using System.Buffers;
using System.Buffers.Binary;
using Google.Protobuf;
using ZB.MOM.WW.OtOpcUa.Core.Scripting.Protocol.V1;

namespace ZB.MOM.WW.OtOpcUa.Core.Scripting.Protocol;

/// <summary>
///     Length-prefixed protobuf framing for the ScriptWorker pipe (lmxopcua#780, design §4.2): a 4-byte little-endian
///     length, then one serialized <see cref="Envelope"/>. Copies mxaccessgw's <c>WorkerFrameProtocol</c> discipline:
///     zero-length, oversize, truncated and unparseable frames are all a <see cref="ScriptWorkerProtocolException"/>,
///     after which the stream is unusable and the peer is dropped.
/// </summary>
public static class ScriptWorkerFrameCodec
{
    /// <summary>The protocol version both ends put on every envelope.</summary>
    public const uint ProtocolVersion = 1;

    /// <summary>Default frame cap: 4 MiB.</summary>
    public const int DefaultMaxFrameBytes = 4 * 1024 * 1024;

    /// <summary>Writes one frame. Not thread-safe: callers serialise writes.</summary>
    /// <param name="stream">The pipe.</param>
    /// <param name="envelope">The envelope.</param>
    /// <param name="maxFrameBytes">The negotiated cap; a larger envelope is refused before anything is written.</param>
    /// <param name="ct">Cancellation.</param>
    /// <returns>A task that completes when the frame is written (not flushed).</returns>
    public static async ValueTask WriteAsync(Stream stream, Envelope envelope, int maxFrameBytes, CancellationToken ct = default)
    {
        ArgumentNullException.ThrowIfNull(stream);
        ArgumentNullException.ThrowIfNull(envelope);
        var size = envelope.CalculateSize();
        if (size == 0) throw new ScriptWorkerProtocolException("refusing to write an empty frame");
        if (size > maxFrameBytes)
            throw new ScriptWorkerProtocolException($"frame of {size} bytes exceeds the {maxFrameBytes}-byte cap");

        var buffer = ArrayPool<byte>.Shared.Rent(size + 4);
        try
        {
            BinaryPrimitives.WriteInt32LittleEndian(buffer, size);
            envelope.WriteTo(new Span<byte>(buffer, 4, size));
            await stream.WriteAsync(buffer.AsMemory(0, size + 4), ct).ConfigureAwait(false);
        }
        finally
        {
            ArrayPool<byte>.Shared.Return(buffer);
        }
    }

    /// <summary>Reads one frame.</summary>
    /// <param name="stream">The pipe.</param>
    /// <param name="maxFrameBytes">The negotiated cap.</param>
    /// <param name="ct">Cancellation.</param>
    /// <returns>The envelope, or null on a clean end of stream at a frame boundary.</returns>
    /// <exception cref="ScriptWorkerProtocolException">A zero-length, oversize, truncated or unparseable frame.</exception>
    public static async ValueTask<Envelope?> ReadAsync(Stream stream, int maxFrameBytes, CancellationToken ct = default)
    {
        ArgumentNullException.ThrowIfNull(stream);
        var header = new byte[4];
        var got = await ReadFullyAsync(stream, header, ct).ConfigureAwait(false);
        if (got == 0) return null;
        if (got < 4) throw new ScriptWorkerProtocolException("truncated frame header");

        var size = BinaryPrimitives.ReadInt32LittleEndian(header);
        CheckLength(size, maxFrameBytes);

        var body = ArrayPool<byte>.Shared.Rent(size);
        try
        {
            var read = await ReadFullyAsync(stream, body.AsMemory(0, size), ct).ConfigureAwait(false);
            if (read < size) throw new ScriptWorkerProtocolException($"truncated frame: {read} of {size} bytes");
            return ParseBody(body.AsSpan(0, size));
        }
        finally
        {
            ArrayPool<byte>.Shared.Return(body);
        }
    }

    /// <summary>
    ///     Whether this platform's pipe should be driven with blocking I/O (lmxopcua#789). On Unix a .NET pipe is a
    ///     socket, and ONE asynchronous operation on it switches it to non-blocking for good, after which even a
    ///     "blocking" read goes through the socket engine (an epoll-thread hand-off and a spinning wait per frame) —
    ///     measured as most of the host's per-evaluation CPU. Kept blocking, a read is a plain kernel wait on a
    ///     dedicated thread. Not on Windows, where an overlapped pipe handle is needed for full duplex and its
    ///     asynchronous path is the efficient one.
    /// </summary>
    public static bool UseBlockingIo => !OperatingSystem.IsWindows();

