CssVariableEvaluator.php

<?php

declare(strict_types=1);

namespace Pelago\Emogrifier\HtmlProcessor;

use Pelago\Emogrifier\Utilities\DeclarationBlockParser;

use function Safe\preg_match;
use function Safe\preg_replace_callback;

/**
 * This class can evaluate CSS custom properties that are defined and used in inline style attributes.
 */
final class CssVariableEvaluator extends AbstractHtmlProcessor
{
    /**
     * temporary collection used by {@see replaceVariablesInDeclarations} and callee methods
     *
     * @var array<non-empty-string, string>
     */
    private $currentVariableDefinitions = [];

    /**
     * Replaces all CSS custom property references in inline style attributes with their corresponding values where
     * defined in inline style attributes (either from the element itself or the nearest ancestor).
     *
     * @return $this
     *
     * @throws \UnexpectedValueException
     */
    public function evaluateVariables(): self
    {
        /**
         * @var list<array{element: \DOMElement, ancestorDefinitions: array<non-empty-string, string>}>
         *      $elementsToEvaluate
         */
        $elementsToEvaluate = [['element' => $this->getHtmlElement(), 'ancestorDefinitions' => []]];

        while (($currentElementData = \array_pop($elementsToEvaluate)) !== null) {
            $currentElement = $currentElementData['element'];
            $currentAncestorDefinitions = $currentElementData['ancestorDefinitions'];

            $style = $currentElement->getAttribute('style');

            // Avoid parsing declarations if none use or define a variable
            if (preg_match('/(?<![\\w\\-])--[\\w\\-]/', $style) !== 0) {
                $declarations = DeclarationBlockParser::parse($style);
                $variableDefinitions =
                    $this->getVariableDefinitionsFromDeclarations($declarations) + $currentAncestorDefinitions;
                $this->currentVariableDefinitions = $variableDefinitions;

                $newDeclarations = $this->replaceVariablesInDeclarations($declarations);
                if ($newDeclarations !== null) {
                    $currentElement->setAttribute('style', $this->getDeclarationsAsString($newDeclarations));
                }
            } else {
                $variableDefinitions = $currentAncestorDefinitions;
            }

            foreach ($currentElement->childNodes as $child) {
                if ($child instanceof \DOMElement) {
                    $elementsToEvaluate[] = ['element' => $child, 'ancestorDefinitions' => $variableDefinitions];
                }
            }
        }

        return $this;
    }

    /**
     * @param array<non-empty-string, string> $declarations
     *
     * @return array<non-empty-string, string>
     */
    private function getVariableDefinitionsFromDeclarations(array $declarations): array
    {
        return \array_filter(
            $declarations,
            static function (string $key): bool {
                return \substr($key, 0, 2) === '--';
            },
            ARRAY_FILTER_USE_KEY
        );
    }

    /**
     * Callback function for {@see replaceVariablesInPropertyValue} performing regular expression replacement.
     *
     * @param array<int, string> $matches
     */
    private function getPropertyValueReplacement(array $matches): string
    {
        $variableName = $matches[1];

        if (isset($this->currentVariableDefinitions[$variableName])) {
            $variableValue = $this->currentVariableDefinitions[$variableName];
        } else {
            $fallbackValueSeparator = $matches[2] ?? '';
            if ($fallbackValueSeparator !== '') {
                $fallbackValue = $matches[3];
                // The fallback value may use other CSS variables, so recurse
                $variableValue = $this->replaceVariablesInPropertyValue($fallbackValue);
            } else {
                $variableValue = $matches[0];
            }
        }

        return $variableValue;
    }

    /**
     * Regular expression based on {@see https://stackoverflow.com/a/54143883/2511031 a StackOverflow answer}.
     */
    private function replaceVariablesInPropertyValue(string $propertyValue): string
    {
        $pattern = '/
                var\\(
                    \\s*+
                    # capture variable name including `--` prefix
                    (
                        --[^\\s\\),]++
                    )
                    \\s*+
                    # capture optional fallback value
                    (?:
                        # capture separator to confirm there is a fallback value
                        (,)\\s*
                        # begin capture with named group that can be used recursively
                        (?<recursable>
                            # begin named group to match sequence without parentheses, except in strings
                            (?<noparentheses>
                                # repeated zero or more times:
                                (?:
                                    # sequence without parentheses or quotes
                                    [^\\(\\)\'"]++
                                    |
                                    # string in double quotes
                                    "(?>[^"\\\\]++|\\\\.)*"
                                    |
                                    # string in single quotes
                                    \'(?>[^\'\\\\]++|\\\\.)*\'
                                )*+
                            )
                            # repeated zero or more times:
                            (?:
                                # sequence in parentheses
                                \\(
                                    # using the named recursable pattern
                                    (?&recursable)
                                \\)
                                # sequence without parentheses, except in strings
                                (?&noparentheses)
                            )*+
                        )
                    )?+
                \\)
            /x';

        $callable = \Closure::fromCallable([$this, 'getPropertyValueReplacement']);
        if (\function_exists('Safe\\preg_replace_callback')) {
            $result = preg_replace_callback($pattern, $callable, $propertyValue);
        } else {
            // @phpstan-ignore-next-line The safe version is only available in "thecodingmachine/safe" for PHP >= 8.1.
            $result = \preg_replace_callback($pattern, $callable, $propertyValue);
        }
        \assert(\is_string($result));

        return $result;
    }

    /**
     * @param array<non-empty-string, string> $declarations
     *
     * @return array<non-empty-string, string>|null `null` is returned if no substitutions were made.
     */
    private function replaceVariablesInDeclarations(array $declarations): ?array
    {
        $substitutionsMade = false;
        $result = \array_map(
            function (string $propertyValue) use (&$substitutionsMade): string {
                $newPropertyValue = $this->replaceVariablesInPropertyValue($propertyValue);
                if ($newPropertyValue !== $propertyValue) {
                    $substitutionsMade = true;
                }
                return $newPropertyValue;
            },
            $declarations
        );

        return $substitutionsMade ? $result : null;
    }

    /**
     * @param array<non-empty-string, string> $declarations
     */
    private function getDeclarationsAsString(array $declarations): string
    {
        $declarationStrings = \array_map(
            static function (string $key, string $value): string {
                return $key . ': ' . $value;
            },
            \array_keys($declarations),
            \array_values($declarations)
        );

        return \implode('; ', $declarationStrings) . ';';
    }
}