//
// WorkoutSessionCoordinator.swift
// Workouts Watch App
//
// Copyright 2025 Rouslan Zenetl. All Rights Reserved.
//

import Foundation
import HealthKit
import SwiftData

/// What the watch should do with its `HKWorkoutSession` given the latest authoritative
/// workout set. `finish` carries the completed run whose metrics we save + forward.
enum SessionEndDecision: Equatable {
    case none
    case discard
    case finish(WorkoutDocument)
}

/// Pure decision seam for the watch's workout-session lifecycle — the session-free core the
/// `WorkoutSessionCoordinator` delegates to, mirroring the `WatchCacheApplier` pattern so the
/// start/end rules are unit-testable without HealthKit or a live cache.
///
/// The end rule reconciles against the *authoritative* workout set (the same set the phone
/// pushes and the watch prunes to), not against any view's lifetime — a running session ends
/// on a genuine non-empty → empty transition of the active set. Comparing against the active
/// set captured at the *previous* reconcile is what makes the launch race safe: at startup the
/// session can be `running` before the run document has synced, but the previous active set is
/// still empty, so we never discard a run we simply haven't heard about yet.
///
/// The start rule makes the watch own its session: whenever the authoritative set holds an
/// active run and no session is running, one should be started. The phone's
/// `startWatchApp(toHandle:)` handoff then becomes a fast-launch optimization rather than the
/// only way a session is ever born — a dropped handoff (watch unreachable at start, a handoff
/// swallowed by `start`'s idempotency guard while the old run's session was still running),
/// a crash/reboot, or a run engaged manually on the wrist all heal at the next reconcile.
enum SessionEndPlanner {
    /// A `.finish` for a session younger than this is demoted to `.discard`: a seconds-old
    /// session has no meaningful samples, and the demotion is what keeps the two stale-data
    /// races from polluting Apple Health with junk workouts attributed to the wrong run —
    /// (a) a transient "old run completed, new run not yet pushed" context arriving right
    /// after the new run's session started, and (b) a self-start off stale cache at launch,
    /// corrected seconds later by the fresh context. In both cases the active set refills
    /// and `shouldStart` brings the session right back.
    static let minimumFinishAge: TimeInterval = 30

    /// Ids of the runs that keep the session alive — not-yet-finished (`notStarted` /
    /// `inProgress`). A `completed` / `skipped` run has dropped off the active set.
    static func activeIDs(in workouts: [WorkoutDocument]) -> Set<String> {
        Set(workouts
            .filter {
                let status = WorkoutStatus(rawValue: $0.status)
                return status == .notStarted || status == .inProgress
            }
            .map(\.id))
    }

    /// True when a session should be running but isn't — the watch self-starts one rather
    /// than depending on the phone's one-shot launch handoff having arrived.
    static func shouldStart(isRunning: Bool, workouts: [WorkoutDocument]) -> Bool {
        !isRunning && !activeIDs(in: workouts).isEmpty
    }

    /// The run whose routine configures a self-started session: the most recently started
    /// active one (ties broken by id for determinism).
    static func runToStart(in workouts: [WorkoutDocument]) -> WorkoutDocument? {
        let ids = activeIDs(in: workouts)
        return workouts
            .filter { ids.contains($0.id) }
            .max { a, b in a.start == b.start ? a.id < b.id : a.start < b.start }
    }

    /// `sessionAge` is seconds since the running session started (nil when unknown — e.g. a
    /// recovered session, which is by definition old enough to finish normally).
    static func decide(isRunning: Bool,
                       sessionAge: TimeInterval?,
                       previouslyActiveIDs: Set<String>,
                       workouts: [WorkoutDocument]) -> SessionEndDecision {
        guard isRunning,
              activeIDs(in: workouts).isEmpty,
              !previouslyActiveIDs.isEmpty else { return .none }
        // A previously-active run that survived as `.completed` is a finish (save + forward
        // metrics); if none survived, every active run vanished (discarded/deleted) → discard.
        // Among several completed survivors (parallel runs), take the most recently started —
        // deterministic, and the one whose lifetime best matches the session's.
        let finished = previouslyActiveIDs
            .compactMap { id in workouts.first { $0.id == id } }
            .filter { WorkoutStatus(rawValue: $0.status) == .completed }
            .max { a, b in a.start == b.start ? a.id < b.id : a.start < b.start }
        guard let finished else { return .discard }
        // Too young to be a real finish — see `minimumFinishAge`.
        if let sessionAge, sessionAge < minimumFinishAge { return .discard }
        return .finish(finished)
    }
}

