import Foundation

struct AutocompleteEngine {

    struct AutocompleteResult {
        let filledSlots: [(slotId: UUID, dishId: UUID)]
        let unfilledCount: Int
    }

    struct RuleViolation {
        let slotId: UUID
        let dayOfWeek: Int
        let mealType: String
        let dishId: UUID
        let dishName: String
        /// Which rules this assignment breaks — the panel turns these into a
        /// plain-language explanation and a suggested fix.
        let reasons: [Reason]

        struct Reason: Hashable {
            enum Kind: Hashable {
                case maxPerWeek
                case noConsecutive
                case noDuplicateInDay
                case mealTypeOnly
                case dayRestriction
                case repeatedInWeek
            }
            let kind: Kind
            let tagId: UUID?
            let limit: Int?
            /// "lunch"/"dinner" for mealTypeOnly, "weekdays"/"weekend" for dayRestriction.
            let restriction: String?

            init(kind: Kind, tagId: UUID? = nil, limit: Int? = nil, restriction: String? = nil) {
                self.kind = kind
                self.tagId = tagId
                self.limit = limit
                self.restriction = restriction
            }
        }
    }

    // recentPlans: up to 12 previous weeks for period detection (weekly/bi-weekly/monthly cycles).
    static func autocomplete(
        emptySlots: [MealSlot],
        currentPlan: WeekPlan,
        allDishes: [Dish],
        allTags: [Tag],
        recentPlans: [WeekPlan] = [],
        rejectionCounts: [UUID: Int] = [:]
    ) -> AutocompleteResult {
        let tagMap = Dictionary(allTags.map { ($0.id, $0) }, uniquingKeysWith: { first, _ in first })
        let dishMap = Dictionary(allDishes.map { ($0.id, $0) }, uniquingKeysWith: { first, _ in first })
        let historyScores = periodScores(dishes: allDishes, recentPlans: recentPlans, currentWeekStart: currentPlan.weekStartDate)
        let ratingScores = Self.ratingScores(dishes: allDishes, recentPlans: recentPlans)

        var filledSlots: [(slotId: UUID, dishId: UUID)] = []
        var unfilledCount = 0
        var pendingSlots = emptySlots.filter { !$0.isEatingOut && !$0.isSkipped }

        // Fixed-slot rules win outright: a dish pinned to a weekday+meal fills
        // that slot before any scoring. Runs before the MRV loop so the pinned
        // dish also counts against no-repeat/tag rules for the rest of the week.
        for slot in pendingSlots {
            guard let fixed = allDishes.first(where: {
                $0.fixedDayOfWeek == slot.dayOfWeek && $0.fixedMealType == slot.mealType
            }) else { continue }
            slot.dishId = fixed.id
            filledSlots.append((slotId: slot.id, dishId: fixed.id))
        }
        pendingSlots.removeAll { $0.dishId != nil }

        // MRV: at each step pick the slot with fewest valid candidates first.
        // Assignments update currentPlan.slotList in-place, so subsequent candidate
        // counts automatically reflect the growing set of committed dishes.
        while !pendingSlots.isEmpty {
            // Score each remaining slot by strict candidate count
            let ranked = pendingSlots
                .map { slot -> (slot: MealSlot, strict: [Dish]) in
                    let strict = allDishes.filter {
                        !violatesRules(dish: $0, slot: slot, plan: currentPlan, tagMap: tagMap, dishMap: dishMap)
                    }
                    return (slot, strict)
                }
                .sorted { $0.strict.count < $1.strict.count }

            let (slot, strictCandidates) = ranked[0]
            pendingSlots.removeAll { $0.id == slot.id }

