420 lines
19 KiB
Swift
420 lines
19 KiB
Swift
import XCTest
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#if SWIFT_PACKAGE
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@testable import FuelBoardShared
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#endif
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/// Worth the Drive calculation audit.
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///
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/// Written against the audit brief (30 MPG imperial, £60 fill-up, baseline
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/// Rontec Shelf at 171.9p, 0.4 miles). The brief's three observed results are
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/// reproduced here exactly, so any future change to the formula breaks a test
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/// instead of silently shifting the numbers on screen.
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///
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/// Deterministic journey distances only — never live MKDirections responses.
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final class FuelSavingsCalculationAuditTests: XCTestCase {
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private func station(_ id: String, price: Double?) -> FuelStation {
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FuelStation(
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id: id,
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name: id,
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brand: "X",
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address: "",
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postcode: "",
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lat: 51.5,
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lng: -0.1,
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prices: price.map { [FuelType.e10: $0] } ?? [:],
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priceUpdated: nil
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)
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}
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/// Journeys are OUT AND BACK (the app's convention): the brief's baseline is
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/// 0.4 miles away, so its journey is 0.8 miles, and "extra driving" of N
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/// miles means a candidate journey of 0.8 + N.
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private func journey(_ miles: Double, basis: FuelSavingsDistanceBasis = .roadRoute) -> FuelSavingsJourney {
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FuelSavingsJourney(miles: miles, basis: basis)
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}
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private let baselineJourneyMiles = 0.8
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private func briefResult(candidatePrice: Double, extraMiles: Double) throws -> FuelSavingsResult {
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try FuelSavingsCalculator.calculate(
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mpg: 30,
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amount: FuelSavingsFillAmount(value: 60, unit: .pounds),
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baselinePricePencePerLitre: 171.9,
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candidatePricePencePerLitre: candidatePrice,
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baselineJourneyMiles: baselineJourneyMiles,
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candidateJourneyMiles: baselineJourneyMiles + extraMiles
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)
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}
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// MARK: - 1. Unit conversion
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func testImperialGallonIsUsedNotTheUSGallon() {
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XCTAssertEqual(FuelSavingsCalculator.imperialGallonLitres, 4.54609)
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XCTAssertNotEqual(FuelSavingsCalculator.imperialGallonLitres, 3.785411784,
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"a US gallon must never be used: it would understate every travel cost")
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// 30 miles at 30 MPG burns exactly one imperial gallon.
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XCTAssertEqual(FuelSavingsCalculator.fuelUsedLitres(distanceMiles: 30, mpg: 30), 4.54609, accuracy: 1e-12)
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// 100 miles at 50 MPG.
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XCTAssertEqual(FuelSavingsCalculator.fuelUsedLitres(distanceMiles: 100, mpg: 50), 9.09218, accuracy: 1e-12)
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// Zero distance burns nothing, and the relationship is linear in distance.
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XCTAssertEqual(FuelSavingsCalculator.fuelUsedLitres(distanceMiles: 0, mpg: 30), 0, accuracy: 1e-12)
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XCTAssertEqual(FuelSavingsCalculator.fuelUsedLitres(distanceMiles: 60, mpg: 30),
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2 * FuelSavingsCalculator.fuelUsedLitres(distanceMiles: 30, mpg: 30),
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accuracy: 1e-12)
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}
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func testMileAndKilometreFactorsAreTheExactOnes() {
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XCTAssertEqual(FuelSavingsCalculator.kilometresPerMile, 1.609344)
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XCTAssertEqual(FuelSavingsCalculator.metresPerMile, 1609.344)
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XCTAssertEqual(FuelSavingsCalculator.metresPerMile,
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FuelSavingsCalculator.kilometresPerMile * 1000,
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accuracy: 1e-9,
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"the metre and kilometre factors must describe the same mile")
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}
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func testDistanceLabelUsesTheSameConversionAsTheModel() {
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// One mile, shown in kilometres, goes through the single factor.
