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@Singleton
class Preferences(context: Context) {
    private val prefs: SharedPreferences = context.getSharedPreferences("Prefs", Context.MODE_PRIVATE)

    companion object {
        const val PREF_CURRENT_STATE = "CurrentState"
        const val PREF_LAST_KNOWN_ERROR = "LastKnownError"
        const val PREF_LAST_ACTION_PERFORMED = "LastActionPerformed"
    }

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/**
 * Keeps track of current and prior application state.
 *
 * @param dataStore storage
 */
@Singleton
class StateMachine(private val dataStore: DataStore) {
    /**
     * Storage implementation for our state machine.
     *

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/**
 * A factory that can serialize/deserialize json.
 */
object GsonUtils {
    /**
     * Handles deserializing the json string into a [T] instance.
     *
     * @param json the json to deserialize into an instance of [T]
     * @throws JsonSyntaxException if there is a failure deserializing then this exception is thrown
     */

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/**
 * A storage implementation that enforces whole class serialization/deserialization.
 *
 * This enforces organization of data that needs to be persisted rather than having generic classes
 * with lots of unrelated data.
 *
 * @param context the caller's context
 */
@Singleton
class DataStore(context: Context) {

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abstract class EncryptedDatabase : RoomDatabase() {
    companion object {
        fun getInstance(passcode: CharArray, context: Context):
            EncryptedDatabase = buildDatabase(passcode, context)
        
        private fun buildDatabase(
            passcode: CharArray,
            context: Context
        ): EncryptedDatabase {
            // DatabaseKeyMgr is a singleton that all of the above code is wrapped into.

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/**
 * Returns the database key suitable for using with Room.
 *
 * @param passcode the user's passcode
 * @param context the caller's context
 */
fun getCharKey(passcode: CharArray, context: Context): CharArray {
    if (dbCharKey == null) {
        initKey(passcode, context)
    }

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/**
 * Decrypts the [Storable] instance using the [passcode].
 *
 * @pararm passcode the user's passcode
 * @param storable the storable instance previously saved with [saveToPrefs]
 * @return the raw byte key previously generated with [generateRandomKey]
 */
fun getRawByteKey(passcode: CharArray, storable: Storable): ByteArray {
    val aesWrappedKey = Base64.decode(storable.key, Base64.DEFAULT)
    val iv = Base64.decode(storable.iv, Base64.DEFAULT)

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/**
 * Retrieves the [Storable] instance from prefs.
 *
 * @param context the caller's context
 * @return the storable instance
 */
fun getStorable(context: Context): Storable? {
    val prefs = context.getSharedPreferences("database",
        Context.MODE_PRIVATE)
    val serialized = prefs.getString("key", null)

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/**
 * Save the storable instance to preferences.
 *
 * @param storable a storable instance
 */
fun saveToPrefs(context: Context, storable: Storable) {
    val serialized = Gson().toJson(storable)
    val prefs = context.getSharedPreferences("database",
        Context.MODE_PRIVATE)
    prefs.edit().putString("key", serialized).apply()

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fun persistRawKey(userPasscode: CharArray) {
    val storable = toStorable(rawByteKey, userPasscode)
    // Implementation explained in next step
    saveToPrefs(storable)
}

/**
 * Returns a [Storable] instance with the db key encrypted using PBE.
 *
 * @param rawDbKey the raw database key