const SIZE = 4;
const boardDiv = document.getElementById('board');
const statusDiv = document.getElementById('status');
const restartBtn = document.getElementById('restart-btn');
let board, won;

function initGame() {
  board = [];
  won = false;
  let numbers = Array.from({length: SIZE*SIZE}, (_, i) => i);
  do {
    shuffle(numbers);
  } while(!isSolvable(numbers));
  // Llenar el tablero
  for (let r=0; r<SIZE; r++) {
    let row = [];
    for (let c=0; c<SIZE; c++)
      row.push(numbers[r*SIZE + c]);
    board.push(row);
  }
  statusDiv.textContent = "Ordena las fichas del 1 al 15";
  renderBoard();
}

function shuffle(array) {
  for (let i = array.length - 1; i > 0; i--) {
    const j = Math.floor(Math.random() * (i + 1));
    [array[i], array[j]] = [array[j], array[i]];
  }
}

// Verifica si el puzzle es resoluble (paridad)
function isSolvable(arr) {
  let invCount = 0;
  for (let i = 0; i < arr.length; i++) {
    if (arr[i] === 0) continue;
    for (let j = i+1; j < arr.length; j++) {
      if (arr[j] !== 0 && arr[i] > arr[j]) invCount++;
    }
  }
  let emptyRow = Math.floor(arr.indexOf(0) / SIZE);
  return (invCount + emptyRow) % 2 === 0;
}

function renderBoard() {
  boardDiv.innerHTML = '';
  for (let r = 0; r < SIZE; r++) {
    for (let c = 0; c < SIZE; c++) {
      const val = board[r][c];
      const tile = document.createElement('div');
      tile.className = 'tile' + (val === 0 ? ' empty' : '');
      tile.textContent = val === 0 ? '' : val;
      if (val !== 0 && !won) {
        tile.onclick = () => moveTile(r, c);
      }
      boardDiv.appendChild(tile);
    }
  }
  if (checkWin() && !won) {
    won = true;
    statusDiv.textContent = "¡Felicidades! Puzzle resuelto 🎉";
  }
}

function moveTile(r, c) {
  let neighbours = [
    [r-1, c],
    [r+1, c],
    [r, c-1],
    [r, c+1]
  ];
  for (let [nr, nc] of neighbours) {
    if (nr >=0 && nr < SIZE && nc >=0 && nc < SIZE && board[nr][nc] === 0) {
      // Intercambia
      [board[nr][nc], board[r][c]] = [board[r][c], board[nr][nc]];
      renderBoard();
      return;
    }
  }
}

function checkWin() {
  let n = 1;
  for (let r = 0; r < SIZE; r++) {
    for (let c = 0; c < SIZE; c++) {
      if (r === SIZE-1 && c === SIZE-1) {
        if (board[r][c] !== 0) return false;
      } else {
        if (board[r][c] !== n++) return false;
      }
    }
  }
  return true;
}

restartBtn.onclick = initGame;
initGame();