import HealthKit

let healthStore = HKHealthStore()

guard HKHealthStore.isHealthDataAvailable() else {
    // HealthKit not available on this device (e.g., iPad)
    return
}

func requestAuthorization() async throws {
    let typesToShare: Set<HKSampleType> = [
        HKQuantityType(.stepCount),
        HKQuantityType(.activeEnergyBurned)
    ]

    let typesToRead: Set<HKObjectType> = [
        HKQuantityType(.stepCount),
        HKQuantityType(.heartRate),
        HKQuantityType(.activeEnergyBurned),
        HKCharacteristicType(.dateOfBirth)
    ]

    try await healthStore.requestAuthorization(
        toShare: typesToShare,
        read: typesToRead
    )
}

let status = healthStore.authorizationStatus(
    for: HKQuantityType(.stepCount)
)

switch status {
case .notDetermined:
    // Haven't requested yet -- safe to call requestAuthorization
    break
case .sharingAuthorized:
    // User granted write access
    break
case .sharingDenied:
    // User denied write access (read denial is indistinguishable from "no data")
    break
@unknown default:
    break
}

func fetchRecentHeartRates() async throws -> [HKQuantitySample] {
    let heartRateType = HKQuantityType(.heartRate)

    let descriptor = HKSampleQueryDescriptor(
        predicates: [.quantitySample(type: heartRateType)],
        sortDescriptors: [SortDescriptor(\.endDate, order: .reverse)],
        limit: 20
    )

    let results = try await descriptor.result(for: healthStore)
    return results
}

// Extracting values from samples:
for sample in results {
    let bpm = sample.quantity.doubleValue(
        for: HKUnit.count().unitDivided(by: .minute())
    )
    print("\(bpm) bpm at \(sample.endDate)")
}

func fetchTodayStepCount() async throws -> Double? {
    let calendar = Calendar.current
    let startOfDay = calendar.startOfDay(for: Date())
    let endOfDay = calendar.date(byAdding: .day, value: 1, to: startOfDay)!

    let predicate = HKQuery.predicateForSamples(
        withStart: startOfDay, end: endOfDay
    )
    let stepType = HKQuantityType(.stepCount)
    let samplePredicate = HKSamplePredicate.quantitySample(
        type: stepType, predicate: predicate
    )

    let query = HKStatisticsQueryDescriptor(
        predicate: samplePredicate,
        options: .cumulativeSum
    )

    let result = try await query.result(for: healthStore)
    return result?.sumQuantity()?.doubleValue(for: .count())
}

func fetchDailySteps(forLast days: Int) async throws -> [(date: Date, steps: Double)] {
    let calendar = Calendar.current
    let endDate = calendar.startOfDay(
        for: calendar.date(byAdding: .day, value: 1, to: Date())!
    )
    let startDate = calendar.date(byAdding: .day, value: -days, to: endDate)!

    let predicate = HKQuery.predicateForSamples(
        withStart: startDate, end: endDate
    )
    let stepType = HKQuantityType(.stepCount)
    let samplePredicate = HKSamplePredicate.quantitySample(
        type: stepType, predicate: predicate
    )

    let query = HKStatisticsCollectionQueryDescriptor(
        predicate: samplePredicate,
        options: .cumulativeSum,
        anchorDate: endDate,
        intervalComponents: DateComponents(day: 1)
    )

    let collection = try await query.result(for: healthStore)
    var dailySteps: [(date: Date, steps: Double)] = []

    collection.statisticsCollection.enumerateStatistics(
        from: startDate, to: endDate
    ) { statistics, _ in
        let steps = statistics.sumQuantity()?
            .doubleValue(for: .count()) ?? 0
        dailySteps.append((date: statistics.startDate, steps: steps))
    }

    return dailySteps
}

let updateStream = query.results(for: healthStore)

