using System;
using System.IO;
using System.Runtime.InteropServices;
using System.Text;

namespace Capnp
{
    /// <summary>
    /// Supports the deserialization of Cap'n Proto messages from a stream (see https://capnproto.org/encoding.html#serialization-over-a-stream).
    /// Packing and compression cannot be handled yet.
    /// </summary>
    public static class Framing
    {
        /// <summary>
        /// Deserializes a message from given stream.
        /// </summary>
        /// <param name="stream">The stream to read from</param>
        /// <returns>The deserialized message</returns>
        /// <exception cref="ArgumentNullException"><paramref name="stream"/> is null.</exception>
        /// <exception cref="ArgumentException">The stream does not support reading, is null, or is already closed.</exception>
        /// <exception cref="EndOfStreamException">The end of the stream is reached.</exception>
        /// <exception cref="ObjectDisposedException">The stream is closed.</exception>
        /// <exception cref="IOException">An I/O error occurs.</exception>
        /// <exception cref="OverflowException">Encountered invalid framing data.</exception>
        /// <exception cref="OutOfMemoryException">Too many or too large segments, probably due to invalid framing data.</exception>
        public static WireFrame ReadSegments(Stream stream)
        {
            using (var reader = new BinaryReader(stream, Encoding.Default, true))
            {
                uint scountm = reader.ReadUInt32();
                uint scount = checked(scountm + 1);
                var buffers = new Memory<ulong>[scount];

                for (uint i = 0; i < scount; i++)
                {
                    uint size = reader.ReadUInt32();
                    buffers[i] = new Memory<ulong>(new ulong[size]);
                }

                if ((scount & 1) == 0)
                {
                    // Padding
                    reader.ReadUInt32();
                }

                for (uint i = 0; i < scount; i++)
                {
                    var buffer = MemoryMarshal.Cast<ulong, byte>(buffers[i].Span);

                    if (reader.Read(buffer) != buffer.Length)
                    {
                        throw new EndOfStreamException("Expected more bytes according to framing header");
                    }
                }

                return new WireFrame(buffers);
            }
        }
    }
}