fuelboard/Shared/FuelSavings.swift

import Foundation

enum FuelSavingsFillUnit: String, Codable, CaseIterable, Identifiable {
    case pounds
    case litres

    var id: String { rawValue }

    var displayName: String {
        switch self {
        case .pounds: return "£"
        case .litres: return "Litres"
        }
    }
}

struct FuelSavingsFillAmount: Codable, Equatable {
    var value: Double
    var unit: FuelSavingsFillUnit

    var isValid: Bool {
        value.isFinite && value > 0 && value <= FuelSavingsCalculator.maximumFillAmount(for: unit)
    }
}

struct FuelSavingsSettings: Codable, Equatable {
    var enabled: Bool
    var mpg: Double
    var preferredAmount: FuelSavingsFillAmount

    static let defaultMPG: Double = 45
    static let defaultPreferredAmount = FuelSavingsFillAmount(value: 40, unit: .pounds)

    static var defaults: FuelSavingsSettings {
        FuelSavingsSettings(enabled: false, mpg: defaultMPG, preferredAmount: defaultPreferredAmount)
    }

    var isConfigured: Bool {
        enabled && FuelSavingsCalculator.isValidMPG(mpg) && preferredAmount.isValid
    }
}

enum FuelSavingsStatus: String, Codable, Equatable {
    case worthTheDrive
    case breakEven
    case notWorthTheDrive
    /// The comparison baseline itself — the station every other saving is
    /// measured against. Shown as "Closest station", never "Break even".
    case baseline

    var displayName: String {
        switch self {
        case .worthTheDrive: return "Worth the drive"
        case .breakEven: return "Break even"
        case .notWorthTheDrive: return "Not worth the drive"
        case .baseline: return FuelSavingsResult.baselineHeadline
        }
    }
}

/// How the journey distances behind an estimate were obtained.
enum FuelSavingsDistanceBasis: String, Codable, Equatable {
    /// Measured road route, out and back, from the routing service.
    case roadRoute
    /// Straight-line out-and-back estimate, used until a route is measured.
    case straightLine

    var isEstimate: Bool { self == .straightLine }
}

struct FuelSavingsResult: Codable, Equatable {
    let fillLitres: Double
    let grossSavingGBP: Double
    let extraDrivingCostGBP: Double
    let extraDistanceMiles: Double
    let netSavingGBP: Double
    let status: FuelSavingsStatus
    let distanceBasis: FuelSavingsDistanceBasis

    init(fillLitres: Double,
         grossSavingGBP: Double,
         extraDrivingCostGBP: Double,
         extraDistanceMiles: Double,
         netSavingGBP: Double,
         status: FuelSavingsStatus,
         distanceBasis: FuelSavingsDistanceBasis = .roadRoute) {
        self.fillLitres = fillLitres
        self.grossSavingGBP = grossSavingGBP
        self.extraDrivingCostGBP = extraDrivingCostGBP
        self.extraDistanceMiles = extraDistanceMiles
        self.netSavingGBP = netSavingGBP
        self.status = status
        self.distanceBasis = distanceBasis
    }
}

/// Money formatting for savings strings, in one place so the exact wording the
/// UI shows is testable.
enum FuelSavingsFormat {
    static func money(_ value: Double) -> String {
        String(format: "£%.2f", value)
    }

    /// A distance stored in MILES, formatted in the display unit. The figures
    /// behind an estimate are stored in miles whatever the user's unit, so this
    /// is the one place that conversion lives.
    static func distance(_ miles: Double, unit: DistanceUnit) -> String {
        // Convert miles→km FIRST, then let the unit format it. `unit.toKM` would
        // convert by the CURRENT unit instead, leaving a km user's figure
        // unlabelled and 1.6× too large.
        unit.format(miles * FuelSavingsCalculator.kilometresPerMile)
    }

    /// Row label for the extra driving a saving has to beat. The stored figure is
    /// always MILES while the row's distance line is one-way, so the wording says
    /// "more driving": the extra is an out-and-back figure and can legitimately
    /// exceed the one-way distance, which read as a contradiction before.
    static func extraTravelLabel(extraDistanceMiles: Double, unit: DistanceUnit) -> String? {
        guard extraDistanceMiles > 0.05 else { return nil }
        return "\(distance(extraDistanceMiles, unit: unit)) more driving than closest"
    }
}

extension FuelSavingsResult {
    /// Baseline row label — required wording.
    static let baselineHeadline = "Closest station"
    /// Neutral result label — required wording.
    static let breakEvenHeadline = "Break even"

