<?php

namespace Drupal\smtp\Plugin\Mail;

use Drupal\Component\Utility\EmailValidatorInterface;
use Drupal\Core\Config\ConfigFactoryInterface;
use Drupal\Core\File\FileSystemInterface;
use Drupal\Core\Logger\LoggerChannelFactoryInterface;
use Drupal\Core\Mail\MailFormatHelper;
use Drupal\Core\Mail\MailInterface;
use Drupal\Core\Messenger\MessengerInterface;
use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
use Drupal\Core\Render\RendererInterface;
use Drupal\Core\Session\AccountProxyInterface;
use Drupal\Core\StringTranslation\StringTranslationTrait;
use PHPMailer\PHPMailer\PHPMailer;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Symfony\Component\HttpFoundation\Session\SessionInterface;
use Symfony\Component\Mime\Header\UnstructuredHeader;
use Symfony\Component\Mime\MimeTypeGuesserInterface;

/**
 * Modify the drupal mail system to use smtp when sending emails.
 *
 * Include the option to choose between plain text or HTML.
 *
 * @Mail(
 *   id = "SMTPMailSystem",
 *   label = @Translation("SMTP Mailer"),
 *   description = @Translation("Sends the message, using SMTP.")
 * )
 */
class SMTPMailSystem implements MailInterface, ContainerFactoryPluginInterface {

  use StringTranslationTrait;

  /**
   * Flag to allow HTML Formatted email.
   *
   * @var bool
   */
  // phpcs:ignore
  protected $AllowHtml;

  /**
   * SMTP Config Object.
   *
   * @var \Drupal\Core\Config\ImmutableConfig
   */
  protected $smtpConfig;

  /**
   * Logger.
   *
   * @var \Drupal\Core\Logger\LoggerChannelFactoryInterface
   */
  protected $logger;

  /**
   * Messenger.
   *
   * @var \Drupal\Core\Messenger\MessengerInterface
   */
  protected $messenger;

  /**
   * Email validator.
   *
   * @var \Drupal\Component\Utility\EmailValidatorInterface
   */
  protected $emailValidator;

  /**
   * The config factory service.
   *
   * @var \Drupal\Core\Config\ConfigFactoryInterface
   */
  protected $configFactory;

  /**
   * The curent user service.
   *
   * @var \Drupal\Core\Session\AccountProxyInterface
   */
  protected $currentUser;

  /**
   * The File System service.
   *
   * @var \Drupal\Core\File\FileSystemInterface
   */
  protected $fileSystem;

  /**
   * The file mime type guesser service.
   *
   * @var \Symfony\Component\Mime\MimeTypeGuesserInterface
   */
  protected $mimeTypeGuesser;

  /**
   * The SMTP object, stored between calls when keep alive is enabled.
   *
   * @var \PHPMailer\PHPMailer\SMTP
   */
  protected $persistentSmtp;

  /**
   * The renderer.
   *
   * @var \Drupal\Core\Render\RendererInterface
   */
  protected $renderer;

  /**
   * The session object.
   *
   * @var \Symfony\Component\HttpFoundation\Session\SessionInterface
   */
  protected $session;

