/**
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program. If not, see <http://www.gnu.org/licenses/>.
 */
import java.awt.GridLayout;
import java.awt.HeadlessException;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JTextField;

/**
 * @author Ing. František Kučera (frantovo.cz)
 */
public class Caesar extends JFrame {

	private JTextField prostýText = new JTextField();
	private JTextField šifrovanýText = new JTextField();
	private JButton šifruj = new JButton("Šifruj");
	private JButton dešifruj = new JButton("Dešifruj");
	private static final char[] ABECEDA_ZNAKY = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".toCharArray();
	private static final List<Character> ABECEDA;
	private static final int SKOK = 3;

	static {
		List<Character> abeceda = new ArrayList<>();
		for (char z : ABECEDA_ZNAKY) {
			abeceda.add(z);
		}
		ABECEDA = Collections.unmodifiableList(abeceda);
	}

	public static void main(String[] args) {
		Caesar c = new Caesar();
		c.setVisible(true);
	}

	public Caesar() throws HeadlessException {
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		setTitle("Caesarova šifra");
		setLocationRelativeTo(null);
		setLayout(new GridLayout(3, 2));
		
		add(new JLabel("Prostý text"));
		add(prostýText);

		add(new JLabel("Šifrovaný text"));
		add(šifrovanýText);

		add(šifruj);
		add(dešifruj);

		šifruj.addActionListener(new ActionListener() {
			@Override
			public void actionPerformed(ActionEvent e) {
				šifrovanýText.setText(posuň(prostýText.getText(), SKOK));
			}
		});

		dešifruj.addActionListener(new ActionListener() {
			@Override
			public void actionPerformed(ActionEvent e) {
				prostýText.setText(posuň(šifrovanýText.getText(), -SKOK));
			}
		});

		pack();
	}

	private static String posuň(String text, int oKolik) {
		StringBuilder posunutý = new StringBuilder(text.length());

		for (char z : text.toCharArray()) {
			posunutý.append(posuň(z, oKolik));
		}

		return posunutý.toString();
	}

	private static char posuň(char znak, int oKolik) {
		int původníPozice = ABECEDA.indexOf(znak);

		if (původníPozice < 0) {
			return znak;
		} else {
			int nováPozice = (původníPozice + oKolik) % ABECEDA_ZNAKY.length;
			nováPozice = nováPozice < 0 ? ABECEDA_ZNAKY.length + nováPozice : nováPozice;
			return ABECEDA.get(nováPozice);
		}
	}
}