//=================================================================================
// File MCalc.java jlouie 6.96 73270.1776@compuserve.com
// This file should be named MCalc.java and compiled with 'javac'/'java compiler'
// Title  * MCalc v1.6 *
// Free if distributed as is with a functioning and unaltered about box.
// Thanks to W. Carlini, T. Ball, R. Scorer, W. Brogden & the folks at Java Support.
// Creates an GUI AWT Mortgage Calculator
// Compiled with:	Sun's Mac JDK 1.02/ System 7.5.2
// Tested with: 	Mac Netscape 2.0javab1 and 3.0b4
// 					Sun's MacJDK Applet Viewer
//
// Known bugs and incompatabilities:
// This Applet does not work with Netscape 3.0b4 (Mac), see workaround in MCalc.init()
// Dialog boxes and Frame menus are not supported by Netscape 2.0jb1(Mac)
// Packed Dialog boxes do not display the last item under Netscape 3.0b4 (Mac)
// Modal Dialog boxes are non_modal under Netscape 2.01 (Win95)
// Key filter isDigit() is buggy with Netscape 3.0b4 (Win95)
// 
// Features not implemented:
// Responding to keyboard equivalents.
// Formatting numeric output (see Format() in "Core Java").
// Online help
//
// "New" Features: 	Corrects an error in MyTextField initialization
//					"Supports" key filters in MyTextField	
//					Utilizes Font() class.
//
// Ref: "Teach Yourself Java in 21 Days", Lemay & Perkins, Sams Net
// Ref: "Java in a Nutshell", David Flanagan, O'Reilly & Associates
// Ref: "Core Java", Cornell & Horstmann, Sunsoft Press
// Ref: "The Java Programming Language" Arnold & Gosling Addison Wesley
// General Ref: "Code Complete" Steve McConnell, Microsoft Press
//=================================================================================

import java.awt.*;

//=====================================================================
//
// Class * MCalc *
//
// Main applet class.  Adds a button to a web page.
//
//======================================================================

public class MCalc extends java.applet.Applet
{	
	Button newCalc;
		
	public void init()  // add a button to the web page
	{
		//NewCalc();  // workaround Mac Netscape 3.0b4
		add(new Label("Mortgage Calculator:"));
		add(newCalc= new Button("New Calculator"));
	}
	
	public void NewCalc()
	{
			new MyFrame("MCalc");  
	}
		
	public boolean action(Event evt, Object obj) 
	{	
		if (evt.target == newCalc)
		{
				NewCalc();
				return true;
		}
		return super.action(evt, obj);	
	}
}


//============================================================================
//
// Class * MyFrame *
//
// Calculator Input/Output Interface
//
//==========================================================================

class MyFrame extends Frame
{
	MyTextField[] 	textArray;  			// Array of four extended text boxes for input/output
	Label			messageBox; 			// Calculator field for user feedback
	MyAbout 		dl;						// About Dialog Box
	MyError			err;					// Error Dialog Box
	double 			dbArray[], dbResult= 0;	// Array of double, stores numeric input
	Point			p;						// Stores location window				
	
	static final int NO_KEY_FILTER= 0;			// keyFilter Constants
	static final int NUMERIC_ONLY= 	1;
	static final int DIGITS_ONLY= 	2;
	
	
	MyFrame(String title)				//  Main Window Constructor
	{
		super(title);
		dbArray= new double[4];
		textArray= new MyTextField[4];
		setLayout(new GridLayout(6,2,10,10));
		add(new Label("A Mortgage Calculator", Label.RIGHT));
		messageBox= new Label("One Entry Must Be 0", Label.LEFT);
		add(messageBox);
		add(new Label("Principle: $", Label.RIGHT));
		textArray[0]= new MyTextField("",20, NUMERIC_ONLY);
		add(textArray[0]);
		add(new Label("Annual Interest (0-100): %", Label.RIGHT));
		textArray[1]= new MyTextField("",20, NUMERIC_ONLY);
		add(textArray[1]);
		add(new Label("Loan Period (Months): ", Label.RIGHT));
		textArray[2]= new MyTextField("",20, DIGITS_ONLY);
		add(textArray[2]);
		add(new Label("Monthly Payments: $", Label.RIGHT));
		textArray[3]= new MyTextField("0",20, NUMERIC_ONLY);
		add(textArray[3]);
		