  /**
   * Constructs a SMPTMailSystem object.
   *
   * @param array $configuration
   *   The configuration array.
   * @param string $plugin_id
   *   The plug-in ID.
   * @param mixed $plugin_definition
   *   The plug-in definition.
   * @param \Drupal\Core\Logger\LoggerChannelFactoryInterface $logger
   *   The logger object.
   * @param \Drupal\Core\Messenger\MessengerInterface $messenger
   *   The messenger object.
   * @param \Drupal\Component\Utility\EmailValidatorInterface $emailValidator
   *   The messenger object.
   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
   *   The config factory object.
   * @param \Drupal\Core\Session\AccountProxyInterface $account
   *   The current user service.
   * @param \Drupal\Core\File\FileSystemInterface $file_system
   *   The file system service.
   * @param \Symfony\Component\Mime\MimeTypeGuesserInterface $mime_type_guesser
   *   The file mime type guesser service.
   * @param \Drupal\Core\Render\RendererInterface $renderer
   *   The renderer.
   * @param \Symfony\Component\HttpFoundation\Session\SessionInterface $session
   *   The session.
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, LoggerChannelFactoryInterface $logger, MessengerInterface $messenger, EmailValidatorInterface $emailValidator, ConfigFactoryInterface $config_factory, AccountProxyInterface $account, FileSystemInterface $file_system, MimeTypeGuesserInterface $mime_type_guesser, RendererInterface $renderer, SessionInterface $session) {
    $this->smtpConfig = $config_factory->get('smtp.settings');
    $this->logger = $logger;
    $this->messenger = $messenger;
    $this->emailValidator = $emailValidator;
    $this->configFactory = $config_factory;
    $this->currentUser = $account;
    $this->fileSystem = $file_system;
    $this->mimeTypeGuesser = $mime_type_guesser;
    $this->renderer = $renderer;
    $this->session = $session;
  }

  /**
   * Creates an instance of the plugin.
   *
   * @param \Symfony\Component\DependencyInjection\ContainerInterface $container
   *   The container to pull out services used in the plugin.
   * @param array $configuration
   *   A configuration array containing information about the plugin instance.
   * @param string $plugin_id
   *   The plugin ID for the plugin instance.
   * @param mixed $plugin_definition
   *   The plugin implementation definition.
   *
   * @return static
   *   Returns an instance of this plugin.
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static(
      $configuration,
      $plugin_id,
      $plugin_definition,
      $container->get('logger.factory'),
      $container->get('messenger'),
      $container->get('email.validator'),
      $container->get('config.factory'),
      $container->get('current_user'),
      $container->get('file_system'),
      $container->get('file.mime_type.guesser'),
      $container->get('renderer'),
      $container->get('session')
    );
  }

  /**
   * Concatenate and wrap the e-mail body for either plain-text or HTML emails.
   *
   * @param array $message
   *   A message array, as described in hook_mail_alter().
   *
   * @return array
   *   The formatted $message.
   */
  public function format(array $message) {
    $this->AllowHtml = $this->smtpConfig->get('smtp_allowhtml');

    // Join the body array into one string.
    $message['body'] = implode("\n\n", $message['body']);
    if (!$this->AllowHtml) {
      // Convert any HTML to plain-text.
      $message['body'] = MailFormatHelper::htmlToText($message['body']);
      // Wrap the mail body for sending.
      $message['body'] = MailFormatHelper::wrapMail($message['body']);
    }

    return $message;
  }

  /**
   * Send the e-mail message.
   *
   * @param array $message
   *   A message array, as described in hook_mail_alter().
   *
   * @return mixed
   *   TRUE if the mail was successfully accepted, otherwise FALSE.
   *
   * @throws \PHPMailer\PHPMailer\Exception
   *
   * @see drupal_mail()
   */
  public function mail(array $message) {
    $logger = $this->logger->get('smtp');

    if (!class_exists(PHPMailer::class)) {
      $logger->error($this->t('Unable to send mail: PHPMailer class was not found.'));
      return FALSE;
    }

    $to = $message['to'];
    $body = $message['body'];
    $headers = array_change_key_case($message['headers']);
    $subject = $message['subject'];

    // Optionally reroute all emails to a single address.
    [$to, $headers] = $this->applyRerouting($to, $headers);

    $mailer = $this->getMailer();
    // Use email.validator due to different validation standard by PHPMailer.
    $mailer::$validator = [$this->emailValidator, 'isValid'];

    // Override PHPMailer default timeout if requested.
    $smtp_timeout = $this->smtpConfig->get('smtp_timeout');
    if (!empty($smtp_timeout)) {
      $mailer->Timeout = $smtp_timeout;
    }

