周期检测算法

f(0) = 0
f(i) = (i - 1) % 4

0 0 1 2 3 0 1 2 3 0 1 2 3

public static Tuple<int, int> ModifiedTortoiseHare(Func<int, int> f, int x0 = 0, int checks = 4)
{
    for (; ; x0++)
    {
        int lam = 0, tortoise, hare;

        do
        {
            lam++;
            tortoise = f(x0 + lam);
            hare = f(x0 + 2 * lam);
        } while (tortoise != hare);

        int mu = -1;

        do
        {
            mu++;
            tortoise = f(x0 + mu);
            hare = f(x0 + mu + lam);
        } while (tortoise != hare);

        if (mu != 0) continue;

        bool correct = true;
        int lamCheckMax = lam * checks;

        for (int x = 0; x < lamCheckMax; x++)
        {
            if (f(x0 + x + mu) != f(x0 + x + mu + lam))
            {
                correct = false;
                if (mu != 0) x0 += mu - 1;
                break;
            }
        }

        if (correct) return Tuple.Create(x0 + mu, lam);
    }
}