README.md

Mobile-specific Best Practices

This guide is aimed at developers for mobile platforms who want to make their native apps more sustainable, that is, both environmentally and socially acceptable. Unlike general rules of thumb, this guide is focused on code smells, that is surface symptoms that suggest there might be a problem, or that there might be a better way of writing the code. Therefore, these low-level best practices offer the great advantage of being detectable by program analysis tools, such as ecoCode Mobile (formerly hosted by Cnumr).

Android Platform

Environmental Code Smells

Name	Detailed Description
Optimized API
Fused Location	The fused location provider is one of the location APIs in Google Play services which combines signals from GPS, Wi-Fi, and cell networks, as well as accelerometer, gyroscope, magnetometer and other sensors. It is officially recommended to maximize battery life. Thus, developer has to set up Google Play Service in her gradle file with a dependency to com.google.android.gms:play-services-location:x.y.z, and then to import from com.google.android.gms.location instead of the android.location package of the SDK.
Bluetooth Low-Energy	In contrast to classic Bluetooth, Bluetooth Low Energy (BLE) is designed to provide significantly lower power consumption. Its purpose is to save energy on both paired devices but very few developers are aware of this alternative API. From the Android client side, it means append android.bluetooth.le.* imports to android.bluetooth.* imports in order to benefits from low-energy features.
Lazy Loading	When displaying scrollable data on screen, the new Jetpack Compose API introduced lazy views instead of ListView, GridView and even RecycleView. These components use the technique of lazy loading, which consists of loading data only when it arrives at the display area. Import androidx.compose.foundation.lazy.* to benefit from objects like LazyColumn, LazyRow, LazyVerticalGrid or LazyHorizontalGrid.
Leakage
Media Leak	Creation of a Media Recorder object with new MediaRecorder() is used to record audio and video, while creation of a Media Player object with new MediaPlayer() can be used to control playback of audio/video files and streams. Both classes own a release() method. In addition to unnecessary resources (such as memory and instances of codecs) being held, failure to call this method immediately if a media object is no longer needed may also lead to continuous battery consumption for mobile devices.
Sensor Leak	Most Android-powered devices have built-in sensors that measure motion, orientation, and various environmental conditions. In addition to these are the image sensor (a.k.a Camera) and the geopositioning sensor (a.k.a GPS). The common point of all these sensors is that they are expensive while in use. Their common bug is to let the sensor unnecessarily process data when the app enters an idle state, typically when paused or stopped. Consequently, calls must be carefully pairwised: SensorManager#registerListener()/unregisterListener() for regular sensors, Camera#open()/Camera#release() for the camera and LocationManager#requestLocationUpdates()/removeUpdates() for the GPS. Failing to do so can drain the battery in just a few hours.
Everlasting Service	If someone calls Context#startService() then the system will retrieve the service (creating it and calling its onCreate() method if needed) and then call its onStartCommand(Intent, int, int) method with the arguments supplied by the client. The service will at this point continue running until Context#stopService() or Service#stopSelf() is called. Failing to call any of these methods can lead to uncontrolled energy leakage.
Bottleneck
Internet In The Loop	Opening and closing internet connection continuously is extremely battery-inefficient since HTTP exchange is the most consuming operation of the network. This bug typically occurs when one obtain a new HttpURLConnection by calling URL#openConnection() within a loop control structure (while, for, do-while, for-each). Also, this bad practice must be early prevented because it is the root of another evil that consists in polling data at regular intervals, instead of using push notifications to save a lot of battery power.
Wifi Multicast Lock	Normally the Wifi stack filters out packets not explicitly addressed to the device. Acquiring a Multicast Lock with WifiManager.MulticastLock#acquire() will cause the stack to receive packets addressed to multicast addresses. Processing these extra packets can cause a noticeable battery drain and must be disabled when not needed with to a call to WifiManager.MulticastLock#release().
Uncompressed Data Transmission	Transmitting a file over a network infrastructure without compressing it consumes more energy than with compression. More precisely, energy efficiency is improved in case the data is compressed at least by 10%, transmitted and decompressed at the other network node. From the Android client side, it means making a post HTTP request using a GZIPOutputStream instead of the classical OutputStream, along with the HttpURLConnection object.
