package com.buginmyhead.chatter.fundamental.architecture

/**
 * Trans-state is a coinage to indicate transition and state together in the state machine concept.
 *
 * Example for [GenericTransState], [GenericTransition], [GenericTransition.Disposal]:
 * ```kotlin
 * sealed interface Event
 * sealed interface TransState : GenericTransState<TransState, State, Event>
 * sealed interface State : TransState
 *
 * data class Transition(
 *     override val state: State,
 *     val sideEffect1: Any,
 *     val sideEffect2: Any,
 * ) : TransState, GenericTransition<TransState, State, Event> {
 *
 *     data object Disposal : Event, GenericTransition.Disposal
 *
 * }
 * ```
 *
 * @param TS trans-state: the runtime type is either [GenericTransState] or [GenericTransition].
 * @param S state
 * @param E event
 */
@Suppress("UNCHECKED_CAST")
interface GenericTransState<TS : GenericTransState<TS, S, E>, S : TS, E> {

    fun transitByEvent(event: E): TS

    companion object {

        /**
         * @return
         * [GenericTransition.state] if the receiver is [GenericTransition],
         *  or the receiver itself otherwise.
         */
        @JvmStatic
        val <TS : GenericTransState<TS, S, E>, S : TS, E> TS.state: S get() =
            (if (this is GenericTransition<*, *, *>) state else this) as S

    }

}


/**
 * Transition in the state machine concept.
 *
 * Read the documentation of [GenericTransState] for an example.
 *
 * @param S state
 * @param E event
 */
interface GenericTransition<TS : GenericTransState<TS, S, E>, S : TS, E> : GenericTransState<TS, S, E> {

    val state: S

    /**
     * @return [state] for [Disposal] event, or `[transitByEvent]` of [state] for other events.
     */
    override fun transitByEvent(event: E): TS =
        if (event is Disposal) state else state.transitByEvent(event)

    /**
     * Implement with the custom event interface.
     * The meaning of this event is to reset side effect state to default
     *  by calling [transitByEvent].
     *
     * Read the documentation of [GenericTransState] for an example.
     */
    interface Disposal

}