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XCTAssertEqual(
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FuelSavingsFormat.distance(1, unit: .kilometers),
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DistanceUnit.kilometers.format(FuelSavingsCalculator.kilometresPerMile)
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)
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XCTAssertEqual(FuelSavingsFormat.distance(1, unit: .miles), DistanceUnit.miles.format(1.609344))
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}
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// MARK: - 2/4. The three stations from the brief
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func testBriefCaseTescoBradfordIsMarginallyWorseOff() throws {
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let result = try briefResult(candidatePrice: 170.9, extraMiles: 2.0)
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XCTAssertEqual(result.fillLitres, 34.904014, accuracy: 1e-6)
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XCTAssertEqual(result.grossSavingGBP, 0.349040, accuracy: 1e-6, "1p/litre cheaper on a 34.90 L fill")
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XCTAssertEqual(result.extraDistanceMiles, 2.0, accuracy: 1e-9)
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XCTAssertEqual(result.extraDrivingCostGBP, 0.517951, accuracy: 1e-6)
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XCTAssertEqual(result.netSavingGBP, -0.168911, accuracy: 1e-6)
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XCTAssertEqual(result.status, .notWorthTheDrive)
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XCTAssertEqual(result.headline(), "£0.17 worse off after travel")
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}
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/// The brief recorded "£0.18 worse off" for this row against a displayed
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/// "2.0 miles" of extra driving; the brief also warns that the displayed
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/// mileage is rounded. This pins that explanation instead of treating it as
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/// a calculation bug: at an extra distance the caption still shows as
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/// "2.0 mi" (2.00-2.05), the headline reads £0.17-£0.18, and the difference
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/// is entirely the rounding of the distance, not of the money.
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func testTheReportedEighteenPenceIsRoundedExtraDistanceNotARoundingBug() throws {
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XCTAssertEqual(try briefResult(candidatePrice: 170.9, extraMiles: 2.00).headline(),
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"£0.17 worse off after travel")
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XCTAssertEqual(try briefResult(candidatePrice: 170.9, extraMiles: 2.05).headline(),
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"£0.18 worse off after travel")
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// 2.05 mi shows as "2.0 mi" at one decimal place — the same caption.
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XCTAssertEqual(FuelSavingsFormat.extraTravelLabel(extraDistanceMiles: 2.05, unit: .miles),
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"2.0 mi more driving than closest")
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XCTAssertEqual(FuelSavingsFormat.extraTravelLabel(extraDistanceMiles: 2.00, unit: .miles),
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"2.0 mi more driving than closest")
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}
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func testBriefCaseMFGHareAndHoundsIsWorseOff() throws {
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let result = try briefResult(candidatePrice: 172.9, extraMiles: 2.8)
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XCTAssertEqual(result.fillLitres, 34.904014, accuracy: 1e-6)
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XCTAssertEqual(result.grossSavingGBP, -0.349040, accuracy: 1e-6, "1p/litre dearer costs 34.90 L × 1p")
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XCTAssertEqual(result.extraDistanceMiles, 2.8, accuracy: 1e-9)
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XCTAssertEqual(result.extraDrivingCostGBP, 0.733618, accuracy: 1e-6)
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XCTAssertEqual(result.netSavingGBP, -1.082658, accuracy: 1e-6)
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XCTAssertEqual(result.status, .notWorthTheDrive)
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XCTAssertEqual(result.headline(), "£1.08 worse off after travel")
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}
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func testBriefCasePGGNapoleonSamePumpPriceIsTravelCostOnly() throws {
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let result = try briefResult(candidatePrice: 171.9, extraMiles: 2.8)
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XCTAssertEqual(result.grossSavingGBP, 0, accuracy: 1e-9, "same pump price saves nothing at the pump")
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XCTAssertEqual(result.extraDrivingCostGBP, 0.729375, accuracy: 1e-6)
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XCTAssertEqual(result.netSavingGBP, -0.729375, accuracy: 1e-6,
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"with no price difference the whole result is the extra driving")
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XCTAssertEqual(result.status, .notWorthTheDrive)
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XCTAssertEqual(result.headline(), "£0.73 worse off after travel")
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}
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// MARK: - 5/6. Classification, tolerance and rounding
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func testSamePriceAndSameDistanceIsBreakEven() throws {
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let result = try briefResult(candidatePrice: 171.9, extraMiles: 0)
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XCTAssertEqual(result.grossSavingGBP, 0, accuracy: 1e-9)
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XCTAssertEqual(result.extraDrivingCostGBP, 0, accuracy: 1e-9)
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XCTAssertEqual(result.netSavingGBP, 0, accuracy: 1e-9)
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XCTAssertEqual(result.status, .breakEven)
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XCTAssertEqual(result.headline(), "Break even")
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}
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func testSmallSavingInsideTheToleranceIsBreakEvenNotPennies() throws {
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// 171.9p vs 170.9p over a 1.23 mi delta: net +£0.0305.
