PartitaIva_ValueObject

/// <summary>
/// Italian fiscal id for companies
/// </summary>
public class PartitaIva: SimpleValueObject<string>
{
    public const int MaxLength = 255;

    public PartitaIva(string value) : base(value) { }

    private static readonly int[] _oddMap = new int[] { 0, 2, 4, 6, 8, 1, 3, 5, 7, 9 };
    private static bool CheckSum(string value)
    {
        // https://github.com/MrBogomips/ScalaExtractorsLib/blob/master/lib/src/main/scala/com/computableideas/lib/extractors/biz/it/PartitaIva.scala
        Debug.Assert(value.Length == 11);

        int c0 = '0';
        int acc = 0;
        for (var pos = 0; pos < 11; ++pos)
        {
            int ci = value[pos] - c0;
            if (pos % 2 == 0) acc += ci;
            else acc += _oddMap[ci];
        }

        return acc % 10 == 0;
    }

    public static Result<PartitaIva> From(string value)
    {
        if (Guard.String.IsNullOrEmpty(nameof(value), value, out var message)) // TODO: use guard 
            return Result.Failure<PartitaIva>(message);
        if (Guard.String.IsNullOrTooLong(nameof(value), value, MaxLength, out message))
            return Result.Failure<PartitaIva>(message);

        if (!value.All(c => char.IsDigit(c)))
            return Result.Failure<PartitaIva>(ErrorMessages.PartitaIva.InvalidFormat(value));
        if  (!CheckSum())
            return Result.Failure<PartitaIva>(ErrorMessages.PartitaIva.InvalidCheckSum(value));
        return new PartitaIva(value);

        bool CheckSum()
        {
            int c0 = '0';
            int acc = 0;
            for (var pos = 0; pos < 11; ++pos)
            {
                int ci = value[pos] - c0;
                if (pos % 2 == 0) acc += ci;
                else acc += _oddMap[ci];
            }

            return acc % 10 == 0;
        }
    }
}