class FundamentalCompatibilityConstraintValidator

class FundamentalCompatibilityConstraintValidator extends ConstraintValidator implements ContainerInjectionInterface {
    use PluginManagerDependentValidatorTrait;
    use TextEditorObjectDependentValidatorTrait;
    
    /**
     * The fundamental CKEditor 5 plugins without which it cannot function.
     *
     * @var string[]
     */
    const FUNDAMENTAL_CKEDITOR5_PLUGINS = [
        'ckeditor5_essentials',
        'ckeditor5_paragraph',
    ];
    
    /**
     * {@inheritdoc}
     *
     * @throws \Symfony\Component\Validator\Exception\UnexpectedTypeException
     *   Thrown when the given constraint is not supported by this validator.
     */
    public function validate($toolbar_item, Constraint $constraint) {
        if (!$constraint instanceof FundamentalCompatibilityConstraint) {
            throw new UnexpectedTypeException($constraint, __NAMESPACE__ . '\\FundamentalCompatibility');
        }
        $text_editor = $this->createTextEditorObjectFromContext();
        // First: the two fundamental checks against the text format. If any of
        // them adds a constraint violation, return early, because it is a
        // fundamental compatibility problem.
        $this->checkNoMarkupFilters($text_editor->getFilterFormat(), $constraint);
        if ($this->context
            ->getViolations()
            ->count() > 0) {
            return;
        }
        $this->checkHtmlRestrictionsAreCompatible($text_editor->getFilterFormat(), $constraint);
        if ($this->context
            ->getViolations()
            ->count() > 0) {
            return;
        }
        // Second: ensure that all tags can actually be created.
        $this->checkAllHtmlTagsAreCreatable($text_editor, $constraint);
        // Finally: ensure the CKEditor 5 configuration's ability to generate HTML
        // markup precisely matches that of the text format.
        $this->checkHtmlRestrictionsMatch($text_editor, $constraint);
    }
    
    /**
     * Checks no TYPE_MARKUP_LANGUAGE filters are present.
     *
     * Two TYPE_MARKUP_LANGUAGE filters are exempted:
     * - filter_autop: pointless but harmless to have enabled
     * - filter_url: not recommended but also harmless to have enabled
     *
     * These two commonly enabled filters with a long history in Drupal are
     * considered to be acceptable to have enabled.
     *
     * @param \Drupal\filter\FilterFormatInterface $text_format
     *   The text format to validate.
     * @param \Drupal\ckeditor5\Plugin\Validation\Constraint\FundamentalCompatibilityConstraint $constraint
     *   The constraint to validate.
     */
    private function checkNoMarkupFilters(FilterFormatInterface $text_format, FundamentalCompatibilityConstraint $constraint) : void {
        $markup_filters = static::getFiltersInFormatOfType($text_format, FilterInterface::TYPE_MARKUP_LANGUAGE);
        foreach ($markup_filters as $markup_filter) {
            if ($markup_filter instanceof FilterAutoP || $markup_filter instanceof FilterUrl) {
                continue;
            }
            $this->context
                ->buildViolation($constraint->noMarkupFiltersMessage)
                ->setParameter('%filter_label', (string) $markup_filter->getLabel())
                ->setParameter('%filter_plugin_id', $markup_filter->getPluginId())
                ->addViolation();
        }
    }
    
    /**
     * Checks that fundamental CKEditor 5 plugins' HTML tags are allowed.
     *
     * @param \Drupal\filter\FilterFormatInterface $text_format
     *   The text format to validate.
     * @param \Drupal\ckeditor5\Plugin\Validation\Constraint\FundamentalCompatibilityConstraint $constraint
     *   The constraint to validate.
     */
    private function checkHtmlRestrictionsAreCompatible(FilterFormatInterface $text_format, FundamentalCompatibilityConstraint $constraint) : void {
        $html_restrictions = HTMLRestrictions::fromTextFormat($text_format);
        if ($html_restrictions->isUnrestricted()) {
            return;
        }
        $fundamental = new HTMLRestrictions($this->pluginManager
            ->getProvidedElements(self::FUNDAMENTAL_CKEDITOR5_PLUGINS));
        if (!$fundamental->diff($html_restrictions)
            ->allowsNothing()) {
            $offending_filter = static::findHtmlRestrictorFilterNotAllowingTags($text_format, $fundamental);
            $this->context
                ->buildViolation($constraint->nonAllowedElementsMessage)
                ->setParameter('%filter_label', (string) $offending_filter->getLabel())
                ->setParameter('%filter_plugin_id', $offending_filter->getPluginId())
                ->addViolation();
        }
    }
    
