Game of Fifteen Tutorial on the SSaurel's Channel

GameOfFifteen.java

private void newGame() {
  do {
    reset(); // reset in intial state
    shuffle(); // shuffle
  } while(!isSolvable()); // make it until grid be solvable

  gameOver = false;
}

private void reset() {
  for (int i = 0; i < tiles.length; i++) {
    tiles[i] = (i + 1) % tiles.length;
  }

  // we set blank cell at the last
  blankPos = tiles.length - 1;
}

private void shuffle() {
  // don't include the blank tile in the shuffle, leave in the solved position
  int n = nbTiles;

  while (n > 1) {
    int r = RANDOM.nextInt(n--);
    int tmp = tiles[r];
    tiles[r] = tiles[n];
    tiles[n] = tmp;
  }
}

// Only half permutations o the puzzle are solvable
// Whenever a tile is preceded by a tile with higher value it counts
// as an inversion. In our case, with the blank tile in the solved position,
// the number of inversions must be even for the puzzle to be solvable
private boolean isSolvable() {
  int countInversions = 0;

  for (int i = 0; i < nbTiles; i++) {
    for (int j = 0; j < i; j++) {
      if (tiles[j] > tiles[i])
        countInversions++;
    }
  }

  return countInversions % 2 == 0;
}

private boolean isSolved() {
  if (tiles[tiles.length - 1] != 0) // if blank tile is not in the solved position ==> not solved
    return false;

  for (int i = nbTiles - 1; i >= 0; i--) {
    if (tiles[i] != i + 1)
      return false;			
  }

  return true;
}