Markov Chain Ruleset Building

foreach (string s in dataSequence)
{
    int rest = data.Length%stepSize;

    for (int i = 0; i < data.Length - rest; i += stepSize)
    {
        string first = data.Substring(i, stepSize);
        string followedBy = "";

        if (i == data.Length - (rest + stepSize))
            followedBy = data.Substring(i + stepSize, rest);
        else
            followedBy = data.Substring(i + stepSize, stepSize);

        if (!_rules.ContainsKey(first))
        {
            _rules.Add(first, new List<string>());
                }

        _rules[first].Add(followedBy);
    }
}