//
//  CocoaMQTTDeliver.swift
//  CocoaMQTT
//
//  Created by HJianBo on 2019/5/2.
//  Copyright © 2019 emqx.io. All rights reserved.
//

import Foundation
import Dispatch

protocol CocoaMQTTDeliverProtocol: AnyObject {

    var delegateQueue: DispatchQueue { get set }

    func deliver(_ deliver: CocoaMQTTDeliver, wantToSend frame: Frame)
}

private struct InflightFrame {

    /// The infligth frame maybe a `FramePublish` or `FramePubRel`
    var frame: Frame

    /// Monotonic time (Dispatch uptime) at which this frame should be retried next.
    var nextRetryAtUptimeNs: UInt64

}

extension Array where Element == InflightFrame {

    func filterMap(isIncluded: (Element) -> (Bool, Element)) -> [Element] {
        var tmp = [Element]()
        for e in self {
            let res = isIncluded(e)
            if res.0 {
                tmp.append(res.1)
            }
        }
        return tmp
    }
}

// CocoaMQTTDeliver
class CocoaMQTTDeliver: NSObject {

    /// The dispatch queue is used by delivering frames in serially
    private var deliverQueue = DispatchQueue.init(label: "deliver.cocoamqtt.emqx", qos: .default)

    weak var delegate: CocoaMQTTDeliverProtocol?

    fileprivate var inflight = [InflightFrame]()

    fileprivate var mqueue = [Frame]()

    var mqueueSize: UInt = 1000

    var inflightWindowSize: UInt = 10

    /// Retry time interval millisecond
    var retryTimeInterval: Double = 5000

    private var awaitingTimer: CocoaMQTTTimer?

    var isQueueEmpty: Bool { mqueue.isEmpty }
    var isQueueFull: Bool { mqueue.count >= mqueueSize }
    var isInflightFull: Bool { inflight.count >= inflightWindowSize }
    var isInflightEmpty: Bool { inflight.isEmpty }

    var storage: CocoaMQTTStorage?

    func recoverSessionBy(_ storage: CocoaMQTTStorage) {
        let frames = storage.readAll()
        // Sync to push the frame to mqueue for avoiding overcommit
        deliverQueue.sync {
            self.storage = storage
            for f in frames {
                mqueue.append(f)
            }
            if !frames.isEmpty {
                printInfo("Deliver recover \(frames.count) msgs")
                printDebug("Recover message \(frames)")
            }
        }

        guard !frames.isEmpty else {
            return
        }

        deliverQueue.async { [weak self] in
            guard let self = self else { return }
            self.tryTransport()
        }
    }

    /// Add a FramePublish to the message queue to wait for sending
    ///
    /// return false means the frame is rejected because of the buffer is full
    func add(_ frame: FramePublish) -> Bool {
        guard !isQueueFull else {
            printError("Sending buffer is full, frame \(frame) has been rejected to add.")
            return false
        }

        // Sync to push the frame to mqueue for avoiding overcommit
        deliverQueue.sync {
            mqueue.append(frame)
            _ = storage?.write(frame)
        }

        deliverQueue.async { [weak self] in
            guard let self = self else { return }
            self.tryTransport()
        }

        return true
    }

    /// Acknowledge a PUBLISH/PUBREL by msgid
    func ack(by frame: Frame) {
        let msgid: UInt16
        if let puback = frame as? FramePubAck {
            msgid = puback.msgid
        } else if let pubrec = frame as? FramePubRec {
            msgid = pubrec.msgid
        } else if let pubcom = frame as? FramePubComp {
            msgid = pubcom.msgid
        } else {
            return
        }

        let ackType = frame.type
        let shouldRemoveFromStorage = frame is FramePubAck || frame is FramePubComp
        let ackFrameDescription = String(describing: frame)

        deliverQueue.async { [weak self] in
            guard let self = self else { return }
            let acked = self.ackInflightFrame(withMsgid: msgid, type: ackType)
            if acked.count == 0 {
                printWarning("Acknowledge by \(ackFrameDescription), but not found in inflight window")
            } else {
                // TODO: ACK DONT DELETE PUBREL
                for f in acked where shouldRemoveFromStorage {
                    self.storage?.remove(f)
                }
                printDebug("Acknowledge frame id \(msgid) success, acked: \(acked)")
                self.tryTransport()
            }
        }
    }

    /// Clean Inflight content to prevent message blocked, when next connection established
    ///
    /// !!Warning: it's a temporary method for hotfix #221
    func cleanAll() {
        deliverQueue.sync { [weak self] in
            guard let self = self else { return }
            self.mqueue.removeAll()
            self.inflight.removeAll()
        }
    }
}

// MARK: Private Funcs
extension CocoaMQTTDeliver {

    // try transport a frame from mqueue to inflight
    private func tryTransport() {
        if isQueueEmpty || isInflightFull { return }

        // take out the earliest frame
        if mqueue.isEmpty { return }
        let frame = mqueue.remove(at: 0)

        deliver(frame)

        // keep trying after a transport
        self.tryTransport()
    }

    /// Try to deliver a frame
    private func deliver(_ frame: Frame) {
        if frame.qos == .qos0 {
            // Send Qos0 message, whatever the in-flight queue is full
            // TODO: A retrict deliver mode is need?
            sendfun(frame)
        } else {

            sendfun(frame)
            let nowUptimeNs = DispatchTime.now().uptimeNanoseconds
            inflight.append(InflightFrame(frame: frame, nextRetryAtUptimeNs: nextRetryDeadline(from: nowUptimeNs)))