Uncached Data Reception	Caching all of the application's HTTP responses to the filesystem (application-specific cache directory) so they may be reused, allow to save energy. To that purpose the class HttpResponseCache supports HttpURLConnection and HttpsURLConnection; there is no platform-provided cache for further clients. Installing the cache at application startup is done with the static method HttpResponseCache.install(File directory, long maxSize).
Sobriety
Dark UI	Developers are allowed to apply native themes for their app, or derive new ones throught inheritence. This decision has a significant impact on energy consumption since displaying dark colors is particularly beneficial for mobile devices with (AM)OLED screens (under certain conditions). By default Android will set a dark style and hence switching to the light theme (parent style Theme.*.Light) within the manifest should be avoided. Of course, custom resources like bright colors values and bitmap images with too high luminance should be avoided too.
Day Night Mode	Dark theme is available in Android 10 (API level 29) and higher. It allows the user to switch its system to a dark mode and the apps ought to adapt accordingly. In order to support Dark theme, the app's theme must inherit from the DayNight theme with parent="Theme.*.DayNight". In addition, developers may provide resources specific to DayNight mode in a directory res/values-night/.
Brightness Override	Introduced in Android 9, the adaptive brightness feature raises or lower the brightness of the screen depending on the light in the current environment. For some reasons, developpers may disable this feature programmatically, setting the field WindowManager.LayoutParams#screenBrightness with the constant BRIGHTNESS_OVERRIDE_FULL. This intelligent feature was introduced to improve battery life. Its deactivation is a very bad idea.
Thrifty Geolocation	Location awareness is one of the most popular features used by apps. The first thing is to try to get the best possible provider (LocationManager#getBestProvider()) based on an energy criteria thanks to Criteria#setPowerRequirement(int level), using constant POWER_LOW instead of POWER_HIGH or POWER_MEDIUM. Next, with a call to LocationManager#requestLocationUpdates (long minTime, float minDistance, Criteria criteria, PendingIntent intent), the provider will only send your application an update when the location has changed by at least minDistance meters, AND at least minTime milliseconds have passed. So minTime should be the primary tool to conserving battery life, and, to a lesser extent, minDistance, these two must be imperatively greater than 0.
Thrifty BLE	With Bluetooth Low Energy technology (see BLE API smell), a Bluetooth Smart Ready device (the master) will establish a link with a Bluetooth Smart device (the slave). Most often, the slave is a GATT server and the master is a GATT client. GATT capable devices can be discovered using BLE scan process. So you should always use BluetoothLeScanner#startScan() with the scan settings set to SCAN_MODE_LOW_POWER (Default scan mode). In addition, invoke BluetoothGatt#requestConnectionPriority(int connectionPriority) with the value CONNECTION_PRIORITY_LOW_POWER. Lastly, the default and preferred advertising mode is ADVERTISE_MODE_LOW_POWER when calling AdvertiseSettings.Builder#setAdvertiseMode(int advertiseMode).
Thrifty Motion Sensor	The rotation vector sensor is the most frequently used sensor for motion detection and monitoring. When using SensorManager#getDefaultSensor(int type), always prefer the constant TYPE_GEOMAGNETIC_ROTATION_VECTOR which is similar to TYPE_ROTATION_VECTOR, but using a magnetometer instead of using a gyroscope. This sensor uses lower power than the other rotation vectors, because it doesn't use the gyroscope. However, it is more noisy and will work best outdoors.
Thrifty Notification	Giving informations to the end-user through notifications is an important aspect of a modern app. However, a notification does not necessarily need to be loud and vibrant to achieve its purpose. Default mode is enough. That is why when building a notification with NotificationCompat.Builder, there should be no extra calls to the methods setVibrate() nor setSound() on the builder object. Please consider NotificationChannel#setVibrationPattern() since API 31.
Vibration-free	Shaking of an Android device is possible in all circumstances with a call to getSystemService(Context.VIBRATOR_SERVICE) (API 26). Behind this effect stands a specific miniature hardware component, motor or actuator, that consumes power. As a consequence, its usage must be discouraged, especially since its added value is not clear. Please consider Context.VIBRATOR_MANAGER_SERVICE since API 31.
Torch-free	Programmatically enabling torch mode with CameraManager#setTorchMode(..., true) should be avoided as the torch will drain the battery unnecessarily.
High Frame Rate	In Android 11 (API level 30) or higher, a call to Surface#setFrameRate(float frameRate, int compatibility) results in a change to the display refresh rate. However, a regular app displays 60 frames per second (60Hz). In order to optimize content refreshes and hence saving energy, this frequency should not be raised to 90Hz or 120Hz, despite this is now supported by many devices. This bad practice can be detected when the value of the frameRate argument is greater than 60f.
Extraneous Animation	Avoiding extraneous animations in the UI is a good practice for saving battery power. This can be checked either in the Java code when an object is instance of Animator (sub)class, or simply through the presence of xml files in the res/animator/ resource directory.
