import XCTest
#if SWIFT_PACKAGE
@testable import FuelBoardShared
#endif

/// Ranking rules for the Worth the Drive sort.
///
/// Regression: the sort used to rank only a shortlist of the nearest stations
/// whose ROAD ROUTES had been measured, and everything else was listed as
/// unavailable. Cheaper stations a few miles further out — the exact stations
/// the sort exists to surface — were dropped from the top of the list, and
/// stations on later pages showed no savings at all.
final class FuelSavingsEstimateTests: XCTestCase {
    private let settings = FuelSavingsSettings(
        enabled: true,
        mpg: 45,
        preferredAmount: FuelSavingsFillAmount(value: 40, unit: .pounds)
    )
    private let amount = FuelSavingsFillAmount(value: 40, unit: .pounds)

    private func station(_ id: String,
                         price: Double?,
                         lat: Double = 51.5,
                         lng: Double = -0.1) -> FuelStation {
        FuelStation(
            id: id,
            name: id,
            brand: "X",
            address: "",
            postcode: "",
            lat: lat,
            lng: lng,
            prices: price.map { [FuelType.e10: $0] } ?? [:],
            priceUpdated: nil
        )
    }

    /// Straight-line out-and-back estimate, as the app builds it when no road
    /// route has been measured yet.
    private func straightLine(_ miles: Double) -> FuelSavingsJourney {
        FuelSavingsJourney(miles: miles, basis: .straightLine)
    }

    func testCheaperStationThreePointFiveMilesAwayIsRankedFirstWithASaving() {
        // Nearest station: 1 mile away at 145.9p. The station the user can see
        // in the Cheapest tab: 3.5 miles away at 138.9p.
        let baseline = station("baseline", price: 145.9)
        let candidate = station("cheaper-3.5", price: 138.9, lat: 51.55)
        let filler = (0..<14).map { station("filler-\($0)", price: 150 + Double($0)) }
        let pool = [baseline, candidate] + filler

        let ranked = FuelSavingsEstimator.ranked(
            pool,
            fuel: .e10,
            baseline: baseline,
            settings: settings,
            amount: amount,
            journeyMiles: { s in
                switch s.id {
                case "baseline": return self.straightLine(2.0)
                case "cheaper-3.5": return self.straightLine(7.0)
                default: return self.straightLine(20.0)
                }
            }
        )

        XCTAssertEqual(ranked.count, pool.count)
        XCTAssertEqual(ranked.first?.id, "cheaper-3.5",
                       "a cheaper station 3.5 miles away must rank above the nearest station")

        let result = FuelSavingsEstimator.availability(
            station: candidate,
            fuel: .e10,
            baseline: baseline,
            settings: settings,
            amount: amount,
            journey: straightLine(7.0),
            baselineJourney: straightLine(2.0)
        )?.result

        XCTAssertNotNil(result, "the station must show a saving, not an unavailable state")
        XCTAssertGreaterThan(result?.netSavingGBP ?? -1, 0)
        XCTAssertEqual(result?.status, .worthTheDrive)
        XCTAssertEqual(result?.distanceBasis, .straightLine,
                       "unmeasured routes are reported as straight-line estimates")
    }

    func testEveryPricedStationInThePoolIsEstimatedNotJustAShortlist() {
        // 40 stations, but only the first one has a measured road route — the
        // old implementation capped the whole ranking to 12 shortlisted
        // stations and returned "Outside route shortlist" for the rest.
        let baseline = station("baseline", price: 145.9)
        let pool = [baseline] + (0..<39).map { station("s\($0)", price: 135 + Double($0)) }

        let ranked = FuelSavingsEstimator.ranked(
            pool,
            fuel: .e10,
            baseline: baseline,
            settings: settings,
            amount: amount,
            journeyMiles: { s in
                s.id == "baseline"
                    ? FuelSavingsJourney(miles: 2.0, basis: .roadRoute)
                    : self.straightLine(2.0 + (Double(s.id.dropFirst()) ?? 6))
            }
        )

