public void Calculate_The_Correct_Result()
{
var sut = new LuhnChecksumCalculator();
sut.Calculate("7992739871").Should().Be(3);
sut.Calculate("123456789").Should().Be(7);
}
public void When_Subject_Is_Null_Throw_ArgumentNullException()
{
var sut = new LuhnChecksumCalculator();
Action action = () => sut.Calculate(null);
action.Should().Throw <ArgumentNullException>();
}