filterToggleMutations.kt

/**
 * Every toggle isExpanded == null should be processed, however every specific request to
 * expand or collapse, should be distinct until changed.
 */
private fun Flow<Action.ToggleFilter>.filterToggleMutations(): Flow<Mutation<State>> =
    map { it.isExpanded }
        .scan(listOf<Boolean?>()) { emissions, isExpanded -> 
            (emissions + isExpanded).takeLast(2) 
        }
        .transformWhile { emissions ->
            when {
                emissions.isEmpty() -> Unit
                emissions.size == 1 -> emit(emissions.first())
                else -> {
                    val (previous, current) = emissions
                    if (current == null || current != previous) emit(current)
                }
            }
            true
        }
        .map { isExpanded ->
            Mutation {
                copy(queryState = queryState.copy(expanded = isExpanded ?: !queryState.expanded))
            }
        }