(() => {
  const {
    Engine,
    Render,
    Runner,
    World,
    Bodies,
    Constraint,
    Events,
    Query,
    Vector,
  } = Matter;

  const config = {
    gravity: 0.78,
    spawnIntervalMs: 720,
    minChain: 3,
    palette: ["#ff595e", "#ffca3a", "#8ac926", "#1982c4", "#6a4c93"],
    ballRadius: 48,
  };

  const sceneEl = document.getElementById("scene-wrapper");
  const activeColorEl = document.getElementById("active-color");
  const chainLenEl = document.getElementById("chain-length");
  const spawnRateEl = document.getElementById("spawn-rate");
  const minLinkEl = document.getElementById("min-link");
  const paletteLegendEl = document.getElementById("palette-legend");
  const scoreEl = document.getElementById("score");
  const gameOverEl = document.getElementById("game-over");
  const finalScoreEl = document.getElementById("final-score");
  const restartBtn = document.getElementById("restart-btn");

  let width = sceneEl.clientWidth;
  let height = sceneEl.clientHeight;

  const engine = Engine.create();
  engine.gravity.y = config.gravity;
  const world = engine.world;

  const render = Render.create({
    element: sceneEl,
    engine,
    options: {
      width,
      height,
      wireframes: false,
      background: "transparent",
      showAngleIndicator: false,
      pixelRatio: window.devicePixelRatio || 1,
    },
  });
  Render.run(render);

  const runner = Runner.create();
  Runner.run(runner, engine);

  // Static boundaries and some obstacles to bounce around.
  let boundaries = [];
  const createBounds = () => {
    boundaries.forEach((b) => World.remove(world, b));
    boundaries = [
      Bodies.rectangle(width / 2, height + 40, width, 80, {
        isStatic: true,
        restitution: 0.8,
      }),
      Bodies.rectangle(-40, height / 2, 80, height * 2, { isStatic: true }),
      Bodies.rectangle(width + 40, height / 2, 80, height * 2, {
        isStatic: true,
      }),
    ];
    boundaries.push(
      Bodies.rectangle(width * 0.25, height * 0.55, 160, 20, {
        isStatic: true,
        angle: -0.3,
      }),
      Bodies.rectangle(width * 0.7, height * 0.4, 220, 24, {
        isStatic: true,
        angle: 0.26,
      }),
    );
    World.add(world, boundaries);
  };
  createBounds();

  const balls = [];
  let spawnTimer = null;
  let score = 0;
  let gameOver = false;

  const chain = {
    active: false,
    color: null,
    bodies: [],
    constraints: [],
    pointer: null,
  };

  const spawnBall = () => {
    if (gameOver) return;
    const color =
      config.palette[Math.floor(Math.random() * config.palette.length)];
    const x = Math.max(
      config.ballRadius + 10,
      Math.min(width - config.ballRadius - 10, Math.random() * width),
    );
    const y = -config.ballRadius * 2;
    const ball = Bodies.circle(x, y, config.ballRadius, {
      restitution: 0.72,
      friction: 0.01,
      frictionAir: 0.015,
      render: {
        fillStyle: color,
        strokeStyle: "#0b1222",
        lineWidth: 2,
      },
    });
    ball.plugin = { color, hasEntered: false, entryCheckId: null };
    balls.push(ball);
    World.add(world, ball);
    ball.plugin.entryCheckId = setTimeout(() => {
      ball.plugin.entryCheckId = null;
      if (gameOver) return;
      if (!ball.plugin.hasEntered && Math.abs(ball.velocity.y) < 0.2) {
        triggerGameOver();
      }
    }, 1500);
  };

  const startSpawner = () => {
    if (spawnTimer) clearInterval(spawnTimer);
    spawnTimer = setInterval(spawnBall, config.spawnIntervalMs);
  };

  const cleanupBall = (ball) => {
    if (ball.plugin && ball.plugin.entryCheckId) {
      clearTimeout(ball.plugin.entryCheckId);
      ball.plugin.entryCheckId = null;
    }
  };

