// The name of the Gmail Label that is to be checked for purging?
var LABELS_TO_DELETE = [
  "label",
  "label-subLabel"
];

// Purge messages in the above label automatically after how many days?
var DELETE_AFTER_DAYS = "15";

// Maximum number of threads to process per run
var PAGE_SIZE = 150;

// If number of threads exceeds page size, resume job after X mins (max execution time is 6 mins)
var RESUME_FREQUENCY = 10;

var TRIGGER_NAME = "dailyDeleteGmail";

/*

IMPLEMENTATION

*/
function Intialize() {
  return;
}

function Install() {
  // First run 2 mins after install
  ScriptApp.newTrigger(TRIGGER_NAME)
           .timeBased()
           .at(new Date((new Date()).getTime() + 1000*60*2))
           .create();
  
  // Run daily there after
  ScriptApp.newTrigger(TRIGGER_NAME)
           .timeBased().everyDays(1).create();
}

function Uninstall() {
  var triggers = ScriptApp.getProjectTriggers();
  for (var i=0; i<triggers.length; i++) {
    ScriptApp.deleteTrigger(triggers[i]);
  }
}

function dailyDeleteGmail() {
  var age = new Date();  
  age.setDate(age.getDate() - DELETE_AFTER_DAYS);    
  
  var purge  = Utilities.formatDate(age, Session.getScriptTimeZone(), "yyyy-MM-dd");
  var search = "(label:" + LABELS_TO_DELETE.join(" OR label:") + ") before:" + purge;
  Logger.log("PURGE: " + purge);
  Logger.log("SEARCH: " + search);
  
  try {
    var threads = GmailApp.search(search, 0, PAGE_SIZE);
    
    // Resume again in 10 minutes
    if (threads.length == PAGE_SIZE) {
      Logger.log("Scheduling follow up job...");
      console.info("Scheduling follow up job...");
      ScriptApp.newTrigger(TRIGGER_NAME)
               .timeBased()
               .at(new Date((new Date()).getTime() + 1000*60*RESUME_FREQUENCY))
               .create();
    }
    
    // Move threads/messages which meet age criteria to trash
    Logger.log("Processing " + threads.length + " threads...");
    console.info("Processing " + threads.length + " threads...");
    for (var i=0; i<threads.length; i++) {
      var thread = threads[i];
      
      if (thread.getLastMessageDate() < age) {
        thread.moveToTrash();
      } else {
        var messages = GmailApp.getMessagesForThread(threads[i]);
        for (var j=0; j<messages.length; j++) {
          var email = messages[j];       
          if (email.getDate() < age) {
            email.moveToTrash();
          }
        }
      }
    }
  } catch (e) {
    Logger.log(e);
    console.error(e);
  }
}