Rendezvous/src/FinalFactory.Rendezvous.Server/Transport/LiteNetNatRequestCodec.cs

using System.Buffers.Binary;
using System.Net;
using System.Text;
using FinalFactory.Rendezvous.Contracts;

namespace FinalFactory.Rendezvous.Server.Transport;

internal static class LiteNetNatRequestCodec
{
    private const byte NatMessageProperty = 17;
    private const int TypeIdentifierLength = 8;
    private const int TokenLengthPrefix = NatPunchRequestTokenCodec.EncodedLength + 1;
    // LiteNetLib 2.1.4's private NatIntroduceRequest type ID. The native socket
    // integration test deliberately fails if a package upgrade changes this wire value.
    private static ReadOnlySpan<byte> RequestTypeIdentifier =>
        [0x88, 0xbe, 0x10, 0x26, 0xbf, 0xb1, 0x66, 0x9c];

    public static bool TryDecode(
        ReadOnlySpan<byte> datagram,
        out IPEndPoint? claimedLocalEndpoint,
        out string? token)
    {
        claimedLocalEndpoint = null;
        token = null;
        if (datagram.Length < 1 + TypeIdentifierLength + 1 + 4 + 2 + 2
            + NatPunchRequestTokenCodec.EncodedLength
            || datagram[0] != NatMessageProperty
            || !datagram.Slice(1, TypeIdentifierLength).SequenceEqual(RequestTypeIdentifier))
        {
            return false;
        }

        int offset = 1 + TypeIdentifierLength;
        int addressLength = datagram[offset++] switch
        {
            0 => 4,
            1 => 16,
            _ => 0,
        };
        int expectedLength = offset + addressLength + 2 + 2
            + NatPunchRequestTokenCodec.EncodedLength;
        if (addressLength == 0 || datagram.Length != expectedLength)
        {
            return false;
        }

        IPAddress localAddress = new(datagram.Slice(offset, addressLength));
        offset += addressLength;
        int localPort = BinaryPrimitives.ReadUInt16LittleEndian(datagram.Slice(offset, 2));
        offset += 2;
        int encodedTokenLength = BinaryPrimitives.ReadUInt16LittleEndian(datagram.Slice(offset, 2));
        offset += 2;
        if (localPort == 0 || encodedTokenLength != TokenLengthPrefix)
        {
            return false;
        }

        ReadOnlySpan<byte> tokenBytes = datagram.Slice(
            offset,
            NatPunchRequestTokenCodec.EncodedLength);
        for (int index = 0; index < tokenBytes.Length; index++)
        {
            if (tokenBytes[index] > 0x7f)
            {
                return false;
            }
        }

        claimedLocalEndpoint = new(localAddress, localPort);
        token = Encoding.ASCII.GetString(tokenBytes);
        return true;
    }
}