Extraneous Init	If possible, avoid initialising anything unnecessarily in your Application class in order to improve start-up latency. Many libraries offer on-demand initialisation, allowing you to call them only when you need them.
Hardware acceleration	Hardware acceleration for 2D rendering is enabled by default if your Target API level is >=14. Because of the increase in resources required to activate hardware acceleration, your application will use more RAM. However, it can be explicitly disabled at the application or activity level with the specific attribute android:hardwareAccelerated="false" in the Android manifest file, or programmatically.
Auto Completion	A special type of text field allows completion suggestions to be displayed automatically as the user types. Unless the data is provided statically, using an <AutoCompleteTextView> element in an XML layout opens the door to numerous requests to a remote server, and should be avoided wherever possible. Sometimes a simple placeholder can effectively guide the user.
Idleness
Keep Screen On	To avoid draining the battery, an Android device that is left idle quickly falls asleep. Hence, keeping the CPU on should be avoided, unless it is absolutely necessary. If so, developers typically use the FLAG_KEEP_SCREEN_ON in their activity. Another way to implement this is in their application's layout XML file, by using the android:keepScreenOn attribute.
Keep CPU On	To avoid draining the battery, an Android device that is left idle quickly falls asleep. Hence, keeping the screen on should be avoided, unless it is absolutely necessary. If so, developers typically use a Power Manager system service feature called wake locks by invoking PowerManager.WakeLock#newWakeLock(int levelAndFlags, String tag), along with the specific permission WAKE_LOCK in their manifest.
Durable Wake Lock	A wake lock is a mechanism to indicate that your application needs to have the device stay on. The general principle is to obtain a wake lock, acquire it and finally release it. Hence, the challenge here is to release the lock as soon as possible to avoid running down the device's battery excessively. Missing call to PowerManager#release() is a built-in check of Android lint (Wakelock check) but that does not prevent abuse of the lock over too long a period of time. This can be avoided by a call to PowerManager.WakeLock#acquire(long timeout) instead of PowerManager.WakeLock#acquire(), because the lock will be released for sure after the given timeout expires.
Rigid Alarm	Applications are strongly discouraged from using exact alarms unnecessarily as they reduce the OS's ability to minimize battery use (i.e. Doze Mode). For most apps prior to API 19, setInexactRepeating() is preferable over setRepeating(). When you use this method, Android synchronizes multiple inexact repeating alarms and fires them at the same time, thus reducing the battery drain. Similarly, setExact() and setExactAndAllowWhileIdle() can significantly impact the power use of the device when idle, so they should be used with care. High-frequency alarms are also bad for battery life but this is already checked by Android lint (ShortAlarm built-in check).
Continuous Rendering	For developers wishing to display OpenGL rendering, when choosing the rendering mode with GLSurfaceView#setRenderMode(int renderMode), using RENDERMODE_WHEN_DIRTY instead of RENDERMODE_CONTINUOUSLY (By default) can improve battery life and overall system performance by allowing the GPU and CPU to idle when the view does not need to be updated.
Keep Voice Awake	During a voice interaction session, VoiceInteractionSession#setKeepAwake(boolean keepAwake) allows to decide whether it will keep the device awake while it is running a voice activity. By default, the system holds a wake lock for it while in this state, so that it can work even if the screen is off. Setting this to false removes that wake lock, allowing the CPU to go to sleep and hence does not let this continue to drain the battery.
Power
Ignore Battery Optimizations	An app holding the Manifest.permission.REQUEST_IGNORE_BATTERY_OPTIMIZATIONS ask the user to allow it to ignore battery optimizations (that is, put them on the whitelist of apps). Most applications should not use this; there are many facilities provided by the platform for applications to operate correctly in the various power saving modes.
Companion in background	A negative effect on the device's battery is when an app is paired with a companion device (over Bluetooth, BLE, or Wi-Fi) and that it has been excluded from battery optimizations (run in the background) using the declaration Manifest.permission.REQUEST_COMPANION_RUN_IN_BACKGROUND.
Charge Awareness	It's always good that an app has different behavior when device is connected/disconnected to a power station, or has different battery levels. One can monitor the changes in charging state with a broadcast receiver registered on the actions ACTION_POWER_CONNECTED and ACTION_POWER_DISCONNECTED, or monitor significant changes in battery level with a broadcast receiver registered on the actions BATTERY_LOW and BATTERY_OKAY.
