package v2ch11.calculator;

import module java.desktop;

/**
 * A panel with calculator buttons and a result display.
 */
public class CalculatorPanel extends JPanel {
    private JButton display;
    private JPanel panel;
    private double result;
    private String lastCommand;
    private boolean start;

    public CalculatorPanel() {
        setLayout(new BorderLayout());

        result = 0;
        lastCommand = "=";
        start = true;

        // add the display

        display = new JButton("0");
        display.setEnabled(false);
        add(display, BorderLayout.NORTH);

        var insert = new InsertAction();
        var command = new CommandAction();

        // add the buttons in a 4 x 4 grid

        panel = new JPanel();
        panel.setLayout(new GridLayout(4, 4));

        addButton("7", insert);
        addButton("8", insert);
        addButton("9", insert);
        addButton("/", command);

        addButton("4", insert);
        addButton("5", insert);
        addButton("6", insert);
        addButton("*", command);

        addButton("1", insert);
        addButton("2", insert);
        addButton("3", insert);
        addButton("-", command);

        addButton("0", insert);
        addButton(".", insert);
        addButton("=", command);
        addButton("+", command);

        add(panel, BorderLayout.CENTER);
    }

    /**
     * Adds a button to the center panel.
     * @param label the button label
     * @param listener the button listener
     */
    private void addButton(String label, ActionListener listener) {
        var button = new JButton(label);
        button.addActionListener(listener);
        panel.add(button);
    }

    /**
     * This action inserts the button action string to the end of the display text.
     */
    private class InsertAction implements ActionListener {
        public void actionPerformed(ActionEvent event) {
            String input = event.getActionCommand();
            if (start) {
                display.setText("");
                start = false;
            }
            display.setText(display.getText() + input);
        }
    }

    /**
     * This action executes the command that the button action string denotes.
     */
    private class CommandAction implements ActionListener {
        public void actionPerformed(ActionEvent event) {
            String command = event.getActionCommand();

            if (start) {
                if (command.equals("-")) {
                    display.setText(command);
                    start = false;
                }
                else
                    lastCommand = command;
            }
            else {
                calculate(Double.parseDouble(display.getText()));
                lastCommand = command;
                start = true;
            }
        }
    }

    /**
     * Carries out the pending calculation.
     * @param x the value to be accumulated with the prior result.
     */
    public void calculate(double x) {
        if (lastCommand.equals("+"))
            result += x;
        else if (lastCommand.equals("-"))
            result -= x;
        else if (lastCommand.equals("*"))
            result *= x;
        else if (lastCommand.equals("/"))
            result /= x;
        else if (lastCommand.equals("=")) result = x;
        display.setText("" + result);
    }
}