    /// The same estimate relabelled as the comparison baseline: nothing to save
    /// or lose against itself, and no extra travel.
    func asBaseline() -> FuelSavingsResult {
        FuelSavingsResult(
            fillLitres: fillLitres,
            grossSavingGBP: 0,
            extraDrivingCostGBP: 0,
            extraDistanceMiles: 0,
            netSavingGBP: 0,
            status: .baseline,
            distanceBasis: distanceBasis
        )
    }

    /// Row headline, e.g. "Save £0.52 after travel", "£0.31 worse off after
    /// travel", "Break even", "Closest station".
    ///
    /// `compact` (list rows) also marks a straight-line estimate with "(est.)";
    /// the detail view spells that out in full instead.
    func headline(compact: Bool = false) -> String {
        let estimate = (compact && distanceBasis.isEstimate) ? " (est.)" : ""
        switch status {
        case .baseline:
            return Self.baselineHeadline
        case .breakEven:
            return Self.breakEvenHeadline
        case .worthTheDrive:
            return "Save \(FuelSavingsFormat.money(netSavingGBP)) after travel\(estimate)"
        case .notWorthTheDrive:
            return "\(FuelSavingsFormat.money(abs(netSavingGBP))) worse off after travel\(estimate)"
        }
    }

    /// Row capsule copy: the headline, plus what the baseline row IS. Without the
    /// suffix the baseline capsule just repeated the status word above it
    /// ("Closest station" twice) and said nothing about the saving being measured
    /// against it.
    func capsuleHeadline() -> String {
        let headline = headline(compact: true)
        return status == .baseline ? "\(headline) · your baseline" : headline
    }
}

/// One station's journey distance, plus how it was obtained.
struct FuelSavingsJourney: Equatable {
    let miles: Double
    let basis: FuelSavingsDistanceBasis
}

enum FuelSavingsAvailability: Equatable {
    case available(FuelSavingsResult)
    case unavailable(String)

    var result: FuelSavingsResult? {
        if case .available(let result) = self { return result }
        return nil
    }
}

struct FuelSavingsRankedStation: Equatable {
    let stationID: String
    let result: FuelSavingsAvailability
    let distanceMiles: Double
    let pricePencePerLitre: Double
}

enum FuelSavingsError: Error, Equatable {
    case invalidMPG
    case invalidAmount
    case invalidPrice
    case invalidDistance
}

enum FuelSavingsCalculator {
    /// 1 UK (imperial) gallon. Must never be the US gallon (3.785411784 L):
    /// UK pumps are priced per litre and drivers quote UK MPG, so a US gallon
    /// would understate every travel cost by 17%.
    static let imperialGallonLitres = 4.54609
    /// Exact mile↔kilometre factors (1 mile = 1.609344 km). Every figure the
    /// savings maths keeps is in MILES, and every displayed or fallback
    /// distance converts through these, so one quantity is never converted with
    /// two different constants. The audit found 1.60934, 1609.34 and 0.621371
    /// mixed in the same path — a ~2 parts-per-million difference, immaterial to
    /// the money, but three constants where there should be one.
    static let kilometresPerMile = 1.609344
    static let metresPerMile = 1609.344
    static let breakEvenToleranceGBP = 0.05
    static let minimumMPG: Double = 5
    static let maximumMPG: Double = 200

    static func maximumFillAmount(for unit: FuelSavingsFillUnit) -> Double {
        switch unit {
        case .pounds: return 500
        case .litres: return 250
        }
    }

    /// Bounds for every fill-up SLIDER (Settings and the in-list adjustment
    /// share them, so the two controls can never offer different ranges). The
    /// validity ceiling above is deliberately wider, so a stored outlier is
    /// displayed rather than destroyed by the control that shows it.
    static func fillAmountSliderRange(for unit: FuelSavingsFillUnit) -> ClosedRange<Double> {
        switch unit {
        case .pounds: return 10...100
        case .litres: return 10...80
        }
    }

    static func fillAmountSliderStep(for unit: FuelSavingsFillUnit) -> Double {
        switch unit {
        case .pounds: return 5
        case .litres: return 1
        }
    }

