let enabled = false;
let steps: Record<string, number> = {};

/** UI-only instrumentation; production calls are just the original calculation. */
export function measureNavigationStep<T>(name: string, compute: () => T): T {
  if (!import.meta.env.DEV || !enabled) return compute();
  const started = performance.now();
  try { return compute(); }
  finally { steps[name] = (steps[name] ?? 0) + performance.now() - started; }
}

/** Opt-in, local development measurement. Never records report content. */
export function observeNavigationPerformance(): void {
  enabled = true;
  let startedAt = 0;
  let target = '';
  let sequence = 0;
  let frames = 0;
  let previousFrame = 0;
  let longestFrame = 0;
  const output = document.createElement('output');
  output.id = 'assessor-navigation-performance';
  output.hidden = true;
  document.body.append(output);
  document.addEventListener('click', event => {
    const button = (event.target as Element).closest('.sidebar-primary-actions button');
    if (!button || button.hasAttribute('disabled')) return;
    target = button.textContent?.trim() ?? '';
    startedAt = performance.now();
    steps = {};
    sequence += 1;
    const current = sequence;
    previousFrame = startedAt;
    longestFrame = 0;
    frames = 0;
    const measure = (now: number) => {
      if (sequence !== current) return;
      longestFrame = Math.max(longestFrame, now - previousFrame);
      previousFrame = now;
      frames += 1;
      if (frames < 3) { requestAnimationFrame(measure); return; }
      output.textContent = JSON.stringify({ area: target, sequence: current,
        paintMs: Math.round(now - startedAt), longestFrameMs: Math.round(longestFrame),
        stepsMs: Object.fromEntries(Object.entries(steps).map(([name, duration]) => [name, Math.round(duration)])) });
    };
    requestAnimationFrame(measure);
  }, true);
}