            // Start a retry timer for resending it if it not receive PUBACK or PUBREC
            if awaitingTimer == nil {
                awaitingTimer = CocoaMQTTTimer.every(retryTimeInterval / 1000.0, name: "awaitingTimer") { [weak self] in
                    guard let self = self else { return }
                    self.deliverQueue.async {
                        self.redeliver()
                    }
                }
            }
        }
    }

    /// Attempt to redeliver in-flight messages
    private func redeliver(nowUptimeNs: UInt64 = DispatchTime.now().uptimeNanoseconds) {
        if isInflightEmpty {
            // Revoke the awaiting timer
            awaitingTimer = nil
            return
        }
        for (idx, frame) in inflight.enumerated() where nowUptimeNs >= frame.nextRetryAtUptimeNs {
            var duplicatedFrame = frame
            duplicatedFrame.frame.dup = true
            duplicatedFrame.nextRetryAtUptimeNs = nextRetryDeadline(after: frame.nextRetryAtUptimeNs, nowUptimeNs: nowUptimeNs)

            inflight[idx] = duplicatedFrame

            printInfo("Re-delivery frame \(duplicatedFrame.frame)")
            sendfun(duplicatedFrame.frame)
        }
    }

    private func retryIntervalNanoseconds() -> UInt64 {
        let intervalNs = retryTimeInterval * 1_000_000
        guard intervalNs.isFinite, intervalNs > 0 else {
            return 1
        }
        if intervalNs >= Double(UInt64.max) {
            return UInt64.max
        }
        return UInt64(intervalNs.rounded())
    }

    private func nextRetryDeadline(from nowUptimeNs: UInt64) -> UInt64 {
        let (nextDeadline, overflow) = nowUptimeNs.addingReportingOverflow(retryIntervalNanoseconds())
        return overflow ? UInt64.max : nextDeadline
    }

    private func nextRetryDeadline(after currentDeadline: UInt64, nowUptimeNs: UInt64) -> UInt64 {
        let intervalNs = retryIntervalNanoseconds()
        guard nowUptimeNs >= currentDeadline else {
            return currentDeadline
        }

        let missedIntervals = ((nowUptimeNs - currentDeadline) / intervalNs) + 1
        let (advance, multiplyOverflow) = intervalNs.multipliedReportingOverflow(by: missedIntervals)
        if multiplyOverflow {
            return UInt64.max
        }
        let (nextDeadline, addOverflow) = currentDeadline.addingReportingOverflow(advance)
        return addOverflow ? UInt64.max : nextDeadline
    }

    @discardableResult
    private func ackInflightFrame(withMsgid msgid: UInt16, type: FrameType) -> [Frame] {
        var ackedFrames = [Frame]()
        inflight = inflight.filterMap { frame in

            // -- ACK for PUBLISH
            if let publish = frame.frame as? FramePublish,
               publish.msgid == msgid {

                if publish.qos == .qos2 && type == .pubrec {  // -- Replace PUBLISH with PUBREL
                    let pubrel = FramePubRel(msgid: publish.msgid)

                    var nframe = frame
                    nframe.frame = pubrel
                    nframe.nextRetryAtUptimeNs = nextRetryDeadline(from: DispatchTime.now().uptimeNanoseconds)

                    _ = storage?.write(pubrel)
                    sendfun(pubrel)

                    ackedFrames.append(publish)
                    return (true, nframe)
                } else if publish.qos == .qos1 && type == .puback {
                    ackedFrames.append(publish)
                    return (false, frame)
                }
            }

            // -- ACK for PUBREL
            if let pubrel = frame.frame as? FramePubRel,
               pubrel.msgid == msgid && type == .pubcomp {

                ackedFrames.append(pubrel)
                return (false, frame)
            }
            return (true, frame)
        }

        return ackedFrames
    }

    private func sendfun(_ frame: Frame) {
        guard let delegate = self.delegate else {
            printError("The deliver delegate is nil!!! the frame will be drop: \(frame)")
            return
        }

        if frame.qos == .qos0 {
            if let p = frame as? FramePublish { storage?.remove(p) }
        }

        delegate.delegateQueue.async {
            delegate.deliver(self, wantToSend: frame)
        }
    }
}

// For tests
extension CocoaMQTTDeliver {

    func t_inflightFrames() -> [Frame] {
        var frames = [Frame]()
        for f in inflight {
            frames.append(f.frame)
        }
        return frames
    }

    func t_queuedFrames() -> [Frame] {
        return mqueue
    }

    @discardableResult
    func t_setInflightNextRetryTime(_ nextRetryAtUptimeNs: UInt64, forMsgid msgid: UInt16) -> Bool {
        return deliverQueue.sync {
            for idx in inflight.indices {
                if let publish = inflight[idx].frame as? FramePublish, publish.msgid == msgid {
                    inflight[idx].nextRetryAtUptimeNs = nextRetryAtUptimeNs
                    return true
                }
                if let pubrel = inflight[idx].frame as? FramePubRel, pubrel.msgid == msgid {
                    inflight[idx].nextRetryAtUptimeNs = nextRetryAtUptimeNs
                    return true
                }
            }
            return false
        }
    }

    func t_retryIntervalNanoseconds() -> UInt64 {
        return deliverQueue.sync {
            retryIntervalNanoseconds()
        }
    }

    func t_redeliver(atUptimeNanoseconds uptimeNs: UInt64) {
        deliverQueue.sync {
            self.redeliver(nowUptimeNs: uptimeNs)
        }
    }
}