Task {
    for try await result in updateStream {
        // result.statisticsCollection contains updated data
    }
}

func saveSteps(count: Double, start: Date, end: Date) async throws {
    let stepType = HKQuantityType(.stepCount)
    let quantity = HKQuantity(unit: .count(), doubleValue: count)

    let sample = HKQuantitySample(
        type: stepType,
        quantity: quantity,
        start: start,
        end: end
    )

    try await healthStore.save(sample)
}

func enableStepCountBackgroundDelivery() async throws {
    let stepType = HKQuantityType(.stepCount)

    try await healthStore.enableBackgroundDelivery(
        for: stepType,
        frequency: .hourly
    )
}

let observerQuery = HKObserverQuery(
    sampleType: HKQuantityType(.stepCount),
    predicate: nil
) { query, completionHandler, error in
    defer { completionHandler() }  // Must call to signal done
    guard error == nil else { return }
    // Fetch new data, update UI, etc.
}
healthStore.execute(observerQuery)

func startWorkout() async throws {
    let configuration = HKWorkoutConfiguration()
    configuration.activityType = .running
    configuration.locationType = .outdoor

    let session = try HKWorkoutSession(
        healthStore: healthStore,
        configuration: configuration
    )
    session.delegate = self

    let builder = session.associatedWorkoutBuilder()
    builder.dataSource = HKLiveWorkoutDataSource(
        healthStore: healthStore,
        workoutConfiguration: configuration
    )

    session.startActivity(with: Date())
    try await builder.beginCollection(at: Date())
}

func endWorkout(
    session: HKWorkoutSession,
    builder: HKLiveWorkoutBuilder
) async throws {
    session.end()
    try await builder.endCollection(at: Date())
    try await builder.finishWorkout()
}

// Basic units
HKUnit.count()                              // Steps, counts
HKUnit.meter()                              // Distance
HKUnit.mile()                               // Distance (imperial)
HKUnit.kilocalorie()                        // Energy
HKUnit.joule(with: .kilo)                   // Energy (SI)
HKUnit.gramUnit(with: .kilo)                // Mass (kg)
HKUnit.pound()                              // Mass (imperial)
HKUnit.percent()                            // Percentage

// Compound units
HKUnit.count().unitDivided(by: .minute())   // Heart rate (bpm)
HKUnit.meter().unitDivided(by: .second())   // Speed (m/s)

// Prefixed units
HKUnit.gramUnit(with: .milli)               // Milligrams
HKUnit.literUnit(with: .deci)               // Deciliters

// App Review will reject this
let allTypes: Set<HKObjectType> = [
    HKQuantityType(.stepCount),
    HKQuantityType(.heartRate),
    HKQuantityType(.bloodGlucose),
    HKQuantityType(.bodyMass),
    HKQuantityType(.oxygenSaturation),
    // ...20 more types the app never uses
]

let neededTypes: Set<HKObjectType> = [
    HKQuantityType(.stepCount),
    HKQuantityType(.activeEnergyBurned)
]

func getSteps() async throws -> Double {
    let result = try await query.result(for: healthStore)
    return result!.sumQuantity()!.doubleValue(for: .count()) // Crashes if denied
}

func getSteps() async throws -> Double {
    let result = try await query.result(for: healthStore)
    return result?.sumQuantity()?.doubleValue(for: .count()) ?? 0
}

let store = HKHealthStore() // Crashes on iPad
try await store.requestAuthorization(toShare: types, read: types)

guard HKHealthStore.isHealthDataAvailable() else {
    showUnsupportedDeviceMessage()
    return
}

// Bad: HKSampleQuery with callback on main thread
// Good: async descriptor
func loadAllData() async throws -> [HKQuantitySample] {
    let descriptor = HKSampleQueryDescriptor(
        predicates: [.quantitySample(type: stepType)],
        sortDescriptors: [SortDescriptor(\.endDate, order: .reverse)],
        limit: 100
    )
    return try await descriptor.result(for: healthStore)
}

let query = HKObserverQuery(sampleType: type, predicate: nil) { _, handler, _ in
    processNewData()
    // Forgot to call handler() -- system won't schedule next delivery
}

let query = HKObserverQuery(sampleType: type, predicate: nil) { _, handler, _ in
    defer { handler() }
    processNewData()
}

// Heart rate is discrete, not cumulative -- this returns nil
let query = HKStatisticsQueryDescriptor(
    predicate: heartRatePredicate,
    options: .cumulativeSum
)

// Use discrete options for discrete types
let query = HKStatisticsQueryDescriptor(
    predicate: heartRatePredicate,
    options: .discreteAverage
)