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let result = try briefResult(candidatePrice: 170.9, extraMiles: 1.23)
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XCTAssertEqual(result.netSavingGBP, 0.030500, accuracy: 1e-6)
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XCTAssertEqual(result.status, .breakEven, "±5p reads as break even rather than a meaningless £0.03")
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XCTAssertEqual(result.headline(), "Break even")
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}
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func testBreakEvenToleranceDecidesTheClassificationAtTheBoundary() throws {
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let net = -0.168911
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// A tolerance wider than the loss → break even; narrower → worse off.
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let lenient = try FuelSavingsCalculator.calculate(
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mpg: 30,
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amount: FuelSavingsFillAmount(value: 60, unit: .pounds),
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baselinePricePencePerLitre: 171.9,
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candidatePricePencePerLitre: 170.9,
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baselineJourneyMiles: baselineJourneyMiles,
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candidateJourneyMiles: baselineJourneyMiles + 2.0,
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breakEvenToleranceGBP: 0.17
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)
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XCTAssertEqual(lenient.netSavingGBP, net, accuracy: 1e-6)
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XCTAssertEqual(lenient.status, .breakEven)
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let strict = try FuelSavingsCalculator.calculate(
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mpg: 30,
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amount: FuelSavingsFillAmount(value: 60, unit: .pounds),
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baselinePricePencePerLitre: 171.9,
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candidatePricePencePerLitre: 170.9,
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baselineJourneyMiles: baselineJourneyMiles,
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candidateJourneyMiles: baselineJourneyMiles + 2.0,
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breakEvenToleranceGBP: 0.16
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)
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XCTAssertEqual(strict.status, .notWorthTheDrive)
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}
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func testMoneyIsRoundedOnlyForDisplay() throws {
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// Full precision retained internally…
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let result = try briefResult(candidatePrice: 172.9, extraMiles: 2.8)
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XCTAssertEqual(result.netSavingGBP, -1.082658, accuracy: 1e-6)
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XCTAssertNotEqual(result.netSavingGBP, -1.08, "the stored value must not be pre-rounded")
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// …and rounded to two places only when shown.
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XCTAssertEqual(FuelSavingsFormat.money(1.082658), "£1.08")
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XCTAssertEqual(FuelSavingsFormat.money(0.729375), "£0.73")
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XCTAssertEqual(FuelSavingsFormat.money(0.1749), "£0.17")
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XCTAssertEqual(FuelSavingsFormat.money(0.176), "£0.18")
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XCTAssertEqual(FuelSavingsFormat.money(0), "£0.00")
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}
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func testSortingUsesTheUnroundedNetSoRoundedTiesStillOrderCorrectly() {
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// Two rows that both DISPLAY "£0.18 worse off" but differ in the raw
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// figure: the sort must use the raw value, never the shown one.
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func ranked(_ id: String, net: Double) -> FuelSavingsRankedStation {
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FuelSavingsRankedStation(
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stationID: id,
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result: .available(FuelSavingsResult(
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fillLitres: 34.9,
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grossSavingGBP: 0,
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extraDrivingCostGBP: abs(net),
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extraDistanceMiles: 2.0,
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netSavingGBP: net,
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status: .notWorthTheDrive
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)),
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distanceMiles: 2.8,
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pricePencePerLitre: 171.9
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)
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}
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let worse = ranked("worse", net: -0.1751)
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let lessWorse = ranked("less-worse", net: -0.1749)
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XCTAssertEqual(FuelSavingsFormat.money(abs(worse.result.result!.netSavingGBP)), "£0.18")
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XCTAssertEqual(FuelSavingsFormat.money(abs(lessWorse.result.result!.netSavingGBP)), "£0.17")
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let sorted = FuelSavingsCalculator.sortedForWorthTheDrive([worse, lessWorse])
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XCTAssertEqual(sorted.first?.stationID, "less-worse",
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"the smaller loss must rank first even when the money rounds to the same figure")
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}
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// MARK: - 3. Distances and travel cost
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func testCheaperFuelFurtherAwayCanStillBeWorthTheDrive() throws {
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// 145.9p nearby vs 138.9p five miles further out, £40 fill, 45 MPG.