    // Turn on debugging, if requested.
    if ($this->smtpConfig->get('smtp_debugging')
      && $this->currentUser->hasPermission('administer smtp module')) {
      $mailer->SMTPDebug = $this->smtpConfig->get('smtp_debug_level');
      $mailer->Debugoutput = function ($message, $debug_level) {
        $debug_logs = $this->session->get('smtp_debug', []);
        $debug_logs[] = [
          'message' => $message,
          'level' => $debug_level,
        ];
        $this->session->set('smtp_debug', $debug_logs);
      };
    }

    // Turn on KeepAlive feature if requested.
    if ($this->smtpConfig->get('smtp_keepalive')) {
      $mailer->SMTPKeepAlive = TRUE;
    }

    // Prefer the from_name from the message.
    if (!empty($message['params']['from_name'])) {
      $from_name = $message['params']['from_name'];
    }

    // If the smtp_fromname is set, it overrides the name that was passed as
    // part of the $from address.
    elseif (!empty($this->smtpConfig->get('smtp_fromname'))) {
      $from_name = $this->smtpConfig->get('smtp_fromname');
    }

    if (empty($from_name)) {
      // If value is not defined in settings, use site_name.
      $from_name = $this->configFactory->get('system.site')->get('name');
    }

    // Set from email.
    if (!empty($this->smtpConfig->get('smtp_from')) && $this->emailValidator->isValid($this->smtpConfig->get('smtp_from'))) {
      $from = $this->smtpConfig->get('smtp_from');
    }

    // If the SMTP module from email has not been provided, use the provided
    // from email.
    elseif (!empty($message['params']['from_mail'])) {
      $from = $message['params']['from_mail'];
    }

    // Alternative way to set from with email.
    elseif (!empty($message['from'])) {
      $from = $message['from'];
    }

    // Set SMTP from the default site mail as a fallback.
    else {
      $from = $this->configFactory->get('system.site')->get('mail');
    }

    // The $from address might contain the "name" part. If it does, split it,
    // since PHPMailer expects $from to be the raw email address.
    $matches = [];
    if (preg_match('/^(.*)\s\<(.*)\>$/', $from, $matches)) {
      $from = $matches[2];
    }

    // Defines the From value to what we expect.
    $mailer->From = $from;
    // phpcs:ignore
    $mailer->FromName = (new UnstructuredHeader('From', $from_name))->getBodyAsString();

    // Sender needs to be set to an email address only to prevent trying to
    // send an email with an invalid sender (where the name is present).
    $mailer->Sender = $from;

    $hostname = $this->smtpConfig->get('smtp_client_hostname');
    if ($hostname != '') {
      $mailer->Hostname = $hostname;
    }

    $helo = $this->smtpConfig->get('smtp_client_helo');
    if ($helo != '') {
      $mailer->Helo = $helo;
    }

    // Create the list of 'To:' recipients.
    $torecipients = explode(',', $to);
    foreach ($torecipients as $torecipient) {
      if (empty($torecipient)) {
        continue;
      }
      $to_comp = $this->getComponents($torecipient);
      $mailer->AddAddress($to_comp['email'], $to_comp['name']);
    }

    // Parse the headers of the message and set the PHPMailer object's settings
    // accordingly.
    foreach ($headers as $key => $value) {
      if ($value == NULL || $value == '') {
        // $value should always be set. If not, remove the header field and
        // skip to the next header field.
        unset($headers[$key]);
        continue;
      }
      switch ($key) {
        case 'content-type':
          // Parse several values on the Content-type header,
          // storing them in an array like.
          // key=value -> $vars['key']='value'.
          $vars = explode(';', $value);

          foreach ($vars as $i => $var) {
            if ($cut = strpos($var, '=')) {
              $new_var = trim(strtolower(substr($var, $cut + 1)));
              $new_key = trim(substr($var, 0, $cut));
              unset($vars[$i]);
              $vars[$new_key] = $new_var;
            }
          }

