export type Event_T = [name:string, payload:any];
export interface Machine_T {
  states: Array<State_T>
}
export interface State_T {
  name: string;
  eventReactionCouplings: Array<EventReactionCouplings_T>;
}
export interface EventReactionCouplings_T {
  eventName: string;
  reactions: Array<Reaction_T>;
};
export type Reaction_T = SideEffect_T | ContextMutation_T | Goto_T;
export interface SideEffect_T {
  type: 'SideEffect';
  fn: SideEffectFunction_T;
};
export type SideEffectFunction_T = (ctx:any,e:Event_T,self:Interpreter_T)=>void;
export interface ContextMutation_T {
  type: 'ContextMutation';
  fn: ContextMutationFunction_T;
};
export type ContextMutationFunction_T = (ctx:any,e:Event_T,self:Interpreter_T)=>any;
export interface Goto_T {
  type: 'Goto';
  targetStateName: string;
};

export const Machine = function(...states:Array<State_T>) : Machine_T { return {states}; };
export const State = function(name:string, ...eventReactionCouplings:Array<EventReactionCouplings_T>) : State_T{ return {name, eventReactionCouplings}; };
export const On = function(eventName:string, ...reactions:Array<Reaction_T>) : EventReactionCouplings_T{ return {eventName, reactions}; };
export const SideEffect = function(fn:SideEffectFunction_T) : SideEffect_T{ return {type:'SideEffect', fn}; };
export const Goto = function(targetStateName:string) : Goto_T { return {type:'Goto', targetStateName} };
export const Context = function(fn:ContextMutationFunction_T) : ContextMutation_T { return {type:'ContextMutation', fn} };





export interface Interpreter_T {
  machine: Machine_T;
  state: string;
  context: any;
  eventQueue:Array<Event_T>;
  subscriptions: Record<string, SubscriptionCallbackFunction_T>;
  isTransitioning: boolean;
  isPaused: boolean;
}

/**
 * Description placeholder
 *
 * @export
 * @param {Machine_T} machine
 * @param {InitialContextFunction_T} initialContextFunction - in the form of a function rather than a direct value, so as to facilitate co-initialization of peer interpreters. Otherwise, the "parent" interpreter will start, but without a reference to a running child interpreter which it might expect to exist.
 * @param {?string} [initialStateName]
 * @returns {Interpreter_T}
 */
export function Interpreter(machine:Machine_T, initialContext:any, initialStateName?:string) : Interpreter_T{
  if(typeof initialStateName === 'undefined'){ initialStateName = machine.states[0].name; }
  const interpreter = {machine, state: initialStateName, context:initialContext, eventQueue:[], isTransitioning:false, subscriptions: {}, isPaused: true}
  send(interpreter, ['entry', null] );
  return interpreter;
}
export function start(interpreter:Interpreter_T){
  interpreter.isPaused = false;
  processEvents(interpreter);
}
export function pause(interpreter:Interpreter_T){
  interpreter.isPaused = true;
}

/**  Helper function for `send()`
 */
function getState(interpreter : Interpreter_T) : State_T{
  return interpreter.machine.states.find((state)=>state.name===interpreter.state) as unknown as State_T;
}
/**  Helper function for `send()`
 */
function getMatchingEventReactionCouplings(state : State_T, event:Event_T) : Array<EventReactionCouplings_T>{
  return state.eventReactionCouplings.filter((eventReactionCoupling)=>eventReactionCoupling.eventName===event[0]);
}
/**  Inject an Event into the Interpreter's "tick queue".
 * 
 * An event can be signify something "new" happening, such that its reactions should run on the next Tick;
 * or it can signify a milestone "within" the current Tick, such that a Tick can be thought of as having 
 * "sub-Ticks".
 * 
 * This distinction is significant for proper ordering of reaction execution, and also for determining
 * whether to run a reaction at all. If an Event is received, and is specified to be applied on a past 
 * Tick, it is discarded.
 */
export function send(interpreter : Interpreter_T, event:Event_T){
  interpreter.eventQueue.push(event);
  if(interpreter.isTransitioning === false){
    processEvents(interpreter);
  }
}
export const enqueue = send;
function processEvents(interpreter:Interpreter_T){
  interpreter.isTransitioning = true;
  while(interpreter.eventQueue.length > 0 && interpreter.isPaused===false){
    processNextEvent(interpreter);
  }
  interpreter.isTransitioning = false;
  // only run subscriptions here, once the machine's state has settled:
  Object.values(interpreter.subscriptions).forEach((subscriptionCallbackFunction)=>{ subscriptionCallbackFunction(interpreter); });
}
function processNextEvent(interpreter:Interpreter_T){
  const nextEvent = interpreter.eventQueue.shift();
  if(typeof nextEvent !== 'undefined'){
    const state = getState(interpreter);
    const eventReactionCouplings = getMatchingEventReactionCouplings(state, nextEvent);
    const reactions = eventReactionCouplings
      .map((eventReactionCoupling)=>eventReactionCoupling.reactions)
      .flat();
    const {sideEffects, contextMutations, goto_} = categorizeReactions(reactions);
    // can process sideEffects in parallel:
    sideEffects.forEach((sideEffect)=>{
      sideEffect.fn(interpreter.context, nextEvent, interpreter);
    });
    // must process contextMutations in-series:
    contextMutations.forEach((contextMutation)=>{
      interpreter.context = contextMutation.fn(interpreter.context, nextEvent, interpreter);
    });
    // processing of `goto` must be last:
    if(goto_ !== null){
      send(interpreter, ['exit', null]);
      interpreter.state = goto_.targetStateName;
      send(interpreter, ['entry', null]);
    }
  }
}
function categorizeReactions(reactions:Array<Reaction_T>) : {sideEffects:Array<SideEffect_T>, contextMutations:Array<ContextMutation_T>, goto_:Goto_T|null}{
  let 
    sideEffects:Array<SideEffect_T> = [], 
    contextMutations:Array<ContextMutation_T> = [], 
    goto_:Goto_T|null = null;
  reactions.forEach((reaction)=>{
    if(reaction.type === 'SideEffect'){
      sideEffects.push(reaction);
    }
    else if(reaction.type === 'ContextMutation'){
      contextMutations.push(reaction);
    }
    else if(reaction.type === 'Goto'){
      goto_ = reaction;
    }
  });
  return {sideEffects, contextMutations, goto_};
}

export type SubscriptionCallbackFunction_T = (self:Interpreter_T)=>void;
let subscriptionId : number = 0;
export function subscribe(interpreter:Interpreter_T, callback:SubscriptionCallbackFunction_T){
  subscriptionId++;
  interpreter.subscriptions[subscriptionId.toString()] = callback;
  return subscriptionId.toString();
}
export function unsubscribe(interpreter:Interpreter_T, subscriptionId:string){
  delete interpreter.subscriptions[subscriptionId.toString()];
}

export const Spawn = function(){};
export const Unspawn = function(){};

/*
export function useMachine(machine, options){
  return useMemo(()=>interpret(AppMachine, {context:{}}),[]);
}
*/