// Written by Amit Agarwal
// Web: ctrlq.org
// Email: amit@labnol.org

function createDraftMessage() {
  var attachments = ['File_ID_1', 'File_ID_2'];

  var message = {
    to: {
      name: 'Google Scripts',
      email: 'amit@labnol.org',
    },
    from: {
      name: 'Amit Agarwal',
      email: 'amit@labnol.org',
    },
    body: {
      text: "Mr hänn is schon lang nümme g'she.",
      html: "Mr hänn is schon **lang nümme** g'she.",
    },
    subject: 'ctrlq, tech à la carte',
    files: getAttachments_(attachments),
  };

  // Compose Gmail message and send immediately
  callGmailAPI_(message);
}

function callGmailAPI_(message) {
  var payload = createMimeMessage_(message);

  var response = UrlFetchApp.fetch('https://www.googleapis.com/upload/gmail/v1/users/me/drafts?uploadType=media', {
    method: 'POST',
    headers: {
      Authorization: 'Bearer ' + ScriptApp.getOAuthToken(),
      'Content-Type': 'message/rfc822',
    },
    muteHttpExceptions: true,
    payload: payload,
  });

  Logger.log(response.getResponseCode());
  Logger.log(response.getContentText());
}

// UTF-8 characters in names and subject
function encode_(subject) {
  var enc_subject = Utilities.base64Encode(subject, Utilities.Charset.UTF_8);
  return '=?utf-8?B?' + enc_subject + '?=';
}

// Insert file attachments from Google Drive
function getAttachments_(ids) {
  var att = [];
  for (var i in ids) {
    var file = DriveApp.getFileById(ids[i]);
    att.push({
      mimeType: file.getMimeType(),
      fileName: file.getName(),
      bytes: Utilities.base64Encode(file.getBlob().getBytes()),
    });
  }
  return att;
}

// Create a MIME message that complies with RFC 2822
function createMimeMessage_(msg) {
  var nl = 'n';
  var boundary = '__ctrlq_dot_org__';

  var mimeBody = [
    'MIME-Version: 1.0',
    'To: ' + encode_(msg.to.name) + '<' + msg.to.email + '>',
    'From: ' + encode_(msg.from.name) + '<' + msg.from.email + '>',
    'Subject: ' + encode_(msg.subject), // takes care of accented characters

    'Content-Type: multipart/alternative; boundary=' + boundary + nl,
    '--' + boundary,

    'Content-Type: text/plain; charset=UTF-8',
    'Content-Transfer-Encoding: base64' + nl,
    Utilities.base64Encode(msg.body.text, Utilities.Charset.UTF_8) + nl,
    '--' + boundary,

    'Content-Type: text/html; charset=UTF-8',
    'Content-Transfer-Encoding: base64' + nl,
    Utilities.base64Encode(msg.body.html, Utilities.Charset.UTF_8) + nl,
  ];

  for (var i = 0; i < msg.files.length; i++) {
    var attachment = [
      '--' + boundary,
      'Content-Type: ' + msg.files[i].mimeType + '; name="' + msg.files[i].fileName + '"',
      'Content-Disposition: attachment; filename="' + msg.files[i].fileName + '"',
      'Content-Transfer-Encoding: base64' + nl,
      msg.files[i].bytes,
    ];

    mimeBody.push(attachment.join(nl));
  }

  mimeBody.push('--' + boundary + '--');

  return mimeBody.join(nl);
}