          // Set the charset based on the provided value,
          // otherwise set it to UTF-8 (which is Drupal's internal default).
          $mailer->CharSet = $vars['charset'] ?? 'UTF-8';

          // If $vars is empty then set an empty value at index 0,
          // to avoid a PHP warning in the next statement.
          $vars[0] = $vars[0] ?? '';

          switch ($vars[0]) {
            case 'text/plain':
              // The message includes only a plain text part.
              $mailer->IsHTML(FALSE);
              $content_type = 'text/plain';
              break;

            case 'text/html':
              // The message includes only an HTML part.
              $mailer->IsHTML(TRUE);
              $content_type = 'text/html';
              break;

            case 'multipart/related':
              // Get the boundary ID from the Content-Type header.
              $boundary = $this->getSubstring($value, 'boundary', '"', '"');

              // The message includes an HTML part w/inline attachments.
              $mailer->ContentType = $content_type = 'multipart/related; boundary="' . $boundary . '"';
              break;

            case 'multipart/alternative':
              // The message includes both a plain text and an HTML part.
              $mailer->ContentType = $content_type = 'multipart/alternative';

              // Get the boundary ID from the Content-Type header.
              $boundary = $this->getSubstring($value, 'boundary', '"', '"');
              break;

            case 'multipart/mixed':
              // The message includes one or more attachments.
              $mailer->ContentType = $content_type = 'multipart/mixed';

              // Get the boundary ID from the Content-Type header.
              $boundary = $this->getSubstring($value, 'boundary', '"', '"');
              break;

            default:
              // Everything else is unsupported by PHPMailer.
              $this->messenger->addMessage($this->t('The %header of your message is not supported by PHPMailer and will be sent as text/plain instead.',
                ['%header' => "Content-Type: $value"]),
                'error');
              $logger->error($this->t('The %header of your message is not supported by PHPMailer and will be sent as text/plain instead.',
                ['%header' => "Content-Type: $value"]));

              // Force the Content-Type to be text/plain.
              $mailer->IsHTML(FALSE);
              $content_type = 'text/plain';
          }
          break;

        case 'reply-to':
          // Only add a "reply-to" if it's not the same as "from".
          $reply_to_comp = $this->getComponents($value);
          $reply_to_email = $reply_to_comp['email'];
          if ($reply_to_email !== $from) {
            $mailer->AddReplyTo($reply_to_email, $reply_to_comp['name']);
          }
          break;

        case 'sender':
          // Header always needs to be set so the mail won't fail to be sent.
          $headers['sender'] = $from;
          // Change the "sender" if it's not the same as "from".
          $sender_comp = $this->getComponents($value);
          $sender_email = $sender_comp['email'];
          if ($sender_email !== $from) {
            $mailer->Sender = $sender_email;
            $headers['sender'] = $sender_email;
          }
          break;

        case 'content-transfer-encoding':
          $mailer->Encoding = $value;
          break;

        case 'return-path':
          $sender_comp = $this->getComponents($value);
          $mailer->Sender = $sender_comp['email'];
          break;

        case 'from':
          // The Drupal mail system specifies the 'From' header, and we do not
          // it to be added again, after our processing above.
          // @todo refactor above "From" handling to here.
          break;

        case 'mime-version':
        case 'x-mailer':
          // Let PHPMailer specify these.
          break;

        case 'errors-to':
          $mailer->AddCustomHeader('Errors-To: ' . $value);
          break;

        case 'cc':
          $cc_recipients = explode(',', $value);
          foreach ($cc_recipients as $cc_recipient) {
            $cc_comp = $this->getComponents($cc_recipient);
            $mailer->AddCC($cc_comp['email'], $cc_comp['name']);
          }
          break;

        case 'bcc':
          $bcc_recipients = explode(',', $value);
          foreach ($bcc_recipients as $bcc_recipient) {
            $bcc_comp = $this->getComponents($bcc_recipient);