    /// Bounds for the average-MPG slider. The validity bounds (5...200) are
    /// wide enough for any stored figure but far too wide to fine-tune by
    /// touch; every real-world UK car sits inside this range.
    static let mpgSliderRange: ClosedRange<Double> = 10...100
    static let mpgSliderStep: Double = 1

    static func isValidMPG(_ mpg: Double) -> Bool {
        mpg.isFinite && mpg >= minimumMPG && mpg <= maximumMPG
    }

    static func fuelUsedLitres(distanceMiles: Double, mpg: Double) -> Double {
        distanceMiles / mpg * imperialGallonLitres
    }

    static func fillLitres(amount: FuelSavingsFillAmount, baselinePricePencePerLitre: Double) throws -> Double {
        guard amount.isValid else { throw FuelSavingsError.invalidAmount }
        switch amount.unit {
        case .litres:
            return amount.value
        case .pounds:
            guard baselinePricePencePerLitre.isFinite, baselinePricePencePerLitre > 0 else {
                throw FuelSavingsError.invalidPrice
            }
            return amount.value / (baselinePricePencePerLitre / 100.0)
        }
    }

    static func convert(amount: FuelSavingsFillAmount,
                        to unit: FuelSavingsFillUnit,
                        referencePricePencePerLitre: Double) throws -> FuelSavingsFillAmount {
        guard referencePricePencePerLitre.isFinite, referencePricePencePerLitre > 0 else {
            throw FuelSavingsError.invalidPrice
        }
        guard amount.isValid else { throw FuelSavingsError.invalidAmount }
        guard amount.unit != unit else { return amount }
        switch (amount.unit, unit) {
        case (.pounds, .litres):
            return FuelSavingsFillAmount(
                value: amount.value / (referencePricePencePerLitre / 100.0),
                unit: .litres
            )
        case (.litres, .pounds):
            return FuelSavingsFillAmount(
                value: amount.value * (referencePricePencePerLitre / 100.0),
                unit: .pounds
            )
        case (.pounds, .pounds), (.litres, .litres):
            return amount
        }
    }

    static func calculate(mpg: Double,
                          amount: FuelSavingsFillAmount,
                          baselinePricePencePerLitre: Double,
                          candidatePricePencePerLitre: Double,
                          baselineJourneyMiles: Double,
                          candidateJourneyMiles: Double,
                          distanceBasis: FuelSavingsDistanceBasis = .roadRoute,
                          breakEvenToleranceGBP: Double = Self.breakEvenToleranceGBP) throws -> FuelSavingsResult {
        guard isValidMPG(mpg) else { throw FuelSavingsError.invalidMPG }
        guard baselinePricePencePerLitre.isFinite, baselinePricePencePerLitre > 0,
              candidatePricePencePerLitre.isFinite, candidatePricePencePerLitre > 0 else {
            throw FuelSavingsError.invalidPrice
        }
        guard baselineJourneyMiles.isFinite, candidateJourneyMiles.isFinite,
              baselineJourneyMiles >= 0, candidateJourneyMiles >= 0 else {
            throw FuelSavingsError.invalidDistance
        }
        let fillLitres = try fillLitres(amount: amount, baselinePricePencePerLitre: baselinePricePencePerLitre)
        let baselinePriceGBP = baselinePricePencePerLitre / 100.0
        let candidatePriceGBP = candidatePricePencePerLitre / 100.0
        let grossSaving = fillLitres * (baselinePriceGBP - candidatePriceGBP)
        let extraDistance = max(0, candidateJourneyMiles - baselineJourneyMiles)
        let extraFuelLitres = fuelUsedLitres(distanceMiles: extraDistance, mpg: mpg)
        let extraFuelCost = extraFuelLitres * candidatePriceGBP
        let netSaving = grossSaving - extraFuelCost
        let status: FuelSavingsStatus
        if netSaving > breakEvenToleranceGBP {
            status = .worthTheDrive
        } else if netSaving < -breakEvenToleranceGBP {
            status = .notWorthTheDrive
        } else {
            status = .breakEven
        }
        return FuelSavingsResult(
            fillLitres: fillLitres,
            grossSavingGBP: grossSaving,
            extraDrivingCostGBP: extraFuelCost,
            extraDistanceMiles: extraDistance,
            netSavingGBP: netSaving,
            status: status,
            distanceBasis: distanceBasis
        )
    }

    static func sortedForWorthTheDrive(_ stations: [FuelSavingsRankedStation]) -> [FuelSavingsRankedStation] {
        stations.sorted { lhs, rhs in
            switch (lhs.result.result, rhs.result.result) {
            case let (l?, r?):
                if l.netSavingGBP != r.netSavingGBP { return l.netSavingGBP > r.netSavingGBP }
                if lhs.distanceMiles != rhs.distanceMiles { return lhs.distanceMiles < rhs.distanceMiles }
                if lhs.pricePencePerLitre != rhs.pricePencePerLitre { return lhs.pricePencePerLitre < rhs.pricePencePerLitre }
                return lhs.stationID < rhs.stationID
            case (_?, nil):
                return true
            case (nil, _?):
                return false
            case (nil, nil):
                if lhs.distanceMiles != rhs.distanceMiles { return lhs.distanceMiles < rhs.distanceMiles }
                return lhs.stationID < rhs.stationID
            }
        }
    }
}