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let result = try FuelSavingsCalculator.calculate(
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mpg: 45,
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amount: FuelSavingsFillAmount(value: 40, unit: .pounds),
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baselinePricePencePerLitre: 145.9,
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candidatePricePencePerLitre: 138.9,
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baselineJourneyMiles: 2.0,
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candidateJourneyMiles: 7.0
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)
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XCTAssertEqual(result.fillLitres, 27.416038, accuracy: 1e-6)
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XCTAssertEqual(result.grossSavingGBP, 1.919123, accuracy: 1e-6)
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XCTAssertEqual(result.extraDrivingCostGBP, 0.701613, accuracy: 1e-6)
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XCTAssertEqual(result.netSavingGBP, 1.217509, accuracy: 1e-6)
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XCTAssertEqual(result.status, .worthTheDrive)
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XCTAssertEqual(result.headline(), "Save £1.22 after travel")
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}
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func testNearerCandidateIsNeverChargedExtraTravel() throws {
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// The baseline is the nearest priced station, so this should not happen
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// in the app — but the clamp must hold if it ever does.
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let result = try FuelSavingsCalculator.calculate(
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mpg: 30,
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amount: FuelSavingsFillAmount(value: 60, unit: .pounds),
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baselinePricePencePerLitre: 171.9,
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candidatePricePencePerLitre: 169.9,
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baselineJourneyMiles: 2.0,
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candidateJourneyMiles: 1.0
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)
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XCTAssertEqual(result.extraDistanceMiles, 0, "a shorter journey is never a negative extra distance")
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XCTAssertEqual(result.extraDrivingCostGBP, 0, accuracy: 1e-12)
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XCTAssertEqual(result.netSavingGBP, result.grossSavingGBP, accuracy: 1e-12)
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}
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func testDifferentMPGValuesScaleOnlyTheTravelCost() throws {
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let sixty = try FuelSavingsCalculator.calculate(
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mpg: 60,
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amount: FuelSavingsFillAmount(value: 60, unit: .pounds),
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baselinePricePencePerLitre: 171.9,
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candidatePricePencePerLitre: 172.9,
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baselineJourneyMiles: 0.8,
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candidateJourneyMiles: 3.6
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)
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XCTAssertEqual(sixty.grossSavingGBP, -0.349040, accuracy: 1e-6, "MPG must not touch the pump saving")
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XCTAssertEqual(sixty.extraDrivingCostGBP, 0.366809, accuracy: 1e-6)
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XCTAssertEqual(sixty.netSavingGBP, -0.715849, accuracy: 1e-6)
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let fortyFive = try FuelSavingsCalculator.calculate(
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mpg: 45,
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amount: FuelSavingsFillAmount(value: 60, unit: .pounds),
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baselinePricePencePerLitre: 171.9,
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candidatePricePencePerLitre: 172.9,
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baselineJourneyMiles: 0.8,
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candidateJourneyMiles: 3.6
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)
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XCTAssertEqual(fortyFive.extraDrivingCostGBP, 0.489078, accuracy: 1e-6)
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XCTAssertEqual(fortyFive.netSavingGBP, -0.838119, accuracy: 1e-6)
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}
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func testDifferentFillBudgetsScaleOnlyThePumpSaving() throws {
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let forty = try FuelSavingsCalculator.calculate(
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mpg: 30,
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amount: FuelSavingsFillAmount(value: 40, unit: .pounds),
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baselinePricePencePerLitre: 171.9,
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candidatePricePencePerLitre: 172.9,
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baselineJourneyMiles: 0.8,
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candidateJourneyMiles: 3.6
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)
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XCTAssertEqual(forty.fillLitres, 23.269343, accuracy: 1e-6, "£40 buys 23.27 L at 171.9p")
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XCTAssertEqual(forty.grossSavingGBP, -0.232693, accuracy: 1e-6)
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XCTAssertEqual(forty.extraDrivingCostGBP, 0.733618, accuracy: 1e-6, "the journey cost is budget-independent")
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XCTAssertEqual(forty.netSavingGBP, -0.966311, accuracy: 1e-6)
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let ninety = try FuelSavingsCalculator.calculate(
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mpg: 30,
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amount: FuelSavingsFillAmount(value: 90, unit: .pounds),
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baselinePricePencePerLitre: 171.9,
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candidatePricePencePerLitre: 172.9,
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baselineJourneyMiles: 0.8,
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candidateJourneyMiles: 3.6
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)
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XCTAssertEqual(ninety.fillLitres, 52.356021, accuracy: 1e-6)
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XCTAssertEqual(ninety.grossSavingGBP, -0.523560, accuracy: 1e-6)
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XCTAssertEqual(ninety.netSavingGBP, -1.257178, accuracy: 1e-6)
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}
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func testFillUpInLitresIsFixedRegardlessOfPrice() throws {
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// 40 L is 40 L: no baseline price is involved in the quantity.