            // phpcs:ignore
            $mailer->AddBCC($bcc_comp['email'], (new UnstructuredHeader('BCC', $bcc_comp['name']))->getBodyAsString());
          }
          break;

        default:
          // The header key is not special - add it as is.
          $mailer->AddCustomHeader($key . ': ' . $value);
      }
    }

    // Add the message's subject.
    $mailer->Subject = $subject;

    // Processes the message's body.
    switch ($content_type) {
      case 'multipart/related':
        $mailer->Body = $body;
        // @todo Figure out if there is anything more to handling this type.
        break;

      case 'multipart/alternative':
        // Split the body based on the boundary ID.
        $body_parts = $this->boundarySplit($body, $boundary);
        foreach ($body_parts as $body_part) {
          // If plain/text within the body part, add it to $mailer->AltBody.
          if (strpos($body_part, 'text/plain')) {
            // Clean up the text.
            $body_part = trim($this->removeHeaders(trim($body_part)));
            // Include it as part of the mail object.
            $mailer->AltBody = $body_part;
          }
          // If plain/html within the body part, add it to $mailer->Body.
          elseif (strpos($body_part, 'text/html')) {
            // Clean up the text.
            $body_part = trim($this->removeHeaders(trim($body_part)));
            // Include it as part of the mail object.
            $mailer->Body = $body_part;
          }
        }
        break;

      case 'multipart/mixed':
        // Split the body based on the boundary ID.
        $body_parts = $this->boundarySplit($body, $boundary);

        // Determine if there is an HTML part.
        $text_html = FALSE;
        foreach ($body_parts as $body_part) {
          if (strpos($body_part, 'text/html')) {
            $text_html = TRUE;
          }
        }

        foreach ($body_parts as $body_part) {
          // If test/plain within the body part, add it to either
          // $mailer->AltBody or $mailer->Body, depending on whether there is
          // also a text/html part ot not.
          if (strpos($body_part, 'multipart/alternative')) {

            // Get boundary ID from the Content-Type header.
            $boundary2 = $this->getSubstring($body_part, 'boundary', '"', '"');
            // Clean up the text.
            $body_part = trim($this->removeHeaders(trim($body_part)));
            // Split the body based on the boundary ID.
            $body_parts2 = $this->boundarySplit($body_part, $boundary2);

            foreach ($body_parts2 as $body_part2) {

              // If plain/text within the body part, add it to $mailer->AltBody.
              if (strpos($body_part2, 'text/plain')) {
                // Clean up the text.
                $body_part2 = trim($this->removeHeaders(trim($body_part2)));
                // Include it as part of the mail object.
                $mailer->AltBody = $body_part2;
                $mailer->ContentType = 'multipart/mixed';
              }
              // If plain/html within the body part, add it to $mailer->Body.
              elseif (strpos($body_part2, 'text/html')) {
                // Get the encoding.
                $body_part2_encoding = trim($this->getSubstring($body_part2, 'Content-Transfer-Encoding', ' ', "\n"));
                // Clean up the text.
                $body_part2 = trim($this->removeHeaders(trim($body_part2)));
                // Check whether the encoding is base64, and if so, decode it.
                if (mb_strtolower($body_part2_encoding) == 'base64') {
                  // Include it as part of the mail object.
                  $mailer->Body = base64_decode($body_part2);
                  // Ensure the whole message is recoded in the base64 format.
                  $mailer->Encoding = 'base64';
                }
                else {
                  // Include it as part of the mail object.
                  $mailer->Body = $body_part2;
                }
                $mailer->ContentType = 'multipart/mixed';
              }
            }
          }

