frontend/src/lib/useIdle.ts

import * as React from "react";

// Idle detection — the gate that makes cloud vision affordable. When the user
// is away (no input for `thresholdMs`) or the tab is hidden, ambient vision
// pauses so the Modal GPU scales to zero and bills nothing.
export function useIdle(thresholdMs = 150_000): boolean {
  const [idle, setIdle] = React.useState(false);
  const last = React.useRef(Date.now());

  React.useEffect(() => {
    const bump = () => {
      last.current = Date.now();
      setIdle(false); // wake immediately on activity (React bails if already false)
    };
    const events = ["mousemove", "mousedown", "keydown", "wheel", "touchstart"] as const;
    for (const e of events) window.addEventListener(e, bump, { passive: true });

    const tick = setInterval(() => {
      setIdle(document.hidden || Date.now() - last.current > thresholdMs);
    }, 5000);

    return () => {
      for (const e of events) window.removeEventListener(e, bump);
      clearInterval(tick);
    };
  }, [thresholdMs]);

  return idle;
}