import Foundation
import NIOCore

/// Bidirectional TCP forwarder. Pairs two channels so bytes flow in both directions.
/// Used for CONNECT tunneling (passthrough mode, no MITM).
final class GlueHandler: ChannelInboundHandler, RemovableChannelHandler {
    typealias InboundIn = ByteBuffer
    typealias OutboundOut = ByteBuffer

    var partner: GlueHandler?
    private var context: ChannelHandlerContext?
    private var pendingRead = false

    func handlerAdded(context: ChannelHandlerContext) {
        self.context = context
    }

    func handlerRemoved(context: ChannelHandlerContext) {
        self.context = nil
        self.partner = nil
    }

    func channelRead(context: ChannelHandlerContext, data: NIOAny) {
        partner?.write(unwrapInboundIn(data))
    }

    func channelReadComplete(context: ChannelHandlerContext) {
        partner?.flush()
    }

    func channelInactive(context: ChannelHandlerContext) {
        partner?.close()
    }

    func errorCaught(context: ChannelHandlerContext, error: Error) {
        context.close(promise: nil)
    }

    func channelWritabilityChanged(context: ChannelHandlerContext) {
        if context.channel.isWritable {
            partner?.read()
        }
    }

    // MARK: - Partner operations

    private func write(_ buffer: ByteBuffer) {
        context?.write(wrapOutboundOut(buffer), promise: nil)
    }

    private func flush() {
        context?.flush()
    }

    private func read() {
        if let context, !pendingRead {
            pendingRead = true
            context.read()
            pendingRead = false
        }
    }

    private func close() {
        context?.close(promise: nil)
    }
}