          // If text/plain within the body part, add it to $mailer->Body.
          elseif (strpos($body_part, 'text/plain')) {
            // Clean up the text.
            $body_part = trim($this->removeHeaders(trim($body_part)));

            if ($text_html) {
              $mailer->AltBody = $body_part;
              $mailer->IsHTML(TRUE);
              $mailer->ContentType = 'multipart/mixed';
            }
            else {
              $mailer->Body = $body_part;
              $mailer->IsHTML(FALSE);
              $mailer->ContentType = 'multipart/mixed';
            }
          }
          // If text/html within the body part, add it to $mailer->Body.
          elseif (strpos($body_part, 'text/html')) {
            // Clean up the text.
            $body_part = trim($this->removeHeaders(trim($body_part)));
            // Include it as part of the mail object.
            $mailer->Body = $body_part;
            $mailer->IsHTML(TRUE);
            $mailer->ContentType = 'multipart/mixed';
          }
          // Add the attachment.
          elseif (strpos($body_part, 'Content-Disposition: attachment;') && !isset($message['params']['attachments'])) {
            $file_path     = $this->getSubstring($body_part, 'filename=', '"', '"');
            $file_name     = $this->getSubstring($body_part, ' name=', '"', '"');
            $file_encoding = $this->getSubstring($body_part, 'Content-Transfer-Encoding', ' ', "\n");
            $file_type     = $this->getSubstring($body_part, 'Content-Type', ' ', ';');

            if (file_exists($file_path)) {
              if (!$mailer->addStringAttachment(file_get_contents($file_path), $file_name, $file_encoding, $file_type)) {
                $this->messenger->addMessage($this->t('Attachment could not be found or accessed.'));
              }
            }
            else {
              // Clean up the text.
              $body_part = trim($this->removeHeaders(trim($body_part)));

              if (mb_strtolower($file_encoding) == 'base64') {
                $attachment = base64_decode($body_part);
              }
              elseif (mb_strtolower($file_encoding) == 'quoted-printable') {
                $attachment = quoted_printable_decode($body_part);
              }
              else {
                $attachment = $body_part;
              }

              $attachment_new_filename = $this->fileSystem->tempnam('temporary://', 'smtp');
              // phpcs:ignore
              $file_path = \Drupal::service('file.repository')->writeData($attachment, $attachment_new_filename, FileSystemInterface::EXISTS_REPLACE); // @phpstan-ignore-line
              $real_path = $this->fileSystem->realpath($file_path->uri);

              if (!$mailer->addStringAttachment(file_get_contents($real_path), $file_name)) {
                $this->messenger->addMessage($this->t('Attachment could not be found or accessed.'));
              }
            }
          }
        }
        break;

      default:
        $mailer->Body = $body;
        break;
    }

    // Process attachments that are passed as params. Both the Mime Mail and
    // Webform modules do this.
    if (!empty($message['params']['attachments'])) {
      foreach ($message['params']['attachments'] as $attachment) {
        // Support either the attachment being specified as a filepath
        // OR loaded into memory.
        if (isset($attachment['filepath'])) {
          $filename = $attachment['filename'] ?? basename($attachment['filepath']);
          $filemime = $attachment['filemime'] ?? $this->mimeTypeGuesser->guessMimeType($attachment['filepath']);
          $mailer->addStringAttachment(file_get_contents($attachment['filepath']), $filename, 'base64', $filemime);
        }
        elseif (isset($attachment['filecontent'])) {
          $mailer->AddStringAttachment($attachment['filecontent'], $attachment['filename'], 'base64', $attachment['filemime']);
        }
      }
    }

    // Set the authentication settings.
    $username = $this->smtpConfig->get('smtp_username');
    $password = $this->smtpConfig->get('smtp_password');

    // If username and password are given, use SMTP authentication.
    if ($username != '' && $password != '') {
      $mailer->SMTPAuth = TRUE;
      $mailer->Username = $username;
      $mailer->Password = $password;
    }

