//
//  TripPlanningEngine.swift
//  SportsTime
//
//  Thin orchestrator that delegates to scenario-specific planners.
//

import Foundation

/// Main entry point for trip planning.
/// Delegates to scenario-specific planners via the ScenarioPlanner protocol.
///
/// - Expected Behavior:
///   - Uses ScenarioPlannerFactory.planner(for:) to select the right planner
///   - Delegates entirely to the selected scenario planner
///   - Applies repeat city filter to successful results
///   - If all options violate repeat city constraint → .failure with .repeatCityViolation
///   - Passes through failures from scenario planners unchanged
///
/// - Invariants:
///   - Never modifies the logic of scenario planners
///   - Always returns a result (success or failure), never throws
///   - Repeat city filter only applied when allowRepeatCities is false
///
final class TripPlanningEngine {

    /// Warnings generated during the last planning run.
    /// Populated when options are filtered out but valid results remain.
    private(set) var warnings: [ConstraintViolation] = []

    /// Plans itineraries based on the request inputs.
    /// Automatically detects which scenario applies and delegates to the appropriate planner.
    ///
    /// - Parameter request: The planning request containing all inputs
    /// - Returns: Ranked itineraries on success, or explicit failure with reason
    func planItineraries(request: PlanningRequest) -> ItineraryResult {

        // Reset warnings from previous run
        warnings = []

        // Warn on empty sports set
        if request.preferences.sports.isEmpty {
            warnings.append(ConstraintViolation(
                type: .missingData,
                description: "No sports selected — results may be empty",
                severity: .warning
            ))
        }

        // Validate date range is not inverted
        if request.preferences.endDate < request.preferences.startDate {
            return .failure(PlanningFailure(
                reason: .missingDateRange,
                violations: [
                    ConstraintViolation(
                        type: .dateRange,
                        description: "End date is before start date",
                        severity: .error
                    )
                ]
            ))
        }

        // Detect scenario and get the appropriate planner
        let planner = ScenarioPlannerFactory.planner(for: request)

        // Delegate to the scenario planner
        let result = planner.plan(request: request)

        // Apply preference filters to successful results
        return applyPreferenceFilters(to: result, request: request)
    }

    // MARK: - Private

    /// Applies allowRepeatCities filter after scenario planners return.
    /// Note: Region filtering is done during game selection in scenario planners.
    /// Tracks excluded options as warnings when valid results remain.
    private func applyPreferenceFilters(
        to result: ItineraryResult,
        request: PlanningRequest
    ) -> ItineraryResult {
        guard case .success(let originalOptions) = result else {
            return result
        }

        var options = originalOptions

        // Filter repeat cities (this is enforced during beam search, but double-check here)
        let preRepeatCount = options.count
        options = RouteFilters.filterRepeatCities(
            options,
            allow: request.preferences.allowRepeatCities
        )

        if options.isEmpty && !request.preferences.allowRepeatCities {
            let violatingCities = RouteFilters.findRepeatCities(in: originalOptions)
            return .failure(PlanningFailure(
                reason: .repeatCityViolation(cities: violatingCities)
            ))
        }

        let repeatCityExcluded = preRepeatCount - options.count
        if repeatCityExcluded > 0 {
            warnings.append(ConstraintViolation(
                type: .general,
                description: "\(repeatCityExcluded) route(s) excluded for visiting the same city on multiple days",
                severity: .warning
            ))
        }

        // Must-stop filter: ensure all must-stop cities appear in routes
        if !request.preferences.mustStopLocations.isEmpty {
            let requiredCities = request.preferences.mustStopLocations
                .map { $0.name.lowercased() }
                .filter { !$0.isEmpty }

            if !requiredCities.isEmpty {
                let preMustStopCount = options.count
                options = options.filter { option in
                    let tripCities = Set(option.stops.map { $0.city.lowercased() })
                    return requiredCities.allSatisfy { tripCities.contains($0) }
                }

                if options.isEmpty {
                    return .failure(PlanningFailure(
                        reason: .noValidRoutes,
                        violations: [
                            ConstraintViolation(
                                type: .mustStop,
                                description: "No routes include all must-stop cities",
                                severity: .error
                            )
                        ]
                    ))
                }

                let mustStopExcluded = preMustStopCount - options.count
                if mustStopExcluded > 0 {
                    let cityList = requiredCities.joined(separator: ", ")
                    warnings.append(ConstraintViolation(
                        type: .mustStop,
                        description: "\(mustStopExcluded) route(s) excluded for missing must-stop cities: \(cityList)",
                        severity: .warning
                    ))
                }
            }
        }

        // Validate travel segments: filter out invalid options
        let preValidCount = options.count
        options = options.filter { $0.isValid }
        if options.isEmpty {
            return .failure(PlanningFailure(
                reason: .noValidRoutes,
                violations: [
                    ConstraintViolation(
                        type: .segmentMismatch,
                        description: "No valid itineraries could be built",
                        severity: .error
                    )
                ]
            ))
        }

        let segmentExcluded = preValidCount - options.count
        if segmentExcluded > 0 {
            warnings.append(ConstraintViolation(
                type: .segmentMismatch,
                description: "\(segmentExcluded) route(s) excluded due to invalid travel segments",
                severity: .warning
            ))
        }

        return .success(options)
    }
}