		// we must instance MyTextFields _before_ this assignment
		textArray[0].nextField = textArray[1];
		textArray[0].prevField = textArray[3];
		textArray[1].nextField = textArray[2];
		textArray[1].prevField = textArray[0];
		textArray[2].nextField = textArray[3];
		textArray[2].prevField = textArray[1];
		textArray[3].nextField = textArray[0];
		textArray[3].prevField = textArray[2];
		Button OKbutton= new Button("OK");
		add(OKbutton);
		add(new Button("Cancel"));
		MenuBar mb= new MenuBar();
		Menu m= new Menu("File");
		m.add(new MenuItem("About MCalc..."));
		m.add(new MenuItem("Close"));
		mb.add(m);
		setMenuBar(mb);	
		p= location();
		this.move(p.x + 20, p.y + 20);
		this.resize(400,400); 	// workaround 
		this.pack();  			// workaround Win95 & Netscape
		Font f= OKbutton.getFont();
		f= new Font(f.getName(), Font.BOLD, f.getSize());
		OKbutton.setFont(f);
		OKbutton.setBackground(Color.white);
		f= null;
		this.setBackground(Color.lightGray);
		this.show();
		textArray[0].requestFocus();
	}
	
	//==================================
	// * ParseString *
	//
	// Strips the input string of commas
	//
	// Use a StringBuffer to avoid multiple 
	// memory allocations
	//
	//==================================
	
	String ParseString(String inString)
	{
		StringBuffer	outString= new StringBuffer(inString.length());

		for (int i=0; i<inString.length(); i++)
		{
			if (inString.charAt(i) != ',')
				{outString.append(inString.charAt(i));}
		}
		return outString.toString();
	}
	
	//==============================
	//
	// * handleEvent *
	//
	// Each interactive panel must handle it's events.
	// Trapping tab here may not be necessary
	// Call super.handleEvent() to call this panels action() method
	//
	//==============================
	
	public boolean handleEvent(Event event) // MyFrame
	{
		MyTextField theField;
		
		switch (event.id)
		{
			case Event.WINDOW_DESTROY:
				if (this.isShowing()) this.hide();
				this.dispose();
				return true;
			
			case Event.KEY_PRESS:  // ? not necessary if MyTextField properly initialized
        		if ((event.target instanceof MyTextField) && (event.key == '\t'))
        		{
					for (int i=0; i<(textArray.length); i++)
					{
						if (event.target == textArray[textArray.length-1])
						{
							textArray[0].requestFocus();
							textArray[0].selectAll();
							break;
						}
						if (event.target == textArray[i])
						{
							textArray[i+1].requestFocus();
							textArray[i+1].selectAll();
							break;
						}
					}
				}
			
		}
		return super.handleEvent(event);  // calls action
	}

	//==============================
	//
	// * action *
	//
	// called by default handleEvent()
	// trapping return key here, workaround
	//
	//==============================
				
	public boolean action(Event evt, Object obj)  // MyFrame
	{
		char ch;
		String str= (String)obj;
		
		if (evt.target instanceof MenuItem)
		{
			ch= str.charAt(0);
			switch (ch)
			{
				case 'C':  			// Cancel
					if (this.isShowing()) this.hide();
					this.dispose();
					return true;
				case 'A':
					DoAboutBox();  	// About
					return true;
			}	
		}
		
		if (evt.target instanceof Button)
		{
			ch= str.charAt(0);
			switch (ch)
			{
				case 'O': 				// OK			
					DoCalc();
					return true;
				case 'C': 				// Cancel
					if (this.isShowing()) this.hide();
					this.dispose();
					return true;
			}
		}
		
		// final attempt to trap return key here, workaround
		
		if (evt.target instanceof MyTextField)  
		{ 
				MyTextField mtf= (MyTextField)evt.target;
				if (mtf != null)
				{
					//if ((evt.modifiers & Event.SHIFT_MASK) == Event.SHIFT_MASK)
					if (evt.shiftDown()) // traps shift and option-shift etc.
					{
						if (mtf.prevField != null)
						{
							mtf.prevField.requestFocus();
							mtf.prevField.selectAll(); 
							return true;		
						}					
					}
					else
					{
						if (mtf.nextField != null) 
						{
							mtf.nextField.requestFocus();
							mtf.nextField.selectAll();
							return true; 		
						}
					}
				}
		}
	
		
		return super.action(evt, obj); 
	}
	
	//================================
	//
	// * gotFocus() *
	//
	// override gotFocus(), not necessary
	//
	//================================

