import java.util.Observable;
import java.util.Stack;


class CalculatorEngine extends Observable
	{

	private int state;
	private Stack ops;
	private Op op;
	

	CalculatorEngine()
		{ this.clear(); }
	

	Stack ops()
		{ return this.ops; }
	
	void ops( Stack aStack )
		{ this.ops = aStack; }


	Op op()
		{ return this.op; }
		
	void op( Op anOp )
		{ this.op = anOp; }


	public void notifyObservers()
		{
		setChanged();
		notifyObservers( ops().peek() );
		}
		

	synchronized void digit( String aString )
		{
		String temp;

		if ( state == 3 )
			{
			ops().push( "0" );
			state = 4;
			}
		if ( state == 7 )
			{
			clear();
			};
			
		temp =  (String) ops().pop();

		if ( ( state == 1 ) || ( state == 5 ) )
			{
			temp = temp + "." + aString;
			if ( state == 1 )
				{
				state = 2;
				}
			else if ( state == 5 )
				{
				state = 6;
				};
			}
		else
			{
			temp = temp + aString;
			};
		
		ops.push( temp );
		notifyObservers();
		}
	
	synchronized void period()
		{
		if ( state == 0 )
			{
			state = 1;
			}
		else if ( state == 3 )
			{
			ops().push( "0" );
			state = 5;
			}
		else if ( state == 4 )
			{
			state = 5;
			}
		else if ( state == 7 )
			{
			clear();
			state = 1;
			};
		}
	
	synchronized void operation( Op anOp )
		{
		if ( anOp.isBinaryOp() )
			{
			binaryOperation( anOp );
			state = 3;
			}
		else
			{
			unaryOperation( anOp );
			};
		notifyObservers();
		}
	
	void binaryOperation( Op anOp )
		{
		if ( ( state == 4 ) || (state == 5) || (state == 6) )
			{
			Double temp1;
			Double temp2;
			 
			temp2 = ( new Double( (String) (ops().pop()) ) );
			temp1 = ( new Double( (String) (ops().pop()) ) );
			ops().push( op().perform( temp1, temp2 ).toString() );	
			};
		op( anOp );
		}
	
	void unaryOperation( Op anOp )
		{
		Double temp;
		
		temp = ( new Double( (String) (ops().pop()) ) );
		ops().push( anOp.perform( temp ).toString() );
		}	
		
	synchronized void clear()
		{
		ops( new Stack() );
		ops().push( "0" );
		op( null );
		state = 0;
		
		notifyObservers();
		}
	
	synchronized void calculate()
		{
		binaryOperation( null );
		state = 7;
		
		notifyObservers();
		}
		
	}