mxaccess/src/MxNativeCodec/NmxObservedFrame.cs

using System.Buffers.Binary;
using System.Text;

namespace MxNativeCodec;

public sealed record NmxObservedEnvelope(
    bool HasLengthPrefix,
    int? TotalLengthPrefix,
    int DeclaredInnerLength,
    int ActualInnerLength,
    ReadOnlyMemory<byte> Header,
    ReadOnlyMemory<byte> InnerBody)
{
    public const int HeaderLength = 46;
    public const int InnerLengthOffset = 2;

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

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

        return new NmxObservedEnvelope(
            false,
            null,
            declaredInnerLength,
            actualInnerLength,
            body[..HeaderLength],
            body.Slice(HeaderLength, actualInnerLength));
    }

    public static NmxObservedEnvelope ParseProcessDataReceivedBody(ReadOnlyMemory<byte> body)
    {
        if (body.Length < sizeof(int) + HeaderLength)
        {
            throw new ArgumentException("NMX ProcessDataReceived body is too short.", nameof(body));
        }

        int totalLengthPrefix = BinaryPrimitives.ReadInt32LittleEndian(body.Span[..sizeof(int)]);
        if (totalLengthPrefix != body.Length)
        {
            throw new ArgumentException("NMX ProcessDataReceived length prefix does not match body size.", nameof(body));
        }

        int headerOffset = sizeof(int);
        int declaredInnerLength = BinaryPrimitives.ReadInt32LittleEndian(
            body.Span.Slice(headerOffset + InnerLengthOffset, sizeof(int)));
        int actualInnerLength = declaredInnerLength - sizeof(int);
        if (actualInnerLength < 0 || headerOffset + HeaderLength + actualInnerLength != body.Length)
        {
            throw new ArgumentException("NMX ProcessDataReceived inner length does not match body size.", nameof(body));
        }

        return new NmxObservedEnvelope(
            true,
            totalLengthPrefix,
            declaredInnerLength,
            actualInnerLength,
            body.Slice(headerOffset, HeaderLength),
            body.Slice(headerOffset + HeaderLength, actualInnerLength));
    }

    public static NmxObservedEnvelope ParseProcessDataReceivedBodyFlexible(ReadOnlyMemory<byte> body)
    {
        if (body.Length >= sizeof(int) + HeaderLength)
        {
            int totalLengthPrefix = BinaryPrimitives.ReadInt32LittleEndian(body.Span[..sizeof(int)]);
            if (totalLengthPrefix == body.Length)
            {
                return ParseProcessDataReceivedBody(body);
            }
        }

        if (body.Length < HeaderLength)
        {
            throw new ArgumentException("NMX ProcessDataReceived body is too short.", nameof(body));
        }

        int declaredInnerLength = BinaryPrimitives.ReadInt32LittleEndian(body.Span.Slice(InnerLengthOffset, sizeof(int)));
        int actualInnerLength = body.Length - HeaderLength;
        if (declaredInnerLength != actualInnerLength)
        {
            throw new ArgumentException("NMX ProcessDataReceived header inner length does not match body size.", nameof(body));
        }

        return new NmxObservedEnvelope(
            false,
            null,
            declaredInnerLength,
            actualInnerLength,
            body[..HeaderLength],
            body.Slice(HeaderLength, actualInnerLength));
    }
}

public sealed record NmxObservedString(int Offset, string Value);

public sealed record NmxObservedMessage(
    byte Command,
    string CommandName,
    byte VersionMajor,
    byte VersionMinor,
    Guid? ItemCorrelationId,
    IReadOnlyList<NmxObservedString> Strings)
{
    public static NmxObservedMessage Parse(ReadOnlySpan<byte> body)
    {
        if (body.Length < 3)
        {
            throw new ArgumentException("NMX message body is too short.", nameof(body));
        }

        byte command = body[0];
        Guid? itemCorrelationId = null;
        if (command is 0x1f or 0x21 && body.Length >= 19)
        {
            itemCorrelationId = new Guid(body.Slice(3, 16));
        }

        return new NmxObservedMessage(
            command,
            GetCommandName(command),
            body[1],
            body[2],
            itemCorrelationId,
            ExtractUtf16Strings(body));
    }

    private static string GetCommandName(byte command)
    {
        return command switch
        {
            0x17 => "MetadataQuery",
            0x1f => "AdviseSupervisory",
            0x21 => "UnAdvise",
            0x32 => "SubscriptionStatus",
            0x33 => "DataUpdate",
            0x37 => "Write",
            0x40 => "MetadataResponse",
            _ => $"Unknown0x{command:X2}",
        };
    }

    private static IReadOnlyList<NmxObservedString> ExtractUtf16Strings(ReadOnlySpan<byte> body)
    {
        List<NmxObservedString> strings = [];
        int offset = 0;
        while (offset + 8 <= body.Length)
        {
            int start = offset;
            int chars = 0;
            while (offset + 1 < body.Length)
            {
                byte lo = body[offset];
                byte hi = body[offset + 1];
                if (lo == 0 && hi == 0)
                {
                    break;
                }

                if (hi != 0 || lo < 0x20 || lo > 0x7e)
                {
                    chars = 0;
                    break;
                }

                chars++;
                offset += 2;
            }

            if (chars >= 3 && offset + 1 < body.Length && body[offset] == 0 && body[offset + 1] == 0)
            {
                string value = Encoding.Unicode.GetString(body.Slice(start, chars * 2));
                strings.Add(new NmxObservedString(start, value));
                offset += 2;
                continue;
            }

            offset = start + 1;
        }

        return strings;
    }
}