import Foundation
import Observation
import SwiftData
import WatchConnectivity

/// Watch side of the iPhone↔Watch bridge. The watch never touches iCloud — it
/// keeps a local SwiftData cache fed only by application-context pushes from the
/// phone, updates it optimistically on local edits, and forwards changed workouts
/// to the phone (which is the sole writer of iCloud Drive).
@Observable
@MainActor
final class WatchConnectivityBridge: NSObject {
    private let container: ModelContainer
    private var session: WCSession?

    /// Last time state was received from the phone (for a sync indicator).
    private(set) var lastSyncDate: Date?

    /// Exclusive-edit lock pushed by the phone. While set, the watch parks the matching
    /// run (popping out of its progress view) and blocks re-entry, so the phone owns the
    /// edit and the watch can't clobber it with a stale optimistic write. `editingWorkoutID`
    /// matches a run by its workout id; `editingSplitID` matches any run by its `splitID`.
    private(set) var editingWorkoutID: String?
    private(set) var editingSplitID: String?

    /// Monotonic sequence stamped on each live-run frame we send. Bumped to stay ahead of any
    /// frame we *receive*, so the two devices share one increasing per-run sequence and either
    /// side can drop a stale / out-of-order delivery (see `LiveProgress.version`).
    private var liveVersion = 0

    /// The latest live-run message we haven't confirmed reached the phone (depth 1, latest-wins).
    /// Staged regardless of reachability and re-sent by `flushLive()` when reachability/activation
    /// returns, so a brief WatchConnectivity drop doesn't desync the mirror. A newer frame — or the
    /// terminal `.ended` — replaces it, so we never deliver stale state. Frames carry an absolute
    /// wall-clock anchor, so a late re-send self-corrects on arrival rather than reading as stale.
    private var pendingLive: PendingLive?

    /// The latest live-run frame the *phone* sent, for the run we currently have open to apply
    /// (ephemeral; nil when the phone isn't driving). The watch's `ExerciseProgressView` reads
    /// this to follow a phone-driven transition; it's never persisted.
    private(set) var liveIncoming: LiveProgress?

    /// The run currently open in the watch's navigated driver. When the incoming frame is for
    /// it, the watch follows inline there and suppresses the follower cover (so it never stacks
    /// on top of a run the user already has open).
    var navigatedRunID: String?

    /// A run the user dismissed the follower cover for; suppressed until that run ends.
    private var mutedLogID: String?

    /// The frame to present as a follower cover when the phone drives a run the watch isn't
    /// already showing: the latest, unless the user dismissed it or has that run open inline.
    var presentable: LiveProgress? {
        guard let f = liveIncoming, f.logID != mutedLogID, f.logID != navigatedRunID else { return nil }
        return f
    }

    /// The user dismissed the follower cover; don't re-present this run until it ends.
    func muteLive() { mutedLogID = liveIncoming?.logID }

    private var context: ModelContext { container.mainContext }

    init(container: ModelContainer) {
        self.container = container
        super.init()
    }

    func activate() {
        guard WCSession.isSupported() else { return }
        let session = WCSession.default
        session.delegate = self
        session.activate()
        self.session = session
        // Apply whatever the phone last pushed, then ask for a fresh push.
        let ctx = session.receivedApplicationContext
        applyState(WCPayload.decodeSplits(ctx), workouts: WCPayload.decodeWorkouts(ctx))
        applySettings(ctx)
        editingWorkoutID = WCPayload.decodeEditingWorkoutID(ctx)
        editingSplitID = WCPayload.decodeEditingSplitID(ctx)
        requestSync()
    }

    func requestSync() {
        guard let session, session.activationState == .activated, session.isReachable else { return }
        session.sendMessage(WCPayload.requestSyncMessage(), replyHandler: nil, errorHandler: nil)
    }

    /// Optimistically applies a workout edit to the local cache and forwards it to
    /// the phone for durable persistence in iCloud Drive.
    func update(workout doc: WorkoutDocument) {
        CacheMapper.upsertWorkout(doc, relativePath: doc.relativePath, into: context)
        try? context.save()
        sendToPhone(doc)
    }

