Rendezvous/src/FinalFactory.Rendezvous.Client/Traversal/DirectConnectionRequestCodec.cs

using System.Text;
using FinalFactory.Rendezvous.Contracts;

namespace FinalFactory.Rendezvous.Client;

public sealed class DirectConnectionRequest
{
    public JoinAttemptId AttemptId { get; set; }
    public string ConnectionTicket { get; set; } = string.Empty;

    public override string ToString() =>
        $"[DirectConnectionRequest {AttemptId}; ticket redacted]";
}

public static class DirectConnectionRequestCodec
{
    public const int EncodedLength = 63;

    private const int MagicLength = 4;
    private const int AttemptIdLength = 16;
    private const int TicketLength = ContractLimits.DerivedCredentialCharacters;
    private static readonly byte[] Magic = [(byte)'R', (byte)'V', (byte)'D', (byte)'1'];

    public static bool IsRendezvousRequest(ReadOnlySpan<byte> encoded) =>
        encoded.Length >= MagicLength && encoded[..MagicLength].SequenceEqual(Magic);

    public static byte[] Encode(JoinAttemptId attemptId, string connectionTicket)
    {
        if (attemptId.Value == Guid.Empty
            || connectionTicket is null
            || connectionTicket.Length != TicketLength
            || !ContractValidation.IsConnectionTicketValid(connectionTicket))
        {
            throw new ArgumentException("The direct connection request fields are invalid.");
        }

        byte[] encoded = new byte[EncodedLength];
        Magic.CopyTo(encoded, 0);
        if (!attemptId.Value.TryWriteBytes(encoded.AsSpan(MagicLength, AttemptIdLength)))
        {
            throw new InvalidOperationException("The join attempt identifier could not be encoded.");
        }

        Encoding.ASCII.GetBytes(
            connectionTicket,
            0,
            connectionTicket.Length,
            encoded,
            MagicLength + AttemptIdLength);
        return encoded;
    }

    public static bool TryDecode(
        ReadOnlySpan<byte> encoded,
        out DirectConnectionRequest? request)
    {
        request = null;
        if (encoded.Length != EncodedLength
            || !encoded[..MagicLength].SequenceEqual(Magic))
        {
            return false;
        }

        Guid attemptId = new(encoded.Slice(MagicLength, AttemptIdLength));
        if (attemptId == Guid.Empty)
        {
            return false;
        }

        string ticket = Encoding.ASCII.GetString(encoded[(MagicLength + AttemptIdLength)..]);
        if (!ContractValidation.IsConnectionTicketValid(ticket))
        {
            return false;
        }

        request = new DirectConnectionRequest
        {
            AttemptId = new JoinAttemptId(attemptId),
            ConnectionTicket = ticket,
        };
        return true;
    }
}