/// The single owner of the watch's workout-session lifecycle — both ends of it. Long-lived
/// (held by `WatchAppServices`, not a view), so it runs on the WatchConnectivity delegate
/// callback — with or without a mounted view, including in the session's own background
/// runtime. Termination here is the fix for the leak where ending a run *from the phone*
/// while the watch app was backgrounded left the `HKWorkoutSession` running (watchOS then
/// re-foregrounded the app on every wrist raise) and skipped the metrics capture entirely.
/// Self-starting here is the counterpart fix: a session is started whenever the authoritative
/// set holds an active run and none is running, so a dropped phone launch handoff (or a
/// crash, or a run engaged manually on the wrist) can't leave a workout sessionless — no HR,
/// no Health save, app suspending wrist-down.
///
/// Same move as the live-mirror's `repairFromDurable`, one layer down: drive the terminal
/// action from the authoritative cache apply rather than a transient UI observer.
@MainActor
final class WorkoutSessionCoordinator {
    private let sessionManager: WorkoutSessionManager
    private let bridge: WatchConnectivityBridge
    private let context: ModelContext

    /// The active set captured at the previous reconcile — the baseline the non-empty → empty
    /// transition is measured against (see `SessionEndPlanner`).
    private var previouslyActiveIDs: Set<String> = []

    /// Guards the async finish so a burst of reconciles can't double-end the session.
    private var isEnding = false

    init(sessionManager: WorkoutSessionManager, bridge: WatchConnectivityBridge, context: ModelContext) {
        self.sessionManager = sessionManager
        self.bridge = bridge
        self.context = context
    }

    /// Re-evaluate the session against the current authoritative cache. Called after every
    /// cache mutation (phone push or the watch's own optimistic edit) and once at bootstrap to
    /// seed the baseline. The watch cache holds only active + recently-pruned runs, so the
    /// fetch-and-map is bounded and cheap.
    func reconcile() {
        guard !isEnding else { return }
        let workouts = ((try? context.fetch(FetchDescriptor<Workout>())) ?? [])
            .map(WorkoutDocument.init(from:))
        let decision = SessionEndPlanner.decide(
            isRunning: sessionManager.isRunning,
            sessionAge: sessionManager.sessionStartDate.map { -$0.timeIntervalSinceNow },
            previouslyActiveIDs: previouslyActiveIDs,
            workouts: workouts)
        previouslyActiveIDs = SessionEndPlanner.activeIDs(in: workouts)
        switch decision {
        case .none:            startIfNeeded(workouts: workouts)
        case .discard:         sessionManager.discard()
        case .finish(let doc): endAndForward(doc)
        }
    }

    /// Self-start a session when an active run has none (see `SessionEndPlanner.shouldStart`).
    /// The activity type comes from the run's routine so the saved Health workout is categorized
    /// the same way a phone-launched one would be; a run whose routine is gone (deleted, or not
    /// yet synced) falls back to traditional strength training.
    private func startIfNeeded(workouts: [WorkoutDocument]) {
        guard SessionEndPlanner.shouldStart(isRunning: sessionManager.isRunning, workouts: workouts),
              let run = SessionEndPlanner.runToStart(in: workouts) else { return }
        let routine = run.routineID.flatMap { id in
            (try? context.fetch(FetchDescriptor<Routine>())).flatMap { $0.first { $0.id == id } }
        }
        let configuration = HKWorkoutConfiguration()
        configuration.activityType = routine?.activityTypeEnum.hkActivityType ?? .traditionalStrengthTraining
        configuration.locationType = .indoor
        sessionManager.start(with: configuration)
    }

    /// Finish the session (saving the `HKWorkout` to Health), attach the captured metrics, and
    /// forward the run back to the phone. Lifted from the old `ActiveWorkoutGateView.endSession`
    /// so the behavior is unchanged — only its trigger moved off the view.
    private func endAndForward(_ doc: WorkoutDocument) {
        isEnding = true
        Task {
            defer { isEnding = false }
            guard var metrics = await sessionManager.finishAndSave() else { return }
            metrics.totalVolume = WorkoutVolume.total(doc.logs)
            var updated = doc
            updated.metrics = metrics
            updated.updatedAt = Date()
            bridge.update(workout: updated)   // optimistic upsert + forward (queued if offline)
        }
    }
}