// Program to draw the Sierpinski Triangle fractal to a given depth

import java.awt.*;
import java.util.*;

public class Sierpinski {
   public static final int PANEL_SIZE = 600;

   public static void main(String[] args) {
      // set up DrawingPanel
      DrawingPanel panel = new DrawingPanel(PANEL_SIZE, PANEL_SIZE);
      panel.setBackground(Color.CYAN);
      Graphics g = panel.getGraphics();
      
      // prompt for depth
      Scanner console = new Scanner(System.in);
      System.out.print("Depth? ");
      int depth = console.nextInt();
      
      // calculate triangle height and begin drawing
      int triHeight = (int)Math.round(Math.sqrt(3.0) * PANEL_SIZE / 2.0);
      sierpinski(new Point(PANEL_SIZE / 2, PANEL_SIZE - triHeight),
                 new Point(0, PANEL_SIZE),
                 new Point(PANEL_SIZE, PANEL_SIZE),
                 depth, g);
   }
   
   // pre: depth >= 1
   //      g is a valid Graphics object from a DrawingPanel
   //
   // draws a sierpinski triangle to the given depth inside a triangle
   //   with vertices at p1, p2, and p3
   public static void sierpinski(Point p1, Point p2, Point p3,
                                 int depth, Graphics g) {
      if (depth == 1) {
         // base case - draw a triangle
         drawTriangle(p1, p2, p3, g);
      } else {
         // recursive case - calculate the midpoints of the sides and draw
         //   three smaller triangles
         Point mid12 = midpoint(p1, p2);
         Point mid23 = midpoint(p2, p3);
         Point mid31 = midpoint(p3, p1);
         
         sierpinski(p1, mid12, mid31, depth - 1, g);
         sierpinski(mid12, p2, mid23, depth - 1, g);
         sierpinski(mid31, mid23, p3, depth - 1, g);
      }
   }
   
   // pre: g is a valid Graphics object from a DrawingPanel
   //
   // draws a triangle with vertices at p1, p2, and p3
   private static void drawTriangle(Point p1, Point p2, Point p3, Graphics g) {
      Polygon tri = new Polygon();
      tri.addPoint(p1.getX(), p1.getY());
      tri.addPoint(p2.getX(), p2.getY());
      tri.addPoint(p3.getX(), p3.getY());
      g.drawPolygon(tri);
   }
   
   // returns the midpoint of p1 and p2
   private static Point midpoint(Point p1, Point p2) {
      return new Point((p1.getX() + p2.getX()) / 2, (p1.getY() + p2.getY()) / 2);
   }
}