Ayende's Tax Problem

using System;
using System.Collections.Generic;
using NUnit.Framework;

namespace TaxRate.Tests.Unit
{
    [TestFixture]
    public class TaxCalculatorTests
    {
        [Test]
        [TestCase(5000, 500)]
        [TestCase(5800, 609.2)]
        [TestCase(9000, 1087.8)]
        [TestCase(15000, 2532.9)]
        [TestCase(50000, 15068.1)]
        public void Calculate_Given15k_ShouldReturnCorrectResult(decimal grossSalary, decimal expectedTax)
        {
            // Arrange
            IList<Rate> rates = new List<Rate>();
            rates.Add(new Rate(5070 - 0, (decimal)0.10));
            rates.Add(new Rate(8660 - 5070, (decimal)0.14));
            rates.Add(new Rate(14070 - 8660, (decimal)0.23));
            rates.Add(new Rate(21240 - 14070, (decimal)0.30));
            rates.Add(new Rate(40230 - 21240, (decimal)0.33));
            rates.Add(new Rate(int.MaxValue - 40230, (decimal)0.45));
            var calculator = new TaxCalculator(rates);

            // Act
            decimal tax = calculator.CalculateTax(grossSalary);

            // Assert
            Assert.AreEqual(expectedTax, tax, "Tax is not correct");
        }
    }

    public interface ITaxCalculator
    {
        decimal CalculateTax(decimal grossSalary);
    }

    public class Rate
    {
        public decimal Amount { get; private set; }
        public decimal Percent { get; private set; }

        public Rate(decimal amount, decimal percent)
        {
            Amount = amount;
            Percent = percent;
        }
    }

    public class TaxCalculator : ITaxCalculator
    {
        private readonly IList<Rate> _rates;

        public TaxCalculator(IList<Rate> rates)
        {
            _rates = rates;
        }

        public decimal CalculateTax(decimal grossSalary)
        {
            decimal totalTax = 0;
            decimal remaining = grossSalary;
            int currentRateId = 0;

            while (remaining > 0)
            {
                var rate = _rates[currentRateId];

                decimal taxable = Math.Min(rate.Amount, remaining);

                decimal taxForThisRate = taxable * rate.Percent;

                totalTax += taxForThisRate;

                remaining -= rate.Amount;
                currentRateId++;
            }

            return totalTax;
        }
    }
}