    // MARK: - Live run mirror (ephemeral; coalesced redelivery)

    /// Broadcast where the run flow currently is, so a propped-up iPhone can mirror it. Staged as
    /// the latest pending frame and sent when the phone is reachable; if it's unreachable the frame
    /// is held and re-sent on reconnect (`flushLive`). Because frames are full state snapshots with
    /// a wall-clock anchor, holding only the newest one (depth 1) and self-correcting its timers on
    /// arrival means a re-send is never stale.
    func sendLiveProgress(_ frame: LiveProgress) {
        guard let session, session.activationState == .activated else { return }
        liveVersion += 1
        var stamped = frame
        stamped.version = liveVersion
        pendingLive = .progress(stamped)
        flushLive()
    }

    /// Tell the phone to stop mirroring this run (the user left the progress flow). Staged like a
    /// frame so a drop at the moment the run ends doesn't strand the phone's follower cover — the
    /// terminal marker supersedes any pending progress and is re-sent on reconnect.
    func sendLiveEnded(workoutID: String, logID: String) {
        guard let session, session.activationState == .activated else { return }
        pendingLive = .ended(workoutID: workoutID, logID: logID)
        flushLive()
    }

    /// (Re)send the staged live-run message if the phone is reachable. Called on each new frame and
    /// whenever reachability/activation is restored. Leaves it staged on failure; a newer frame or
    /// `.ended` supersedes it, so we never deliver stale state. The error handler runs on
    /// WatchConnectivity's background queue, so it must be nonisolated (@Sendable) — an
    /// inherited-@MainActor closure would trap (swift_task_checkIsolated) there.
    private func flushLive() {
        guard let session, let pending = pendingLive,
              session.activationState == .activated, session.isReachable else { return }
        let payload: [String: Any]
        switch pending {
        case .progress(let frame):
            payload = WCPayload.encodeLiveProgress(frame)
        case .ended(let workoutID, let logID):
            payload = WCPayload.encodeLiveEnded(workoutID: workoutID, logID: logID)
        }
        session.sendMessage(payload, replyHandler: nil, errorHandler: { @Sendable _ in })
    }

    /// Apply a live-run frame the phone sent. Drops a stale one for the same run, and catches
    /// our send counter up so the next frame we send outranks it (shared per-run sequence).
    private func applyIncomingLive(_ frame: LiveProgress) {
        liveVersion = max(liveVersion, frame.version)
        if let current = liveIncoming, current.logID == frame.logID, frame.version < current.version { return }
        liveIncoming = frame
    }

    /// The phone left the run — stop following it (and clear any dismiss for it).
    private func endIncomingLive(logID: String) {
        if liveIncoming?.logID == logID { liveIncoming = nil }
        if mutedLogID == logID { mutedLogID = nil }
    }

    // MARK: - Internal

    private func sendToPhone(_ doc: WorkoutDocument) {
        guard let session, session.activationState == .activated else { return }
        let payload = WCPayload.encodeWorkoutUpdate(doc)
        if session.isReachable {
            // The error handler runs on WatchConnectivity's background queue, so it must be
            // nonisolated (@Sendable) or it would trap (swift_task_checkIsolated). Hop back to the
            // MainActor to fall back to guaranteed delivery; `doc` is Sendable, the payload isn't.
            session.sendMessage(payload, replyHandler: nil, errorHandler: { @Sendable _ in
                Task { @MainActor in
                    _ = self.session?.transferUserInfo(WCPayload.encodeWorkoutUpdate(doc))
                }
            })
        } else {
            session.transferUserInfo(payload)
        }
    }

    private func applySettings(_ dict: [String: Any]) {
        if let rest = WCPayload.decodeRestSeconds(dict) {
            UserDefaults.standard.set(rest, forKey: WCPayload.restSecondsKey)
        }
        if let done = WCPayload.decodeDoneCountdownSeconds(dict) {
            UserDefaults.standard.set(done, forKey: WCPayload.doneCountdownSecondsKey)
        }
    }

