//
//  GroceryStateSyncManager.swift
//  Nextcloud Cookbook iOS Client
//

import Foundation
import OSLog

@MainActor
class GroceryStateSyncManager {
    private weak var appState: AppState?
    private weak var groceryManager: GroceryListManager?

    private var debounceTimers: [String: Task<Void, Never>] = [:]
    private let debounceInterval: TimeInterval = 2.0
    private var isReconciling = false

    private let dataStore = DataStore()
    private let pendingRemovalPath = "grocery_pending_removals.data"
    private(set) var pendingRemovalRecipeIds: Set<String> = []

    init(appState: AppState, groceryManager: GroceryListManager) {
        self.appState = appState
        self.groceryManager = groceryManager
    }

    // MARK: - Push Flow

    /// Debounced sync trigger. Waits `debounceInterval` seconds then pushes state for the recipe.
    func scheduleSync(forRecipeId recipeId: String) {
        guard UserSettings.shared.grocerySyncEnabled else { return }
        guard !isReconciling else { return }

        debounceTimers[recipeId]?.cancel()
        debounceTimers[recipeId] = Task { [weak self] in
            try? await Task.sleep(nanoseconds: UInt64(2_000_000_000))
            guard !Task.isCancelled else { return }
            await self?.pushGroceryState(forRecipeId: recipeId)
        }
    }

    /// Builds local grocery state, fetches server recipe, merges, and PUTs back.
    func pushGroceryState(forRecipeId recipeId: String) async {
        guard let appState, let groceryManager else { return }
        guard let recipeIdInt = Int(recipeId) else { return }

        // Fetch latest recipe from server first so we can detect deletions
        guard let serverRecipe = await appState.getRecipe(id: recipeIdInt, fetchMode: .onlyServer) else {
            Logger.data.error("Grocery sync: failed to fetch recipe \(recipeId) from server")
            return
        }

        let serverState = serverRecipe.groceryState

        // Build local state, passing server state so deleted items can be marked .removed
        let localState = buildLocalState(forRecipeId: recipeId, groceryManager: groceryManager, serverState: serverState)

        // Merge local state with server state
        let merged = mergeStates(local: localState, server: serverState)

        // Upload merged state
        var updatedRecipe = serverRecipe
        updatedRecipe.groceryState = merged
        let (_, alert) = await appState.uploadRecipe(recipeDetail: updatedRecipe, createNew: false)
        if let alert {
            Logger.data.error("Grocery sync: failed to push state for recipe \(recipeId): \(String(describing: alert))")
        }
    }

    // MARK: - Pull Flow

    /// Reconciles server grocery state with local grocery data. Called when a recipe is loaded.
    func reconcileFromServer(serverState: GroceryState?, recipeId: String, recipeName: String) {
        guard let groceryManager else { return }
        guard let serverState, !serverState.items.isEmpty else { return }

        isReconciling = true
        defer { isReconciling = false }

        let localItems = Set(
            groceryManager.groceryDict[recipeId]?.items.map(\.name) ?? []
        )

        for (itemName, itemState) in serverState.items {
            switch itemState.status {
            case .added:
                if !localItems.contains(itemName) {
                    groceryManager.addItem(itemName, toRecipe: recipeId, recipeName: recipeName)
                }
            case .removed:
                if localItems.contains(itemName) {
                    groceryManager.deleteItem(itemName, fromRecipe: recipeId)
                }
            case .completed:
                // Don't re-add completed items; leave local state as-is
                break
            }
        }
    }

    // MARK: - Initial Sync

    /// Pushes any local-only items and reconciles server items on app launch.
    func performInitialSync() async {
        guard let appState, let groceryManager else { return }

        await loadPendingRemovals()

        let recipeIds = Array(groceryManager.groceryDict.keys)
        for recipeId in recipeIds {
            guard let recipeIdInt = Int(recipeId) else { continue }

            // Push local state to server
            await pushGroceryState(forRecipeId: recipeId)

