//===----------------------------------------------------------------------===//
// Copyright © 2025 Apple Inc. and the container project authors. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//   https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//===----------------------------------------------------------------------===//

import NIOCore

final class GlueHandler {

    private var partner: GlueHandler?

    private var context: ChannelHandlerContext?

    private var pendingRead: Bool = false

    private init() {}
}

extension GlueHandler {
    static func matchedPair() -> (GlueHandler, GlueHandler) {
        let first = GlueHandler()
        let second = GlueHandler()

        first.partner = second
        second.partner = first

        return (first, second)
    }
}

extension GlueHandler {
    private func partnerWrite(_ data: NIOAny) {
        self.context?.write(data, promise: nil)
    }

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

    private func partnerWriteEOF() {
        self.context?.close(mode: .output, promise: nil)
    }

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

    private func partnerBecameWritable() {
        if self.pendingRead {
            self.pendingRead = false
            self.context?.read()
        }
    }

    private var partnerWritable: Bool {
        self.context?.channel.isWritable ?? false
    }
}

extension GlueHandler: ChannelDuplexHandler {
    typealias InboundIn = NIOAny
    typealias OutboundIn = NIOAny
    typealias OutboundOut = NIOAny

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

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

    func channelRead(context: ChannelHandlerContext, data: NIOAny) {
        self.partner?.partnerWrite(data)
    }

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

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

    func userInboundEventTriggered(context: ChannelHandlerContext, event: Any) {
        if let event = event as? ChannelEvent, case .inputClosed = event {
            // We have read EOF.
            self.partner?.partnerWriteEOF()
        }
    }

    func errorCaught(context: ChannelHandlerContext, error: Error) {
        self.partner?.partnerCloseFull()
    }

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

    func read(context: ChannelHandlerContext) {
        if let partner = self.partner, partner.partnerWritable {
            context.read()
        } else {
            self.pendingRead = true
        }
    }
}