// Slowly drifting, twinkling starfield with the odd shooting star.

export function startStarfield(canvas) {
  const ctx = canvas.getContext('2d');
  const reduce = window.matchMedia('(prefers-reduced-motion: reduce)');
  const tints = ['#ffffff', '#ffffff', '#ffffff', '#9ff8ff', '#ffb3f0'];
  let w = 0;
  let h = 0;
  let stars = [];
  let shooters = [];
  let nextShot = 4000;
  let last = performance.now();
  let raf = 0;

  function resize() {
    const dpr = Math.min(window.devicePixelRatio || 1, 2);
    w = window.innerWidth;
    h = window.innerHeight;
    canvas.width = Math.round(w * dpr);
    canvas.height = Math.round(h * dpr);
    ctx.setTransform(dpr, 0, 0, dpr, 0, 0);
    const n = Math.min(320, Math.round((w * h) / 5500));
    stars = Array.from({ length: n }, () => ({
      x: Math.random() * w,
      y: Math.random() * h,
      z: 0.25 + Math.random() * 0.75,
      phase: Math.random() * Math.PI * 2,
      speed: 0.6 + Math.random() * 1.8,
      tint: tints[Math.floor(Math.random() * tints.length)],
    }));
    draw(performance.now());
  }

  function draw(now) {
    ctx.clearRect(0, 0, w, h);
    for (const s of stars) {
      const tw = 0.5 + 0.5 * Math.sin(now / 1000 * s.speed + s.phase);
      ctx.globalAlpha = reduce.matches ? 0.5 * s.z : (0.25 + 0.75 * tw) * s.z;
      ctx.fillStyle = s.tint;
      const r = 0.4 + s.z * 1.3;
      ctx.fillRect(s.x, s.y, r, r);
    }
    for (const s of shooters) {
      const g = ctx.createLinearGradient(s.x, s.y, s.x - s.vx * 9, s.y - s.vy * 9);
      g.addColorStop(0, 'rgba(255,255,255,0.95)');
      g.addColorStop(1, 'rgba(255,58,216,0)');
      ctx.globalAlpha = Math.min(1, s.life);
      ctx.strokeStyle = g;
      ctx.lineWidth = 1.5;
      ctx.beginPath();
      ctx.moveTo(s.x, s.y);
      ctx.lineTo(s.x - s.vx * 9, s.y - s.vy * 9);
      ctx.stroke();
    }
    ctx.globalAlpha = 1;
  }

  function frame(now) {
    const dt = Math.min(64, now - last);
    last = now;
    for (const s of stars) {
      s.x -= s.z * 0.012 * dt;
      if (s.x < -2) { s.x = w + 2; s.y = Math.random() * h; }
    }
    nextShot -= dt;
    if (nextShot <= 0) {
      nextShot = 7000 + Math.random() * 9000;
      shooters.push({ x: Math.random() * w * 0.8 + w * 0.2, y: Math.random() * h * 0.4, vx: -(6 + Math.random() * 4), vy: 2 + Math.random() * 2, life: 1.4 });
    }
    for (const s of shooters) {
      s.x += s.vx * dt / 16;
      s.y += s.vy * dt / 16;
      s.life -= dt / 900;
    }
    shooters = shooters.filter((s) => s.life > 0);
    draw(now);
    raf = requestAnimationFrame(frame);
  }

  function sync() {
    cancelAnimationFrame(raf);
    if (!reduce.matches) raf = requestAnimationFrame((t) => { last = t; frame(t); });
    else draw(performance.now());
  }

  window.addEventListener('resize', resize);
  reduce.addEventListener('change', sync);
  resize();
  sync();
}