    /**
     * Checks the HTML restrictions match the enabled CKEditor 5 plugins' output.
     *
     * @param \Drupal\editor\EditorInterface $text_editor
     *   The text editor to validate.
     * @param \Drupal\ckeditor5\Plugin\Validation\Constraint\FundamentalCompatibilityConstraint $constraint
     *   The constraint to validate.
     */
    private function checkHtmlRestrictionsMatch(EditorInterface $text_editor, FundamentalCompatibilityConstraint $constraint) : void {
        $html_restrictor_filters = static::getFiltersInFormatOfType($text_editor->getFilterFormat(), FilterInterface::TYPE_HTML_RESTRICTOR);
        $enabled_plugins = array_keys($this->pluginManager
            ->getEnabledDefinitions($text_editor));
        $provided_elements = $this->pluginManager
            ->getProvidedElements($enabled_plugins, $text_editor);
        $provided = new HTMLRestrictions($provided_elements);
        foreach ($html_restrictor_filters as $filter_plugin_id => $filter) {
            $allowed = HTMLRestrictions::fromFilterPluginInstance($filter);
            $diff_allowed = $allowed->diff($provided);
            $diff_elements = $provided->diff($allowed);
            if (!$diff_allowed->allowsNothing()) {
                $this->context
                    ->buildViolation($constraint->notSupportedElementsMessage)
                    ->setParameter('@list', implode(' ', $provided->toCKEditor5ElementsArray()))
                    ->setParameter('@diff', implode(' ', $diff_allowed->toCKEditor5ElementsArray()))
                    ->atPath("filters.{$filter_plugin_id}")
                    ->addViolation();
            }
            if (!$diff_elements->allowsNothing()) {
                $this->context
                    ->buildViolation($constraint->missingElementsMessage)
                    ->setParameter('@list', implode(' ', $provided->toCKEditor5ElementsArray()))
                    ->setParameter('@diff', implode(' ', $diff_elements->toCKEditor5ElementsArray()))
                    ->atPath("filters.{$filter_plugin_id}")
                    ->addViolation();
            }
        }
    }
    
    /**
     * Checks all HTML tags supported by enabled CKEditor 5 plugins are creatable.
     *
     * @param \Drupal\editor\EditorInterface $text_editor
     *   The text editor to validate.
     * @param \Drupal\ckeditor5\Plugin\Validation\Constraint\FundamentalCompatibilityConstraint $constraint
     *   The constraint to validate.
     */
    private function checkAllHtmlTagsAreCreatable(EditorInterface $text_editor, FundamentalCompatibilityConstraint $constraint) : void {
        $enabled_definitions = $this->pluginManager
            ->getEnabledDefinitions($text_editor);
        $enabled_plugins = array_keys($enabled_definitions);
        // When arbitrary HTML is supported, all tags are creatable.
        if (in_array('ckeditor5_arbitraryHtmlSupport', $enabled_plugins, TRUE)) {
            return;
        }
        $tags_and_attributes = new HTMLRestrictions($this->pluginManager
            ->getProvidedElements($enabled_plugins, $text_editor));
        $creatable_tags = new HTMLRestrictions($this->pluginManager
            ->getProvidedElements($enabled_plugins, $text_editor, FALSE, TRUE));
        $needed_tags = $tags_and_attributes->extractPlainTagsSubset();
        $non_creatable_tags = $needed_tags->diff($creatable_tags);
        if (!$non_creatable_tags->allowsNothing()) {
            foreach ($non_creatable_tags->toCKEditor5ElementsArray() as $non_creatable_tag) {
                // Find the plugin which has a non-creatable tag.
                $needle = HTMLRestrictions::fromString($non_creatable_tag);
                $matching_plugins = array_filter($enabled_definitions, function (CKEditor5PluginDefinition $d) use ($needle, $text_editor) {
                    if (!$d->hasElements()) {
                        return FALSE;
                    }
                    $haystack = new HTMLRestrictions($this->pluginManager
                        ->getProvidedElements([
                        $d->id(),
                    ], $text_editor, FALSE, FALSE));
                    return !$haystack->extractPlainTagsSubset()
                        ->intersect($needle)
                        ->allowsNothing();
                });
                assert(count($matching_plugins) === 1);
                $plugin_definition = reset($matching_plugins);
                assert($plugin_definition instanceof CKEditor5PluginDefinition);
                // Compute which attributes it would be able to create on this tag.
                $provided_elements = new HTMLRestrictions($this->pluginManager
                    ->getProvidedElements([
                    $plugin_definition->id(),
                ], $text_editor, FALSE, FALSE));
                $attributes_on_tag = $provided_elements->intersect(new HTMLRestrictions(array_fill_keys(array_keys($needle->getAllowedElements()), TRUE)));
                $violation = $this->context
                    ->buildViolation($constraint->nonCreatableTagMessage)
                    ->setParameter('@non_creatable_tag', $non_creatable_tag)
                    ->setParameter('%plugin', $plugin_definition->label())
                    ->setParameter('@attributes_on_tag', implode(', ', $attributes_on_tag->toCKEditor5ElementsArray()));
                // If this plugin has a configurable subset, associate the violation
                // with the property path pointing to this plugin's settings form.
                if (is_a($plugin_definition->getClass(), CKEditor5PluginElementsSubsetInterface::class, TRUE)) {
                    $violation->atPath(sprintf('settings.plugins.%s', $plugin_definition->id()));
                }
                elseif ($plugin_definition->hasToolbarItems()) {
                    $toolbar_items = $plugin_definition->getToolbarItems();
                    $active_toolbar_items = array_intersect($text_editor->getSettings()['toolbar']['items'], array_keys($toolbar_items));
                    $violation->atPath(sprintf('settings.toolbar.items.%d', array_keys($active_toolbar_items)[0]));
                }
                $violation->addViolation();
            }
        }
    }
    
