namespace New.Domain.ValueObjects;

/// <summary>
/// A Dutch "burgerservicenummer" - always exactly 9 digits, validated with the
/// eleven-proof (elfproef) checksum.
///
/// This value object deliberately does NOT trim or otherwise massage its
/// input. The legacy source stores BSNs as a space-padded CHAR(9), and it is
/// the legacy mapper's job (New.Infrastructure.Legacy.LegacyAanvraagMapper)
/// to trim before handing the raw value to this constructor - if it forgets,
/// this constructor throws, which is the point: silently accepting padded
/// input here would hide that mapping bug instead of surfacing it.
/// </summary>
public sealed record Bsn
{
    public string Value { get; }

    public Bsn(string value)
    {
        if (string.IsNullOrEmpty(value) || value.Length != 9 || !value.All(char.IsDigit))
        {
            throw new DomainInvariantViolationException(
                "Bsn.Format",
                $"A BSN must be exactly 9 digits. Got '{value}'.");
        }

        if (!PassesElevenProof(value))
        {
            throw new DomainInvariantViolationException(
                "Bsn.ElevenProof",
                $"'{value}' does not pass the eleven-proof (elfproef) checksum.");
        }

        // All-zero digits trivially satisfy the eleven-proof formula (every
        // weighted term is zero) but "000000000" has never been an issued
        // BSN - real BSN validation excludes it explicitly, not just via the
        // checksum.
        if (value == "000000000")
        {
            throw new DomainInvariantViolationException(
                "Bsn.ElevenProof",
                "'000000000' is not a valid BSN.");
        }

        Value = value;
    }

    /// <summary>
    /// (9*d1 + 8*d2 + 7*d3 + 6*d4 + 5*d5 + 4*d6 + 3*d7 + 2*d8 - 1*d9) % 11 == 0
    /// </summary>
    private static bool PassesElevenProof(string digits)
    {
        var sum = 0;
        for (var i = 0; i < 8; i++)
        {
            var weight = 9 - i;
            sum += weight * (digits[i] - '0');
        }

        sum -= digits[8] - '0';

        return sum % 11 == 0;
    }

    public override string ToString() => Value;
}