import NIOCore
import NIOPosix

/// A fake MySQL server speaking just enough of the wire protocol to complete a plaintext handshake.
///
/// Its greeting advertises the `mysql_native_password` plugin but **not** the `CLIENT_SSL` capability, and
/// it answers the client's handshake response with a bare OK packet — so a client asking for TLS can only
/// end up connected in plaintext. This is what the `prefer` fallback test connects to, proving the driver
/// downgrades to plaintext rather than refusing the connection.
final class PlaintextMySQLServer {

    // MARK: Properties

    /// The port the server listens on, assigned by the system at bind time.
    let port: Int

    /// The listening channel the server accepts connections through.
    private let channel: Channel

    // MARK: Initializers

    private init(
        channel: Channel,
        port: Int
    ) {
        self.channel = channel
        self.port = port
    }

    // MARK: Functions

    /// Starts a server on the loopback interface, on a system-assigned port.
    /// - Returns: the running server, ready to be connected to at ``port``.
    static func start() async throws -> PlaintextMySQLServer {
        let channel = try await ServerBootstrap(group: MultiThreadedEventLoopGroup.singleton)
            .childChannelInitializer { channel in
                channel.eventLoop.makeCompletedFuture {
                    try channel.pipeline.syncOperations.addHandler(Handler())
                }
            }
            .bind(host: "127.0.0.1", port: 0)
            .get()

        guard let port = channel.localAddress?.port else {
            throw ChannelError.unknownLocalAddress
        }

        return .init(
            channel: channel,
            port: port
        )
    }

    /// Stops the server, closing its listening channel.
    func stop() async throws {
        try await channel.close().get()
    }

}

// MARK: - Handlers

private extension PlaintextMySQLServer {

    /// Greets a freshly accepted connection, accepts whatever authentication response arrives,
    /// and closes on anything after that (e.g. a `COM_QUIT`).
    final class Handler: ChannelInboundHandler {

        // MARK: Type aliases

        typealias InboundIn = ByteBuffer
        typealias OutboundOut = ByteBuffer

        // MARK: Properties

        /// Whether the client's handshake response has already been answered with an OK packet.
        private var didAuthenticate = false

        // MARK: Functions

        func channelActive(context: ChannelHandlerContext) {
            context.writeAndFlush(
                wrapOutboundOut(Self.greeting(allocator: context.channel.allocator)),
                promise: nil
            )
        }

        func channelRead(
            context: ChannelHandlerContext,
            data: NIOAny
        ) {
            guard didAuthenticate else {
                didAuthenticate = true

                context.writeAndFlush(
                    wrapOutboundOut(Self.ok(allocator: context.channel.allocator)),
                    promise: nil
                )

                return
            }

            context.close(promise: nil)
        }

        // MARK: Helpers

        /// The `HandshakeV10` greeting, framed and ready to send as the connection's first packet.
        ///
        /// The advertised capabilities are `CLIENT_LONG_PASSWORD`, `CLIENT_PROTOCOL_41`,
        /// `CLIENT_SECURE_CONNECTION`, and `CLIENT_PLUGIN_AUTH` — deliberately **not** `CLIENT_SSL`,
        /// so the client cannot upgrade the connection to TLS.
        private static func greeting(allocator: ByteBufferAllocator) -> ByteBuffer {
            var payload = allocator.buffer(capacity: 80)

            payload.writeInteger(10, endianness: .little, as: UInt8.self)     // protocol version
            payload.writeNullTerminatedString("8.0.0")                       // server version
            payload.writeInteger(1, endianness: .little, as: UInt32.self)    // connection id
            payload.writeBytes([1, 2, 3, 4, 5, 6, 7, 8])                     // auth plugin data, part 1
            payload.writeInteger(0, endianness: .little, as: UInt8.self)     // filler
            payload.writeInteger(0x8201, endianness: .little, as: UInt16.self) // capabilities, lower: LONG_PASSWORD | PROTOCOL_41 | SECURE_CONNECTION
            payload.writeInteger(0x21, endianness: .little, as: UInt8.self)  // character set (utf8)
            payload.writeInteger(0x0002, endianness: .little, as: UInt16.self) // status flags (autocommit)
            payload.writeInteger(0x0008, endianness: .little, as: UInt16.self) // capabilities, upper: PLUGIN_AUTH
            payload.writeInteger(21, endianness: .little, as: UInt8.self)    // auth plugin data length
            payload.writeBytes([UInt8](repeating: 0, count: 10))             // reserved
            payload.writeBytes([9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 0]) // auth plugin data, part 2
            payload.writeNullTerminatedString("mysql_native_password")       // auth plugin name

            return framed(payload, sequence: 0, allocator: allocator)
        }

        /// A bare OK packet, framed as the reply to the client's handshake response.
        private static func ok(allocator: ByteBufferAllocator) -> ByteBuffer {
            var payload = allocator.buffer(capacity: 8)

            payload.writeBytes([0x00, 0x00, 0x00, 0x02, 0x00, 0x00, 0x00])  // OK, no rows, autocommit, no warnings

            return framed(payload, sequence: 2, allocator: allocator)
        }

        /// Wraps a payload in the MySQL packet frame: a 3-byte little-endian length and a sequence byte.
        private static func framed(
            _ payload: ByteBuffer,
            sequence: UInt8,
            allocator: ByteBufferAllocator
        ) -> ByteBuffer {
            var packet = allocator.buffer(capacity: payload.readableBytes + 4)
            var payload = payload

            packet.writeInteger(UInt8(payload.readableBytes & 0xff))
            packet.writeInteger(UInt8((payload.readableBytes >> 8) & 0xff))
            packet.writeInteger(UInt8((payload.readableBytes >> 16) & 0xff))
            packet.writeInteger(sequence)
            packet.writeBuffer(&payload)

            return packet
        }

    }

}