        XCTAssertEqual(ranked.count, pool.count)

        for station in pool where station.id != "baseline" {
            let availability = FuelSavingsEstimator.availability(
                station: station,
                fuel: .e10,
                baseline: baseline,
                settings: settings,
                amount: amount,
                journey: straightLine(10.0),
                baselineJourney: FuelSavingsJourney(miles: 2.0, basis: .roadRoute)
            )
            XCTAssertNotNil(availability?.result,
                            "\(station.id) should carry an estimated saving, not an unavailable state")
        }
    }

    func testMeasuredRoadRoutesAreReportedAsRoadRouteBasis() {
        let baseline = station("baseline", price: 145.9)
        let candidate = station("candidate", price: 138.9)

        let result = FuelSavingsEstimator.availability(
            station: candidate,
            fuel: .e10,
            baseline: baseline,
            settings: settings,
            amount: amount,
            journey: FuelSavingsJourney(miles: 7.0, basis: .roadRoute),
            baselineJourney: FuelSavingsJourney(miles: 2.0, basis: .roadRoute)
        )?.result

        XCTAssertEqual(result?.distanceBasis, .roadRoute)
        XCTAssertFalse(FuelSavingsDistanceBasis.roadRoute.isEstimate)
    }

    func testMixedBasisIsReportedAsAnEstimate() {
        let baseline = station("baseline", price: 145.9)
        let candidate = station("candidate", price: 138.9)

        // Baseline measured, candidate not: the comparison is only as good as
        // its weakest leg, so it must be labelled an estimate.
        let result = FuelSavingsEstimator.availability(
            station: candidate,
            fuel: .e10,
            baseline: baseline,
            settings: settings,
            amount: amount,
            journey: straightLine(7.0),
            baselineJourney: FuelSavingsJourney(miles: 2.0, basis: .roadRoute)
        )?.result

        XCTAssertEqual(result?.distanceBasis, .straightLine)
    }

    func testBaselineStationUsesTheSameJourneyValuesAsSavings() {
        let geographicallyNearest = station("geographically-nearest", price: 145.9)
        let roadNearest = station("road-nearest", price: 146.9)
        let noPrice = station("no-price", price: nil)

        let baseline = FuelSavingsEstimator.baselineStation(
            [geographicallyNearest, roadNearest, noPrice],
            fuel: .e10,
            journeyMiles: { station in
                switch station.id {
                case "geographically-nearest": return FuelSavingsJourney(miles: 8.0, basis: .roadRoute)
                case "road-nearest": return FuelSavingsJourney(miles: 4.0, basis: .roadRoute)
                case "no-price": return FuelSavingsJourney(miles: 1.0, basis: .roadRoute)
                default: return nil
                }
            }
        )

        XCTAssertEqual(baseline?.id, "road-nearest",
                       "the Best Value baseline must be the shortest usable journey, not a separate straight-line/display-distance choice")
    }

    func testBaselineStationFallsBackThenChangesWhenRoadRoutesArrive() {
        let stationA = station("a", price: 145.9)
        let stationB = station("b", price: 146.9)

        let noRoadRoutes = FuelSavingsEstimator.baselineStation(
            [stationA, stationB],
            fuel: .e10,
            journeyMiles: { station in
                switch station.id {
                case "a": return FuelSavingsJourney(miles: 2.0, basis: .straightLine)
                case "b": return FuelSavingsJourney(miles: 3.0, basis: .straightLine)
                default: return nil
                }
            }
        )
        XCTAssertEqual(noRoadRoutes?.id, "a")

        let partialRoadRoutes = FuelSavingsEstimator.baselineStation(
            [stationA, stationB],
            fuel: .e10,
            journeyMiles: { station in
                switch station.id {
                case "a": return FuelSavingsJourney(miles: 5.0, basis: .roadRoute)
                case "b": return FuelSavingsJourney(miles: 3.0, basis: .straightLine)
                default: return nil
                }
            }
        )
        XCTAssertEqual(partialRoadRoutes?.id, "b",
                       "baseline must be recalculated from the latest journey values when routes arrive")
    }

    func testRankingToleratesDuplicateStationIDs() {
        let baseline = station("baseline", price: 145.9)
        let duplicateA = station("duplicate", price: 138.9, lat: 51.55)
        let duplicateB = station("duplicate", price: 139.9, lat: 51.56)
        let pool = [baseline, duplicateA, duplicateB]

        let ranked = FuelSavingsEstimator.ranked(
            pool,
            fuel: .e10,
            baseline: baseline,
            settings: settings,
            amount: amount,
            journeyMiles: { s in
                s.id == "baseline" ? self.straightLine(2.0) : self.straightLine(7.0)
            }
        )

        XCTAssertEqual(ranked.count, pool.count)
        XCTAssertEqual(ranked.filter { $0.id == "duplicate" }.count, 2)
    }

    func testStationWithoutAPriceIsUnavailable() {
        let baseline = station("baseline", price: 145.9)
        let noPrice = station("no-price", price: nil)

        let availability = FuelSavingsEstimator.availability(
            station: noPrice,
            fuel: .e10,
            baseline: baseline,
            settings: settings,
            amount: amount,
            journey: straightLine(4.0),
            baselineJourney: straightLine(2.0)
        )

