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 }
}

/// Whether a Best Value comparison is backed by complete road-route data.
enum JourneyConfidence: String, Codable, Equatable {
    /// Both baseline and candidate have measured outward and return road routes.
    case confirmed
    /// At least one journey is straight-line, missing, failed, or using a legacy
    /// one-way road cache whose return leg is inferred rather than measured.
    case estimated

    var isEstimated: Bool { self == .estimated }
}

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

    init(fillLitres: Double,
         grossSavingGBP: Double,
         extraFuelLitres: Double = 0,
         extraDrivingCostGBP: Double,
         extraDistanceMiles: Double,
         netSavingGBP: Double,
         status: FuelSavingsStatus,
         distanceBasis: FuelSavingsDistanceBasis = .roadRoute,
         journeyConfidence: JourneyConfidence? = nil) {
        self.fillLitres = fillLitres
        self.grossSavingGBP = grossSavingGBP
        self.extraFuelLitres = extraFuelLitres
        self.extraDrivingCostGBP = extraDrivingCostGBP
        self.extraDistanceMiles = extraDistanceMiles
        self.netSavingGBP = netSavingGBP
        self.status = status
        self.distanceBasis = distanceBasis
        self.journeyConfidence = journeyConfidence ?? (distanceBasis.isEstimate ? .estimated : .confirmed)
    }
}

/// 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 names
    /// the RETURN journey: the figure is an out-and-back DELTA between two
    /// journeys and can legitimately exceed the one-way distance, which read as a
    /// contradiction when it said "more driving than closest".
    static func extraTravelLabel(extraDistanceMiles: Double,
                                 unit: DistanceUnit,
                                 confidence: JourneyConfidence = .confirmed) -> String? {
        guard let distanceText = extraTravelDistanceLabel(extraDistanceMiles: extraDistanceMiles,
                                                         unit: unit,
                                                         confidence: confidence) else { return nil }
        return "\(distanceText) extra return travel"
    }

    /// The extra-travel FIGURE on its own, with its confidence marker: "4.3 mi"
    /// or "Approx. 4.3 mi".
    ///
    /// A row form ("Extra return travel | 4.3 mi") names the journey in its own
    /// label, so this is the same figure and the same "Approx." rule as
    /// `extraTravelLabel`, from the same inputs — never rebuilt at a call site,
    /// or one surface marks an estimate while another drops the marker.
    static func extraTravelDistanceLabel(extraDistanceMiles: Double,
                                         unit: DistanceUnit,
                                         confidence: JourneyConfidence = .confirmed) -> String? {
        guard extraDistanceMiles > 0.05 else { return nil }
        let distanceText = distance(extraDistanceMiles, unit: unit)
        return confidence == .estimated ? "Approx. \(distanceText)" : distanceText
    }

    /// Compact comparison copy kept outside the coloured verdict capsule.
    /// "Closest" refers to the existing shortest-journey baseline selection;
    /// it does not claim a straight-line comparison.
    static func comparisonLabel(status: FuelSavingsStatus) -> String {
        status == .baseline ? "your baseline" : "vs closest station"
    }

    /// VoiceOver form of `extraTravelLabel`. The abbreviation and the unit suffix
    /// are spoken as words ("Approximately 6.4 miles extra return travel") because
    /// synthesizers render "Approx." and "mi" inconsistently.
    static func extraTravelSpokenLabel(extraDistanceMiles: Double,
                                       unit: DistanceUnit,
                                       confidence: JourneyConfidence = .confirmed) -> String? {
        guard extraDistanceMiles > 0.05 else { return nil }
        let distanceText = spokenDistance(extraDistanceMiles, unit: unit)
        return confidence == .estimated
            ? "Approximately \(distanceText) extra return travel"
            : "\(distanceText) extra return travel"
    }

    /// A distance stored in MILES, spoken: "6.4 miles" / "10.3 kilometres". The
    /// miles→km hop is deliberate: the stored figure is miles while
    /// `DistanceUnit.spoken` speaks a KM value, the same boundary `distance(_:unit:)`
    /// crosses.
    static func spokenDistance(_ miles: Double, unit: DistanceUnit) -> String {
        unit.spoken(miles * FuelSavingsCalculator.kilometresPerMile)
    }
}

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,
            extraFuelLitres: 0,
            extraDrivingCostGBP: 0,
            extraDistanceMiles: 0,
            netSavingGBP: 0,
            status: .baseline,
            distanceBasis: distanceBasis,
            journeyConfidence: journeyConfidence
        )
    }

