// Chapter 10
// Functional Programming
/*
 * 1. Immutability > when no change is needed in the state of the object ex. tuples
 * 2. Pure Functions > no side effect on the return "you have the same output given the input
 * 3. High Order Functions > functions that are passed as args
 * 4. Lambda Expression > one operation or exp assigned
 * 5. LINQ - standardized query syntax
 * 
 */

// using statement
using System; // core C# library
using System.Linq; // LINQ


// CreditCardProcessing Class
public class CreditCardProcessing
{
    // Immutable CreditCard class
    public class CreditCard
    {
        public string CardNumber
        {
            get;
        }
        public string ExpiryDate
        {
            get;
        }
        public string Cvc
        {
            get;
        }

        public CreditCard(string cardNumber, string expiryDate, string cvc)
        {
            CardNumber = cardNumber;
            ExpiryDate = expiryDate;
            Cvc = cvc;
        }

    } // Immutable CreditCard Class

    // Function to process the credit card

    public static IEnumerable<CreditCard> ProcessCreditCards(IEnumerable<CreditCard> creditCards)
    {
        // foreach loop structural
        // foreach loop, that happens AFTER you call the function to loop thru the tuples is being replaced with an Enumerator the happens WHILE the function is being initialized. 

        // here we have already prepped the data 

        // enumerator functional iterator
        // LINQ where() to validate the credit card info

        // Option 1 for LINQ High Order Function
        /*
        * var validCards = creditCards.Where(isCreditCardValid);
        */

        // Option 2 for LINQ Lambda Expression "Arrow Function"
        var validCards = creditCards.Where(
            // Lambda Exp (params) => exp or op;
            (card) => !string.IsNullOrEmpty(card.CardNumber) && !string.IsNullOrEmpty(card.ExpiryDate) && !string.IsNullOrWhiteSpace(card.Cvc)
            ); // where

        return validCards; // return does not change   
    
    } // ProcessCreditCards

    // boolean pure function to validate the credit card / use in the Where() LINQ statment
    // naming with an is prefix for bool > best practice
    public static bool isCreditCardValid(CreditCard card)
    {
        // do something to validate
        // card # has to be 16 digits
        // exp date is in mm/yy format
        // etc.
        // check for nulls
        // LINQ IsNullOrEmpty() built in query to check for nulls or empty or whitespace
        return !string.IsNullOrEmpty(card.CardNumber) && !string.IsNullOrEmpty(card.ExpiryDate) && !string.IsNullOrWhiteSpace(card.Cvc);
        // return true for valid cards
    
    } // isCreditCardValid


} // CreditCardProcessing Class


// Program Class
class Program
{

    static void Main(string[] args)
    {
        // Instances of Credit Cards
        var creditCards = new List<CreditCardProcessing.CreditCard>
        {
            new CreditCardProcessing.CreditCard("", "12/25", "123"),  // false
            new CreditCardProcessing.CreditCard("4444111122223333", "11/28", "123"), // true
            new CreditCardProcessing.CreditCard("","",""), // false
            new CreditCardProcessing.CreditCard("4444111122225555", "11/28", "123") //true
        }; // List of Credit Cards

        // use a foreach to loop thru the list and vaildate
        var validCards = CreditCardProcessing.ProcessCreditCards(creditCards); // List

        foreach (var creditCard in validCards) // true
        {
            // scope local var creditCard
            Console.WriteLine($"Valid Credit Card: {creditCard.CardNumber}, Expiration Date: {creditCard.ExpiryDate}, Security Code: {creditCard.Cvc}.");

        } // foreach
    
    } // Main

} // Program Class