const storageKey = "ddc-notifications";
const finishedTitles = {
  ready: "Download ready",
  completed: "Download moved",
  failed: "Download failed",
};
const previousStatuses = new Map();

function notificationsSupported() {
  return "Notification" in window;
}

function readPreference() {
  try {
    return localStorage.getItem(storageKey) === "on";
  } catch {
    return false;
  }
}

function storePreference(enabled) {
  try {
    localStorage.setItem(storageKey, enabled ? "on" : "off");
  } catch {
    return;
  }
}

export function notificationsEnabled() {
  return notificationsSupported() && Notification.permission === "granted" && readPreference();
}

export async function setNotificationsEnabled(enabled) {
  if (!enabled) {
    storePreference(false);
    return;
  }
  if (!notificationsSupported()) throw new Error("This browser does not support notifications.");
  const permission = await Notification.requestPermission();
  if (permission !== "granted") throw new Error("Notifications are blocked for this site in the browser settings.");
  storePreference(true);
}

export function collectTransitions(jobs) {
  const transitions = [];
  const seenIds = new Set();
  for (const job of jobs) {
    seenIds.add(job.id);
    const previous = previousStatuses.get(job.id);
    if (previous && previous !== job.status) transitions.push({ job, previous });
    previousStatuses.set(job.id, job.status);
  }
  for (const id of previousStatuses.keys()) {
    if (!seenIds.has(id)) previousStatuses.delete(id);
  }
  return transitions;
}

export function notifyTransitions(transitions) {
  if (!notificationsEnabled()) return;
  for (const { job, previous } of transitions) {
    const title = finishedTitles[job.status];
    const wasWorking = previous === "queued" || previous === "downloading" || previous === "extracting" || previous === "moving";
    if (!title || !wasWorking) continue;
    if (job.status === "ready" && previous === "moving") continue;
    new Notification(title, { body: job.fileName || job.sourceUrl, tag: `ddc-${job.id}` });
  }
}