stringEntropy

public static int Entropy(this string s)
{
    if (string.IsNullOrEmpty(s)) return 0;
    
    var array = ArrayPool<char>.Shared.Rent(s.Length);
    
    s.CopyTo(0, array, 0, s.Length);
    var span = array.AsSpan(0, s.Length);
    span.Sort();

    var last = -1;
    var count = 0;
    foreach (var c in span)
    {
        if (c == last) continue;
        
        last = c;
        count++;
    }
    
    ArrayPool<char>.Shared.Return(array);
    
    return count;
}