	/*
	public boolean gotFocus(Event evt, Object obj)
	{
		if ((evt.target instanceof MyTextField) && (evt.target != null))
			{((MyTextField)evt.target).selectAll();}
		return super.gotFocus(evt, obj);
	}
	*/
	

	
	//==================================
	// * DoAlgo * CalcInt *
	//
	// The actual amortization algorithm:
	//
	//	m= P*i/(1-(1+i)^-N)
	//	i=r/1200
	//
	// solving for i is non_trivial
	// Thanks to Dr. W. Carlini (& Euclid) for this formula!
	//
	//==================================
	
	// Euclid's theorem to the rescue.
	
	String CalcInt(double P, double N, double m)
	{
		String outString;
		
		double temp=(m/P), answer=(m/P), diff=100, num=0, den=0, accuracy=.00001;
		int i, maxIterations= 1000;
		
		try
		{
			for (i=0; ((diff>accuracy) && (i<maxIterations)); i++)
			{
				temp= answer;
				num= (P*temp/m)+Math.pow((1+temp), -N)-1;
				den= (P/m)-N*Math.pow((1+temp),(-N-1));
				//if (den==0) throw new ArithmeticException(); // not needed
				answer= temp- (num/den);
				diff= answer- temp;
				if (diff<0) diff= -diff;
			}
			if ((answer < 0) || (Double.isNaN(answer)) || (i == maxIterations))
				{throw new ArithmeticException();}
			outString= Double.toString((1200*answer)); // annual interest (0-100)
		}
		catch (ArithmeticException e) {outString= "Invalid Input";}
		return outString;
	}
	
	String DoAlgo(int field)
	{
		String 	outString;
		double	db, temp= 0;
		double 	P= dbArray[0];			// principle
		double 	i= (dbArray[1]/1200);  	// monthly percentage rate
		double 	N= dbArray[2]; 			// loan period months
		double 	m= dbArray[3];			// monthly payments
		
		outString= "Error";
		if (field<4)
		{
			try
			{	
				switch (field)
				{
					case 0: // Principle
						db= 1+i;
						db= 1/Math.pow(db, N);
						db= ((1-db)/i)*m;
						outString= Double.toString(db);
						break;
					case 1:  // Annual Interest
						outString= CalcInt(P, N, m);
						break;
					case 2:  // Loan Period
						db= (1-(P*i/m));
						db= Math.log(db);
						temp= 1+i;
						db= -db/Math.log(temp);
						outString= Double.toString(db);
						break;
					case 3:  // Monthly Payments
						db= 1+i;
						db= 1/Math.pow(db, N);
						db= (P*i)/(1-db);
						outString= Double.toString(db);
						break;
					default:;
					}
			}
			catch (ArithmeticException e) {;} // outString= "Error"
		}
		return outString;	
	}
	
	//==================================
	// * DoCalc *
	//
	// Check for valid input, update the
	// window fields and then DoAlgo()
	//
	//==================================
	
	void DoCalc()
	{
		String	strArray[], strResult= "", tempString="";
		Double 	db;
		int		count= 0, field= 0;
		boolean	isError= false;
		
		strArray= new String[4];
		
		for (int i=0; i<4; i++)
		{	
			strArray[i]= textArray[i].getText();		
			try
			{
				tempString= ParseString(strArray[i]); // strip commas
				db= Double.valueOf(tempString);  // get Double, may throw exception
				dbArray[i]= db.doubleValue();	  // convert Double to double 			
				if (dbArray[i]<0) throw new NumberFormatException();  // negative #
				
				switch (i)
				{
					case 0:  // Principle
					break;
					case 1:  // Annual Interest
						if (dbArray[i]>100) 
							{throw new NumberFormatException();} // over 100%
					break;
					case 2:  // Months
						if ((int)dbArray[i] != dbArray[i])
							{throw new NumberFormatException();}  // not an integer
					break;
					case 3:  // Monthly Payment
					break;
				}
				textArray[i].setText(Double.toString(dbArray[i])); // echo changes
			}
			catch (NumberFormatException e)
			{
				dbArray[i]= 0;
				isError= true;
				textArray[i].setText("Error");
			}
				
