG DOM does not have the
 * necessary information (it's a UI property).
 *
 * \param pw            Pointer to the current SVG parser state; unused
 * \param node          Libdom SVG node to check; unused
 * \param is_target     Pointer to the boolean return value
 *
 * \return Always returns CSS_OK
 */
css_error node_is_target(void *pw, void *node, bool *is_target)
{
        UNUSED(pw);
        UNUSED(node);
        *is_target = false;
        return CSS_OK;
}


/**
 * Check if the given node is the given language
 *
 * This test is corresponds to the CSS :lang() selector and is not
 * fully implemented yet: it looks only for "lang" attributes on the
 * given element and its parents, and performs a simple substring
 * check. This results in a partial implementation of CSS Level 3 for
 * SVG 2.0. In particular, it ignores all "xml:lang" attributes in
 * favor of the "lang" attribute that is defined only in SVG 2.0.
 *
 * \param pw            Pointer to the current SVG parser state; unused
 * \param node          Libdom SVG node to check
 * \param lang          The language to match
 * \param is_lang       Pointer to the boolean return value
 *
 * \return CSS_OK on success, or CSS_NOMEM if anything goes wrong
 */
static css_error node_is_lang(void *pw, void *node,
                lwc_string *lang, bool *is_lang)
{
        UNUSED(pw);
        /* SVG2 elements support both "lang" and "xml:lang"
         * attributes; earlier versions have only the XML
         * attribute. It would not be too hard to add support for
         * xml:lang" here. The main difficulty standing in the way of
         * a full Level 4 implementation is the complexity of the
         * :lang() selector:
         *
         *   https://www.w3.org/TR/selectors-4/#the-lang-pseudo
         *
         */

        css_error c_err;
        dom_exception d_err;
        dom_node *n; /* current node */
        dom_node *p; /* parent node */
        bool match;  /* retval from node_has_attribute_substring() */

        /* Define the attribute name "lang" that we're looking for.
         * We only use a css_qname here because that's what the
         * node_has_attribute_substring() takes; the namespace
         * portion of it is irrelevant. */
        css_qname attr;
        attr.ns = NULL;

        if (lwc_intern_string("lang", 4, &attr.name) != lwc_error_ok) {
                return CSS_NOMEM;
        }

        *is_lang = false; /* default to no match */
        n = (dom_node *)node;

        /* Loop through all parents of the given node looking for a
         * substring match */
        while (n != NULL) {
                c_err = _node_has_attribute_substring(pw, (void *)n, &attr,
                                                lang, &match, true);
                if (c_err != CSS_OK) {
                        lwc_string_destroy(attr.name);
                        return c_err;
                }
                if (match) {
                        /* matched this element; we're done */
                        lwc_string_destroy(attr.name);
                        *is_lang = true;
                        return CSS_OK;
                }

                /* no match on this element, try its parent */
                d_err = dom_node_get_parent_node(n, &p);
                if (d_err != DOM_NO_ERR) {
                        lwc_string_destroy(attr.name);
                        return CSS_NOMEM;
                }
                n = p;
        }

        /* If we never find a match we may wind up here */
        lwc_string_destroy(attr.name);
        return CSS_OK;
}