//
//  MealPlanSyncManager.swift
//  Nextcloud Cookbook iOS Client
//

import Foundation
import OSLog

@MainActor
class MealPlanSyncManager {
    private weak var appState: AppState?
    private weak var mealPlanManager: MealPlanManager?

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

    init(appState: AppState, mealPlanManager: MealPlanManager) {
        self.appState = appState
        self.mealPlanManager = mealPlanManager
    }

    // MARK: - Push Flow

    func scheduleSync(forRecipeId recipeId: String) {
        guard UserSettings.shared.mealPlanSyncEnabled 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?.pushMealPlanState(forRecipeId: recipeId)
        }
    }

    func pushMealPlanState(forRecipeId recipeId: String) async {
        guard let appState, let mealPlanManager else { return }
        guard let recipeIdInt = Int(recipeId) else { return }

        let localAssignment = mealPlanManager.assignment(forRecipeId: recipeId)

        guard let serverRecipe = await appState.getRecipe(id: recipeIdInt, fetchMode: .onlyServer) else {
            Logger.data.error("Meal plan sync: failed to fetch recipe \(recipeId) from server")
            return
        }

        let merged = mergeAssignments(local: localAssignment, server: serverRecipe.mealPlanAssignment)

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

    // MARK: - Pull Flow

    func reconcileFromServer(serverAssignment: MealPlanAssignment?, recipeId: String, recipeName: String) {
        guard let mealPlanManager else { return }
        mealPlanManager.reconcileFromServer(serverAssignment: serverAssignment, recipeId: recipeId, recipeName: recipeName)
    }

    // MARK: - Full Sync

    func performSync() async {
        guard let appState, let mealPlanManager else { return }

        // Phase 1: Push locally-known meal plan state
        let localRecipeIds = Array(Set(
            mealPlanManager.entriesByDate.values.flatMap { $0 }.map(\.recipeId)
        ))
        for recipeId in localRecipeIds {
            await pushMealPlanState(forRecipeId: recipeId)
        }

        // Phase 2: Discover meal plan assignments from server
        let allRecipes = await appState.getRecipes()
        let lastSync = UserSettings.shared.lastMealPlanSyncDate

        // Filter to recipes modified since last sync
        let recipesToCheck: [Recipe]
        if let lastSync {
            let formatter = DateFormatter()
            formatter.dateFormat = "yyyy-MM-dd HH:mm:ss"
            formatter.timeZone = TimeZone(secondsFromGMT: 0)
            recipesToCheck = allRecipes.filter { recipe in
                guard let dateStr = recipe.dateModified,
                      let date = formatter.date(from: dateStr) else { return true }
                return date > lastSync
            }
        } else {
            recipesToCheck = allRecipes  // First sync: check all
        }

        // Fetch details concurrently (max 5 parallel)
        await withTaskGroup(of: (String, String, MealPlanAssignment?)?.self) { group in
            var iterator = recipesToCheck.makeIterator()
            let maxConcurrent = 5
            var active = 0

            while active < maxConcurrent, let recipe = iterator.next() {
                active += 1
                group.addTask {
                    guard let detail = await appState.getRecipe(
                        id: recipe.recipe_id, fetchMode: .onlyServer
                    ) else { return nil }
                    return (String(recipe.recipe_id), detail.name, detail.mealPlanAssignment)
                }
            }

            for await result in group {
                if let (recipeId, recipeName, assignment) = result,
                   let assignment, !assignment.dates.isEmpty {
                    mealPlanManager.reconcileFromServer(
                        serverAssignment: assignment,
                        recipeId: recipeId,
                        recipeName: recipeName
                    )
                }
                if let recipe = iterator.next() {
                    group.addTask {
                        guard let detail = await appState.getRecipe(
                            id: recipe.recipe_id, fetchMode: .onlyServer
                        ) else { return nil }
                        return (String(recipe.recipe_id), detail.name, detail.mealPlanAssignment)
                    }
                }
            }
        }

        UserSettings.shared.lastMealPlanSyncDate = Date()
    }

    // MARK: - Merge Logic

    private func mergeAssignments(local: MealPlanAssignment?, server: MealPlanAssignment?) -> MealPlanAssignment {
        guard let local else { return server ?? MealPlanAssignment() }
        guard let server else { return local }

        var merged = local.dates
        for (dayStr, serverEntry) in server.dates {
            if let localEntry = merged[dayStr] {
                let localDate = MealPlanDate.date(from: localEntry.modifiedAt) ?? .distantPast
                let serverDate = MealPlanDate.date(from: serverEntry.modifiedAt) ?? .distantPast
                if serverDate > localDate {
                    merged[dayStr] = serverEntry
                }
            } else {
                merged[dayStr] = serverEntry
            }
        }

        // Prune all date entries older than 30 days
        let cutoff = Calendar.current.date(byAdding: .day, value: -30, to: Calendar.current.startOfDay(for: Date()))!
        merged = merged.filter { dayStr, _ in
            guard let date = MealPlanDate.dateFromDay(dayStr) else { return true }
            return date >= cutoff
        }

        return MealPlanAssignment(
            lastModified: MealPlanDate.now(),
            dates: merged
        )
    }
}