import CoreBluetooth

final class BluetoothManager: NSObject, CBCentralManagerDelegate {
    private var centralManager: CBCentralManager!
    private var discoveredPeripheral: CBPeripheral?

    override init() {
        super.init()
        centralManager = CBCentralManager(delegate: self, queue: nil)
    }

    func centralManagerDidUpdateState(_ central: CBCentralManager) {
        switch central.state {
        case .poweredOn:
            startScanning()
        case .poweredOff:
            // Bluetooth is off -- prompt user to enable
            break
        case .unauthorized:
            // App not authorized for Bluetooth
            break
        case .unsupported:
            // Device does not support BLE
            break
        case .resetting, .unknown:
            break
        @unknown default:
            break
        }
    }
}

let heartRateServiceUUID = CBUUID(string: "180D")

func startScanning() {
    centralManager.scanForPeripherals(
        withServices: [heartRateServiceUUID],
        options: [CBCentralManagerScanOptionAllowDuplicatesKey: false]
    )
}

func centralManager(
    _ central: CBCentralManager,
    didDiscover peripheral: CBPeripheral,
    advertisementData: [String: Any],
    rssi RSSI: NSNumber
) {
    guard RSSI.intValue > -70 else { return } // Filter weak signals

    // IMPORTANT: Retain the peripheral -- it will be deallocated otherwise
    discoveredPeripheral = peripheral
    centralManager.stopScan()
    centralManager.connect(peripheral, options: nil)
}

func centralManager(
    _ central: CBCentralManager,
    didConnect peripheral: CBPeripheral
) {
    peripheral.delegate = self
    peripheral.discoverServices([heartRateServiceUUID])
}

func centralManager(
    _ central: CBCentralManager,
    didFailToConnect peripheral: CBPeripheral,
    error: Error?
) {
    // Handle connection failure -- retry or inform user
    discoveredPeripheral = nil
}

func centralManager(
    _ central: CBCentralManager,
    didDisconnectPeripheral peripheral: CBPeripheral,
    timestamp: CFAbsoluteTime,
    isReconnecting: Bool,
    error: Error?
) {
    if isReconnecting {
        // System is automatically reconnecting
        return
    }
    // Handle disconnection -- optionally reconnect
    discoveredPeripheral = nil
}

extension BluetoothManager: CBPeripheralDelegate {
    func peripheral(
        _ peripheral: CBPeripheral,
        didDiscoverServices error: Error?
    ) {
        guard let services = peripheral.services else { return }
        for service in services {
            peripheral.discoverCharacteristics(nil, for: service)
        }
    }

    func peripheral(
        _ peripheral: CBPeripheral,
        didDiscoverCharacteristicsFor service: CBService,
        error: Error?
    ) {
        guard let characteristics = service.characteristics else { return }
        for characteristic in characteristics {
            if characteristic.properties.contains(.notify) {
                peripheral.setNotifyValue(true, for: characteristic)
            }
            if characteristic.properties.contains(.read) {
                peripheral.readValue(for: characteristic)
            }
        }
    }
}

let heartRateMeasurementUUID = CBUUID(string: "2A37")
let batteryLevelUUID = CBUUID(string: "2A19")

func peripheral(
    _ peripheral: CBPeripheral,
    didUpdateValueFor characteristic: CBCharacteristic,
    error: Error?
) {
    guard let data = characteristic.value else { return }

    switch characteristic.uuid {
    case CBUUID(string: "2A37"):
        let heartRate = parseHeartRate(data)
        print("Heart rate: \(heartRate) bpm")
    case CBUUID(string: "2A19"):
        let batteryLevel = data.first.map { Int($0) } ?? 0
        print("Battery: \(batteryLevel)%")
    default:
        break
    }
}

private func parseHeartRate(_ data: Data) -> Int {
    let flags = data[0]
    let is16Bit = (flags & 0x01) != 0
    if is16Bit {
        return Int(data[1]) | (Int(data[2]) << 8)
    } else {
        return Int(data[1])
    }
}

func writeValue(_ data: Data, to characteristic: CBCharacteristic,
                on peripheral: CBPeripheral) {
    if characteristic.properties.contains(.writeWithoutResponse) {
        peripheral.writeValue(data, for: characteristic, type: .withoutResponse)
    } else if characteristic.properties.contains(.write) {
        peripheral.writeValue(data, for: characteristic, type: .withResponse)
    }
}

// Confirmation callback for .withResponse writes
func peripheral(
    _ peripheral: CBPeripheral,
    didWriteValueFor characteristic: CBCharacteristic,
    error: Error?
) {
    if let error {
        print("Write failed: \(error.localizedDescription)")
    }
}

