/**
 * ─────────────────────────────────────────────
 *  SPB Drive — Personal Cloud Drive
 * ─────────────────────────────────────────────
 *  Author  : Simon-Pierre Boucher
 *  Contact : contact@spboucher.ai
 *  File    : src/preview/queue.mjs
 *  Purpose : Tiny in-process job queue (concurrency 2) for transcodes/conversions
 *  License : MIT © Simon-Pierre Boucher
 * ─────────────────────────────────────────────
 */

const CONCURRENCY = 2;

/** @type {Map<string, {promise: Promise<any>, status: string}>} */
const jobs = new Map();
const waiting = [];
let running = 0;

function pump() {
  while (running < CONCURRENCY && waiting.length > 0) {
    const next = waiting.shift();
    running += 1;
    next.entry.status = 'running';
    Promise.resolve()
      .then(next.fn)
      .then(
        (val) => { next.entry.status = 'done'; next.resolve(val); },
        (err) => { next.entry.status = 'error'; next.entry.error = String(err?.message ?? err); next.reject(err); },
      )
      .finally(() => {
        running -= 1;
        setTimeout(() => jobs.delete(next.key), 60_000);
        pump();
      });
  }
}

/**
 * Enqueue work keyed by a cache key — duplicate keys share one promise,
 * so a conversion runs at most once per unique blob.
 * @param {string} key e.g. `office:<sha>`
 * @param {() => Promise<any>} fn
 * @returns {Promise<any>}
 */
export function enqueue(key, fn) {
  const existing = jobs.get(key);
  if (existing) return existing.promise;
  const entry = { status: 'queued', error: null };
  entry.promise = new Promise((resolve, reject) => {
    waiting.push({ key, fn, resolve, reject, entry });
  });
  entry.promise.catch(() => {}); // observed by callers; avoid unhandled warnings
  jobs.set(key, entry);
  pump();
  return entry.promise;
}

/** Status for a key: 'queued' | 'running' | 'done' | 'error' | null. */
export function jobStatus(key) {
  const entry = jobs.get(key);
  return entry ? { status: entry.status, error: entry.error ?? null } : null;
}

/** Current queue depth (for /healthz). */
export function queueDepth() {
  return waiting.length + running;
}