    // Set the protocol prefix for the smtp host.
    switch ($this->smtpConfig->get('smtp_protocol')) {
      case 'ssl':
        $mailer->SMTPSecure = 'ssl';
        break;

      case 'tls':
        $mailer->SMTPSecure = 'tls';
        break;

      default:
        $mailer->SMTPSecure = '';
    }

    $mailer->SMTPAutoTLS = $this->smtpConfig->get('smtp_autotls');

    // Set other connection settings.
    $mailer->Host = $this->smtpConfig->get('smtp_host') . ';' . $this->smtpConfig->get('smtp_hostbackup');
    $mailer->Port = $this->smtpConfig->get('smtp_port');
    $mailer->Mailer = 'smtp';

    $mailerArr = [
      'mailer' => $mailer,
      'to' => $to,
      'from' => $from,
      'mail_system' => $this,
    ];
    if ($this->smtpConfig->get('smtp_queue')) {
      $logger->info($this->t('Queue sending mail to: @to (subject: %subject)', ['@to' => $to, '%subject' => $subject]));
      smtp_send_queue($mailerArr);
    }
    else {
      return $this->smtpMailerSend($mailerArr);
    }

    return TRUE;
  }

  /**
   * Splits the input into parts based on the given boundary.
   *
   * Swiped from Mail::MimeDecode, with modifications based on Drupal's coding
   * standards and this bug report: http://pear.php.net/bugs/bug.php?id=6495
   *
   * @param string $input
   *   A string containing the body text to parse.
   * @param string $boundary
   *   A string with the boundary string to parse on.
   *
   * @return array
   *   An array containing the resulting mime parts
   */
  protected function boundarySplit($input, $boundary) {
    $parts       = [];
    $bs_possible = substr($boundary, 2, -2);
    $bs_check    = '\"' . $bs_possible . '\"';

    if ($boundary == $bs_check) {
      $boundary = $bs_possible;
    }

    $tmp = explode('--' . $boundary, $input);

    for ($i = 1; $i < count($tmp); $i++) {
      if (trim($tmp[$i])) {
        $parts[] = $tmp[$i];
      }
    }

    return $parts;
  }

  /**
   * Strips the headers from the body part.
   *
   * @param string $input
   *   A string containing the body part to strip.
   *
   * @return string
   *   A string with the stripped body part.
   */
  protected function removeHeaders($input) {
    $part_array = explode("\n", $input);

    // Will strip these headers according to RFC2045.
    $headers_to_strip = [
      'Content-Type',
      'Content-Transfer-Encoding',
      'Content-ID',
      'Content-Disposition',
    ];
    $pattern = '/^(' . implode('|', $headers_to_strip) . '):/';

    while (count($part_array) > 0) {

      // Ignore trailing spaces/newlines.
      $line = rtrim($part_array[0]);

      // If the line starts with a known header string.
      if (preg_match($pattern, $line)) {
        $line = rtrim(array_shift($part_array));

        // Remove line containing matched header.
        // If line ends in a ';' and the next line starts with four spaces,
        // it's a continuation of the header split onto the next line.
        // Continue removing lines while we have this condition.
        while (substr($line, -1) == ';' && count($part_array) > 0 && substr($part_array[0], 0, 4) == '    ') {
          $line = rtrim(array_shift($part_array));
        }
      }
      else {
        // No match header, must be past headers; stop searching.
        break;
      }
    }

    $output = implode("\n", $part_array);
    return $output;
  }

  /**
   * Returns a string that is contained within another string.
   *
   * Returns the string from within $source that is some where after $target
   * and is between $beginning_character and $ending_character.
   *
   * @param string $source
   *   A string containing the text to look through.
   * @param string $target
   *   A string containing the text in $source to start looking from.
   * @param string $beginning_character
   *   A string containing the character just before the sought after text.
   * @param string $ending_character
   *   A string containing the character just after the sought after text.
   *
   * @return string
   *   A string with the text found between the $beginning_character and the
   *   $ending_character.
   */
  protected function getSubstring($source, $target, $beginning_character, $ending_character) {
    $search_start     = strpos($source, $target) + 1;
    $first_character  = strpos($source, $beginning_character, $search_start) + 1;
    $second_character = strpos($source, $ending_character, $first_character) + 1;
    $substring        = substr($source, $first_character, $second_character - $first_character);
    $string_length    = strlen($substring) - 1;