            // Fallback: relax the implicit no-repeat rule if strict set is empty
            let candidates: [Dish]
            if strictCandidates.isEmpty {
                candidates = allDishes.filter {
                    !violatesExplicitRules(dish: $0, slot: slot, plan: currentPlan, tagMap: tagMap, dishMap: dishMap)
                }
            } else {
                candidates = strictCandidates
            }

            if candidates.isEmpty {
                unfilledCount += 1
                continue
            }

            if let picked = weightedRandomPick(candidates: candidates, plan: currentPlan, historyScores: historyScores, ratingScores: ratingScores, rejectionCounts: rejectionCounts) {
                slot.dishId = picked.id
                filledSlots.append((slotId: slot.id, dishId: picked.id))
            } else {
                unfilledCount += 1
            }
        }

        return AutocompleteResult(filledSlots: filledSlots, unfilledCount: unfilledCount)
    }

    static func findViolations(plan: WeekPlan, allDishes: [Dish], allTags: [Tag]) -> [RuleViolation] {
        let dishMap = Dictionary(allDishes.map { ($0.id, $0) }, uniquingKeysWith: { first, _ in first })
        let tagMap = Dictionary(allTags.map { ($0.id, $0) }, uniquingKeysWith: { first, _ in first })
        // Computed from the current plan instead of the stored
        // `isRuleOverridden` flag: that flag went stale (it listed meals whose
        // conflict had already been resolved, with nothing to explain) and it
        // missed violations that only appear when a rule is edited after
        // planning. Slots the user explicitly dismissed stay hidden.
        return plan.slotList.compactMap { slot in
            guard let dishId = slot.dishId,
                  let dish = dishMap[dishId],
                  !slot.isEatingOut,
                  !slot.isSkipped,
                  !slot.isRuleIgnored else { return nil }
            let found = reasons(dish: dish, slot: slot, plan: plan, tagMap: tagMap, dishMap: dishMap)
            guard !found.isEmpty else { return nil }
            return RuleViolation(
                slotId: slot.id,
                dayOfWeek: slot.dayOfWeek,
                mealType: slot.mealType,
                dishId: dishId,
                dishName: dish.name,
                reasons: found
            )
        }
    }

    /// Every rule the dish breaks in this slot. Mirrors `violatesExplicitRules`
    /// but collects instead of short-circuiting, so the UI can explain itself.
    static func reasons(
        dish: Dish,
        slot: MealSlot,
        plan: WeekPlan,
        tagMap: [UUID: Tag],
        dishMap: [UUID: Dish]
    ) -> [RuleViolation.Reason] {
        var found: [RuleViolation.Reason] = []

        if plan.slotList.contains(where: { $0.id != slot.id && $0.dishId == dish.id }) {
            found.append(.init(kind: .repeatedInWeek))
        }

        for tagId in dish.tagIds {
            guard let tag = tagMap[tagId] else { continue }

            if let maxPerWeek = tag.maxPerWeek {
                var count = 0
                for s in plan.slotList {
                    guard let did = s.dishId, let d = dishMap[did] else { continue }
                    if d.tagIds.contains(tagId) { count += 1 }
                }
                if count > maxPerWeek {
                    found.append(.init(kind: .maxPerWeek, tagId: tagId, limit: maxPerWeek))
                }
            }

            if tag.noConsecutive {
                for adjSlot in adjacentSlots(of: slot, in: plan) {
                    guard let did = adjSlot.dishId, let d = dishMap[did] else { continue }
                    if d.tagIds.contains(tagId) {
                        found.append(.init(kind: .noConsecutive, tagId: tagId))
                        break
                    }
                }
            }

            if tag.noDuplicateInDay {
                for sdSlot in plan.slotList where sdSlot.dayOfWeek == slot.dayOfWeek && sdSlot.id != slot.id {
                    guard let did = sdSlot.dishId, let d = dishMap[did] else { continue }
                    if d.tagIds.contains(tagId) {
                        found.append(.init(kind: .noDuplicateInDay, tagId: tagId))
                        break
                    }
                }
            }

            if let restriction = tag.mealTypeRestriction, slot.mealType != restriction {
                found.append(.init(kind: .mealTypeOnly, tagId: tagId, restriction: restriction))
            }

            if let dayRestriction = tag.dayRestriction {
                let broken = (dayRestriction == "weekdays" && slot.dayOfWeek > 4) ||
                             (dayRestriction == "weekend" && slot.dayOfWeek < 5)
                if broken {
                    found.append(.init(kind: .dayRestriction, tagId: tagId, restriction: dayRestriction))
                }
            }
        }
        return found
    }

    static func validateDrop(
        dish: Dish,
        slot: MealSlot,
        plan: WeekPlan,
        allTags: [Tag],
        allDishes: [Dish]
    ) -> Bool {
        let tagMap = Dictionary(allTags.map { ($0.id, $0) }, uniquingKeysWith: { first, _ in first })
        let dishMap = Dictionary(allDishes.map { ($0.id, $0) }, uniquingKeysWith: { first, _ in first })
        return !violatesRules(dish: dish, slot: slot, plan: plan, tagMap: tagMap, dishMap: dishMap)
    }