        XCTAssertEqual(availability, .unavailable("No price"))
    }

    func testFeatureOffProducesNoSavingsRow() {
        let baseline = station("baseline", price: 145.9)
        let candidate = station("candidate", price: 138.9)

        let availability = FuelSavingsEstimator.availability(
            station: candidate,
            fuel: .e10,
            baseline: baseline,
            settings: .defaults,
            amount: amount,
            journey: straightLine(4.0),
            baselineJourney: straightLine(2.0)
        )

        XCTAssertNil(availability)
    }

    // MARK: - Required wording and baseline labelling

    private var briefSettings: FuelSavingsSettings {
        FuelSavingsSettings(enabled: true,
                            mpg: 30,
                            preferredAmount: FuelSavingsFillAmount(value: 60, unit: .pounds))
    }

    private var briefAmount: FuelSavingsFillAmount {
        FuelSavingsFillAmount(value: 60, unit: .pounds)
    }

    /// The scenario from the brief: 30 MPG, £60 fill, baseline Rontec Shelf
    /// (171.9p, 0.4 mi), candidate Shell Whetley Hill (167.9p, 3.8 mi).
    ///
    /// Whetley Hill is 4p/litre cheaper, yet it is WORSE OFF — and that is
    /// correct, not a calculation bug: £60 buys 35.74 L at 167.9p, so the
    /// cheaper pump saves 35.74 × 4p = £1.43, while the extra 6.8 return miles
    /// burn 6.8/30 × 4.54609 = 1.03 L at 167.9p = £1.73. Net ≈ −£0.30.
    func testCheaperStationFurtherAwayIsWorseOffAfterTravel() {
        let baseline = station("rontec-shelf", price: 171.9)
        let candidate = station("shell-whetley-hill", price: 167.9)

        let availability = FuelSavingsEstimator.availability(
            station: candidate,
            fuel: .e10,
            baseline: baseline,
            settings: briefSettings,
            amount: briefAmount,
            journey: straightLine(7.6),        // 3.8 mi out and back
            baselineJourney: straightLine(0.8) // 0.4 mi out and back
        )

        guard let result = availability?.result else {
            return XCTFail("expected an estimate, got \(String(describing: availability))")
        }
        XCTAssertEqual(result.fillLitres, 35.736, accuracy: 0.005,
                       "the £60 budget buys litres at the candidate pump price")
        XCTAssertEqual(result.grossSavingGBP, 1.429, accuracy: 0.005)
        XCTAssertEqual(result.extraDistanceMiles, 6.8, accuracy: 0.001,
                       "only the return-trip DELTA counts — the shared journey is never counted twice")
        XCTAssertEqual(result.extraDrivingCostGBP, 1.730, accuracy: 0.005)
        XCTAssertEqual(result.netSavingGBP, -0.301, accuracy: 0.005)
        XCTAssertEqual(result.status, .notWorthTheDrive)
        XCTAssertEqual(result.journeyConfidence, .estimated)
        XCTAssertEqual(result.headline(compact: true), "Approx. Costs £0.30 more")
    }

    func testMeasuredRoadRoutesDropTheEstimateMarker() {
        let baseline = station("baseline", price: 171.9)
        let candidate = station("candidate", price: 167.9)

        let availability = FuelSavingsEstimator.availability(
            station: candidate,
            fuel: .e10,
            baseline: baseline,
            settings: briefSettings,
            amount: briefAmount,
            journey: FuelSavingsJourney(miles: 7.6, basis: .roadRoute),
            baselineJourney: FuelSavingsJourney(miles: 0.8, basis: .roadRoute)
        )

        XCTAssertEqual(availability?.result?.distanceBasis, .roadRoute)
        XCTAssertEqual(availability?.result?.journeyConfidence, .confirmed)
        XCTAssertEqual(availability?.result?.headline(compact: true), "Costs £0.30 more")
        XCTAssertEqual(availability?.result?.headline(), "Costs £0.30 more")
    }

    func testBaselineRowIsLabelledClosestStation() {
        let baseline = station("nearest", price: 171.9)

        let availability = FuelSavingsEstimator.availability(
            station: baseline,
            fuel: .e10,
            baseline: baseline,
            settings: briefSettings,
            amount: briefAmount,
            journey: straightLine(0.8),
            baselineJourney: straightLine(0.8)
        )

