Ayende's Tax Rate Problem

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

namespace Spikes.AyendesTaxProblem
{
    [TestFixture]
    // http://ayende.com/blog/108545/the-tax-calculation-challenge
    public class TaxCalculatorTests
    {
        private TaxCalculator calculator;

        [SetUp]
        public void SetUp()
        {
            var rates = new[]
            {
                new TaxRate(     0,             5070, 10 ),
                new TaxRate(  5071,             8660, 14 ),
                new TaxRate(  8661,            14070, 23 ),
                new TaxRate( 14071,            21240, 30 ),
                new TaxRate( 21241,            40230, 33 ),
                new TaxRate( 40231, decimal.MaxValue, 45 ),
            };
            calculator = new TaxCalculator(rates);
        }

        [TestCase(  5000.0,   500.0 )]
        [TestCase(  5800.0,   609.2 )]
        [TestCase(  9000.0,  1087.8 )]
        [TestCase( 15000.0,  2532.9 )]
        [TestCase( 50000.0, 15068.1 )]
        public void CalculateTax_SpecifiedEarnings_EqualsResult(decimal earnings, decimal expected)
        {
            var actual = calculator.CalculateTax(earnings);
            Assert.That(actual, Is.EqualTo(expected));
        }
    }

    public class TaxRate
    {
        public decimal LowerBound  { get; private set; }
        public decimal UpperBound  { get; private set; }
        public decimal RatePercent { get; private set; }

        public TaxRate(decimal lowerBound, decimal upperBound, decimal rate)
        {
            LowerBound  = lowerBound;
            UpperBound  = upperBound;
            RatePercent = rate;
        }
    }

    public class TaxCalculator
    {
        private readonly IEnumerable<TaxRate> rates;

        public TaxCalculator(IEnumerable<TaxRate> rates)
        {
            if (rates == null) throw new ArgumentNullException("rates");
            this.rates = rates;
        }

        public decimal CalculateTax(decimal earnings)
        {
            return rates
                .Where(rate => rate.LowerBound <= earnings)
                .Select(rate => rate.RatePercent * getTaxableAmount(rate, earnings) / 100m)
                .Sum();
        }

        private decimal getTaxableAmount(TaxRate rate, decimal gross)
        {
            // dirty hack for unspecified inter-rate gap.
            var min = Math.Max(0m, rate.LowerBound - 1);

            var max = Math.Min(gross, rate.UpperBound);

            var taxableAmt = max - min;

            return taxableAmt;
        }
    }
}