    if ($substring[$string_length] == $ending_character) {
      $substring = substr($substring, 0, $string_length);
    }

    return $substring;
  }

  /**
   * Returns an array of name and email address from a string.
   *
   * @param string $input
   *   A string that contains different possible combinations of names and
   *   email address.
   *
   * @return array
   *   An array containing a name and an email address.
   */
  protected function getComponents($input) {
    $input = trim($input);
    $components = [
      'input' => $input,
      'name' => '',
      'email' => '',
    ];

    // If the input is a valid email address in its entirety,
    // then there is nothing to do, just return that.
    if ($this->emailValidator->isValid($input)) {
      $components['email'] = $input;
      return $components;
    }

    // Check if $input has one of the following formats, extract what we can:
    // some name <address@example.com>.
    // "another name" <address@example.com>.
    // <address@example.com>.
    if (preg_match('/^"?([^"\t\n]*)"?\s*<([^>\t\n]*)>$/', $input, $matches)) {
      $components['name'] = trim($matches[1]);
      $components['email'] = trim($matches[2]);
    }

    return $components;
  }

  /**
   * Wrapper around global static call to increase testability.
   *
   * @param array $mailerArr
   *   Variables to send email.
   *
   * @return bool
   *   TRUE if email was sent. FALSE otherwise.
   *
   * @see _smtp_mailer_send()
   */
  public function smtpMailerSend(array $mailerArr) {
    return _smtp_mailer_send($mailerArr);
  }

  /**
   * Applies rerouting, if an address is set for that purpose.
   *
   * @param string $to
   *   Original $to address.
   * @param array $headers
   *   Original headers for the message.
   *
   * @return array
   *   Keyed array, with new values for $to and $headers, after potentially
   *   applying rerouting.
   */
  protected function applyRerouting($to, array $headers) {
    $new_to = $to;
    $new_headers = $headers;

    $reroute_address = $this->smtpConfig->get('smtp_reroute_address');
    if (!empty($reroute_address)) {
      $new_to = $reroute_address;
      // Remove any CC and BCC headers that might have been set.
      unset($new_headers['cc']);
      unset($new_headers['bcc']);
    }

    return [$new_to, $new_headers];
  }

  /**
   * Get (and, if applicable, build) the PHPMailer object.
   *
   * @return \PHPMailer\PHPMailer\PHPMailer
   *   The PHPMailer Library.
   */
  protected function getMailer() {
    if ($this->smtpConfig->get('smtp_keepalive')) {
      $mailer = new PHPMailer(TRUE);
      if ($this->persistentSmtp) {
        $mailer->setSMTPInstance($this->persistentSmtp);
      }
      else {
        $this->persistentSmtp = $mailer->getSMTPInstance();
      }
      return $mailer;
    }
    else {
      return new PHPMailer(TRUE);
    }
  }

  /**
   * Log debug messages.
   */
  public function debug() {
    $logger = $this->logger->get('smtp');
    $debug_logs = $this->session->get('smtp_debug', []);
    if ($this->smtpConfig->get('smtp_debugging') && $this->currentUser->hasPermission('administer smtp module') && !empty($debug_logs)) {
      $item_list = [
        '#theme' => 'item_list',
        '#items' => [],
      ];
      foreach ($debug_logs as $debug_log) {
        $item_list['#items'][] = $debug_log['message'];
      }
      $debug_logs_message = $this->renderer->render($item_list);
      $logger->log('debug', $debug_logs_message);
      $this->session->remove('smtp_debug');
    }
  }

}