/**
 * @file loop-delay.js
 * @description How long this server's event loop was held up — what an
 * operator sees as a laggy dashboard or terminal: every request, hook and
 * keystroke echo waits for it. One `monitorEventLoopDelay` histogram for the
 * whole process, sampled every 20 ms and reset each minute, so `loopDelay()`
 * describes the last full minute (the running one until the first minute
 * ends). Read by `GET /api/metrics` (Prometheus gauges) and
 * `GET /api/settings/info` (the System Health score).
 * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
 */

const { monitorEventLoopDelay } = require("perf_hooks");

const WINDOW_MS = 60_000;
const histogram = monitorEventLoopDelay({ resolution: 20 });
histogram.enable();

const read = () => ({
  p50: histogram.percentile(50),
  p99: histogram.percentile(99),
  max: histogram.max,
});

let lastWindow = null; // the last full minute, reported until the next one ends
setInterval(() => {
  lastWindow = read();
  histogram.reset();
}, WINDOW_MS).unref();

/** `{p50, p99, max}` in nanoseconds over the last minute. */
function loopDelay() {
  return lastWindow || read();
}

module.exports = { loopDelay, WINDOW_MS };