        XCTAssertEqual(availability?.result?.status, .baseline)
        XCTAssertEqual(availability?.result?.headline(compact: true), "Closest station")
        XCTAssertEqual(FuelSavingsStatus.baseline.displayName, "Closest station")
        XCTAssertEqual(availability?.result?.netSavingGBP, 0)
        XCTAssertEqual(availability?.result?.extraDistanceMiles, 0)
    }

    func testPositiveResultHeadlineSaysSaveAfterTravel() {
        // 4p cheaper AND almost the same distance, so the saving survives travel.
        let baseline = station("baseline", price: 160.0)
        let candidate = station("candidate", price: 150.0)

        let availability = FuelSavingsEstimator.availability(
            station: candidate,
            fuel: .e10,
            baseline: baseline,
            settings: briefSettings,
            amount: briefAmount,
            journey: straightLine(6.4),
            baselineJourney: straightLine(6.0)
        )

        guard let result = availability?.result else {
            return XCTFail("expected an estimate, got \(String(describing: availability))")
        }
        XCTAssertEqual(result.status, .worthTheDrive)
        // £60 buys 40 L at 150p; 10p/litre cheaper saves £4.00; 0.4 extra
        // return miles cost ~£0.09. Net ≈ £3.91.
        XCTAssertEqual(result.netSavingGBP, 3.909, accuracy: 0.005)
        XCTAssertEqual(result.headline(compact: true), "Approx. Save £3.91")
    }

    func testNeutralResultHeadlineIsBreakEven() {
        // 0.1p cheaper over the same distance: inside the break-even tolerance.
        let baseline = station("baseline", price: 150.0)
        let candidate = station("candidate", price: 149.9)

        let availability = FuelSavingsEstimator.availability(
            station: candidate,
            fuel: .e10,
            baseline: baseline,
            settings: briefSettings,
            amount: briefAmount,
            journey: straightLine(6.0),
            baselineJourney: straightLine(6.0)
        )

        XCTAssertEqual(availability?.result?.status, .breakEven)
        XCTAssertEqual(availability?.result?.headline(compact: true), "Break even")
        XCTAssertEqual(availability?.result?.headline(), "Break even")
    }

    /// The exact strings the brief asks for.
    func testHeadlineWordingMatchesTheBrief() {
        XCTAssertEqual(FuelSavingsResult.baselineHeadline, "Closest station")
        XCTAssertEqual(FuelSavingsResult.breakEvenHeadline, "Break even")

        let saving = FuelSavingsResult(fillLitres: 34.9,
                                       grossSavingGBP: 1.40,
                                       extraDrivingCostGBP: 0.88,
                                       extraDistanceMiles: 3.5,
                                       netSavingGBP: 0.52,
                                       status: .worthTheDrive,
                                       distanceBasis: .roadRoute)
        XCTAssertEqual(saving.headline(), "Save £0.52")

        let loss = FuelSavingsResult(fillLitres: 34.9,
                                     grossSavingGBP: 1.40,
                                     extraDrivingCostGBP: 1.71,
                                     extraDistanceMiles: 6.8,
                                     netSavingGBP: -0.31,
                                     status: .notWorthTheDrive,
                                     distanceBasis: .roadRoute)
        XCTAssertEqual(loss.headline(), "Costs £0.31 more")
    }

    /// The sort must order by NET saving, not pump price: a pricier station can
    /// win when the cheaper one is far enough away.
    func testSortingRanksByNetSavingNotPumpPrice() {
        let baseline = station("baseline", price: 171.9)
        let cheapButFar = station("cheap-but-far", price: 167.9)
        let pricierButNear = station("pricier-but-near", price: 169.9)

        let ranked = FuelSavingsEstimator.ranked(
            [baseline, cheapButFar, pricierButNear],
            fuel: .e10,
            baseline: baseline,
            settings: briefSettings,
            amount: briefAmount,
            journeyMiles: { s in
                switch s.id {
                case "baseline": return self.straightLine(0.8)
                case "cheap-but-far": return self.straightLine(7.6)
                default: return self.straightLine(1.2)
                }
            }
        )

        XCTAssertEqual(ranked.first?.id, "pricier-but-near",
                       "net saving decides the order, not the pump price")
        XCTAssertEqual(ranked.last?.id, "cheap-but-far")
    }

