SpanWriter.cs

    public ref struct SpanWriter
    {
        private readonly Span<byte> _buffer;
        private int _position;

        public int Remaining => Count - Position;
        public int Count { get; }

        public int Position
        {
            get => _position;
            private set
            {
                if (value > Count || value < 0)
                {
                    throw new InvalidOperationException(
                        $"Cannot set position at {value} because < 0 or > count:{Count} !");
                }
                _position = value;
            }
        }

        public SpanWriter(Span<byte> buffer)
        {
            Count = buffer.Length;
            _buffer = buffer;
            _position = 0;
            Position = 0;
        }

        public void Seek(int position) => Position = position;
        public void Advance(int count) => Position += count;

        public Span<byte> GetSpan(int count = 0)
            => count == 0
                ? _buffer.Slice(Position)
                : _buffer.Slice(Position, count);
    }