Save Mode Awareness	Taking into account when the device is entering or exiting the power save mode is higly desirable for the battery life. It implies the existence of a broadcast receiver registered on the action ACTION_POWER_SAVE_MODE_CHANGED, or programmaticaly with a call to PowerManager#isPowerSaveMode().
Battery-constrained Work	You can use the WorkManager library to perform work on an efficient schedule that considers whether specific conditions are met, such as power status. A worker can be scheduled to run, provided the device's battery isn't low for exemple. This can be checked by a call to WorkRequest.Builder#setConstraints() where the constraint object built involves Constraints.Builder#setRequiresBatteryNotLow(true) or, in its extremist form, Constraints.Builder#setRequiresCharging(true).
Batch
Service@Boot-time	Services are long-living operations, as components of the apps. However, they can be started in isolation each time the device is next started, without the user's acknowledgement. This technique should be discouraged because the accumulation of these silent services results in excessive battery depletion that remains unexplained from the end-user's point of view. In addition, end-users know how to kill applications, but more rarely how to kill services. Thus, any developer should avoid having a call to Context#startService() from a Broadcast Receiver component that has specified an intent-filter for the BOOT_COMPLETED or QUICKBOOT_POWERON action in the manifest.
Sensor Coalesce	With SensorManager#registerListener(SensorEventListener, Sensor, int) the events are delivered as soon as possible. Instead, SensorManager#registerListener(SensorEventListener, Sensor, int, int maxReportLatencyUs) allows events to stay temporarily in the hardware FIFO (queue) before being delivered. The events can be stored in the hardware FIFO up to maxReportLatencyUs microseconds. Once one of the events in the FIFO needs to be reported, all of the events in the FIFO are reported sequentially. Setting maxReportLatencyUs to a positive value allows to reduce the number of interrupts the AP (Application Processor) receives, hence reducing power consumption, as the AP can switch to a lower power state while the sensor is capturing the data.
Job Coalesce	The Android 5.0 Lollipop (API 21) release introduces a job scheduler API via the JobScheduler class. Compared to a custom Sync Adapter or the alarm manager, the Job Scheduler supports batch scheduling of jobs. The Android system can combine jobs so that battery consumption is reduced. It means that at least one job scheduler exists through a call to the method JobScheduler#shedule(JobInfo job).
Release
Same dependencies	This occurs when equivalent libraries are incorporated into the app. If they do quite the same things, choose only one. This can be checked in the dependencies section of build.gradle along with a human-curated list of equivalent libraries for Android (e.g., Volley≈Retrofit≈okHttp≈android-async-http≈Ktor).
Duplicate dependencies	This occurs when several versions of the same library are incorporated into the app. The weight of the app is fatally increased without changing the features. This can be checked in the dependencies section of build.gradle.
Convert to WebP	WebP is an image format developed by Google, presenting smaller yet more visually-pleasing pictures. Typically, WebP compresses images by an average of 30 percent more than JPEG with no loss in quality. From Android 4.0 (API level 14) and higher, using this image format instead of .gif, .png or .jpeg in the /res/drawable/ folder is a good practice since it allows to decrease the size of apks. Notice that Android Studio allows you to easily convert images to the WebP format.
Clear cache	A good (but quite rare) practice in the long term is to delete the whole cache directory of the app onto the target device. This requires the source code of the app to contains at least one call to the method Context.cacheDir#deleteRecursively().
Shrink Resources	For those that are still not publishing with the Android App Bundle format, it is possible to minimize the app's size via the build.gradle file, by using the following lines. Especially useful for popular apps, it reduces the amount of downloaded data required for installation and updating, across millions of devices. Check buildTypes { release { shrinkResources true } }
Dynamic Feature	Play Feature Delivery uses advanced app bundle functionality to enable conditional or on-demand distribution of certain features of your application. Hence, you can reduce the size of downloaded apps by using the android.dynamicFeatures property in the build.gradle file.
Longevity
Aging devices	The minSdkVersion set in the build.gradle file determines which APIs are available at build time, and determines the minimum version of the OS that the code will be compatible with. The lower the better so as not to exclude owners of older devices.
Fat app	When an app exceeds the limit of 65 536 method references, the configuration multidex must be enabled with multiDexEnabled true in the defaultConfig section of build.gradle. Fat apps rarely install on space-limited devices.
Low Memory	Methods such as ActivityManager#isLowRamDevice() and ActivityManager#getMemoryClass() help you determine memory constraints at runtime. This information allows you to reduce memory usage. Compatibility with low-memory devices is good practice to help them last.