            // Fetch back and reconcile
            if let serverRecipe = await appState.getRecipe(id: recipeIdInt, fetchMode: .onlyServer) {
                let recipeName = groceryManager.groceryDict[recipeId]?.name ?? serverRecipe.name
                reconcileFromServer(
                    serverState: serverRecipe.groceryState,
                    recipeId: recipeId,
                    recipeName: recipeName
                )
            }
        }

        // Push deletion state for recipes whose items were fully removed
        for recipeId in pendingRemovalRecipeIds {
            guard !recipeIds.contains(recipeId) else {
                // Recipe was re-added locally since removal was tracked; clear it
                pendingRemovalRecipeIds.remove(recipeId)
                continue
            }
            await pushGroceryState(forRecipeId: recipeId)
            pendingRemovalRecipeIds.remove(recipeId)
        }
        savePendingRemovals()
    }

    // MARK: - Merge Logic

    /// Merges local and server states using per-item last-writer-wins on `modifiedAt`.
    private func mergeStates(local: GroceryState, server: GroceryState?) -> GroceryState {
        guard let server else { return local }

        var merged = local.items
        for (itemName, serverItem) in server.items {
            if let localItem = merged[itemName] {
                // Both have the item — keep the one with the later modifiedAt
                let localDate = GroceryStateDate.date(from: localItem.modifiedAt) ?? .distantPast
                let serverDate = GroceryStateDate.date(from: serverItem.modifiedAt) ?? .distantPast
                if serverDate > localDate {
                    merged[itemName] = serverItem
                }
            } else {
                // Only server has this item — keep it
                merged[itemName] = serverItem
            }
        }

        // Garbage collection: remove items that are removed/completed and older than 30 days
        let thirtyDaysAgo = Date().addingTimeInterval(-30 * 24 * 60 * 60)
        merged = merged.filter { _, item in
            if item.status == .added { return true }
            guard let modDate = GroceryStateDate.date(from: item.modifiedAt) else { return true }
            return modDate > thirtyDaysAgo
        }

        return GroceryState(
            lastModified: GroceryStateDate.now(),
            items: merged
        )
    }

    // MARK: - Build Local State

    /// Builds a `GroceryState` from the current local grocery data for a recipe.
    /// When `serverState` is provided, any server item with `.added` status that is
    /// absent locally is emitted as `.removed` so the deletion propagates to the server.
    private func buildLocalState(forRecipeId recipeId: String, groceryManager: GroceryListManager, serverState: GroceryState?) -> GroceryState {
        var items: [String: GroceryItemState] = [:]
        let now = GroceryStateDate.now()

        // Existing local items
        if let groceryRecipe = groceryManager.groceryDict[recipeId] {
            for item in groceryRecipe.items {
                let status: GroceryItemState.Status = item.isChecked ? .completed : .added
                items[item.name] = GroceryItemState(status: status, addedAt: now, modifiedAt: now)
            }
        }

        // Mark items that exist on server as .added but are absent locally as .removed
        if let serverState {
            for (itemName, serverItem) in serverState.items {
                if items[itemName] == nil && serverItem.status == .added {
                    items[itemName] = GroceryItemState(
                        status: .removed,
                        addedAt: serverItem.addedAt,
                        modifiedAt: now
                    )
                }
            }
        }

        return GroceryState(lastModified: now, items: items)
    }

    // MARK: - Pending Removal Tracking

    /// Records a recipe ID whose grocery items were fully removed, so that
    /// `performInitialSync` can push the deletion even after the key disappears
    /// from `groceryDict`.
    func trackPendingRemoval(recipeId: String) {
        pendingRemovalRecipeIds.insert(recipeId)
        savePendingRemovals()
    }

    func clearPendingRemoval(recipeId: String) {
        guard pendingRemovalRecipeIds.remove(recipeId) != nil else { return }
        savePendingRemovals()
    }

    private func loadPendingRemovals() async {
        guard let loaded: Set<String> = try? await dataStore.load(fromPath: pendingRemovalPath) else { return }
        pendingRemovalRecipeIds = loaded
    }

    private func savePendingRemovals() {
        Task {
            await dataStore.save(data: pendingRemovalRecipeIds, toPath: pendingRemovalPath)
        }
    }
}