mxaccess/src/MxNativeCodec/NmxTransferEnvelopeTemplate.cs

using System.Buffers.Binary;

namespace MxNativeCodec;

public sealed class NmxTransferEnvelopeTemplate
{
    public const int HeaderLength = 46;
    public const int InnerLengthOffset = 2;

    private readonly byte[] _header;

    private NmxTransferEnvelopeTemplate(byte[] header)
    {
        _header = header;
    }

    public static NmxTransferEnvelopeTemplate FromObserved(ReadOnlySpan<byte> observedTransferBody)
    {
        if (observedTransferBody.Length < HeaderLength)
        {
            throw new ArgumentException("Observed TransferData body is too short.", nameof(observedTransferBody));
        }

        int innerLength = BinaryPrimitives.ReadInt32LittleEndian(observedTransferBody.Slice(InnerLengthOffset, sizeof(int)));
        if (innerLength != observedTransferBody.Length - HeaderLength)
        {
            throw new ArgumentException("Observed TransferData body does not contain the expected inner length.", nameof(observedTransferBody));
        }

        return new NmxTransferEnvelopeTemplate(observedTransferBody[..HeaderLength].ToArray());
    }

    public byte[] Encode(ReadOnlySpan<byte> innerPutRequestBody)
    {
        byte[] body = new byte[HeaderLength + innerPutRequestBody.Length];
        _header.CopyTo(body, 0);
        BinaryPrimitives.WriteInt32LittleEndian(body.AsSpan(InnerLengthOffset, sizeof(int)), innerPutRequestBody.Length);
        innerPutRequestBody.CopyTo(body.AsSpan(HeaderLength));
        return body;
    }

    public ReadOnlyMemory<byte> DecodeInner(ReadOnlyMemory<byte> transferBody)
    {
        if (transferBody.Length < HeaderLength)
        {
            throw new ArgumentException("TransferData body is too short.", nameof(transferBody));
        }

        int innerLength = BinaryPrimitives.ReadInt32LittleEndian(transferBody.Span.Slice(InnerLengthOffset, sizeof(int)));
        if (innerLength != transferBody.Length - HeaderLength)
        {
            throw new ArgumentException("TransferData body inner length does not match the body size.", nameof(transferBody));
        }

        return transferBody[HeaderLength..];
    }
}