    private func applyState(_ splits: [SplitDocument], workouts: [WorkoutDocument]) {
        guard !splits.isEmpty || !workouts.isEmpty else { return }
        var liveSplitIDs = Set<String>()
        for s in splits {
            CacheMapper.upsertSplit(s, relativePath: s.relativePath, into: context)
            liveSplitIDs.insert(s.id)
        }
        var liveWorkoutIDs = Set<String>()
        for w in workouts {
            CacheMapper.upsertWorkout(w, relativePath: w.relativePath, into: context)
            liveWorkoutIDs.insert(w.id)
        }
        // Both are authoritative sets → prune anything the phone no longer sends. For
        // workouts that set is every active run plus recently-completed ones (~24h), so a
        // run that was discarded/deleted on the phone (or aged out of the window) drops out
        // of the push and is pruned here — which empties the active list and ends the
        // session. The watch never originates a workout, so pruning can't lose local data.
        if let allSplits = try? context.fetch(FetchDescriptor<Split>()) {
            for s in allSplits where !liveSplitIDs.contains(s.id) { context.delete(s) }
        }
        if let allWorkouts = try? context.fetch(FetchDescriptor<Workout>()) {
            for w in allWorkouts where !liveWorkoutIDs.contains(w.id) { context.delete(w) }
        }
        try? context.save()
        lastSyncDate = Date()
    }
}

// MARK: - WCSessionDelegate

extension WatchConnectivityBridge: WCSessionDelegate {
    nonisolated func session(_ session: WCSession, activationDidCompleteWith activationState: WCSessionActivationState, error: Error?) {
        Task { @MainActor in
            self.requestSync()
            self.flushLive()   // deliver any frame staged before the session was ready
        }
    }

    nonisolated func sessionReachabilityDidChange(_ session: WCSession) {
        if session.isReachable {
            Task { @MainActor in self.flushLive() }   // catch the phone up on the latest run state after a reconnect
        }
    }

    /// Live-run frames arrive as messages (reachable-only), distinct from the latest-wins
    /// application context that carries durable state.
    nonisolated func session(_ session: WCSession, didReceiveMessage message: [String: Any]) {
        switch message[WCPayload.typeKey] as? String {
        case WCPayload.liveProgressType:
            if let frame = WCPayload.decodeLiveProgress(message) {
                Task { @MainActor in self.applyIncomingLive(frame) }
            }
        case WCPayload.liveEndedType:
            if let logID = message[WCPayload.lpLogIDKey] as? String {
                Task { @MainActor in self.endIncomingLive(logID: logID) }
            }
        default:
            break
        }
    }

    nonisolated func session(_ session: WCSession, didReceiveApplicationContext applicationContext: [String: Any]) {
        let splits = WCPayload.decodeSplits(applicationContext)
        let workouts = WCPayload.decodeWorkouts(applicationContext)
        let rest = WCPayload.decodeRestSeconds(applicationContext)
        let done = WCPayload.decodeDoneCountdownSeconds(applicationContext)
        let editingWorkoutID = WCPayload.decodeEditingWorkoutID(applicationContext)
        let editingSplitID = WCPayload.decodeEditingSplitID(applicationContext)
        Task { @MainActor in
            self.applyState(splits, workouts: workouts)
            if let rest { UserDefaults.standard.set(rest, forKey: WCPayload.restSecondsKey) }
            if let done { UserDefaults.standard.set(done, forKey: WCPayload.doneCountdownSecondsKey) }
            // Absent keys mean "not editing" — set unconditionally so the lock clears.
            self.editingWorkoutID = editingWorkoutID
            self.editingSplitID = editingSplitID
        }
    }
}

/// The single staged live-run message awaiting (re)delivery to the phone — see `pendingLive`.
/// One slot, latest-wins: a newer progress frame or the terminal `.ended` replaces whatever's held.
private enum PendingLive {
    case progress(LiveProgress)
    case ended(workoutID: String, logID: String)
}