    /// <summary>Writes one frame synchronously (the blocking-I/O handshake). Not thread-safe.</summary>
    /// <param name="stream">The pipe.</param>
    /// <param name="envelope">The envelope.</param>
    /// <param name="maxFrameBytes">The cap.</param>
    public static void Write(Stream stream, Envelope envelope, int maxFrameBytes)
    {
        ArgumentNullException.ThrowIfNull(stream);
        ArgumentNullException.ThrowIfNull(envelope);
        var size = envelope.CalculateSize();
        if (size == 0) throw new ScriptWorkerProtocolException("refusing to write an empty frame");
        if (size > maxFrameBytes)
            throw new ScriptWorkerProtocolException($"frame of {size} bytes exceeds the {maxFrameBytes}-byte cap");
        var buffer = ArrayPool<byte>.Shared.Rent(size + 4);
        try
        {
            BinaryPrimitives.WriteInt32LittleEndian(buffer, size);
            envelope.WriteTo(new Span<byte>(buffer, 4, size));
            stream.Write(buffer, 0, size + 4);
        }
        finally
        {
            ArrayPool<byte>.Shared.Return(buffer);
        }
    }

    /// <summary>
    ///     Reads exactly one frame synchronously, never past it (the blocking-I/O handshake: what follows belongs to the
    ///     connection's own reader).
    /// </summary>
    /// <param name="stream">The pipe.</param>
    /// <param name="maxFrameBytes">The cap.</param>
    /// <returns>The envelope, or null on a clean end of stream at a frame boundary.</returns>
    public static Envelope? Read(Stream stream, int maxFrameBytes)
    {
        ArgumentNullException.ThrowIfNull(stream);
        Span<byte> header = stackalloc byte[4];
        var got = stream.ReadAtLeast(header, 4, throwOnEndOfStream: false);
        if (got == 0) return null;
        if (got < 4) throw new ScriptWorkerProtocolException("truncated frame header");
        var size = BinaryPrimitives.ReadInt32LittleEndian(header);
        CheckLength(size, maxFrameBytes);
        var body = ArrayPool<byte>.Shared.Rent(size);
        try
        {
            var read = stream.ReadAtLeast(body.AsSpan(0, size), size, throwOnEndOfStream: false);
            if (read < size) throw new ScriptWorkerProtocolException($"truncated frame: {read} of {size} bytes");
            return ParseBody(body.AsSpan(0, size));
        }
        finally
        {
            ArrayPool<byte>.Shared.Return(body);
        }
    }

    /// <summary>Validates a frame's length prefix against the cap.</summary>
    /// <param name="size">The decoded length.</param>
    /// <param name="maxFrameBytes">The negotiated cap.</param>
    /// <exception cref="ScriptWorkerProtocolException">A zero, negative or oversize length.</exception>
    internal static void CheckLength(int size, int maxFrameBytes)
    {
        if (size == 0) throw new ScriptWorkerProtocolException("zero-length frame");
        if (size < 0 || size > maxFrameBytes)
            throw new ScriptWorkerProtocolException($"frame length {size} is outside 1..{maxFrameBytes}");
    }

    /// <summary>Parses and validates one frame body (everything after the length prefix).</summary>
    /// <param name="body">The body bytes.</param>
    /// <returns>The envelope.</returns>
    /// <exception cref="ScriptWorkerProtocolException">Unparseable, the wrong protocol version, or no body.</exception>
    internal static Envelope ParseBody(ReadOnlySpan<byte> body)
    {
        Envelope envelope;
        try
        {
            envelope = Envelope.Parser.ParseFrom(body);
        }
        catch (InvalidProtocolBufferException ex)
        {
            throw new ScriptWorkerProtocolException($"unparseable frame: {ex.Message}", ex);
        }
        if (envelope.ProtocolVersion != ProtocolVersion)
            throw new ScriptWorkerProtocolException(
                $"protocol version {envelope.ProtocolVersion} is not the supported {ProtocolVersion}");
        if (envelope.BodyCase == Envelope.BodyOneofCase.None)
            throw new ScriptWorkerProtocolException("frame carries no body");
        return envelope;
    }

    private static async ValueTask<int> ReadFullyAsync(Stream stream, Memory<byte> buffer, CancellationToken ct)
    {
        var total = 0;
        while (total < buffer.Length)
        {
            var n = await stream.ReadAsync(buffer[total..], ct).ConfigureAwait(false);
            if (n == 0) break;
            total += n;
        }
        return total;
    }
}

/// <summary>A framing or schema violation on the ScriptWorker pipe; the connection is dropped.</summary>
public sealed class ScriptWorkerProtocolException : Exception
{
    /// <summary>Initializes the exception.</summary>
    /// <param name="message">What was wrong.</param>
    /// <param name="inner">The underlying parser error, if any.</param>
    public ScriptWorkerProtocolException(string message, Exception? inner = null) : base(message, inner) { }
}