namespace Fleet.Rpc;

/// <summary>
/// 发件箱二进制帧：<c>u8 channel | 16 字节 msg_id (UUIDv7) | 正文</c>。
/// 服务端按 msg_id 去重，持久化之后才回复 <c>fleet.outbox.ack</c>。
/// </summary>
public static class OutboxFrame
{
    public const int HeaderBytes = 1 + 16;

    public static bool TryParse(ReadOnlyMemory<byte> frame, out byte channel, out Guid msgId, out ReadOnlyMemory<byte> payload)
    {
        if (frame.Length < HeaderBytes)
        {
            channel = 0;
            msgId = Guid.Empty;
            payload = default;
            return false;
        }

        channel = frame.Span[0];
        msgId = new Guid(frame.Span.Slice(1, 16), bigEndian: true);
        payload = frame[HeaderBytes..];
        return true;
    }

    public static byte[] Build(byte channel, Guid msgId, ReadOnlySpan<byte> payload)
    {
        byte[] frame = new byte[HeaderBytes + payload.Length];
        frame[0] = channel;
        msgId.TryWriteBytes(frame.AsSpan(1, 16), bigEndian: true, out _);
        payload.CopyTo(frame.AsSpan(HeaderBytes));
        return frame;
    }
}