"use client";

import { useEffect, useState } from "react";

function formatElapsed(ms: number): string {
  const totalSec = Math.max(0, Math.floor(ms / 1000));
  const h = Math.floor(totalSec / 3600);
  const m = Math.floor((totalSec % 3600) / 60);
  const s = totalSec % 60;
  const pad = (n: number) => String(n).padStart(2, "0");
  return h > 0 ? `${h}:${pad(m)}:${pad(s)}` : `${m}:${pad(s)}`;
}

export function SessionTimer({ startedAt }: { startedAt: string }) {
  const [now, setNow] = useState(() => Date.now());

  useEffect(() => {
    const timer = setInterval(() => setNow(Date.now()), 1000);
    return () => clearInterval(timer);
  }, []);

  return <span className="font-mono-num">{formatElapsed(now - new Date(startedAt).getTime())}</span>;
}