import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class CirclePanelDemo {

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> new CirclePanelDemo().createAndShowGUI());
    }

    /* ----------------------------- */
    /* 1. Finestra principale       */
    /* ----------------------------- */
    private void createAndShowGUI() {
        JFrame frame = new JFrame("Circle – click, drag & delete");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLayout(new BorderLayout());

        CirclePanel circlePanel = new CirclePanel();
        frame.add(circlePanel, BorderLayout.CENTER);

        JButton clearBtn = new JButton("Cancella cerchio");
        clearBtn.addActionListener(e -> circlePanel.clearCircle());
        JPanel btnPanel = new JPanel(new FlowLayout(FlowLayout.RIGHT));
        btnPanel.add(clearBtn);
        frame.add(btnPanel, BorderLayout.SOUTH);

        frame.setSize(600, 400);
        frame.setLocationRelativeTo(null);   // centra la finestra
        frame.setVisible(true);
    }

    /* ----------------------------- */
    /* 2. JPanel che disegna il cerchio */
    /* ----------------------------- */
    private static class CirclePanel extends JPanel {

        private final int RADIUS = 30;          // raggio fisso

        private int circleX = -1;              // centro X (-1 → nessun cerchio)
        private int circleY = -1;              // centro Y

        /* variabili per il drag */
        private boolean dragging = false;
        private int offsetX, offsetY;

        CirclePanel() {
            setBackground(Color.WHITE);

            MouseAdapter ma = new MouseAdapter() {

                @Override
                public void mousePressed(MouseEvent e) {
                    int mx = e.getX();
                    int my = e.getY();

                    /* Se il cerchio esiste */
                    if (circleX != -1 && circleY != -1) {

                        /* Se il click è dentro il cerchio → inizia drag */
                        if (isInsideCircle(mx, my)) {
                            dragging = true;
                            offsetX = mx - circleX;
                            offsetY = my - circleY;
                        } else {
                            // Click fuori dal cerchio: non fare nulla
                            dragging = false;
                        }
                    } else {
                        /* Nessun cerchio → creiamo un nuovo */
                        setCircle(mx, my);
                    }
                }

                @Override
                public void mouseReleased(MouseEvent e) {
                    dragging = false;
                }

                @Override
                public void mouseDragged(MouseEvent e) {
                    if (dragging && circleX != -1) {
                        int mx = e.getX();
                        int my = e.getY();

                        setCircle(mx - offsetX, my - offsetY);
                    }
                }
            };

            addMouseListener(ma);
            addMouseMotionListener(ma);
        }

        /* ---------------------------------- */
        /* 3. Utilità                           */
        /* ---------------------------------- */

        /** Verifica se il punto (x,y) è dentro il cerchio */
        private boolean isInsideCircle(int x, int y) {
            if (circleX == -1 || circleY == -1) return false;
            int dx = x - circleX;
            int dy = y - circleY;
            return dx * dx + dy * dy <= RADIUS * RADIUS;
        }

        /** Imposta il centro del cerchio e ridisegna */
        private void setCircle(int x, int y) {
            circleX = x;
            circleY = y;
            repaint();
        }

        /** Cancella il cerchio (se presente) */
        public void clearCircle() {
            circleX = -1;
            circleY = -1;
            repaint();
        }

        /* ---------------------------------- */
        /* 4. Disegno del pannello             */
        /* ---------------------------------- */

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);

            if (circleX == -1 || circleY == -1)
                return;   // nessun cerchio da disegnare

            Graphics2D g2 = (Graphics2D) g.create();
            try {
                g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                        RenderingHints.VALUE_ANTIALIAS_ON);

                int topLeftX = circleX - RADIUS;
                int topLeftY = circleY - RADIUS;

                g2.setColor(Color.RED);
                g2.fillOval(topLeftX, topLeftY, 2 * RADIUS, 2 * RADIUS);

                // bordo bianco
                g2.setStroke(new BasicStroke(2f));
                g2.setColor(Color.WHITE);
                g2.drawOval(topLeftX, topLeftY, 2 * RADIUS, 2 * RADIUS);
            } finally {
                g2.dispose();
            }
        }
    }
}