    // MARK: - Rule checks

    private static func violatesRules(
        dish: Dish,
        slot: MealSlot,
        plan: WeekPlan,
        tagMap: [UUID: Tag],
        dishMap: [UUID: Dish]
    ) -> Bool {
        violatesDefaultNoRepeatRule(dish: dish, slot: slot, plan: plan) ||
        violatesExplicitRules(dish: dish, slot: slot, plan: plan, tagMap: tagMap, dishMap: dishMap)
    }

    private static func violatesDefaultNoRepeatRule(dish: Dish, slot: MealSlot, plan: WeekPlan) -> Bool {
        plan.slotList.contains { $0.id != slot.id && $0.dishId == dish.id }
    }

    private static func violatesExplicitRules(
        dish: Dish,
        slot: MealSlot,
        plan: WeekPlan,
        tagMap: [UUID: Tag],
        dishMap: [UUID: Dish]
    ) -> Bool {
        for tagId in dish.tagIds {
            guard let tag = tagMap[tagId] else { continue }

            if let maxPerWeek = tag.maxPerWeek {
                var count = 0
                for s in plan.slotList {
                    guard let did = s.dishId, let d = dishMap[did] else { continue }
                    if d.tagIds.contains(tagId) { count += 1 }
                }
                if count >= maxPerWeek { return true }
            }

            if tag.noConsecutive {
                for adjSlot in adjacentSlots(of: slot, in: plan) {
                    guard let did = adjSlot.dishId, let d = dishMap[did] else { continue }
                    if d.tagIds.contains(tagId) { return true }
                }
            }

            if tag.noDuplicateInDay {
                for sdSlot in plan.slotList where sdSlot.dayOfWeek == slot.dayOfWeek && sdSlot.id != slot.id {
                    guard let did = sdSlot.dishId, let d = dishMap[did] else { continue }
                    if d.tagIds.contains(tagId) { return true }
                }
            }

            if let restriction = tag.mealTypeRestriction, slot.mealType != restriction {
                return true
            }

            if let dayRestriction = tag.dayRestriction {
                switch dayRestriction {
                case "weekdays" where slot.dayOfWeek > 4: return true
                case "weekend" where slot.dayOfWeek < 5: return true
                default: break
                }
            }
        }
        return false
    }

    private static func adjacentSlots(of slot: MealSlot, in plan: WeekPlan) -> [MealSlot] {
        var result: [MealSlot] = []
        if slot.dayOfWeek > 0,
           let prev = plan.slotList.first(where: { $0.dayOfWeek == slot.dayOfWeek - 1 && $0.mealType == slot.mealType }) {
            result.append(prev)
        }
        if slot.dayOfWeek < 6,
           let next = plan.slotList.first(where: { $0.dayOfWeek == slot.dayOfWeek + 1 && $0.mealType == slot.mealType }) {
            result.append(next)
        }
        return result
    }

    // MARK: - Period-based scoring

    // Weight multiplier per dish based on its detected recurrence cycle.
    // Values > 1.0 boost a dish that is "due"; < 1.0 suppress one used too recently.
    // Requires recentPlans covering ≥ 8–12 weeks to reliably detect monthly cycles.
    private static func periodScores(
        dishes: [Dish],
        recentPlans: [WeekPlan],
        currentWeekStart: Date
    ) -> [UUID: Double] {
        guard !recentPlans.isEmpty else { return [:] }
        let calendar = Calendar.current
        var scores: [UUID: Double] = [:]

        for dish in dishes {
            // Week offsets: how many weeks ago did this dish appear?
            var offsets: [Int] = []
            for plan in recentPlans {
                guard plan.slotList.contains(where: { $0.dishId == dish.id }) else { continue }
                let diff = calendar.dateComponents([.weekOfYear], from: plan.weekStartDate, to: currentWeekStart).weekOfYear ?? 0
                if diff > 0 { offsets.append(diff) }
            }
            guard !offsets.isEmpty else { continue }

            let weeksSinceLast = offsets.min()!

