// Exercise 3: records, value equality, and `with` expressions
//
// Implement:
//   - record Money(string Currency, decimal Amount) with a method Add()
//     that returns a NEW Money using a `with` expression (records are
//     conceptually immutable — never mutate in place).
//   - A plain (non-record) class BankNote for contrast, to show reference
//     equality still applies to ordinary classes.

using System;

record Money(string Currency, decimal Amount)
{
    public Money Add(decimal amount)
    {
        // TODO: return a new Money with Amount increased by `amount`,
        // using a `with` expression. Do not mutate `this`.
        throw new NotImplementedException();
    }
}

class BankNote
{
    public decimal Amount { get; init; }
}

class Program
{
    static void Main()
    {
        int pass = 0, total = 0;

        var m1 = new Money("USD", 10m);
        var m2 = new Money("USD", 10m);

        total++;
        if (m1 == m2) { Console.WriteLine("PASS: records compare by value"); pass++; }
        else Console.WriteLine("FAIL: records compare by value");

        total++;
        if (!ReferenceEquals(m1, m2)) { Console.WriteLine("PASS: records are still distinct heap objects"); pass++; }
        else Console.WriteLine("FAIL: records are still distinct heap objects");

        var m3 = m1.Add(5m);
        total++;
        if (m3.Amount == 15m && m1.Amount == 10m) { Console.WriteLine("PASS: Add() returns new instance, original unchanged"); pass++; }
        else Console.WriteLine("FAIL: Add() returns new instance, original unchanged");

        var n1 = new BankNote { Amount = 10m };
        var n2 = new BankNote { Amount = 10m };
        total++;
        if (n1 != n2) { Console.WriteLine("PASS: plain classes use reference equality by default"); pass++; }
        else Console.WriteLine("FAIL: plain classes use reference equality by default");

        Console.WriteLine($"--- {pass}/{total} checks passed ---");
    }
}