import java.awt.image.BufferedImage;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileWriter;
import java.io.IOException;

import javax.imageio.ImageIO;
import javax.swing.JButton;
import javax.swing.JCheckBox;
import javax.swing.JFileChooser;
import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class ImageBrightnessDetector {
    private JFrame frame = new JFrame("ImageBrightnessDetector");
    private JFileChooser fileChooser = new JFileChooser();
    private JCheckBox checkBox;
    private File[] files;

    public static void main(String[] args) {
        ImageBrightnessDetector ibd = new ImageBrightnessDetector();
        ibd.frame.setVisible(true);
    }

    public ImageBrightnessDetector() {
        try {
            UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        } catch (ClassNotFoundException | InstantiationException | IllegalAccessException
                | UnsupportedLookAndFeelException e) {
            e.printStackTrace();
        }
        initialize();
    }

    private void initialize() {
        frame.setBounds(400, 300, 400, 400);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().setLayout(null);

        checkBox = new JCheckBox("Add File names to output");
        checkBox.setBounds(50, 100, 200, 20);
        checkBox.setVisible(true);
        frame.getContentPane().add(checkBox);

        JButton buttonInput = new JButton("Select Input");
        buttonInput.setBounds(50, 200, 100, 30);
        buttonInput.addActionListener(e -> files = getInputFiles());
        buttonInput.setVisible(true);
        frame.getContentPane().add(buttonInput);

        JButton buttonOutput = new JButton("Save Result");
        buttonOutput.setBounds(250, 200, 100, 30);
        buttonOutput.addActionListener(e -> {
            File outputFile = getOutputFile();
            if (outputFile == null) return;

            int[] avgLuminance = new int[files.length];
            try {
                for (int i = 0; i < files.length; i++)
                    avgLuminance[i] = calcAverageBrightness(ImageIO.read(files[i]));
                writeOutput(outputFile, avgLuminance);
            } catch (IOException exc) {
                exc.printStackTrace();
                JOptionPane.showConfirmDialog(frame, "An error occured: " + exc.getMessage(), "Error", JOptionPane.OK_OPTION, JOptionPane.ERROR_MESSAGE);
                return;
            }
        });
        buttonOutput.setVisible(true);
        frame.getContentPane().add(buttonOutput);
    }

    private void writeOutput(File outputFile, int[] values) throws IOException {
        BufferedWriter writer = new BufferedWriter(new FileWriter(outputFile));
        for (int i = 0; i < values.length; i++) {
            if (checkBox.isSelected())
                writer.write(files[i].getName() + ";" + values[i]);
            else
                writer.write(values[i] + "");
            writer.newLine();
        }
        writer.close();
    }

    private int calcAverageBrightness(BufferedImage img) {
        long totalLuminance = 0;
        for (int color : img.getRGB(0, 0, img.getWidth(), img.getHeight(), null, 0, img.getWidth())) {
            int red   = (color >>> 16) & 0xFF;
            int green = (color >>>  8) & 0xFF;
            int blue  = (color >>>  0) & 0xFF;

            totalLuminance += (red * 0.2126f + green * 0.7152f + blue * 0.0722f) / 255 * 100;
        }
        return (int) (totalLuminance / (img.getWidth() * img.getHeight()));
    }

    private File getOutputFile() {
        fileChooser.setMultiSelectionEnabled(false);
        if (fileChooser.showSaveDialog(frame) == JFileChooser.APPROVE_OPTION)
            return fileChooser.getSelectedFile();
        else return null;
    }

    private File[] getInputFiles() {
        fileChooser.setMultiSelectionEnabled(true);
        if (fileChooser.showOpenDialog(frame) == JFileChooser.APPROVE_OPTION)
            return fileChooser.getSelectedFiles();
        else return new File[0];
    }
}