/** 
 * Matt Kretchmar
 * March 10, 2006
 * LoanCalculator.java
 * This program computes monthly payments for an automobile loan.
 * The user specifies the principal (amount borrowed) and the
 * interest rate.  The term is set at 48 months.
 */
import java.util.Scanner;

class LoanCalculator
{
    public static void main ( String args[] )
    {
        double principal;
        double rate;
        double payment;
        
        principal = getData("principal",10000,20000);
        rate = getData("interest rate",7.0,10.0);
        payment = getData("monthly payment",0,1000);

        double residual = computeResidual(rate,principal,payment,48);
        System.out.println("Residual = "+ residual);
    
        // This part of the program now searches for the optimal monthly payment.
        // We start with an upper and lower bound, then use a binary search procedure
        // to reduce the interval in half each time.  We repeat until we narrow in on
        // an upper and lower bound that are 1 cent apart.  
        // Initial lower bound: principal / term
        // Initial upper bound: principal * rate^term / term
        
        double amount;
        double remaining;
        int upperBound;
        int lowerBound;
        int middle;
        
         lowerBound = (int)(principal*100) / 48;
         upperBound = (int)(principal*100 * Math.pow( 1.0+rate/1200, 48)) / 48;
         System.out.println("(" + (upperBound/100.0) + ", " + (lowerBound/100.0) + ")");

         while ( upperBound - lowerBound > 1)
         {
             System.out.print("(" + (upperBound/100.0) + ", " + (lowerBound/100.0) + ")");
             middle = ( lowerBound + upperBound ) / 2;
             amount = middle / 100.0;
             remaining = computeResidual(rate,principal,amount,48);
             System.out.println("  middle = " + middle + ", res = " + remaining);
             if ( remaining > 0 )
                 lowerBound = (int)(amount * 100);
             else
                 upperBound = (int)(amount * 100);
         }
         System.out.println("(" + (upperBound/100.0) + ", " + (lowerBound/100.0) + ")");
         residual = computeResidual(rate,principal,upperBound/100.0,47);
         residual = Math.round(residual*100) / 100.0;
         System.out.println("47 payments of " + (upperBound/100.0) + " and 1 payment of " + residual );
    }
    
    /**
     * This method prompts the user to enter data between lowest and highest.
     * The user's input is tested for this range and they are reprompted until
     * the data entered falls within the range.
     */
    public static double getData ( String msg, double lowest, double highest )
    {
        double p = 0;
        do 
        {
            Scanner keyboard = new Scanner(System.in);
            System.out.println("Enter " + msg + " (" + lowest + " to " + highest + "): ");
            p = keyboard.nextDouble();
        } while ( p < lowest || p > highest );
        return p;
    }
            
    /**
     * This method accepts the rate, principal, payment, and term (number of payments)
     * as input parameters for an automobile loan.  The residual amount of the principal is
     * computed and returned at the end of the car loan.
     */
    public static double computeResidual ( double rate, double principal, double payment, int numPayments )
    {
        // rate adjusted to montly value
        rate = rate / 100;
        rate = rate / 12;
       
        for ( int i = 0; i < numPayments; i++ )
        {
            // add interest
            principal = principal + principal * rate;
            // subtract payment
            principal = principal - payment;
        }
        return principal;
    }
}