/// Turns stations into savings estimates.
///
/// Pure and injectable (no MapKit, CoreLocation or SwiftUI) so the ranking
/// rules are unit tested directly.
enum FuelSavingsEstimator {
    /// Savings for a single station against `baseline`, or nil when there is
    /// nothing to show at all (feature off).
    static func availability(station: FuelStation,
                             fuel: FuelType,
                             baseline: FuelStation?,
                             settings: FuelSavingsSettings,
                             amount: FuelSavingsFillAmount,
                             journey: FuelSavingsJourney?,
                             baselineJourney: FuelSavingsJourney?) -> FuelSavingsAvailability? {
        guard settings.isConfigured else { return nil }
        guard let baseline, let baselineJourney else { return .unavailable("No comparison station") }
        guard let baselinePrice = baseline.prices[fuel] else { return .unavailable("No comparison station") }
        guard let price = station.prices[fuel] else { return .unavailable("No price") }
        guard let journey else { return .unavailable("Distance unavailable") }
        // Both legs must be measured for the estimate to count as a road-route
        // one; otherwise it is reported as a straight-line estimate.
        let basis: FuelSavingsDistanceBasis =
            (journey.basis == .roadRoute && baselineJourney.basis == .roadRoute) ? .roadRoute : .straightLine
        do {
            let result = try FuelSavingsCalculator.calculate(
                mpg: settings.mpg,
                amount: amount,
                baselinePricePencePerLitre: baselinePrice,
                candidatePricePencePerLitre: price,
                baselineJourneyMiles: baselineJourney.miles,
                candidateJourneyMiles: journey.miles,
                distanceBasis: basis
            )
            // The baseline compares against itself: label it as the closest
            // station rather than a meaningless "Break even".
            return .available(station.id == baseline.id ? result.asBaseline() : result)
        } catch {
            return .unavailable("Distance unavailable")
        }
    }

    /// Ranks a whole pool for the Best Value sort.
    ///
    /// EVERY priced station in the pool gets an estimate, not just the handful
    /// whose road routes happen to have been measured: journeys that have no
    /// measured route fall back to a straight-line out-and-back estimate (see
    /// `FuelSavingsDistanceBasis`). Capping this to a shortlist dropped cheaper
    /// stations that were a few miles further away — the exact stations the
    /// sort exists to surface — and left later pages with no savings at all.
    static func ranked(_ stations: [FuelStation],
                       fuel: FuelType,
                       baseline: FuelStation?,
                       settings: FuelSavingsSettings,
                       amount: FuelSavingsFillAmount,
                       journeyMiles: (FuelStation) -> FuelSavingsJourney?) -> [FuelStation] {
        var journeys: [String: FuelSavingsJourney] = [:]
        journeys.reserveCapacity(stations.count)
        for station in stations {
            if let journey = journeyMiles(station) {
                journeys[station.id] = journey
            }
        }
        let baselineJourney = baseline.flatMap { journeys[$0.id] }
        let ranked = stations.map { station -> FuelSavingsRankedStation in
            let journey = journeys[station.id]
            let availability = availability(station: station,
                                            fuel: fuel,
                                            baseline: baseline,
                                            settings: settings,
                                            amount: amount,
                                            journey: journey,
                                            baselineJourney: baselineJourney) ?? .unavailable("Unavailable")
            return FuelSavingsRankedStation(
                stationID: station.id,
                result: availability,
                distanceMiles: journey?.miles ?? .greatestFiniteMagnitude,
                pricePencePerLitre: station.prices[fuel] ?? .infinity
            )
        }
        let order = Dictionary(
            uniqueKeysWithValues: FuelSavingsCalculator.sortedForWorthTheDrive(ranked)
                .enumerated()
                .map { ($0.element.stationID, $0.offset) }
        )
        return stations.sorted { lhs, rhs in
            let left = order[lhs.id] ?? Int.max
            let right = order[rhs.id] ?? Int.max
            if left != right { return left < right }
            return lhs.id < rhs.id
        }
    }
}