  const triggerGameOver = () => {
    if (gameOver) return;
    gameOver = true;
    resetChainVisuals();
    if (spawnTimer) clearInterval(spawnTimer);
    finalScoreEl.textContent = score;
    gameOverEl.classList.add("visible");
  };

  const restartGame = () => {
    gameOver = false;
    score = 0;
    resetChainVisuals();
    balls.forEach((ball) => {
      cleanupBall(ball);
      World.remove(world, ball);
    });
    balls.length = 0;
    gameOverEl.classList.remove("visible");
    updateHud();
    startSpawner();
  };

  const setHighlight = (body, on) => {
    body.render.lineWidth = on ? 4 : 2;
    body.render.strokeStyle = on ? "#f8fafc" : "#0b1222";
  };

  const resetChainVisuals = () => {
    chain.bodies.forEach((b) => setHighlight(b, false));
    chain.constraints.forEach((c) => World.remove(world, c));
    chain.active = false;
    chain.color = null;
    chain.bodies = [];
    chain.constraints = [];
    chain.pointer = null;
    updateHud();
  };

  const removeLastFromChain = () => {
    const removedConstraint = chain.constraints.pop();
    if (removedConstraint) {
      World.remove(world, removedConstraint);
    }
    const removedBody = chain.bodies.pop();
    if (removedBody) setHighlight(removedBody, false);
    updateHud();
  };

  const addToChain = (body) => {
    const last = chain.bodies[chain.bodies.length - 1];
    const dist = Vector.magnitude(Vector.sub(last.position, body.position));
    const constraint = Constraint.create({
      bodyA: last,
      bodyB: body,
      length: dist,
      stiffness: 0.9,
      render: {
        strokeStyle: chain.color,
        lineWidth: 3,
      },
    });
    chain.constraints.push(constraint);
    chain.bodies.push(body);
    setHighlight(body, true);
    World.add(world, constraint);
    updateHud();
  };

  const spawnScorePopup = (point, amount, color) => {
    if (!point) return;
    const el = document.createElement("div");
    el.className = "floating-score";
    el.textContent = `+${amount}`;
    el.style.left = `${point.x}px`;
    el.style.top = `${point.y}px`;
    el.style.color = color || "#e0f2fe";
    sceneEl.appendChild(el);
    setTimeout(() => el.remove(), 950);
  };

  const finishChain = (releasePoint) => {
    if (!chain.active || gameOver) return;
    if (chain.bodies.length >= config.minChain) {
      const gain = 10 * Math.pow(chain.bodies.length, 2);
      score += gain;
      spawnScorePopup(releasePoint || chain.pointer, gain, chain.color);
      chain.constraints.forEach((c) => World.remove(world, c));
      chain.bodies.forEach((body) => {
        cleanupBall(body);
        World.remove(world, body);
      });
      // Remove cleared balls from tracking list.
      for (let i = balls.length - 1; i >= 0; i -= 1) {
        if (chain.bodies.includes(balls[i])) {
          balls.splice(i, 1);
        }
      }
    } else {
      chain.constraints.forEach((c) => World.remove(world, c));
      chain.bodies.forEach((b) => setHighlight(b, false));
    }
    chain.active = false;
    chain.color = null;
    chain.bodies = [];
    chain.constraints = [];
    chain.pointer = null;
    updateHud();
  };

  const pickBody = (point) => {
    const hits = Query.point(balls, point);
    return hits[0];
  };

  const getPointerPosition = (evt) => {
    const rect = render.canvas.getBoundingClientRect();
    const clientX = evt.touches ? evt.touches[0].clientX : evt.clientX;
    const clientY = evt.touches ? evt.touches[0].clientY : evt.clientY;
    return {
      x: clientX - rect.left,
      y: clientY - rect.top,
    };
  };

  const handlePointerDown = (evt) => {
    if (gameOver) return;
    const point = getPointerPosition(evt);
    const body = pickBody(point);
    if (!body) return;
    chain.active = true;
    chain.color = body.plugin.color;
    chain.bodies = [body];
    chain.constraints = [];
    chain.pointer = point;
    setHighlight(body, true);
    updateHud();
  };

