/*
ACTION 8-1 (ACT08_01.CPP)
Output:  A program menu that prompts the user to select a 
	monthly payment, total interest, or total loan amount
	calculation option.
	Invalid entry messages as required.
	The monthly loan payment, total loan interest,
	or total loan amount, depending on the program option that the
	user selects.
	Invalid entry messages as required.

Input:	A user response to the menu (P, I, T, or Q).
	If P is selected: User enters the loan amount, interest rate, and term.
	If I is selected: User enters the loan amount, interest rate, and term.
	If R is selected: User enters the loan amount, interest rate, and term.
	If Q is selected: Terminate program.

Processing:  Calculate the selected option as follows: 
	Case V:	payment = principle * rate/(1 - (1+rate) -term)
	Case I:	interest = term * payment - principle
	Case T:	total = principle + interest
	Case Q: Quit the program.

	where: 	principle is the amount of the loan.
	rate is a monthly interest rate in decimal form.
	term is the number of months of the loan.
*/

//PREPROCESSOR DIRECTIVES
#include <iostream.h>
#include <math.h>

//FUNCTION PROTOTYPES
char displayMenu();
void loanCalculator(char choice);
double calculatePayment();
double calculateInterest();
double calculateTotalLoanAmount();

//MAIN FUNCTION
int main()
{
	//DEFINE VARIABLES
	char choice = 'Q';        	//USER MENU ENTRY

	//DISPLAY PROGRAM DESCRIPTION MESSAGE
	cout << "\nThis program will calculate a monthly loan interest\n"
	     << "payment, total loan interest, or total loan amount." << endl;

  //MENU CONTROL LOOP
	do
	{
		choice = displayMenu();
		loanCalculator(choice);
	} //END DO/WHILE
	while ((choice != 'q') && (choice != 'Q'));

	//RETURN
	return 0;
}//END main()

//DISPLAY MENU FUNCTION
char displayMenu()
{
    //DEFINE LOCAL CHOICE VARIABLE
    char choice = 'Q';

   cout  << "\n\n\t\t\tEnter P to get monthly payment" 
            << "\n\t\t\tEnter I to get total loan interest"
            << "\n\t\t\tEnter T to get total loan amount"
            << "\n\t\t\tEnter Q to quit" << endl;

    cout << "\n\n\tPlease enter your choice:  ";
  
    //READ USER CHOICE
    cin >> choice;


   //RETURN CHOICE
   return choice;
} //END displayMenu()


//LOAN CALCULATION FUNCTION
void loanCalculator(char choice)
{
    switch (choice)
    {
     case 'p':  	//CALL calculatePayment() FUNCTION
     case 'P':  	cout << "The monthly loan payment is $"
                             << calculatePayment() << endl;
                	break;

     case 'i':  	//CALL calculateInterest() FUNCTION
     case 'I' : 	cout << "The monthly loan payment is $"
                             << calculateInterest() << endl;
                	break;

     case 't':  	//CALL calculateTotalLoanAmount() FUNCTION
     case 'T':  	cout << "The monthly loan payment is $"
                            << calculateTotalLoanAmount() << endl;
                	break;

     case 'q':  	//TERMINATE PROGRAM
     case 'Q': 	cout << "Program terminated." << endl;
                	break;

                	//DISPLAY INVALID ENTRY MESSAGE
     default :  	cout << "\n\nThis is an invalid entry." << endl;
    } //END SWITCH
}//END loanCalculator()

//CALCULATE PAYMENT FUNCTION
double calculatePayment() 
{
  //DEFINE LOCAL VARIABLES
	double principle = 0.0;		//LOAN PRINCIPLE
  double rate = 0.0;       	//ANNUAL INTEREST RATE
  int term = 0;            	//TERM OF LOAN IN MONTHS

  //GET LOAN DATA FROM USER
  cout << "\nEnter the amount of the loan: $";
  cin >> principle;
  cout << "\nEnter the duration of the loan in months: ";
  cin >> term;
  cout << "\nEnter the annual interest rate: ";
  cin >> rate;

  //CHECK FOR INVALID RATE
  while ((rate <= 0) || (rate > 100))
  {
    //DISPLAY INVALID ENTRY MESSAGE
    cout << "\n\nThis is an invalid entry." << endl;
    cout << "\nEnter the annual interest rate: ";
    cin >> rate;
  }//END WHILE

  //CALCULATE PAYMENT
  rate = rate/12/100;
	return principle * rate/(1-pow((1+rate),-term));
}//END calculatePayment()

//CALCULATE INTEREST FUNCTION
double calculateInterest() 
{
  //DEFINE LOCAL VARIABLES
	double payment = 0.0;     //MONTHLY PAYMENT
	double principle = 0.0;	  //LOAN PRINCIPLE
  double rate = 0.0;       	//ANNUAL INTEREST RATE
  int term = 0;            	//TERM OF LOAN IN MONTHS

	//GET LOAN DATA FROM USER
  cout << "\nEnter the amount of the loan: $";
  cin >> principle;
  cout << "\nEnter the duration of the loan in months: ";
  cin >> term;
  cout << "\nEnter the annual interest rate: ";
  cin >> rate;

  //CHECK FOR INVALID RATE
  while ((rate <= 0) || (rate > 100))
  {
    //DISPLAY INVALID ENTRY MESSAGE
    cout << "\n\nThis is an invalid entry." << endl;
    cout << "\nEnter the annual interest rate: ";
    cin >> rate;
  }//END WHILE

  //CALCULATE INTEREST
  rate = rate/12/100;
	payment = principle * rate/(1-pow((1+rate),-term));
  return term * payment - principle;
}//END calculateInterest()

//CALCULATE TOTAL LOAN AMOUNT FUNCTION
double calculateTotalLoanAmount() 
{
  //DEFINE LOCAL VARIABLES
  double payment = 0.0;     	//MONTHLY PAYMENT
  double interest = 0.0;    	//TOTAL INTEREST FOR LIFE OF LOAN
	double principle = 0.0;		  //LOAN PRINCIPLE
  double rate = 0.0;       		//ANNUAL INTEREST RATE
  int term = 0;            		//TERM OF LOAN IN MONTHS

	//GET LOAN DATA FROM USER
	cout << "\nEnter the amount of the loan: $";
  cin >> principle;
  cout << "\nEnter the duration of the loan in months: ";
  cin >> term;
	cout << "\nEnter the annual interest rate: ";
  cin >> rate;

  //CHECK FOR INVALID RATE
  while ((rate <= 0) || (rate > 100))
  {
    //DISPLAY INVALID ENTRY MESSAGE
    cout << "\n\nThis is an invalid entry." << endl;
    cout << "\nEnter the annual interest rate: ";
    cin >> rate;
  }//END WHILE

	//CALCULATE TOTAL LOAN AMOUNT
	rate = rate/12/100;
	payment = principle * rate/(1-pow((1+rate),-term));
	interest = term * payment - principle;
	return principle + interest;
 }//END calculateTotalLoanAmount()