    /// Row headline, e.g. "£1.22 better off after travel", "£0.31 worse off after
    /// travel", "Break even", "Closest station".
    ///
    /// Estimated comparisons are prefixed "Approx. " for positive/negative money
    /// results — the WORD, never a "~": a tilde reads as decoration (and assistive
    /// tech may skip it) instead of saying "this is an estimate". Break-even and
    /// baseline stay plain words, with the estimate context carried by the
    /// extra-distance caption or the detail explanation.
    func headline(compact: Bool = false) -> String {
        let prefix = journeyConfidence.isEstimated ? "Approx. " : ""
        switch status {
        case .baseline:
            return Self.baselineHeadline
        case .breakEven:
            return Self.breakEvenHeadline
        case .worthTheDrive:
            return "\(prefix)Save \(FuelSavingsFormat.money(netSavingGBP))"
        case .notWorthTheDrive:
            return "\(prefix)Costs \(FuelSavingsFormat.money(abs(netSavingGBP))) more"
        }
    }

    /// VoiceOver form of `headline()`: "Approx." is spelled out so a synthesizer
    /// cannot read the abbreviation as "approx" or drop it entirely.
    func spokenHeadline() -> String {
        headline().replacingOccurrences(of: "Approx. ", with: "Approximately ")
    }

    /// Row capsule copy. The baseline relationship is shown as secondary text
    /// beside the capsule in the station listing and detail summary, so the
    /// capsule itself contains only the verdict.
    func capsuleHeadline() -> String {
        headline(compact: true)
    }
}

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

    init(miles: Double,
         basis: FuelSavingsDistanceBasis,
         confidence: JourneyConfidence? = nil) {
        self.miles = miles
        self.basis = basis
        self.confidence = confidence ?? (basis.isEstimate ? .estimated : .confirmed)
    }
}

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
}

/// Which recommendation badge a station listing row shows, if any.
///
/// Presentation only — no new rules: the BEST VALUE qualification reuses the
/// app's existing Best Value gate (the user's alert threshold, the same figure
/// `FuelSavingsCalculator.bestValueAlertCandidate` is given) and CHEAPEST
/// reuses the caller's existing "cheapest in range" flag (the station behind
/// `topStationID`). One badge per row: a station that is both the cheapest and
/// worth the drive shows BEST VALUE, the stronger of the two.
enum StationRecommendationBadge: Equatable {
    case bestValue
    case cheapest

    static func badge(netSavingGBP: Double?,
                      isCheapest: Bool,
                      bestValueThresholdGBP: Double) -> StationRecommendationBadge? {
        if let netSavingGBP, netSavingGBP.isFinite, bestValueThresholdGBP.isFinite,
           netSavingGBP >= bestValueThresholdGBP {
            return .bestValue
        }
        return isCheapest ? .cheapest : nil
    }
}

struct BestValueAlertCandidate: Equatable {
    let station: FuelStation
    let result: FuelSavingsResult
}

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

    /// Changing between a per-candidate monetary budget and a fixed litre
    /// quantity has no lossless conversion across all stations. Start a new
    /// comparison at that mode's stated default; the UI must tell the user
    /// explicitly that the comparison basis changed and invite adjustment.
    static func newComparisonAmount(afterSwitching amount: FuelSavingsFillAmount,
                                    to unit: FuelSavingsFillUnit) -> FuelSavingsFillAmount {
        guard amount.unit != unit else { return amount }
        switch unit {
        case .pounds: return FuelSavingsFillAmount(value: 40, unit: .pounds)
        case .litres: return FuelSavingsFillAmount(value: 30, unit: .litres)
        }
    }

    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,
                          journeyConfidence: JourneyConfidence? = nil,
                          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, pricePencePerLitre: candidatePricePencePerLitre)
        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,
            extraFuelLitres: extraFuelLitres,
            extraDrivingCostGBP: extraFuelCost,
            extraDistanceMiles: extraDistance,
            netSavingGBP: netSaving,
            status: status,
            distanceBasis: distanceBasis,
            journeyConfidence: journeyConfidence
        )
    }

