Project Euler 372 - v1

    [TestFixture]
    public class P372v1
    {
        public long GetIntegerArea(long min, long max, long number)
        {
            long sum = 0;
            double sqrt = Math.Sqrt(number);
            for (long x = min; x <= max / sqrt; x++)
            {
                sum += max - (long)Math.Ceiling(x * sqrt) + 1;
            }
            return sum;
        }

        public long Calculate(long min, long max)
        {
            double maxN = (double)max * max / (min * min);
            long sum = 0;
            for (int i = 1; i <= maxN; i++)
            {
                sum += (i % 2 == 0 ? -1 : +1) * GetIntegerArea(min, max, i);
            }

            return sum;
        }

        [Test]
        public void Run5()
        {
            Assert.That(GetIntegerArea(1, 5, 1), Is.EqualTo(15));
            Assert.That(GetIntegerArea(1, 5, 2), Is.EqualTo(8));
            Assert.That(GetIntegerArea(1, 5, 4), Is.EqualTo(6));

            Assert.That(Calculate(1, 5), Is.EqualTo(9));
        }

        [Test]
        public void Run100()
        {
            Assert.That(Calculate(1, 100), Is.EqualTo(3019));
        }

        [Test]
        public void Run10K()
        {
            Assert.That(Calculate(101, 10000), Is.EqualTo(29750422));
        }

        [Test]
        public void GetResult()
        {
            Console.WriteLine(Calculate(2000001, 1000000000));
        }
    }