package co.electriccoin.zcash.preference.model.entry

import co.electriccoin.zcash.preference.api.PreferenceProvider
import kotlinx.coroutines.flow.Flow
import kotlinx.coroutines.flow.distinctUntilChanged
import kotlinx.coroutines.flow.map

/**
 * An entry represents a key and a default value for a preference.  By using a Default object,
 * multiple parts of the code can fetch the same preference without duplication or accidental
 * variation in default value.  Clients define the key and default value together, rather than just
 * the key.
 */
/*
 * API note: the default value is not available through the public interface in order to prevent
 * clients from accidentally using the default value instead of the preference value.
 *
 * Implementation note: although primitives would be nice, Objects don't increase memory usage much.
 * The autoboxing cache solves Booleans, and Strings are already objects, so that just leaves Integers.
 * Overall the number of Integer preference entries is expected to be low compared to Booleans,
 * and perhaps many Integer values will also fit within the autoboxing cache.
 */
interface PreferenceDefault<T> {

    val key: PreferenceKey

    /**
     * @param preferenceProvider Provides actual preference values.
     * @return The value in the preference, or the default value if no preference exists.
     */
    suspend fun getValue(preferenceProvider: PreferenceProvider): T

    /**
     * @param preferenceProvider Provides actual preference values.
     * @param newValue New value to write.
     */
    suspend fun putValue(preferenceProvider: PreferenceProvider, newValue: T)

    /**
     * @param preferenceProvider Provides actual preference values.
     * @return Flow that emits preference changes.  Note that implementations should emit an initial value
     * indicating what was stored in the preferences, in addition to subsequent updates.
     */
    fun observe(preferenceProvider: PreferenceProvider): Flow<T> = preferenceProvider.observe(key)
        .map { getValue(preferenceProvider) }
        .distinctUntilChanged()
}