class FrontMatter

final class FrontMatter {
    
    /**
     * The separator used to indicate front matter data.
     *
     * @var string
     */
    const FRONT_MATTER_SEPARATOR = '---';
    
    /**
     * The regular expression used to extract the YAML front matter content.
     *
     * @var string
     */
    const FRONT_MATTER_REGEXP = "{^(?:" . self::FRONT_MATTER_SEPARATOR . ")[\r\n|\n]*(.*?)[\r\n|\n]+(?:" . self::FRONT_MATTER_SEPARATOR . ")[\r\n|\n]*(.*)\$}s";
    
    /**
     * The parsed source.
     *
     * @var array
     */
    protected $parsed;
    
    /**
     * A serializer class.
     *
     * @var string
     */
    protected $serializer;
    
    /**
     * The source.
     *
     * @var string
     */
    protected $source;
    
    /**
     * FrontMatter constructor.
     *
     * @param string $source
     *   A string source.
     * @param string $serializer
     *   A class that implements
     *   \Drupal\Component\Serialization\SerializationInterface.
     */
    public function __construct($source, $serializer = '\\Drupal\\Component\\Serialization\\Yaml') {
        assert(is_string($source), '$source must be a string');
        assert(is_string($serializer), '$serializer must be a string');
        if (!is_subclass_of($serializer, '\\Drupal\\Component\\Serialization\\SerializationInterface')) {
            throw new \InvalidArgumentException('The $serializer parameter must reference a class that implements \\Drupal\\Component\\Serialization\\SerializationInterface.');
        }
        $this->serializer = $serializer;
        $this->source = $source;
    }
    
    /**
     * Creates a new FrontMatter instance.
     *
     * @param string $source
     *   A string source.
     * @param string $serializer
     *   A class that implements
     *   \Drupal\Component\Serialization\SerializationInterface.
     *
     * @return static
     */
    public static function load($source, $serializer = '\\Drupal\\Component\\Serialization\\Yaml') {
        return new static($source, $serializer);
    }
    
    /**
     * Parses the source.
     *
     * @return array
     *   An associative array containing:
     *   - code: The real source code.
     *   - data: The front matter data extracted and decoded.
     *   - line: The line number where the real source code starts.
     *
     * @throws \Drupal\Component\Serialization\Exception\InvalidDataTypeException
     *   Exception thrown when the Front Matter cannot be parsed.
     */
    private function parse() {
        if (!$this->parsed) {
            $this->parsed = [
                'code' => $this->source,
                'data' => [],
                'line' => 1,
            ];
            // Check for front matter data.
            $len = strlen(static::FRONT_MATTER_SEPARATOR);
            $matches = [];
            if (substr($this->parsed['code'], 0, $len + 1) === static::FRONT_MATTER_SEPARATOR . "\n" || substr($this->parsed['code'], 0, $len + 2) === static::FRONT_MATTER_SEPARATOR . "\r\n") {
                preg_match(static::FRONT_MATTER_REGEXP, $this->parsed['code'], $matches);
                $matches = array_map('trim', $matches);
            }
            // Immediately return if the code doesn't contain front matter data.
            if (empty($matches)) {
                return $this->parsed;
            }
            // Set the extracted source code.
            $this->parsed['code'] = $matches[2];
            // Set the extracted front matter data. Do not catch any exceptions here
            // as doing so would only obfuscate any errors found in the front matter
            // data. Typecast to an array to ensure top level scalars are in an array.
            if ($matches[1]) {
                $this->parsed['data'] = (array) $this->serializer::decode($matches[1]);
            }
            // Determine the real source line by counting newlines from the data and
            // then adding 2 to account for the front matter separator (---) wrappers
            // and then adding 1 more for the actual line number after the data.
            $this->parsed['line'] = count(preg_split('/\\r\\n|\\n/', $matches[1])) + 3;
        }
        return $this->parsed;
    }
    
    /**
     * Retrieves the extracted source code.
     *
     * @return string
     *   The extracted source code.
     *
     * @throws \Drupal\Component\Serialization\Exception\InvalidDataTypeException
     *   Exception thrown when the Front Matter cannot be parsed.
     */
    public function getCode() {
        return $this->parse()['code'];
    }
    
    /**
     * Retrieves the extracted front matter data.
     *
     * @return array
     *   The extracted front matter data.
     *
     * @throws \Drupal\Component\Serialization\Exception\InvalidDataTypeException
     *   Exception thrown when the Front Matter cannot be parsed.
     */
    public function getData() {
        return $this->parse()['data'];
    }
    
    /**
     * Retrieves the line where the source code starts, after any data.
     *
     * @return int
     *   The source code line.
     *
     * @throws \Drupal\Component\Serialization\Exception\InvalidDataTypeException
     *   Exception thrown when the Front Matter cannot be parsed.
     */
    public function getLine() {
        return $this->parse()['line'];
    }

}