Social Code Smells

Name	Detailed Description
Privacy
Crashlytics automatic opt-in	By default, Crashlytics automatically collects error reports for all application users. In order to let the choice to the users, disable the automatic data collection. It means adding a meta-data in the manifest with the attributes android:name="firebase_crashlytics_collection_enabled" and android:value="false". From the source code, you may have the call FirebaseCrashlytics.getInstance().setCrashlyticsCollectionEnabled(false).
Google Tracker	Importing the com.google.android.gms.analytics.Tracker class means that the app sends hits to Google Analytics. It is not necessarily sensitive information, but it is a first step towards Google Ads and hence this practice should be discouraged at early stage.
Hidden Tracker Risk	An empirical evidence is that the more a project imports third party libraries, the more likely it is that there are hidden trackers. This requires to check the number of non-official dependencies throughtout the directives implementation or api in the build.gradle file. A non-official dependency refers to a package beyond the scope of android.*, androidx.* and to a lesser extent, com.android.* and com.google.*.
Tracking Id	For some use cases, it might be necessary to get a unique device identifier by a call to TelephonyManager#getDeviceId() (returns IMEI on GSM, MEID for CDMA). However, this raises privacy concerns and it is not recommended. Alternatively, you may use android.provider.Settings.Secure.ANDROID_ID.
Explain Permission	Users are increasingly suspicious about the permissions requested by an app. A good practice is to test if ActivityCompat#shouldShowRequestPermissionRationale() returns True, so you can show an educational UI to the user. In this UI, describe why the feature, which the user wants to enable, needs a particular permission.
GDPR
Google consent	To support publishers in meeting their duties under the EU User Consent Policy, Google offers a Consent SDK. Hence, importing classes from com.google.android.ads.consent is considered as a good practice.
Inclusion
Grammatical gender	With Android 14, Grammatical Inflection API provides a more personalized experience for users. Among available inflections, grammatical gender is the most salient. Basically, this can be checked with the presence of the values-?-feminine/, values-?-masculine/ and values-?-neuter/ directories.

