2017-12-07_10-48-54.cs

int Part1(int[] array)
{
	var set = new HashSet<string>();
	var n = 0;

	var d = array.GetDictionary();

	while (true)
	{
		d = Redistribute(d);
		n++;

		var s = d.GetString();
		if (set.Contains(s))
			return n;
		set.Add(s);
	}

}

int Part2(int[] array)
{
	var set = new HashSet<string>();
	var list = new List<string>();
	var n = 0;

	var d = array.GetDictionary();

	while (true)
	{
		d = Redistribute(d);
		n++;

		var s = d.GetString();
		if (set.Contains(s))
			return n - list.IndexOf(s) - 1;
		set.Add(s);
		list.Add(s);
	}
}

IDictionary<int, int> Redistribute(IDictionary<int, int> banks)
{
	var op = banks.Clone();
	var top = op
		.OrderByDescending(p => p.Value)
		.ThenBy(p => p.Key)
		.First();
	
	var redist = top.Value;
	op[top.Key] = 0;
	
	foreach(var i in GetNaturalNumbers(top.Key + 1))
	{
		op[i % op.Count]++;
		if (--redist == 0)
			break;
	}
	
	return op;
}



static IEnumerable<int> GetNaturalNumbers(int start)
{
	while (true) yield return start++;
}

static class ext
{

	static IEnumerable<int> GetNaturalNumbers(int start)
	{
		while (true) yield return start++;
	}
	
	public static IDictionary<int, int> GetDictionary(this int[] d) => Enumerable.Range(0, d.Length)
		.ToDictionary(k => k, k => d[k]);
	
	public static string GetString(this IDictionary<int, int> d) => d.Select(x => x.Value).JoinString("-");
	
	public static Dictionary<K, V> Clone<K,V>(this IDictionary<K,V> d) => d.ToDictionary(p => p.Key, p=> p.Value);
	
	public static IDictionary<V, int> GetOccurrences<V>(this IEnumerable<V> e)
	{
		var output = new Dictionary<V, int>();
		foreach (var p in e)
			output[p] = output.GetOrDefault(p) + 1;
		return output;
	}

	public static V GetOrDefault<K, V>(this IDictionary<K, V> d, K k, V def = default(V)) => d.TryGetValue(k, out V v) ? v : def;

	public static string JoinString<T>(this IEnumerable<T> t, string delimiter = "") => string.Join(delimiter, t);
}