// Calculator.java
// p. 673 exercise
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class Calculator extends JApplet implements ActionListener {

    private JTextField displayField;
    private JButton buttons[];
    private BorderLayout layout;
    private JPanel buttonPanel;
    private double sum;    
    private double val1;    
    private char pendingOp;
    private boolean newNumStart;
    private boolean gettingSecondNum;

    public void init() {
	String[] buttonNames = {"7","8","9","/",
				"4","5","6","*",
				"1","2","3","-",
				"0",".","=","+"};
	
      layout = new BorderLayout( 5, 5 ); // 5 pixel gaps

      // get content pane and set its layout
      Container container = getContentPane();
      container.setLayout( layout );

      displayField = new JTextField( "", 20 );
      displayField.setFont( new Font ("Courier",Font.BOLD,20) );
      displayField.setEditable( false );
      //actionListener
      container.add( displayField, BorderLayout.NORTH );

      buttons = new JButton[ 16 ];
      // set up panel and set its layout
      buttonPanel = new JPanel();
      buttonPanel.setLayout( new GridLayout( 4, 4, 3, 3 ) );
      // create and add buttons
      for ( int i = 0; i < buttons.length; i++ ) {
	  buttons[ i ] = new JButton( buttonNames[ i ] );
	  buttons[i].setFont( new Font ("Courier",Font.BOLD,30) );
	  buttonPanel.add( buttons[ i ] );
	  buttons[ i ].addActionListener( this );
      }
      toggleButton( '=', false );  //disable at start of next number
      toggleButton( '+', false );  //disable
      toggleButton( '-', false );  //disable
      toggleButton( '*', false );  //disable
      toggleButton( '/', false );  //disable
      container.add( buttonPanel, BorderLayout.CENTER );

      newNumStart = true;
      gettingSecondNum = false;
   } 

   // handle button events
   public void actionPerformed( ActionEvent event )
   {
       char ch;
       ch = event.getActionCommand().charAt(0);

       if (Character.isDigit(ch)) {
	   if ( newNumStart ) {
	       displayField.setText( "" );
	       newNumStart = false;
	   }
	   //redundantly, but lets .d work
	   if ( gettingSecondNum ) {
	       toggleButton( '=', true );  
	       toggleButton( '+', false );  
	       toggleButton( '-', false );  
	       toggleButton( '*', false  );  
	       toggleButton( '/', false  );  
	   }
	   else {
	       toggleButton( '+', true );  
	       toggleButton( '-', true );  
	       toggleButton( '*', true );  
	       toggleButton( '/', true );  
	   }
	   displayField.setText(displayField.getText()+event.getActionCommand());

       }
       else if (ch == '=') {  //do current operation

	   double val2 = Double.parseDouble(displayField.getText());

	   sum = val1;   //change this later to allow running
	   switch (pendingOp) {
	   case '+' : sum += val2;
	       break;
	   case '-' : sum -= val2;
	       break;
	   case '*' : sum *= val2;
	       break;
	   case '/' : sum /= val2;
	       break;
	   default:
	   }
	   displayField.setText( "" + sum );
	   newNumStart = true;
	   gettingSecondNum = false;
	   toggleButton( '=', false );  //disable at start of next number
	   toggleButton( '+', false );  //disable
	   toggleButton( '-', false );  //disable
	   toggleButton( '*', false );  //disable
	   toggleButton( '/', false );  //disable

	   toggleButton( '.', true );  //enable it
       }
       else if (ch == '.') {    //check if not already decimal point
	   toggleButton( '.', false );  //disable it
	   if ( newNumStart ) {
	       displayField.setText( "" );
	       newNumStart = false;
	   }
	   displayField.setText(displayField.getText() + ".");
       }

       else {    //operation.
	   pendingOp = ch;
	   val1 = Double.parseDouble(displayField.getText());
	   gettingSecondNum = true;

	   displayField.setText( "" );
	   newNumStart = true;

	   toggleButton( '+', false );  //disable
	   toggleButton( '-', false );  //disable
	   toggleButton( '*', false );  //disable
	   toggleButton( '/', false );  //disable

	   toggleButton( '.', true );  //enable it

       }
	    
   }

    private void toggleButton( char button, boolean enableDisable ) {
	//warning: index will have to change if buttons change
	switch (button) {
	case '+' : buttons[15].setEnabled(enableDisable);
	    break;
	case '-' : buttons[11].setEnabled(enableDisable);
	    break;
	case '*' : buttons[7].setEnabled(enableDisable);
	    break;
	case '/' : buttons[3].setEnabled(enableDisable);
	    break;
	case '=' : buttons[14].setEnabled(enableDisable);
	    break;
	case '.' : buttons[13].setEnabled(enableDisable);
	    break;
	default:
	}
	
    }

}