.NET Newsletter #1 - Challenge

Maths.cs

using System;
using System.Numerics;

public class Program
{
    public void Main(params string[] args)
    {
        foreach (var n in Maths.Fib().Take(100))
        {
            Console.WriteLine(n);
        }
    }
}

public static class Maths
{
    /// <summary>
    /// Produces an infinite stream of numbers following the Fibonacci sequence.
    /// CAUTION: Do not try and consume the entire stream with ToList or ToArray, etc.,
    ///          without first calling Take to restrict the number of elements produced,
    ///          otherwise it will never return. This kills the program.
    /// </summary>
    public static IEnumerable<BigInteger> Fib()
    {
        var a = BigInteger.Zero; // F0
        var b = BigInteger.One;  // F1
        var c = a + b;           // F2 / first step
        
        // The sequence always starts with F0, F1
        yield return a;
        yield return b;
        // Values are produced by shifting b -> a, c -> b, then
        // adding a + b to get the next value in the sequence
        while (true)
        {
            yield return c;
            a = b; b = c; c = a + b;
        }
    }
}