pars-elevator/src/components/Counter.tsx

"use client";

import { useEffect, useRef, useState } from "react";

type Props = {
  value: number;
  className?: string;
};

export function Counter({ value, className = "" }: Props) {
  const [n, setN] = useState(0);
  const ref = useRef<HTMLSpanElement>(null);
  const started = useRef(false);

  useEffect(() => {
    const el = ref.current;
    if (!el || started.current) return;

    const run = () => {
      if (started.current) return;
      started.current = true;
      const duration = 1400;
      const start = performance.now();
      const tick = (now: number) => {
        const t = Math.min(1, (now - start) / duration);
        const eased = 1 - Math.pow(1 - t, 3);
        setN(Math.round(value * eased));
        if (t < 1) requestAnimationFrame(tick);
        else setN(value);
      };
      requestAnimationFrame(tick);
    };

    const isVisible = () => {
      const rect = el.getBoundingClientRect();
      const vh = window.innerHeight || document.documentElement.clientHeight;
      return rect.top < vh * 0.92 && rect.bottom > vh * 0.08;
    };

    if (isVisible()) {
      run();
      return;
    }

    const observer = new IntersectionObserver(
      ([entry]) => {
        if (entry.isIntersecting) {
          run();
          observer.disconnect();
        }
      },
      { threshold: 0.2 },
    );

    observer.observe(el);
    return () => observer.disconnect();
  }, [value]);

  return (
    <span ref={ref} className={className} dir="ltr">
      {n.toLocaleString("en-US")}
    </span>
  );
}