The tests
[TestFixture]
public class TaxCalculatorTests
{
[Test]
public void Tax_for_0()
{
var calculator = new TaxCalculator();
var tax = calculator.TaxFor(0);
Assert.That(tax, Is.EqualTo(0));
}
[Test]
public void Tax_for_5000()
{
var calculator = new TaxCalculator();
var tax = calculator.TaxFor(5000);
Assert.That(tax, Is.EqualTo(500));
}
[Test]
public void Tax_for_5800()
{
var calculator = new TaxCalculator();
var tax = calculator.TaxFor(5800);
Assert.That(tax, Is.EqualTo(609.2).Within(0.001));
}
[Test]
public void Tax_for_9000()
{
var calculator = new TaxCalculator();
var tax = calculator.TaxFor(9000);
Assert.That(tax, Is.EqualTo(1087.8).Within(0.001));
}
[Test]
public void Tax_for_15000()
{
var calculator = new TaxCalculator();
var tax = calculator.TaxFor(15000);
Assert.That(tax, Is.EqualTo(2532.9).Within(0.001));
}
[Test]
public void Tax_for_50000()
{
var calculator = new TaxCalculator();
var tax = calculator.TaxFor(50000);
Assert.That(tax, Is.EqualTo(15068.1).Within(0.001));
}
}
And the implementationpublic class TaxCalculator
{
private readonly Tuple<decimal, decimal>[] slices =
{
new Tuple<decimal, decimal>(40230, 0.45M),
new Tuple<decimal, decimal>(21240, 0.33M),
new Tuple<decimal, decimal>(14070, 0.30M),
new Tuple<decimal, decimal>(8660, 0.23M),
new Tuple<decimal, decimal>(5070, 0.14M),
new Tuple<decimal, decimal>(0, 0.10M),
};
public decimal TaxFor(decimal sum)
{
if (sum == 0)
{
return 0;
}
var tax = 0M;
foreach (var slice in slices)
{
if (sum > slice.Item1)
{
var amount = sum - slice.Item1;
tax += amount * slice.Item2;
sum -= amount;
}
}
return tax;
}
}
