/**
 * [Gitea #158] Local-clock-vs-homeserver skew detection.
 *
 * Incident 2026-09-17: a wrong Windows clock made every MatrixRTC membership
 * look expired locally (calls failed, media keys rejected) while the server
 * answered 200 to everything, and nothing in the UI hinted at the cause.
 *
 * Measurement needs no extra requests and no CORS-exposed headers: every event
 * a `/sync` delivers carries `origin_server_ts` (stamped by its origin server)
 * and `unsigned.age` (= OUR server's `now − origin_server_ts` at the moment it
 * built the response). So `origin_server_ts + age` is our server's clock at
 * response time, and `receivedAt − (origin_server_ts + age)` is our skew plus
 * the download latency (tens of ms; ignored). matrix-js-sdk already computes
 * `localTimestamp = Date.now() − age` at event construction, so a sample is
 * simply `localTimestamp − origin_server_ts`.
 *
 * Only LIVE events count (`RoomEvent.Timeline` data.liveEvent, which the SDK
 * already sets false for events replayed from the IndexedDB cache — those carry
 * a stale `age` that would read as hours of skew). The initial network sync's
 * events qualify, so a wrong clock is flagged within seconds of startup.
 */

export const SKEW_WARN_MS = 30_000;
export const SKEW_CLEAR_MS = 15_000;
export const SKEW_MIN_SAMPLES = 3;
/** Samples older than this are forgotten. */
export const SKEW_WINDOW_MS = 5 * 60 * 1000;
export const SKEW_MAX_SAMPLES = 30;
/**
 * "Ahead" must hold across samples received at least this far apart.
 *
 * Incident 2026-09-29: the homeserver's host ran out of memory and stalled for
 * ~2 minutes; the /sync that finally went out carried events whose `age` was
 * computed ~30 s before it arrived, so every client read "your clock is 30 s
 * ahead" — while the real problem was the server. A late delivery can only make
 * the local clock look AHEAD (never behind), so the estimate is the LOWEST
 * recent sample (the one delivered fastest), and "ahead" has to persist across
 * a minute of fresh samples before it is reported. "Behind" can't come from a
 * delay and is reported as soon as there are enough samples.
 */
export const SKEW_AHEAD_SPAN_MS = 60_000;
/**
 * Sanity cap on `age`. Old events are still valid samples (the server computes
 * `age` at response time, so `ts + age` is its clock regardless of the event's
 * own age) — this only rejects garbage.
 */
export const SKEW_MAX_AGE_MS = 30 * 24 * 60 * 60 * 1000;

export type ClockSkewState = {
  /** Median of the recent samples, ms; positive = local clock is AHEAD. */
  skewMs: number | null;
  /** Over the threshold (with hysteresis). */
  warning: boolean;
};

/** A wall-clock change larger than this (vs the monotonic clock) resets the samples. */
export const CLOCK_JUMP_MS = 5_000;

type Sample = { skew: number; at: number };

const currentClock = (): { wall: number; mono: number } => {
  const wall = Date.now();
  const mono = typeof performance !== 'undefined' ? performance.now() : wall;
  return { wall, mono };
};

export class ClockSkewMonitor {
  private samples: Sample[] = [];

  private clockOffset: number | undefined;

  private state: ClockSkewState = { skewMs: null, warning: false };

  private listeners = new Set<(state: ClockSkewState) => void>();

  public getState(): ClockSkewState {
    return this.state;
  }

  public subscribe(cb: (state: ClockSkewState) => void): () => void {
    this.listeners.add(cb);
    return () => {
      this.listeners.delete(cb);
    };
  }