            guard offsets.count >= 2 else {
                // Single sighting: simple recency penalty, no cycle to detect
                switch weeksSinceLast {
                case 1:  scores[dish.id] = 0.20
                case 2:  scores[dish.id] = 0.50
                case 3:  scores[dish.id] = 0.75
                default: break  // ≥4 weeks ago → neutral (1.0)
                }
                continue
            }

            // Estimate recurrence period via median of consecutive intervals
            let sorted = offsets.sorted()
            var intervals: [Int] = []
            for i in 0..<sorted.count - 1 {
                intervals.append(sorted[i + 1] - sorted[i])
            }
            let period = medianDouble(intervals)

            // Score by how well weeksSinceLast aligns with the detected period
            let ratio = Double(weeksSinceLast) / period
            let score: Double
            switch ratio {
            case ..<0.50:     score = 0.10  // too soon — strongly suppress
            case 0.50..<0.75: score = 0.30  // a bit early
            case 0.75..<1.25: score = 3.00  // right on schedule — boost
            case 1.25..<1.75: score = 2.00  // slightly overdue
            case 1.75..<2.50: score = 1.50  // overdue
            default:          score = 1.00  // very overdue or pattern changed
            }
            scores[dish.id] = score
        }
        return scores
    }

    private static func medianDouble(_ values: [Int]) -> Double {
        let s = values.sorted()
        let n = s.count
        return n % 2 == 1 ? Double(s[n / 2]) : Double(s[n / 2 - 1] + s[n / 2]) / 2.0
    }

    // MARK: - Rating-based scoring

    // Boost dishes that appeared in liked weeks; penalise those from disliked weeks.
    // Signal is intentionally modest (±0.5 max) so period scoring dominates.
    private static func ratingScores(dishes: [Dish], recentPlans: [WeekPlan]) -> [UUID: Double] {
        let ratedPlans = recentPlans.filter { $0.userRating != 0 }
        guard !ratedPlans.isEmpty else { return [:] }
        var scores: [UUID: Double] = [:]
        for dish in dishes {
            var liked = 0, disliked = 0
            for plan in ratedPlans {
                guard plan.slotList.contains(where: { $0.dishId == dish.id }) else { continue }
                if plan.userRating > 0 { liked += 1 } else { disliked += 1 }
            }
            guard liked + disliked > 0 else { continue }
            let boost   = min(0.50, Double(liked)    * 0.20)
            let penalty = min(0.50, Double(disliked) * 0.25)
            scores[dish.id] = max(0.50, 1.0 + boost - penalty)
        }
        return scores
    }

    // MARK: - Weighted pick

    private static func weightedRandomPick(
        candidates: [Dish],
        plan: WeekPlan,
        historyScores: [UUID: Double] = [:],
        ratingScores: [UUID: Double] = [:],
        rejectionCounts: [UUID: Int] = [:]
    ) -> Dish? {
        guard !candidates.isEmpty else { return nil }
        let weights: [Double] = candidates.map { dish in
            let usage = plan.slotList.filter { $0.dishId == dish.id }.count
            let priorityBoost: Double = dish.isPriority ? 2.5 : 1.0
            let patternScore    = historyScores[dish.id] ?? 1.0
            let ratingScore     = ratingScores[dish.id] ?? 1.0
            let rejectionScore  = rejectionCounts[dish.id].map { max(0.30, 1.0 - 0.25 * Double($0)) } ?? 1.0
            let baseWeight: Double
            switch usage {
            case 0: baseWeight = 3.0
            case 1: baseWeight = 2.0
            default: baseWeight = 1.0
            }
            return baseWeight * priorityBoost * patternScore * ratingScore * rejectionScore
        }
        let total = weights.reduce(0, +)
        var r = Double.random(in: 0..<total)
        for (i, w) in weights.enumerated() {
            r -= w
            if r <= 0 { return candidates[i] }
        }
        return candidates.last
    }
}