    /**
     * Gets the filters of the given type in this text format.
     *
     * @param \Drupal\filter\FilterFormatInterface $text_format
     *   A text format whose filters to get.
     * @param int $filter_type
     *   One of FilterInterface::TYPE_*.
     * @param callable|null $extra_requirements
     *   An optional callable that can check a filter of this type for additional
     *   conditions to be met. Must return TRUE when it meets the conditions,
     *   FALSE otherwise.
     *
     * @return iterable|\Drupal\filter\Plugin\FilterInterface[]
     *   An iterable of matched filter plugins.
     */
    private static function getFiltersInFormatOfType(FilterFormatInterface $text_format, int $filter_type, ?callable $extra_requirements = NULL) : iterable {
        assert(in_array($filter_type, [
            FilterInterface::TYPE_MARKUP_LANGUAGE,
            FilterInterface::TYPE_HTML_RESTRICTOR,
            FilterInterface::TYPE_TRANSFORM_REVERSIBLE,
            FilterInterface::TYPE_TRANSFORM_IRREVERSIBLE,
        ]));
        foreach ($text_format->filters() as $id => $filter) {
            if ($filter->status && $filter->getType() === $filter_type && ($extra_requirements === NULL || $extra_requirements($filter))) {
                (yield $id => $filter);
            }
        }
    }
    
    /**
     * Analyzes a text format to find the filter not allowing required tags.
     *
     * @param \Drupal\filter\FilterFormatInterface $text_format
     *   A text format whose filters to check for compatibility.
     * @param \Drupal\ckeditor5\HTMLRestrictions $required
     *   A set of HTML restrictions, listing required HTML tags.
     *
     * @return \Drupal\filter\Plugin\FilterInterface
     *   The filter plugin instance not allowing the required tags.
     *
     * @throws \InvalidArgumentException
     */
    private static function findHtmlRestrictorFilterNotAllowingTags(FilterFormatInterface $text_format, HTMLRestrictions $required) : FilterInterface {
        // Get HTML restrictor filters that actually restrict HTML.
        $filters = static::getFiltersInFormatOfType($text_format, FilterInterface::TYPE_HTML_RESTRICTOR, function (FilterInterface $filter) {
            return $filter->getHTMLRestrictions() !== FALSE;
        });
        foreach ($filters as $filter) {
            // Return any filter not allowing >=1 of the required tags.
            if (!$required->diff(HTMLRestrictions::fromFilterPluginInstance($filter))
                ->allowsNothing()) {
                return $filter;
            }
        }
        throw new \InvalidArgumentException('This text format does not have a "tags allowed" restriction that excludes the required tags.');
    }

}