//
//  ItineraryBuilder.swift
//  SportsTime
//
//  Shared utility for building itineraries with travel segments.
//  Used by all scenario planners to convert stops into complete itineraries.
//

import Foundation

/// Result of building an itinerary from stops.
struct BuiltItinerary {
    let stops: [ItineraryStop]
    let travelSegments: [TravelSegment]
    let totalDrivingHours: Double
    let totalDistanceMiles: Double
}

/// Shared logic for building itineraries across all scenario planners.
enum ItineraryBuilder {

    /// Validation that can be performed on each travel segment.
    /// Return `true` if the segment is valid, `false` to reject the itinerary.
    typealias SegmentValidator = (TravelSegment, _ fromStop: ItineraryStop, _ toStop: ItineraryStop) -> Bool

    /// Builds a complete itinerary with travel segments between consecutive stops.
    ///
    /// Algorithm:
    ///   1. Handle edge case: single stop = no travel needed
    ///   2. For each consecutive pair of stops, estimate travel
    ///   3. Optionally validate each segment with custom validator
    ///   4. Accumulate driving hours and distance
    ///   5. Verify invariant: travelSegments.count == stops.count - 1
    ///
    /// - Parameters:
    ///   - stops: The stops to connect with travel segments
    ///   - constraints: Driving constraints (drivers, max hours per day)
    ///   - logPrefix: Prefix for log messages (e.g., "[ScenarioA]")
    ///   - segmentValidator: Optional validation for each segment
    ///
    /// - Returns: Built itinerary if successful, nil if any segment fails
    ///
    static func build(
        stops: [ItineraryStop],
        constraints: DrivingConstraints,
        logPrefix: String = "[ItineraryBuilder]",
        segmentValidator: SegmentValidator? = nil
    ) -> BuiltItinerary? {

        // ──────────────────────────────────────────────────────────────────
        // Edge case: Single stop or empty = no travel needed
        // ──────────────────────────────────────────────────────────────────
        if stops.count <= 1 {
            return BuiltItinerary(
                stops: stops,
                travelSegments: [],
                totalDrivingHours: 0,
                totalDistanceMiles: 0
            )
        }

        // ──────────────────────────────────────────────────────────────────
        // Build travel segments between consecutive stops
        // ──────────────────────────────────────────────────────────────────
        var travelSegments: [TravelSegment] = []
        var totalDrivingHours: Double = 0
        var totalDistance: Double = 0

        for index in 0..<(stops.count - 1) {
            let fromStop = stops[index]
            let toStop = stops[index + 1]

            // Estimate travel for this segment
            guard let segment = TravelEstimator.estimate(
                from: fromStop,
                to: toStop,
                constraints: constraints
            ) else {
                print("\(logPrefix) Failed to estimate travel: \(fromStop.city) -> \(toStop.city)")
                return nil
            }

            // Run optional validator (e.g., arrival time check for Scenario B)
            if let validator = segmentValidator {
                if !validator(segment, fromStop, toStop) {
                    print("\(logPrefix) Segment validation failed: \(fromStop.city) -> \(toStop.city)")
                    return nil
                }
            }

            travelSegments.append(segment)
            totalDrivingHours += segment.estimatedDrivingHours
            totalDistance += segment.estimatedDistanceMiles
        }

        // ──────────────────────────────────────────────────────────────────
        // Verify invariant: segments = stops - 1
        // ──────────────────────────────────────────────────────────────────
        guard travelSegments.count == stops.count - 1 else {
            print("\(logPrefix) Invariant violated: \(travelSegments.count) segments for \(stops.count) stops")
            return nil
        }

        return BuiltItinerary(
            stops: stops,
            travelSegments: travelSegments,
            totalDrivingHours: totalDrivingHours,
            totalDistanceMiles: totalDistance
        )
    }

    // MARK: - Common Validators

    /// Validator that ensures travel duration allows arrival before game start.
    /// Used by Scenario B where selected games have fixed start times.
    ///
    /// This checks if the travel duration is short enough that the user could
    /// theoretically leave after the previous game and arrive before the next.
    ///
    /// - Parameter bufferSeconds: Time buffer before game start (default 1 hour)
    /// - Returns: Validator closure
    ///
    static func arrivalBeforeGameStart(bufferSeconds: TimeInterval = 3600) -> SegmentValidator {
        return { segment, fromStop, toStop in
            guard let gameStart = toStop.firstGameStart else {
                return true  // No game = no constraint
            }

            // Check if there's enough time between departure point and game start
            // Departure assumed after previous day's activities (use departure date as baseline)
            let earliestDeparture = fromStop.departureDate
            let travelDuration = segment.durationSeconds
            let earliestArrival = earliestDeparture.addingTimeInterval(travelDuration)
            let deadline = gameStart.addingTimeInterval(-bufferSeconds)

            if earliestArrival > deadline {
                print("[ItineraryBuilder] Cannot arrive in time: earliest arrival \(earliestArrival) > deadline \(deadline)")
                return false
            }
            return true
        }
    }
}