/** The name of the Gmail Label that is to be checked for purging */
const LABELS_TO_DELETE = [
  "auto_delete"
];

/** Labels to never delete */
const LABELS_TO_NOT_DELETE = [
  "do_not_delete"
]

const TRIGGER_NAME = "dailyDeleteGmail";

/** Purge messages in the above label automatically after how many days? */
const DELETE_AFTER_DAYS = "30";

/** Maximum number of threads to process per run */
const PAGE_SIZE = 300;

/** If number of threads exceeds page size, resume job after X mins (max execution time is 6 mins) */
const RESUME_FREQUENCY = 15;

/** what hour to run the daily trigger at */
const DAILY_RUN_HOUR = 10;

/**
 * Creates an initial trigger to run the project,
 * subsequent runs are automatically created
 */
function Install() {
  // First run 1 minute after install
  console.log("In Install, creating trigger to run after 1 minute");
  ScriptApp.newTrigger(TRIGGER_NAME)
      .timeBased()
      .after(1000*60*1)
      .create();
}

/**
 * Removes any lingering triggers. 
 * During normal execution new triggers are created,
 * To stop/remove this project, run this manually
 */
function Uninstall() {
  const triggers = ScriptApp.getProjectTriggers();
  console.log(`Found ${triggers.length} triggers to remove`);
  triggers.forEach(trigger => {
    ScriptApp.deleteTrigger(trigger);
  })
}

function dailyDeleteGmail(ev) {
  console.log(`Running for ${Session.getActiveUser().getEmail() ?? "unknown"}`);
  const labels = LABELS_TO_DELETE.map(label => `label:${label}`).join(" OR ");
  const not_labels = LABELS_TO_NOT_DELETE.map(label => `label:${label}`).join(" OR ");
  const search = `(${labels}) NOT (${not_labels}) older_than:${DELETE_AFTER_DAYS}d`;
  console.log(`search query: ${search}`);
  
  try {
    Uninstall();
    
    const threads = GmailApp.search(search, 0, PAGE_SIZE);
    
    if (threads.length === PAGE_SIZE) {
      const followupTime = 1000*60*RESUME_FREQUENCY;
      console.log(`More to do, scheduling a followup trigger to run after ${RESUME_FREQUENCY} minutes`);
      ScriptApp.newTrigger(TRIGGER_NAME)
        .timeBased()
        .after(followupTime)
        .create();
    } else {
      console.log("Caught up, creating a followup trigger for tomorrow.");
      const nextRunTime = new Date();
      nextRunTime.setDate(nextRunTime.getDate() + 1);
      nextRunTime.setHours(DAILY_RUN_HOUR);
      ScriptApp.newTrigger(TRIGGER_NAME)
        .timeBased()
        .at(nextRunTime)
        .create();
    }
    
    console.log("Processing " + threads.length + " threads...");
    const age = new Date();  
    age.setDate(age.getDate() - DELETE_AFTER_DAYS);
    let deleted = 0;
    threads.forEach(thread => {
      if (thread.getLastMessageDate() < age) {
        thread.moveToTrash();
        deleted += thread.getMessageCount();
      } else {
        const messages = GmailApp.getMessagesForThread(thread);
        messages.forEach(email => {
          if (email.getDate() < age) {
            email.moveToTrash();
            deleted++
          }
        })
      }
    })
    console.log(`Run completed and moved ${deleted} mails to trash.`)
  } catch (e) {
    console.error(e); 
  }
}