			if (dbArray[i]==0) 
			{
				count +=1; 	//count zero entries
				field= i;	//last zero text field
			}
		}
		
		
		if (isError) 
		{
			DoDialogError("Invalid Entry");
			messageBox.setText("Invalid Entry");
		}
		else
		{
			if (count>1) 
			{
				DoDialogError("Too Many Zero Entries");
				messageBox.setText("Too Many Zero Entries");
			}
			else
			{
				if (count<1) 
				{
					DoDialogError("One Entry Must Be 0");
					messageBox.setText("One Entry Must Be 0");
				}
				else
				{
					strResult= DoAlgo(field);  // finally, do the math!
					textArray[field].setText(strResult);
					messageBox.setText("Done");
				}
			}
		}
	}
	
	//==========================
	//
	// * DoAboutBox *
	//
	// Create an about box
	// We create a reference to the about box to avoid multiple instances
	// This workaround is not necessary when Netscape supports modal dialogs
	//
	//==========================
	
	void DoAboutBox()
	{
		if (dl != null) // workaround for non-modal dialog Win95 netscape 2.01
				{
					if (dl.isShowing()) dl.hide();
					dl.dispose();
					dl= null;
				}
		dl= new MyAbout(this, "About Box", true); 
	}
	
	//==========================
	//
	// * DoDialogError *
	//
	// Create an error dialog
	//
	//==========================
	
	void DoDialogError(String inString)
	{
		messageBox.setText("Error");  // respond to user while dialog loads
		if (err != null) //workaround for non-modal Win95 netscape
					{
						if (err.isShowing()) err.hide();
						err.dispose();
						err= null;
					}
		err= new MyError(this,"Error Box", true, inString);
	}
}
	
	
//=====================================================================
//
// class * MyTextField *
//
// we extend TextField to implement proper tab key behavior
// implements NO_KEY_FILTER, DIGITS_ONLY or NUMERIC_ONLY key filters
// isDigit() is buggy with Netscape 3.0b4 (Win95)
//
//=====================================================================

class MyTextField extends TextField
{
	MyTextField nextField= null, prevField= null;
	private int keyFilter;
	
	static final int NO_KEY_FILTER= 0;			// keyFilter Constants
	static final int NUMERIC_ONLY= 	1;
	static final int DIGITS_ONLY= 	2;
	static final int MIN_KEY=		0;
	static final int MAX_KEY=		DIGITS_ONLY;
	
	public MyTextField(String str, int col, int keyFilter)
	{
		super(str, col);				
		if ((keyFilter<MIN_KEY) || (keyFilter>MAX_KEY))
			{this.keyFilter = NO_KEY_FILTER;}
		else this.keyFilter= keyFilter;
	}
	
	
	//  overloaded constructor, default NO_KEY_FILTER
	
	public MyTextField(String str, int col)
	{
		super(str, col);
		this.keyFilter= NO_KEY_FILTER;	
	}

	//================================
	//
	// * keyDown *
	//
	// override keyDown to trap tabs
	//
	//================================
	
	public boolean keyDown(Event evt, int key)
	{
		switch (key)
		{
			case '\b':		
				break;		// pass to super.keyDown()
			case '\n':		// ** fall through **
			case '\r':		// ** fall through **
			case '\t':						
				//if ((evt.modifiers & Event.SHIFT_MASK) == Event.SHIFT_MASK)
				if (evt.shiftDown()) // traps shift and option-shift etc.
				{
					if (prevField != null)
					{
						prevField.requestFocus();
						prevField.selectAll(); 
						return true;		
					}					
				}
				else
				{
					if (nextField != null) 
					{
						nextField.requestFocus();
						nextField.selectAll();
						return true; 		
					}
				}
				break;  // not handled, pass to super
			default: // not tab, return or backspace
				switch(keyFilter)
				{
					case DIGITS_ONLY:
						if (! IsDigit(key)) return true;  	// handle & ignore key input
						break;								// pass to super
					case NUMERIC_ONLY:
						if (! IsNumeric(key)) return true; 	// handle & ignore key input
						break;										
					// NO_KEY_FILTER, pass to super
				} 
		}
		return super.keyDown(evt, key);
	}
	
	//===============================
	//
	// * isDigit *
	//
	//	returns true if key is digits 0-9
	// 	buggy with Netscape 3.0b4 (Win95)
	//
	//
	//===============================
	
	public boolean IsDigit(int key)
	{
		if (((key < '0') || (key >'9')) ) return false;
		else return true;
	}
	
