(() => {
  const board = document.getElementById("board");
  const ctx = board.getContext("2d");
  const scoreEl = document.getElementById("score");
  const statusEl = document.getElementById("status");
  const restartBtn = document.getElementById("restart");

  const gridSize = 20;
  const cellSize = board.width / gridSize;
  const tickMs = 120;

  const directions = {
    ArrowUp: { x: 0, y: -1 },
    ArrowDown: { x: 0, y: 1 },
    ArrowLeft: { x: -1, y: 0 },
    ArrowRight: { x: 1, y: 0 },
    w: { x: 0, y: -1 },
    s: { x: 0, y: 1 },
    a: { x: -1, y: 0 },
    d: { x: 1, y: 0 },
  };

  function isOpposite(a, b) {
    return a.x + b.x === 0 && a.y + b.y === 0;
  }

  function createInitialState() {
    const start = { x: 9, y: 10 };
    const snake = [start, { x: 8, y: 10 }, { x: 7, y: 10 }];
    const dir = { x: 1, y: 0 };
    return {
      snake,
      dir,
      nextDir: dir,
      food: placeFood(snake),
      score: 0,
      over: false,
    };
  }

  function placeFood(snake) {
    const occupied = new Set(snake.map((seg) => `${seg.x},${seg.y}`));
    const empty = [];
    for (let y = 0; y < gridSize; y += 1) {
      for (let x = 0; x < gridSize; x += 1) {
        const key = `${x},${y}`;
        if (!occupied.has(key)) {
          empty.push({ x, y });
        }
      }
    }
    if (empty.length === 0) return null;
    const pick = empty[Math.floor(Math.random() * empty.length)];
    return pick;
  }

  function nextHead(head, dir) {
    return { x: head.x + dir.x, y: head.y + dir.y };
  }

  function outOfBounds(pos) {
    return pos.x < 0 || pos.x >= gridSize || pos.y < 0 || pos.y >= gridSize;
  }

  function hitSelf(pos, snake) {
    return snake.some((seg) => seg.x === pos.x && seg.y === pos.y);
  }

  function step(state) {
    if (state.over) return state;

    const dir = state.nextDir;
    const head = state.snake[0];
    const next = nextHead(head, dir);
    const ateFood = state.food && next.x === state.food.x && next.y === state.food.y;
    const bodyToCheck = ateFood ? state.snake : state.snake.slice(0, -1);

    if (outOfBounds(next) || hitSelf(next, bodyToCheck)) {
      return { ...state, over: true };
    }
    const nextSnake = [next, ...state.snake];
    if (!ateFood) {
      nextSnake.pop();
    }

    const nextFood = ateFood ? placeFood(nextSnake) : state.food;

    return {
      ...state,
      snake: nextSnake,
      dir,
      nextDir: dir,
      food: nextFood,
      score: state.score + (ateFood ? 1 : 0),
    };
  }

  function drawGrid() {
    ctx.clearRect(0, 0, board.width, board.height);
    ctx.fillStyle = "#fdfdfb";
    ctx.fillRect(0, 0, board.width, board.height);

    ctx.strokeStyle = "#e5e2dc";
    ctx.lineWidth = 1;

    for (let i = 0; i <= gridSize; i += 1) {
      const p = i * cellSize;
      ctx.beginPath();
      ctx.moveTo(p, 0);
      ctx.lineTo(p, board.height);
      ctx.stroke();

      ctx.beginPath();
      ctx.moveTo(0, p);
      ctx.lineTo(board.width, p);
      ctx.stroke();
    }
  }

  function drawSnake(snake) {
    snake.forEach((seg, index) => {
      ctx.fillStyle = index === 0 ? "#2f5d50" : "#4a7a6f";
      ctx.fillRect(seg.x * cellSize + 1, seg.y * cellSize + 1, cellSize - 2, cellSize - 2);
    });
  }

  function drawFood(food) {
    if (!food) return;
    ctx.fillStyle = "#b93b3b";
    ctx.beginPath();
    const cx = food.x * cellSize + cellSize / 2;
    const cy = food.y * cellSize + cellSize / 2;
    ctx.arc(cx, cy, cellSize / 2.6, 0, Math.PI * 2);
    ctx.fill();
  }

  function render(state) {
    drawGrid();
    drawFood(state.food);
    drawSnake(state.snake);

    scoreEl.textContent = String(state.score);
    statusEl.textContent = state.over ? "Game over. Press Restart or Enter." : "";
  }

  let state = createInitialState();
  let intervalId = null;

  function startLoop() {
    if (intervalId) clearInterval(intervalId);
    intervalId = setInterval(() => {
      state = step(state);
      render(state);
    }, tickMs);
  }

  function restart() {
    state = createInitialState();
    render(state);
    startLoop();
  }

  function handleKey(event) {
    const key = event.key;
    if (key === "Enter" && state.over) {
      restart();
      return;
    }
    if (!Object.prototype.hasOwnProperty.call(directions, key)) return;

    const candidate = directions[key];
    if (isOpposite(candidate, state.dir)) return;

    state = { ...state, nextDir: candidate };
  }

  document.addEventListener("keydown", handleKey);
  restartBtn.addEventListener("click", restart);

  render(state);
  startLoop();
})();