export interface CaptureClock {
  now(): number;
}

export interface CaptureClockSources {
  wallNow: () => number;
  monotonicNow: () => number;
}

/**
 * Create an epoch-millisecond clock that advances from a monotonic source.
 *
 * Date.now() establishes the epoch once. Subsequent reads use the monotonic
 * delta, so an operating-system clock correction cannot move captured
 * evidence backwards or introduce a discontinuity between an event and media.
 */
export function createCaptureClock(sources: CaptureClockSources): CaptureClock {
  const epochAnchorMs = sources.wallNow();
  const monotonicAnchorMs = sources.monotonicNow();
  return {
    now: () =>
      Math.round(epochAnchorMs + (sources.monotonicNow() - monotonicAnchorMs)),
  };
}

function monotonicNow(): number {
  return typeof performance !== "undefined" &&
    typeof performance.now === "function"
    ? performance.now()
    : Date.now();
}

/** One shared browser clock for events, recordings, and snapshots. */
export const systemCaptureClock = createCaptureClock({
  wallNow: Date.now,
  monotonicNow,
});