using System.ComponentModel.DataAnnotations;
using System.Numerics;
using System.Text;
using System.Text.RegularExpressions;

namespace NejCommon.Validation;

public class IbanAttribute : RegexAttribute{
    public const string IbanRegex = @"^[A-Z]{2}\d{2}[A-Z0-9]{11,30}$";
    public IbanAttribute() : base(IbanRegex,"CZ4622500000000104816258"){

    }

    protected override ValidationResult IsValid(object value, ValidationContext ctx)
    {
        if (value is null)
            return ValidationResult.Success!;
        var res = base.IsValid(value, ctx);
        if (res != ValidationResult.Success)
            return res;
        
        string iban = value as string;

        string rearranged = iban.Substring(4) + iban.Substring(0, 4);

        // Convert letters to numbers (A=10, B=11, ..., Z=35)
        StringBuilder numericIban = new StringBuilder();
        foreach (char ch in rearranged)
        {
            if (char.IsLetter(ch))
            {
                int charVal = ch - 'A' + 10;
                numericIban.Append(charVal);
            }
            else
            {
                numericIban.Append(ch);
            }
        }

        // Convert to BigInteger and compute mod 97
        BigInteger ibanNumber = BigInteger.Parse(numericIban.ToString());

        if(ibanNumber % 97 == 1)
            return ValidationResult.Success;
        else
            return new ValidationResult("Invalid checksum of IBAN");
    }
}