  const handlePointerMove = (evt) => {
    if (!chain.active) return;
    if (gameOver) return;
    const point = getPointerPosition(evt);
    chain.pointer = point;
    const body = pickBody(point);
    if (!body) return;
    const alreadyInChain = chain.bodies.includes(body);
    if (alreadyInChain) {
      const targetIndex = chain.bodies.indexOf(body);
      if (chain.bodies.length > 1 && targetIndex === chain.bodies.length - 2) {
        removeLastFromChain();
      }
      return;
    }
    if (body.plugin.color !== chain.color) return;
    addToChain(body);
  };

  const handlePointerUp = () => finishChain(chain.pointer);

  render.canvas.addEventListener("mousedown", handlePointerDown);
  render.canvas.addEventListener("mousemove", handlePointerMove);
  window.addEventListener("mouseup", handlePointerUp);
  render.canvas.addEventListener(
    "touchstart",
    (e) => {
      e.preventDefault();
      handlePointerDown(e);
    },
    { passive: false },
  );
  render.canvas.addEventListener(
    "touchmove",
    (e) => {
      e.preventDefault();
      handlePointerMove(e);
    },
    { passive: false },
  );
  render.canvas.addEventListener(
    "touchend",
    (e) => {
      e.preventDefault();
      handlePointerUp(e);
    },
    { passive: false },
  );

  const updateHud = () => {
    spawnRateEl.textContent = `${config.spawnIntervalMs} ms`;
    minLinkEl.textContent = config.minChain;
    chainLenEl.textContent = chain.bodies.length;
    scoreEl.textContent = score;
    if (chain.color) {
      activeColorEl.textContent = "";
      activeColorEl.style.display = "inline-block";
      activeColorEl.style.width = "14px";
      activeColorEl.style.height = "14px";
      activeColorEl.style.borderRadius = "50%";
      activeColorEl.style.background = chain.color;
      activeColorEl.style.border = "1px solid rgba(255,255,255,0.3)";
    } else {
      activeColorEl.removeAttribute("style");
      activeColorEl.textContent = "—";
    }
  };

  const buildLegend = () => {
    paletteLegendEl.innerHTML = "";
    config.palette.forEach((color) => {
      const swatch = document.createElement("span");
      swatch.style.background = color;
      paletteLegendEl.appendChild(swatch);
    });
  };

  const handleResize = () => {
    width = sceneEl.clientWidth;
    height = sceneEl.clientHeight;
    render.canvas.width = width;
    render.canvas.height = height;
    render.options.width = width;
    render.options.height = height;
    Render.lookAt(render, {
      min: { x: 0, y: 0 },
      max: { x: width, y: height },
    });
    createBounds();
  };

  Events.on(engine, "afterUpdate", () => {
    // Keep stray balls within the play area horizontally.
    balls.forEach((ball) => {
      if (
        !ball.plugin.hasEntered &&
        ball.position.y > config.ballRadius * 1.5
      ) {
        ball.plugin.hasEntered = true;
      }
      if (
        ball.position.x < -100 ||
        ball.position.x > width + 100 ||
        ball.position.y > height + 500
      ) {
        cleanupBall(ball);
        ball.plugin.hasEntered = true;
        Matter.Body.setPosition(ball, { x: Math.random() * width, y: -40 });
        Matter.Body.setVelocity(ball, { x: 0, y: 0 });
      }
    });
  });

  Events.on(render, "afterRender", () => {
    if (!chain.active || !chain.pointer || chain.bodies.length === 0) return;
    const last = chain.bodies[chain.bodies.length - 1];
    const ctx = render.context;
    ctx.save();
    ctx.strokeStyle = chain.color || "#fff";
    ctx.lineWidth = 3;
    ctx.setLineDash([6, 6]);
    ctx.beginPath();
    ctx.moveTo(last.position.x, last.position.y);
    ctx.lineTo(chain.pointer.x, chain.pointer.y);
    ctx.stroke();
    ctx.restore();
  });

  window.addEventListener("resize", handleResize);
  restartBtn.addEventListener("click", restartGame);
  buildLegend();
  updateHud();
  startSpawner();
})();