    // MARK: Row copy

    /// The row's extra-driving caption must follow the DISPLAY unit: the stored
    /// figure is always miles, and converting by the current unit instead would
    /// leave a km user's figure unlabelled and 1.6× too large.
    func testExtraTravelLabelFollowsTheDisplayUnit() {
        XCTAssertEqual(
            FuelSavingsFormat.extraTravelLabel(extraDistanceMiles: 19.9, unit: .miles),
            "19.9 mi extra return travel"
        )
        XCTAssertEqual(
            FuelSavingsFormat.extraTravelLabel(extraDistanceMiles: 19.9, unit: .kilometers),
            "32.0 km extra return travel"
        )
        XCTAssertNil(FuelSavingsFormat.extraTravelLabel(extraDistanceMiles: 0, unit: .miles),
                     "the baseline row has no extra travel to report")
        // The same conversion backs the detail sheet's extra-distance figure,
        // which used to be hardcoded miles ("%.1f miles") for km users too.
        XCTAssertEqual(FuelSavingsFormat.distance(19.9, unit: .miles), "19.9 mi")
        XCTAssertEqual(FuelSavingsFormat.distance(19.9, unit: .kilometers), "32.0 km")
    }

    /// The baseline capsule has to say what it IS: it previously repeated the
    /// status word above it ("Closest station" twice) and never explained that
    /// the savings are measured against it.
    func testCapsuleHeadlineMarksTheBaselineAndKeepsTheEstimateMarker() {
        let baseline = FuelSavingsResult(
            fillLitres: 30,
            grossSavingGBP: 0,
            extraDrivingCostGBP: 0,
            extraDistanceMiles: 0,
            netSavingGBP: 0,
            status: .baseline
        )
        XCTAssertEqual(baseline.capsuleHeadline(), "Closest station · your baseline")

        let saving = FuelSavingsResult(
            fillLitres: 30,
            grossSavingGBP: 2.40,
            extraDrivingCostGBP: 1.40,
            extraDistanceMiles: 5.2,
            netSavingGBP: 1.00,
            status: .worthTheDrive,
            distanceBasis: .straightLine
        )
        XCTAssertEqual(saving.capsuleHeadline(), "Approx. Save £1.00")

        let worse = FuelSavingsResult(
            fillLitres: 30,
            grossSavingGBP: 1.40,
            extraDrivingCostGBP: 1.70,
            extraDistanceMiles: 6.7,
            netSavingGBP: -0.31,
            status: .notWorthTheDrive
        )
        XCTAssertEqual(worse.capsuleHeadline(), "Costs £0.31 more")
    }

    // MARK: - Estimate wording (Approx. labels)

    /// The estimate marker is the WORD "Approx.", never a "~", and it is applied to
    /// both directions of the money result.
    func testEstimatedHeadlinesUseTheWordApproxForBothDirections() {
        let saving = FuelSavingsResult(
            fillLitres: 30,
            grossSavingGBP: 4.00,
            extraDrivingCostGBP: 2.75,
            extraDistanceMiles: 6.4,
            netSavingGBP: 1.25,
            status: .worthTheDrive,
            distanceBasis: .straightLine
        )
        XCTAssertEqual(saving.headline(), "Approx. Save £1.25")

        let loss = FuelSavingsResult(
            fillLitres: 30,
            grossSavingGBP: 1.40,
            extraDrivingCostGBP: 2.17,
            extraDistanceMiles: 6.4,
            netSavingGBP: -0.77,
            status: .notWorthTheDrive,
            distanceBasis: .straightLine
        )
        XCTAssertEqual(loss.headline(), "Approx. Costs £0.77 more")
    }

    /// A confirmed road-route comparison must never carry the marker — in either
    /// direction, and not as a stray tilde either.
    func testConfirmedHeadlinesCarryNoApproxMarker() {
        let saving = FuelSavingsResult(
            fillLitres: 30,
            grossSavingGBP: 4.00,
            extraDrivingCostGBP: 2.75,
            extraDistanceMiles: 6.4,
            netSavingGBP: 1.25,
            status: .worthTheDrive,
            distanceBasis: .roadRoute,
            journeyConfidence: .confirmed
        )
        XCTAssertEqual(saving.headline(), "Save £1.25")

        let loss = FuelSavingsResult(
            fillLitres: 30,
            grossSavingGBP: 1.40,
            extraDrivingCostGBP: 2.17,
            extraDistanceMiles: 6.4,
            netSavingGBP: -0.77,
            status: .notWorthTheDrive,
            distanceBasis: .roadRoute,
            journeyConfidence: .confirmed
        )
        XCTAssertEqual(loss.headline(), "Costs £0.77 more")

        for headline in [saving.headline(), loss.headline(), saving.capsuleHeadline()] {
            XCTAssertFalse(headline.contains("Approx"), "\"\(headline)\" must not claim to be an estimate")
            XCTAssertFalse(headline.contains("~"), "\"\(headline)\" must use a word, not a symbol")
        }
    }