  /**
   * Feed one live event. `originServerTs` + `age` come from the event;
   * `localTimestamp` is the SDK's `Date.now() − age` at construction; `clock`
   * is when the sample was taken: wall clock and a monotonic clock
   * (performance.now()), so samples are aged by real elapsed time and a change
   * of the local clock (someone fixing it) starts the measurement afresh.
   * Only feed events stamped by OUR homeserver: another server's
   * `origin_server_ts` carries that server's clock.
   * Returns the new state (unchanged object when nothing moved).
   */
  public sample(
    originServerTs: number,
    age: number | undefined,
    localTimestamp: number,
    clock: { wall: number; mono: number } = currentClock(),
  ): ClockSkewState {
    if (age === undefined || !Number.isFinite(age) || age < 0 || age > SKEW_MAX_AGE_MS) {
      return this.state;
    }
    if (!Number.isFinite(originServerTs) || !Number.isFinite(localTimestamp)) return this.state;
    const now = clock.mono;
    const offset = clock.wall - clock.mono;
    if (this.clockOffset !== undefined && Math.abs(offset - this.clockOffset) > CLOCK_JUMP_MS) {
      // The local clock was changed: earlier samples measured the old clock.
      this.reset();
    }
    this.clockOffset = offset;
    this.samples.push({ skew: localTimestamp - originServerTs, at: now });
    this.samples = this.samples.filter((s) => now - s.at <= SKEW_WINDOW_MS);
    if (this.samples.length > SKEW_MAX_SAMPLES) this.samples.shift();
    if (this.samples.length < SKEW_MIN_SAMPLES) return this.state;

    // Delivery delay only ever adds to a sample: the smallest is the truest.
    const skewMs = Math.min(...this.samples.map((s) => s.skew));
    const abs = Math.abs(skewMs);
    let warning: boolean;
    if (this.state.warning) warning = abs >= SKEW_CLEAR_MS;
    else if (skewMs < -SKEW_WARN_MS) warning = true;
    else if (skewMs > SKEW_WARN_MS) {
      // Ahead: only if the fastest-delivered samples stayed high for a minute.
      const span = now - Math.min(...this.samples.map((s) => s.at));
      warning = span >= SKEW_AHEAD_SPAN_MS;
    } else warning = false;
    if (skewMs === this.state.skewMs && warning === this.state.warning) return this.state;
    this.state = { skewMs, warning };
    this.listeners.forEach((cb) => cb(this.state));
    return this.state;
  }

  public reset(): void {
    this.samples = [];
    this.clockOffset = undefined;
    if (this.state.skewMs !== null || this.state.warning) {
      this.state = { skewMs: null, warning: false };
      this.listeners.forEach((cb) => cb(this.state));
    }
  }
}

/** "14 minutes ahead" / "2 hours behind" / "45 seconds ahead". */
export const formatSkew = (skewMs: number): string => {
  const abs = Math.abs(skewMs);
  const dir = skewMs > 0 ? 'ahead' : 'behind';
  const unit = (n: number, word: string) => `${n} ${word}${n === 1 ? '' : 's'}`;
  if (abs >= 36 * 60 * 60 * 1000)
    return `${unit(Math.round(abs / (24 * 60 * 60 * 1000)), 'day')} ${dir}`;
  if (abs >= 90 * 60 * 1000) return `${unit(Math.round(abs / (60 * 60 * 1000)), 'hour')} ${dir}`;
  if (abs >= 90 * 1000) return `${unit(Math.round(abs / 60_000), 'minute')} ${dir}`;
  return `${unit(Math.round(abs / 1000), 'second')} ${dir}`;
};

/** "14 minutes ahead of the server" / "3 hours behind the server". */
export const describeSkewVsServer = (skewMs: number): string =>
  formatSkew(skewMs)
    .replace(/ ahead$/, ' ahead of the server')
    .replace(/ behind$/, ' behind the server');

export type ClockFixPlatform = 'windows' | 'mac' | 'linux' | 'ios' | 'android' | 'other';

export const detectClockFixPlatform = (ua: string): ClockFixPlatform => {
  if (/iPhone|iPad|iPod/i.test(ua)) return 'ios';
  if (/Android/i.test(ua)) return 'android';
  if (/Windows/i.test(ua)) return 'windows';
  if (/Mac OS X|Macintosh/i.test(ua)) return 'mac';
  if (/Linux|X11/i.test(ua)) return 'linux';
  return 'other';
};

export const clockFixHint = (platform: ClockFixPlatform): string => {
  switch (platform) {
    case 'windows':
      return 'Windows: Settings → Time & language → Date & time → turn on "Set time automatically", then Sync now.';
    case 'mac':
      return 'macOS: System Settings → General → Date & Time → turn on "Set time and date automatically".';
    case 'linux':
      return "Linux: enable NTP (e.g. `timedatectl set-ntp true`) or your desktop's Date & Time → Automatic.";
    case 'ios':
      return 'iOS: Settings → General → Date & Time → Set Automatically.';
    case 'android':
      return 'Android: Settings → System → Date & time → Set time automatically.';
    default:
      return "Turn on automatic (network) time in your operating system's date & time settings.";
  }
};