Math Is Fun Forum

package com.circle.main;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionAdapter;

import javax.swing.JFrame;
import javax.swing.SwingUtilities;

public class Main {

	public static class Vector2f {

		float x, y;

		public static Vector2f sub(Vector2f v1, Vector2f v2) {
			return new Vector2f(v1.x - v2.x, v1.y - v2.y);
		}

		public static float distance(Vector2f v1, Vector2f v2) {
			float x = v1.x - v2.x;
			float y = v1.y - v2.y;
			return (float) Math.sqrt(x * x + y * y);
		}

		public Vector2f(float x, float y) {
			this.x = x;
			this.y = y;
		}
	}

	public static class Circle {

		public Vector2f position;
		public float radius;
		public int drawMode;

		public Circle(float x, float y, float radius) {
			position = new Vector2f(x, y);
			this.radius = radius;
			this.drawMode = 0;
		}

		public boolean isInside(float x, float y) {
			return Vector2f.distance(position, new Vector2f(x, y)) < radius;
		}

		public void draw(Graphics g) {
			if (drawMode == 0) {
				g.drawOval((int) (position.x - radius), (int) (position.y - radius), (int) (radius * 2),(int) (radius * 2));
			} else {
				g.fillOval((int) (position.x - radius),	(int) (position.y - radius), (int) (radius * 2),(int) (radius * 2));
			}
		}
	}

	public static class Result {
		public Vector2f v1, v2;

		public Result(Vector2f v1, Vector2f v2) {
			this.v1 = v1;
			this.v2 = v2;
		}
	}

	public static class MainFrame extends JFrame {

		public Circle cp1, cp2, circle;

		public MainFrame() {
			cp1 = new Circle(100, 100, 10);
			cp2 = new Circle(200, 200, 10);
			circle = new Circle(400, 300, 100);

			cp1.drawMode = cp2.drawMode = 1;

			addMouseMotionListener(new MouseMotionAdapter() {
				public void mouseDragged(MouseEvent evt) {
					int x = evt.getX();
					int y = evt.getY();
					if (cp1.isInside(x, y)) {
						cp1.position.x = x;
						cp1.position.y = y;
						MainFrame.this.repaint();
					}
					if (cp2.isInside(x, y)) {
						cp2.position.x = x;
						cp2.position.y = y;
						MainFrame.this.repaint();
					}
				}
			});
		}

		public Result intersectionTest() {
			Vector2f p1 = cp1.position;
			Vector2f p2 = cp2.position;

			Vector2f p = Vector2f.sub(p1, p2);

			float a = circle.position.x;
			float b = circle.position.y;
			float r = circle.radius;

			float m = p.y / p.x;
			float c = p1.y - m * p1.x;

			float aa = m * m + 1;
			float bb = -2 * a + 2 * m * c - 2 * m * b;
			float cc = a * a + c * c - 2 * b * c + b * b - r * r;

			float d = bb * bb - 4 * aa * cc;

			float x1 = (-bb + (float) Math.sqrt(d)) / (2 * aa);
			float x2 = (-bb - (float) Math.sqrt(d)) / (2 * aa);

			aa = 1;
			bb = -2 * b;
			cc = x1 * x1 - 2 * a * x1 + a * a + b * b - r * r;

			d = bb * bb - 4 * aa * cc;
			float y1 = (-bb + (float) Math.sqrt(d)) / (2 * aa);

			cc = x2 * x2 - 2 * a * x2 + a * a + b * b - r * r;

			d = bb * bb - 4 * aa * cc;
			float y2 = (-bb + (float) Math.sqrt(d)) / (2 * aa);

			return new Result(new Vector2f(x1, y1), new Vector2f(x2, y2));
		}

		public void paint(Graphics g) {
			Graphics2D g2 = (Graphics2D) g;

			super.paint(g2);

			cp1.draw(g2);
			cp2.draw(g2);
			g2.drawLine((int) cp1.position.x, (int) cp1.position.y,	(int) cp2.position.x, (int) cp2.position.y);
			circle.draw(g2);

			Result r = intersectionTest();

			Color c = g2.getColor();
			g2.setColor(Color.RED);
			g2.fillRect((int) r.v1.x - 2, (int) r.v1.y - 2, 4, 4);
			g2.fillRect((int) r.v2.x - 2, (int) r.v2.y - 2, 4, 4);
			g2.setColor(c);
		}
	}

	public static void main(String args[]) {
		final MainFrame frame = new MainFrame();
		frame.setSize(800, 600);
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		SwingUtilities.invokeLater(new Runnable() {
			public void run() {
				frame.setVisible(true);
			}
		});
	}
}