import java.util.Scanner;

/* Car Loan Estimator (version A)
**
** Java application that prints a table showing the monthly payments for 
** car loan with annual interest rates ranging from 1% to 10% and terms
** ranging from 12 months to 72 months.
** This version improves upon the previous one (lacking the 'A' in its
** name) in the following ways:
**
** --The principal of the loan is read from the keyboard rather than
**   being "hard-coded".
** --the methods are parameterized (to some degree) so that the only
**   global variable used is that holding the principal of the loan.
**   (Global constants are still used for indicating the range of 
**   terms and range of interest rates to be considered.)
**
** CMPS 134  Fall 2025  Prog. Assg. #2
** Authors: P.M.J. and R.W.M. 
**
*/

public class CarLoanEstimatorA {

   // GLOBAL CONSTANTS
   static final int TERM_START = 12;                      //Shortest term in months
   static final int TERM_LIMIT = 72;                      //Longest term in months
   static final int TERM_INCREMENT = 12;                  //difference between terms
   static final double ANNUAL_INTEREST_RATE_START = 1.0;  //Lowest Annual Interest Rate
   static final double ANNUAL_INTEREST_RATE_LIMIT = 10.0; //Hightest Annual Interest Rate
 
   // GLOBAL VARIABLE
   static double principal;    //Principal to be borrowed


   public static void main( String [] args ) {
      System.out.println("Welcome to the Car Loan Estimator Program...");

      Scanner keyboard = new Scanner(System.in);
      System.out.print("Enter the principal of the loan: ");
      principal = keyboard.nextDouble();

      System.out.printf("Monthly payments for a $%8.2f loan%n", principal); 
      printHeading();
      printTable();
      System.out.println("Done!!");
   }
   
   public static void printHeading() {
      String underline = "---||";
      System.out.print("   ||  ");
      for (double annualInterestRate = ANNUAL_INTEREST_RATE_START; 
           annualInterestRate <= ANNUAL_INTEREST_RATE_LIMIT; 
           annualInterestRate = annualInterestRate + 1.0)
      {
         System.out.printf("%6.2f %% |  ", annualInterestRate);
         underline = underline + "-----------|";
      }
      System.out.println();
      System.out.println(underline);      
   }
   
   /* Prints the body of the table.
   */
   public static void printTable() {
      for(int term = TERM_START; term <= TERM_LIMIT; term = term + TERM_INCREMENT) {
         printRow(term);
      }
   }
   

   /* Prints one row of the table, specifically the one showing the
   ** monthly payment due for a loan having the specified term.
   */
   public static void printRow(int term) {
      System.out.printf("%2d ||  ",term);
      for(double annualInterestRate = ANNUAL_INTEREST_RATE_START; 
             annualInterestRate <= ANNUAL_INTEREST_RATE_LIMIT; 
             annualInterestRate = annualInterestRate + 1.0)
      {
         double payment = calculatePayment(principal, term, annualInterestRate);
         System.out.printf("%8.2f |  ",payment);
      }
      System.out.println();
   }
   
   /* Given the principal, term (in months), and annual interest rate of a loan
   ** (in percent, as in 5.6%), returns the monthly payment that would need to be 
   ** made in order to complete repayment by the end of the term.
   ** An explanation of the formula used can be found at: 
   ** https://www.youtube.com/watch?v=p8f8XP2tmvE
   */
   public static double calculatePayment(double principal, int term, double intRate) {
      double monthlyInterestRate = (intRate / 100) / 12;

      double numerator = principal * monthlyInterestRate;
      double denominator = 1 - Math.pow(1 + monthlyInterestRate, -term);
      return numerator / denominator;
   }

}