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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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/**
 * 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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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

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private val HEX_CHARS = "0123456789ABCDEF".toCharArray()

/**
 * Extension function that converts a ByteArray to a hex encoded CharArray.
 */
fun ByteArray.toHex(): CharArray {
    val result = StringBuilder()
    forEach {
        val octet = it.toInt()
        val firstIndex = (octet and 0xF0).ushr(4)

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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.