    /// The extra-travel caption names the RETURN journey in both states. The
    /// estimated caption repeats "Approx." deliberately: it describes a different
    /// value from the verdict above it.
    func testExtraTravelNamesTheReturnJourney() {
        XCTAssertEqual(
            FuelSavingsFormat.extraTravelLabel(extraDistanceMiles: 6.4, unit: .miles),
            "6.4 mi extra return travel"
        )
        XCTAssertEqual(
            FuelSavingsFormat.extraTravelLabel(extraDistanceMiles: 6.4, unit: .miles, confidence: .estimated),
            "Approx. 6.4 mi extra return travel"
        )
    }

    /// VoiceOver gets words, not abbreviations: "Approx." → "Approximately" and
    /// "mi"/"km" → "miles"/"kilometres".
    func testSpokenLabelsSpellOutTheAbbreviations() {
        let estimated = FuelSavingsResult(
            fillLitres: 30,
            grossSavingGBP: 1.40,
            extraDrivingCostGBP: 2.17,
            extraDistanceMiles: 6.4,
            netSavingGBP: -0.77,
            status: .notWorthTheDrive,
            distanceBasis: .straightLine
        )
        XCTAssertEqual(estimated.spokenHeadline(), "Approximately Costs £0.77 more")

        XCTAssertEqual(
            FuelSavingsFormat.extraTravelSpokenLabel(extraDistanceMiles: 6.4, unit: .miles, confidence: .estimated),
            "Approximately 6.4 miles extra return travel"
        )
        XCTAssertEqual(
            FuelSavingsFormat.extraTravelSpokenLabel(extraDistanceMiles: 19.9, unit: .kilometers),
            "32.0 kilometres extra return travel"
        )

        let confirmed = FuelSavingsResult(
            fillLitres: 30,
            grossSavingGBP: 1.40,
            extraDrivingCostGBP: 2.17,
            extraDistanceMiles: 6.4,
            netSavingGBP: -0.77,
            status: .notWorthTheDrive,
            distanceBasis: .roadRoute,
            journeyConfidence: .confirmed
        )
        XCTAssertEqual(confirmed.spokenHeadline(), confirmed.headline(),
                       "a confirmed comparison has no abbreviation to spell out")
    }

    // MARK: - Slider bounds (Settings and the in-list control share these)

    /// Both fill-up controls are slider-only now, so every position the slider
    /// can reach must be a usable amount — otherwise the control could sit on a
    /// figure the calculator rejects.
    func testFillAmountSliderBoundsAreAlwaysValidAmounts() {
        for unit in FuelSavingsFillUnit.allCases {
            let range = FuelSavingsCalculator.fillAmountSliderRange(for: unit)
            let step = FuelSavingsCalculator.fillAmountSliderStep(for: unit)
            XCTAssertLessThan(range.lowerBound, range.upperBound, "\(unit) range must be usable")
            XCTAssertGreaterThan(step, 0, "\(unit) step must advance")
            XCTAssertGreaterThan(range.lowerBound, 0, "\(unit) must never offer a zero fill")
            XCTAssertLessThanOrEqual(range.upperBound,
                                     FuelSavingsCalculator.maximumFillAmount(for: unit),
                                     "\(unit) slider must stay inside the valid ceiling")
            for position in [range.lowerBound, range.upperBound, (range.lowerBound + range.upperBound) / 2] {
                XCTAssertTrue(FuelSavingsFillAmount(value: position, unit: unit).isValid,
                              "\(position) \(unit) must be a valid amount")
            }
        }
    }

    func testMPGSliderBoundsAreInsideTheValidMPGRange() {
        let range = FuelSavingsCalculator.mpgSliderRange
        XCTAssertLessThan(range.lowerBound, range.upperBound)
        XCTAssertGreaterThan(FuelSavingsCalculator.mpgSliderStep, 0)
        XCTAssertTrue(FuelSavingsCalculator.isValidMPG(range.lowerBound))
        XCTAssertTrue(FuelSavingsCalculator.isValidMPG(range.upperBound))
        XCTAssertTrue(FuelSavingsCalculator.isValidMPG(45), "a typical UK figure must be reachable")
    }
}