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let result = try FuelSavingsCalculator.calculate(
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mpg: 30,
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amount: FuelSavingsFillAmount(value: 40, unit: .litres),
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baselinePricePencePerLitre: 171.9,
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candidatePricePencePerLitre: 165.9,
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baselineJourneyMiles: 0.8,
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candidateJourneyMiles: 3.6
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)
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XCTAssertEqual(result.fillLitres, 40, accuracy: 1e-9)
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XCTAssertEqual(result.grossSavingGBP, 40 * 0.06, accuracy: 1e-9)
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}
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// MARK: - 7. Failures never invent a saving
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func testUnavailableRouteProducesNoSavingAtAll() {
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let baseline = station("baseline", price: 171.9)
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let candidate = station("candidate", price: 170.9)
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let settings = FuelSavingsSettings(
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enabled: true,
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mpg: 30,
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preferredAmount: FuelSavingsFillAmount(value: 60, unit: .pounds)
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)
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let noRoute = FuelSavingsEstimator.availability(
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station: candidate,
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fuel: .e10,
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baseline: baseline,
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settings: settings,
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amount: FuelSavingsFillAmount(value: 60, unit: .pounds),
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journey: nil,
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baselineJourney: journey(0.8)
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)
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XCTAssertEqual(noRoute, .unavailable("Distance unavailable"))
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let noBaselineRoute = FuelSavingsEstimator.availability(
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station: candidate,
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fuel: .e10,
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baseline: baseline,
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settings: settings,
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amount: FuelSavingsFillAmount(value: 60, unit: .pounds),
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journey: journey(2.8),
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baselineJourney: nil
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)
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XCTAssertEqual(noBaselineRoute, .unavailable("No comparison station"))
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// With both legs present it produces the saving — no silent zero.
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let both = FuelSavingsEstimator.availability(
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station: candidate,
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fuel: .e10,
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baseline: baseline,
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settings: settings,
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amount: FuelSavingsFillAmount(value: 60, unit: .pounds),
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journey: journey(2.8),
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baselineJourney: journey(0.8)
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)
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XCTAssertEqual(both?.result?.netSavingGBP ?? 0, -0.168911, accuracy: 1e-6)
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}
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func testMixedRouteBasisIsReportedAsAnEstimate() {
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let baseline = station("baseline", price: 171.9)
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let candidate = station("candidate", price: 170.9)
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let settings = FuelSavingsSettings(
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enabled: true,
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mpg: 30,
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preferredAmount: FuelSavingsFillAmount(value: 60, unit: .pounds)
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)
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let mixed = FuelSavingsEstimator.availability(
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station: candidate,
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fuel: .e10,
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baseline: baseline,
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settings: settings,
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amount: FuelSavingsFillAmount(value: 60, unit: .pounds),
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journey: journey(2.8, basis: .straightLine),
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baselineJourney: journey(0.8, basis: .roadRoute)
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)
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XCTAssertEqual(mixed?.result?.distanceBasis, .straightLine,
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"one unmeasured leg makes the whole estimate an estimate")
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XCTAssertEqual(mixed?.result?.journeyConfidence, .estimated)
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XCTAssertEqual(mixed?.result?.headline(compact: true), "~£0.17 worse off after travel")
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}
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func testBaselineStationIsLabelledClosestStationNotBreakEven() {
|
||
let baseline = station("baseline", price: 171.9)
|
||
let settings = FuelSavingsSettings(
|
||
enabled: true,
|
||
mpg: 30,
|
||
preferredAmount: FuelSavingsFillAmount(value: 60, unit: .pounds)
|
||
)
|
||
let result = FuelSavingsEstimator.availability(
|
||
station: baseline,
|
||
fuel: .e10,
|
||
baseline: baseline,
|
||
settings: settings,
|
||
amount: FuelSavingsFillAmount(value: 60, unit: .pounds),
|
||
journey: journey(0.8),
|
||
baselineJourney: journey(0.8)
|
||
)?.result
|
||
XCTAssertEqual(result?.status, .baseline)
|
||
XCTAssertEqual(result?.netSavingGBP, 0)
|
||
XCTAssertEqual(result?.headline(), "Closest station")
|
||
}
|
||
}
|