// Subscribe
peripheral.setNotifyValue(true, for: characteristic)

// Unsubscribe
peripheral.setNotifyValue(false, for: characteristic)

// Confirmation
func peripheral(
    _ peripheral: CBPeripheral,
    didUpdateNotificationStateFor characteristic: CBCharacteristic,
    error: Error?
) {
    if characteristic.isNotifying {
        print("Now receiving notifications for \(characteristic.uuid)")
    }
}

final class BLEPeripheralManager: NSObject, CBPeripheralManagerDelegate {
    private var peripheralManager: CBPeripheralManager!
    private let serviceUUID = CBUUID(string: "12345678-1234-1234-1234-123456789ABC")
    private let charUUID = CBUUID(string: "12345678-1234-1234-1234-123456789ABD")

    override init() {
        super.init()
        peripheralManager = CBPeripheralManager(delegate: self, queue: nil)
    }

    func peripheralManagerDidUpdateState(_ peripheral: CBPeripheralManager) {
        guard peripheral.state == .poweredOn else { return }
        setupService()
    }

    private func setupService() {
        let characteristic = CBMutableCharacteristic(
            type: charUUID,
            properties: [.read, .notify],
            value: nil,
            permissions: [.readable]
        )

        let service = CBMutableService(type: serviceUUID, primary: true)
        service.characteristics = [characteristic]
        peripheralManager.add(service)
    }

    func peripheralManager(
        _ peripheral: CBPeripheralManager,
        didAdd service: CBService,
        error: Error?
    ) {
        guard error == nil else { return }
        peripheralManager.startAdvertising([
            CBAdvertisementDataServiceUUIDsKey: [serviceUUID],
            CBAdvertisementDataLocalNameKey: "MyDevice"
        ])
    }
}

// 1. Create with a restoration identifier
centralManager = CBCentralManager(
    delegate: self,
    queue: nil,
    options: [CBCentralManagerOptionRestoreIdentifierKey: "myCentral"]
)

// 2. Implement the restoration delegate method
func centralManager(
    _ central: CBCentralManager,
    willRestoreState dict: [String: Any]
) {
    if let peripherals = dict[CBCentralManagerRestoredStatePeripheralsKey]
        as? [CBPeripheral] {
        for peripheral in peripherals {
            // Re-assign delegate and retain
            peripheral.delegate = self
            discoveredPeripheral = peripheral
        }
    }
}

peripheralManager = CBPeripheralManager(
    delegate: self,
    queue: nil,
    options: [CBPeripheralManagerOptionRestoreIdentifierKey: "myPeripheral"]
)

func peripheralManager(
    _ peripheral: CBPeripheralManager,
    willRestoreState dict: [String: Any]
) {
    // Restore published services, advertising state, etc.
}

// WRONG: Scanning immediately -- manager may not be ready
let manager = CBCentralManager(delegate: self, queue: nil)
manager.scanForPeripherals(withServices: nil) // May silently fail

// CORRECT: Wait for poweredOn in the delegate
func centralManagerDidUpdateState(_ central: CBCentralManager) {
    if central.state == .poweredOn {
        central.scanForPeripherals(withServices: [serviceUUID])
    }
}

// WRONG: No strong reference kept
func centralManager(_ central: CBCentralManager,
                    didDiscover peripheral: CBPeripheral, ...) {
    central.connect(peripheral) // peripheral may be deallocated
}

// CORRECT: Retain the peripheral
func centralManager(_ central: CBCentralManager,
                    didDiscover peripheral: CBPeripheral, ...) {
    self.discoveredPeripheral = peripheral // Strong reference
    central.connect(peripheral)
}

// WRONG: Discovers every BLE device in range -- drains battery
centralManager.scanForPeripherals(withServices: nil)

// CORRECT: Specify the service UUIDs you need
centralManager.scanForPeripherals(withServices: [targetServiceUUID])

// WRONG: Assuming immediate connection
centralManager.connect(peripheral)
discoverServicesNow() // Peripheral not connected yet

// CORRECT: Discover services in the didConnect callback
func centralManager(_ central: CBCentralManager,
                    didConnect peripheral: CBPeripheral) {
    peripheral.delegate = self
    peripheral.discoverServices([serviceUUID])
}

// WRONG: Crashes or silently fails if write is unsupported
peripheral.writeValue(data, for: characteristic, type: .withResponse)

// CORRECT: Check properties first
if characteristic.properties.contains(.write) {
    peripheral.writeValue(data, for: characteristic, type: .withResponse)
} else if characteristic.properties.contains(.writeWithoutResponse) {
    peripheral.writeValue(data, for: characteristic, type: .withoutResponse)
}