	//===============================
	//
	// * isNumeric *
	//
	//	returns true if key is period, comma, or digits 0-9
	//
	//===============================
	
	public boolean IsNumeric(int key)
	{	
		if ((key=='.') || (key==',') || ((key>='0') & (key<='9')) )
			 {return true;}
		else return false;
	}

}

//=====================================================================
//
// class * MyAbout *
//
// Create our About Dialog Box
//
//======================================================================

class MyAbout extends Dialog
{
	Button 	OKbutton;
	Font 	f;
	
	MyAbout(Frame inFrame, String inString, boolean inBoolean)
	{
		super(inFrame, inString, inBoolean);		
		setLayout(new GridLayout(3,1));
		add(new Label("A Java AWT Applet! jlouie", Label.CENTER));
		OKbutton= new Button("OK");
		Panel pl= new Panel();
		pl.setLayout(new FlowLayout(FlowLayout.CENTER,15,15));
		pl.add(OKbutton);
		this.add(pl);
		this.add(new Label("73270.1776@compuserve.com", Label.CENTER));
		// will not display under Netscape 3.0b4 (Mac)
		Point p=location();
		this.move(p.x +20, p.y +20);
		this.resize(200,200); // workaround Win95 & Netscape
		this.pack();  // workaround Win95 & Netscape
		f= OKbutton.getFont();
		f= new Font(f.getName(), Font.BOLD, f.getSize());
		OKbutton.setFont(f);
		f= null;
		OKbutton.setBackground(Color.white);
		this.setBackground(Color.lightGray);
		this.show();
		this.requestFocus();	
	}
	
	public boolean handleEvent(Event event)
	{
		switch (event.id)
		{
			case Event.WINDOW_DESTROY:
				this.hide(); // work around Win95 non_modal dialog
				return true;
			/*case Event.KEY_PRESS:	// buggy
				if (event.key == '\n') 
				{
					this.hide();
					return true;
				}*/
			default:
				return super.handleEvent(event);  // calls action
		}
	}
	
	public boolean action(Event evt, Object obj)
	{
		if (evt.target == OKbutton)
		{
			if (this.isShowing()) this.hide();
			// DoDialogError will this.dispose()
		}
		return true;  // trap all actions
	}
}

//=====================================================================
//
// class * MyError *
//
// Create our error dialog box
// We create a reference to the error box to avoid multiple instances
// This workaround is not necessary when Netscape supports modal dialogs
//
//======================================================================

class MyError extends Dialog
{
	Button 	OKbutton;
	Font	f;
	
	MyError(Frame inFrame, String inString, boolean inBoolean, String errString)
	{
		super(inFrame, inString, inBoolean);		
		setLayout(new GridLayout(3,1));
		//setBackground(Color.blue);
		this.add(new Label(errString,Label.CENTER));
		OKbutton= new Button("OK");
		Panel pl= new Panel();
		pl.setLayout(new FlowLayout(FlowLayout.CENTER,15,15));
		pl.add(OKbutton);
		this.add(pl);
		this.add(new Label("ERROR", Label.CENTER));	
		// will not display under Netscape 3.0b4 (Mac)
		Point p=location();
		this.move(p.x +20, p.y +20);
		this.resize(200,200);  // workaround
		this.pack();  // workaround Win95 & Netscape
		f= OKbutton.getFont();
		f= new Font(f.getName(), Font.BOLD, f.getSize());
		OKbutton.setFont(f);
		f= null;
		OKbutton.setBackground(Color.white);
		this.setBackground(Color.lightGray);
		this.show();	
	}
	
	public boolean handleEvent(Event event)
	{
		switch (event.id)
		{
			case Event.WINDOW_DESTROY:
				this.hide(); // work around Win95 non_model dialog
				// DoError will this.dispose()
				return true;
			/*case Event.KEY_PRESS:	// buggy
				if (event.key == '\n') 
				{
					this.hide();
					return true;
				}*/
			default:
				return super.handleEvent(event); // calls action
		}
	}
	
	public boolean action(Event evt, Object obj)
	{
		if (evt.target == OKbutton)
		{
			if (this.isShowing()) this.hide();
			// DoError will this.dispose()	
		}
		return true;  // modal dialog traps all actions
	}	
}