class MessageConverter

final class MessageConverter {
    
    /**
     * @throws RuntimeException when unable to convert the message to an email
     */
    public static function toEmail(Message $message) : Email {
        if ($message instanceof Email) {
            return $message;
        }
        // try to convert to a "simple" Email instance
        $body = $message->getBody();
        if ($body instanceof TextPart) {
            return self::createEmailFromTextPart($message, $body);
        }
        if ($body instanceof AlternativePart) {
            return self::createEmailFromAlternativePart($message, $body);
        }
        if ($body instanceof RelatedPart) {
            return self::createEmailFromRelatedPart($message, $body);
        }
        if ($body instanceof MixedPart) {
            $parts = $body->getParts();
            if ($parts[0] instanceof RelatedPart) {
                $email = self::createEmailFromRelatedPart($message, $parts[0]);
            }
            elseif ($parts[0] instanceof AlternativePart) {
                $email = self::createEmailFromAlternativePart($message, $parts[0]);
            }
            elseif ($parts[0] instanceof TextPart) {
                $email = self::createEmailFromTextPart($message, $parts[0]);
            }
            else {
                throw new RuntimeException(\sprintf('Unable to create an Email from an instance of "%s" as the body is too complex.', get_debug_type($message)));
            }
            return self::addParts($email, \array_slice($parts, 1));
        }
        throw new RuntimeException(\sprintf('Unable to create an Email from an instance of "%s" as the body is too complex.', get_debug_type($message)));
    }
    private static function createEmailFromTextPart(Message $message, TextPart $part) : Email {
        if ('text' === $part->getMediaType() && 'plain' === $part->getMediaSubtype()) {
            return (new Email(clone $message->getHeaders()))
                ->text($part->getBody(), $part->getPreparedHeaders()
                ->getHeaderParameter('Content-Type', 'charset') ?: 'utf-8');
        }
        if ('text' === $part->getMediaType() && 'html' === $part->getMediaSubtype()) {
            return (new Email(clone $message->getHeaders()))
                ->html($part->getBody(), $part->getPreparedHeaders()
                ->getHeaderParameter('Content-Type', 'charset') ?: 'utf-8');
        }
        throw new RuntimeException(\sprintf('Unable to create an Email from an instance of "%s" as the body is too complex.', get_debug_type($message)));
    }
    private static function createEmailFromAlternativePart(Message $message, AlternativePart $part) : Email {
        $parts = $part->getParts();
        if (2 === \count($parts) && $parts[0] instanceof TextPart && 'text' === $parts[0]->getMediaType() && 'plain' === $parts[0]->getMediaSubtype() && $parts[1] instanceof TextPart && 'text' === $parts[1]->getMediaType() && 'html' === $parts[1]->getMediaSubtype()) {
            return (new Email(clone $message->getHeaders()))
                ->text($parts[0]->getBody(), $parts[0]->getPreparedHeaders()
                ->getHeaderParameter('Content-Type', 'charset') ?: 'utf-8')
                ->html($parts[1]->getBody(), $parts[1]->getPreparedHeaders()
                ->getHeaderParameter('Content-Type', 'charset') ?: 'utf-8');
        }
        throw new RuntimeException(\sprintf('Unable to create an Email from an instance of "%s" as the body is too complex.', get_debug_type($message)));
    }
    private static function createEmailFromRelatedPart(Message $message, RelatedPart $part) : Email {
        $parts = $part->getParts();
        if ($parts[0] instanceof AlternativePart) {
            $email = self::createEmailFromAlternativePart($message, $parts[0]);
        }
        elseif ($parts[0] instanceof TextPart) {
            $email = self::createEmailFromTextPart($message, $parts[0]);
        }
        else {
            throw new RuntimeException(\sprintf('Unable to create an Email from an instance of "%s" as the body is too complex.', get_debug_type($message)));
        }
        return self::addParts($email, \array_slice($parts, 1));
    }
    private static function addParts(Email $email, array $parts) : Email {
        foreach ($parts as $part) {
            if (!$part instanceof DataPart) {
                throw new RuntimeException(\sprintf('Unable to create an Email from an instance of "%s" as the body is too complex.', get_debug_type($email)));
            }
            $email->addPart($part);
        }
        return $email;
    }

}