15 puzzle - OpenProcessing

SlidingPuzzle
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int n = 4; // size of the puzzle
float pieceSize = 730 / n;
Piece[] pieces = new Piece[n][n];
float xOffset = (width - (n * pieceSize)) / 2;
float yOffset = (height - (n * pieceSize)) / 2;
EmptyPiece emptyPiece = new EmptyPiece(n);
​
void setup() {
  fullscreen();
  textAlign(CENTER, CENTER);
  textSize(pieceSize / 3.2);
  for(int y = 0; y < pieces.length; y++) {
    for(int x = 0; x < pieces[y].length; x++) {
      if((x == y) && (x == (n-1))) {
        pieces[x][y] = emptyPiece;
      } else {
        pieces[x][y] = new Piece(x, y);
      }
    }
  }
  xOffset = (width - (n * pieceSize)) / 2;
  yOffset = (height - (n * pieceSize)) / 2;
}
​
void draw() {
  background(255);
  for(int y = 0; y < pieces.length; y++) {
    for(int x = 0; x < pieces[y].length; x++) {
      pieces[x][y].draw(x, y);
    }
  }
}
​
void mouseMoved() {
  int x = int(floor((mouseX - xOffset) / pieceSize));
  int y = int(floor((mouseY - yOffset) / pieceSize));
  if((x >= 0) && (x < n) && (y >= 0) && (y < n)) {
    int xDiff = abs(x - emptyPiece.x);
    int yDiff = abs(y - emptyPiece.y);
    if(((xDiff == 1) && (yDiff == 0)) || ((xDiff == 0) && (yDiff == 1))) {
      pieces[x][y].move();
    }
  }
}
​
class Piece {
  final int initialX, initialY;
  int x, y;
  String name;
  Color drawColor;
  
  Piece(int x, int y) {
    this.initialX = x;
    this.initialY = y;
    this.x = x;
    this.y = y;
    this.name = x + (y * n) + 1;
    this.drawColor = color(255 / n * x, 255 / n * y, 200);
  }
  
  void draw(int x, int y) {
    float xCoord = xOffset + (x * pieceSize);
    float yCoord = yOffset + (y * pieceSize);
    fill(drawColor);
    rect(xCoord + 5, yCoord + 5, pieceSize - 5, pieceSize - 5);
    fill(0);
    text(name, xCoord + 5, yCoord + 5, pieceSize - 5, pieceSize - 5);
  }
  
  void move() {
    int emptyX = emptyPiece.x;
    int emptyY = emptyPiece.y;
    
    // move empty piece
    pieces[x][y] = emptyPiece;
    emptyPiece.x = x;
    emptyPiece.y = y;
    
    // move this piece
    pieces[emptyX][emptyY] = this;
    x = emptyX;
    y = emptyY;
  }
}
​
class EmptyPiece extends Piece {
  EmptyPiece(int n) {
    super(n - 1, n - 1);
  }
  
  void draw(int x, int y) { }
}