public sealed class Isin
{
    private string m_cc, m_nsin;
    private int m_digest;

    public static bool LooksLike(string isin)
    {
        if (string.IsNullOrEmpty(isin))
            return false;
        isin = isin.Trim();
        if (string.IsNullOrEmpty(isin))
            return false;
        Isin obj;
        return Isin.TryParse(isin, out obj);
    }

    public static bool TryParse(string isin, out Isin obj)
    {
        bool retval = false;

        Isin i = new Isin();
        if (retval = i.ParseInternal(isin)) obj = i;
        else obj = null;

        return retval;
    }

    public static Isin Parse(string isin)
    {
        return new Isin(isin);
    }
            
    public Isin(string isin)
    {
        if (!this.ParseInternal(isin))
            throw new FormatException("Not a isin.");
    }

    private Isin() { }

    private bool ParseInternal(string isin)
    {
        bool retval = false;
        do
        {
            isin = isin.Trim().ToUpperInvariant();
            if (isin.Length != 12)
                break;
            retval = isin.StartsWith("IX") && isin.EndsWith("X"); // special case
            if (retval || (retval = (Isin.DoubleAddDouble(isin) == 0)))
            {
                m_cc = isin.Substring(0, 2);
                m_nsin = isin.Substring(1, 10);
                if (!int.TryParse(isin.Substring(11), out m_digest))
                    m_digest = int.MaxValue;                    
            }                
        }
        while(false);

        return retval;
    }

    private static int DoubleAddDouble(string src)
    {
        int s = 0;
        int a = src.Length == 12 ? 1 : 2;
        for (int i = src.Length - 1; i >= 0; i--)
        {
            int c = src[i];
            if (c > '9') // letter
            {
                c -= ('A' - 10);
                s += (3 - a) * (c / 10) + a * c + (a - 1) * (c % 10) / 5;
            }
            else // number
            {
                c -= '0';
                s += a * c + (a - 1) * (c / 5);
                a = 3 - a;
            }
        }
        s %= 10;
        return (10 - s % 10) % 10;
    }

    public string CountryCode
    {
        get { return m_cc; }
    }

    public string Nsin
    {
        get { return m_nsin; }
    }

    public int Digest
    {
        get { return m_digest; }
    }

    public override string ToString()
    {
        if (this.Digest == int.MaxValue)
            return string.Format("{0}{1}X", this.CountryCode, this.Nsin);
        else
            return string.Format("{0}{1}{2}", this.CountryCode, this.Nsin, this.Digest);
    }        
}