Ayende's Tax Thing

TaxCalculator.cs

	public class TaxCalculator
	{
		private static readonly IList<TaxBracket> TaxBrackets;

		static TaxCalculator()
		{
			TaxBrackets = setupTaxBrackets();
		}

		public decimal CalculateTax( decimal grossIncome )
		{
			var taxBracket = TaxBrackets.First( tax => tax.UpperBoundary >= grossIncome );
			var taxBracketIndex = TaxBrackets.IndexOf( taxBracket );

			var totalTax = (taxBracketIndex == 0) 
				? grossIncome * taxBracket.Rate 
				: (grossIncome - TaxBrackets[taxBracketIndex - 1].UpperBoundary) * taxBracket.Rate 
					+ TaxBrackets[taxBracketIndex - 1].TaxOnUpperBoundary;

			return totalTax;
		}

		private static IList<TaxBracket> setupTaxBrackets()
		{
			var taxBrackets = new List<TaxBracket> { 
				new TaxBracket( 0.1m, 5070m )
				, new TaxBracket( 0.14m, 8660m )
				, new TaxBracket( 0.23m, 14070m )
				, new TaxBracket( 0.3m, 21240m )
				, new TaxBracket( 0.33m, 40230m )
				, new TaxBracket( 0.45m, decimal.MaxValue ) 
			};

			for (int counter = 1; counter < taxBrackets.Count; counter++)
			{	// Update boundary tax amounts to reflect previous upper boundary tax amounts.
				taxBrackets[counter].TaxOnUpperBoundary = taxBrackets[counter - 1].TaxOnUpperBoundary 
					+ (taxBrackets[counter].UpperBoundary - taxBrackets[counter - 1].UpperBoundary) * taxBrackets[counter].Rate;
			}
			return taxBrackets;
		}

		private class TaxBracket
		{
			public decimal UpperBoundary
			{
				get;
				private set;
			}

			public decimal Rate
			{
				get;
				private set;
			}

			public decimal TaxOnUpperBoundary
			{
				get;
				set;
			}

			public TaxBracket( decimal rate, decimal upperBounds )
			{
				Rate = rate;
				UpperBoundary = upperBounds;
				TaxOnUpperBoundary = rate * upperBounds;
			}
		}
	}

Ooh, syntax highlighting based on file extension.