    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
            }
        }
    }

    static func bestValueAlertCandidate(
        stations: [FuelStation],
        fuel: FuelType,
        settings: FuelSavingsSettings,
        amount: FuelSavingsFillAmount,
        minimumNetSavingGBP: Double,
        journeyMiles: (FuelStation) -> FuelSavingsJourney?
    ) -> BestValueAlertCandidate? {
        guard settings.isConfigured, minimumNetSavingGBP.isFinite else { return nil }
        let sellers = stations.filter { $0.prices[fuel] != nil }
        guard !sellers.isEmpty else { return nil }
        let baseline = FuelSavingsEstimator.baselineStation(sellers, fuel: fuel, journeyMiles: journeyMiles)
        var best: BestValueAlertCandidate?
        var bestDistance = Double.greatestFiniteMagnitude
        var bestPrice = Double.infinity
        var baselineJourneyCache: FuelSavingsJourney?
        var baselineJourneyLoaded = false
        for station in sellers {
            let journey = journeyMiles(station)
            let baselineJourney: FuelSavingsJourney? = {
                guard let baseline else { return nil }
                if baseline.id == station.id { return journey }
                if !baselineJourneyLoaded {
                    baselineJourneyCache = journeyMiles(baseline)
                    baselineJourneyLoaded = true
                }
                return baselineJourneyCache
            }()
            guard case .available(let result) = FuelSavingsEstimator.availability(
                station: station,
                fuel: fuel,
                baseline: baseline,
                settings: settings,
                amount: amount,
                journey: journey,
                baselineJourney: baselineJourney
            ), result.status == .worthTheDrive,
               result.netSavingGBP >= minimumNetSavingGBP else { continue }
            let distance = journey?.miles ?? .greatestFiniteMagnitude
            let price = station.prices[fuel] ?? .infinity
            guard let current = best else {
                best = BestValueAlertCandidate(station: station, result: result)
                bestDistance = distance
                bestPrice = price
                continue
            }
            if result.netSavingGBP > current.result.netSavingGBP ||
                (result.netSavingGBP == current.result.netSavingGBP && distance < bestDistance) ||
                (result.netSavingGBP == current.result.netSavingGBP && distance == bestDistance && price < bestPrice) ||
                (result.netSavingGBP == current.result.netSavingGBP && distance == bestDistance && price == bestPrice && station.id < current.station.id) {
                best = BestValueAlertCandidate(station: station, result: result)
                bestDistance = distance
                bestPrice = price
            }
        }
        return best
    }
}

/// Turns stations into savings estimates.
///
/// Pure and injectable (no MapKit, CoreLocation or SwiftUI) so the ranking
/// rules are unit tested directly.
enum FuelSavingsEstimator {
    /// Baseline station for Best Value: the station with the shortest journey
    /// among stations that sell the selected fuel, using the SAME journey source
    /// the savings calculation will use. This keeps the row labelled "Closest
    /// station" and every "extra return travel" value tied to one
    /// reference, instead of mixing a separate straight-line baseline with
    /// road-route savings.
    static func baselineStation(_ stations: [FuelStation],
                                fuel: FuelType,
                                journeyMiles: (FuelStation) -> FuelSavingsJourney?) -> FuelStation? {
        var bestStation: FuelStation?
        var bestMiles = Double.infinity
        for station in stations where station.prices[fuel] != nil {
            guard let journey = journeyMiles(station) else { continue }
            let miles = journey.miles
            guard miles.isFinite, miles >= 0 else { continue }
            if miles < bestMiles {
                bestMiles = miles
                bestStation = station
            }
        }
        return bestStation
    }

    /// 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 journeys must be measured, complete road routes for the comparison
        // to be confirmed. Legacy one-way caches keep the road-route basis for
        // distance arithmetic but are still provisional because the return leg is
        // inferred rather than measured.
        let basis: FuelSavingsDistanceBasis =
            (journey.basis == .roadRoute && baselineJourney.basis == .roadRoute) ? .roadRoute : .straightLine
        let confidence: JourneyConfidence =
            (journey.confidence == .confirmed && baselineJourney.confidence == .confirmed) ? .confirmed : .estimated
        do {
            let result = try FuelSavingsCalculator.calculate(
                mpg: settings.mpg,
                amount: amount,
                baselinePricePencePerLitre: baselinePrice,
                candidatePricePencePerLitre: price,
                baselineJourneyMiles: baselineJourney.miles,
                candidateJourneyMiles: journey.miles,
                distanceBasis: basis,
                journeyConfidence: confidence
            )
            // 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
            }
        }
        // The baseline is normally in the pool, so this is the same lookup it has
        // always been; asking for its journey directly covers the case where the
        // list being ranked is filtered tighter than the baseline was chosen from,
        // which would otherwise mark every row unavailable.
        let baselineJourney = baseline.flatMap { journeys[$0.id] ?? journeyMiles($0) }
        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
            )
        }
        var order: [String: Int] = [:]
        for (offset, rankedStation) in FuelSavingsCalculator.sortedForWorthTheDrive(ranked).enumerated() {
            if order[rankedStation.stationID] == nil {
                order[rankedStation.stationID] = 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
        }
    }
}