iOS Platform

Environmental Code Smells

Name	Detailed Description
Leakage
Sensor Leak	Most iOS devices come equipped with a variety of sensors that measure motion, orientation, and various environmental conditions. Additionally, these devices include advanced sensors such as the image sensor (commonly referred to as the Camera) and the geo-positioning sensor (commonly referred to as GPS). The common point of all these sensors is that they are power-intensive while in use. A typical issue arises when these sensors continue to process data unnecessarily after the application enters an idle state, like when it is backgrounded or the user stops interacting with it. As a result, calls to manage these sensors must be carefully paired: AVCaptureSession.startRunning() and AVCaptureSession.stopRunning() for the camera, , CMMotionManager.startAccelerometerUpdates(:) and CMMotionManager.stopAccelerometerUpdates() for Accelerometer, CLLocationManager.startUpdatingLocation() and CLLocationManager.stopUpdatingLocation() for the GPS. Failure to properly manage these calls can lead to significant battery drain within a few hours.
Idleness
Idle Timer	To avoid draining the battery, an iOS device that is left idle quickly falls asleep. Hence, keeping the screen on should be avoided, unless it is absolutely necessary. Developers typically set the UIApplication#isIdleTimerDisabled property to true, to avert system sleep.
Rigid Alarm	Setting a tolerance for timers will allow them to fire later than the scheduled fire date. The system will use this flexibility to shift the execution of timers by small amounts of time, within their tolerances, increasing the ability to optimize power savings. Your app can set the Timer#tolerance property to specify a tolerance for a timer. Using this approach dramatically increases the amount of time that the processor spends idling while users detect no change in system responsiveness.
Power
Charge Awareness	It's always good that an app has different behavior when device is connected/disconnected to a power station, or has different battery levels. Your app can query the UIDevice#batteryLevel and UIDevice#batteryState properties at any time to determine the level of charge and the state of the battery. Your app can also register to receive notifications when the battery level or state changes, using batteryLevelDidChangeNotification and batteryStateDidChangeNotification.
Save Mode Awareness	Use device API to check if low power mode is enabled, and adapt resources usage accordingly. For example, you can reduce frequency of data update if low power mode is enabled. Your app can query the ProcessInfo.processInfo.isLowPowerModeEnabled property at any time to determine whether Low Power Mode is active. Your app can also register to receive notifications when the Low Power Mode state of a device changes, using NSProcessInfoPowerStateDidChange.
Sobriety
Disabled Location Updates Pause	Allowing the system to pause location updates, at times when the location data is unlikely to change, can improve battery life without sacrificing location data. This is the default behavior. For example, if the user stops for food while using a navigation app, the location manager might pause updates for a period of time. It is possible to disable this behavior, by setting the CLLocationManager#pausesLocationUpdatesAutomatically property to false, but applications are strongly discouraged of doing it.
Thrifty Geolocation	Location awareness is one of the most popular features used by apps. By default standard location updates run with the best accuracy level, but unless your app really needs to know the user’s position within a few meters, this level of accuracy isn't needed. Requesting higher accuracy than you need causes the system to power up additional hardware and waste power for unnecessary precision. You can specify a degree of accuracy by setting the CLLocationManager#desiredAccuracy property. Also, setting the CLLocationManager#activityType property will let the system knows what type of location activity your app is performing and helps it determine the most appropriate time to perform location updates. Finally, if your app just needs a quick fix on the user’s location, it’s best to call the CLLocationManager#requestLocation method, that will deliver a single location update.
Motion Sensor Update Rate	Using the Core Motion API, your app can receive continuous motion updates in the form of accelerometer, gyroscope, and device motion (rotation, acceleration, and more) events. Yet you don't often need these updates as soon as they are generated. Before registering to receive these recurring motion updates, you can specify an interval that meets your app’s needs, using CMMotionManager#accelerometerUpdateInterval, CMMotionManager#gyroUpdateInterval, CMMotionManager#deviceMotionUpdateInterval and CMMotionManager#magnetometerUpdateInterval properties. The larger the interval, the fewer events are delivered to your app, improving battery life.
Disabled Dark Mode	Dark Mode was introduced in iOS 13. It allows the user to switch its system to a dark mode and the apps inherits it automatically by default. Yet is possible to overrides it to light mode by setting the value Light for the key UIUserInterfaceStyle in the Information Property List. This should be avoided since displaying dark colors has a significant impact on energy consumption for mobile devices with OLED screens.
Brightness Override	The adaptive brightness feature raises or lower the brightness of the screen depending on the light in the current environment. For some reasons, developpers may disable, temporarily, this feature programmatically, by setting the field UIScreen.main.brightness with a custom value. This intelligent feature was introduced to improve battery life. Its deactivation is a very bad idea, unless you absolutely need it (eg: to display a QRCode).
Vibration-free	Shaking or vibrating an iOS device is possible using UIFeedbackGenerator. Behind this effect stands a specific hardware component, the Haptic Engine, which consumes power. As a consequence, its usage should be carefully considered, especially if its added value is not clear.
Torch-free	Turning on the torch mode programmatically with AVCaptureDevice#torchMode or AVCaptureDevice#setTorchModeOn(level:) must absolutely be avoided because the flashlight is one of the most energy-intensive component.
Animation-free	Animations play a crucial role in creating a dynamic and engaging user interface in iOS applications, however they must be avoided. Animations can be detected by checking the use of functions like withAnimation(::), the animation(_:value:) view modifier, and the binding’s animation(_:) method in SwiftUI, as well as checking the use of UIKit's animation methods.

Licence

This guide is maintained by Dr. Olivier Le Goaër and is protected by CC BY-NC-ND 4.0