import { BehaviorSubject, Observable, Subscription, of } from 'rxjs';
import type { StatePart } from './smartstate.classes.statepart.js';
import type { StateAction } from './smartstate.classes.stateaction.js';

export type TProcessStatus = 'idle' | 'running' | 'paused' | 'disposed';
export type TAutoPause = 'visibility' | Observable<boolean> | false;

export interface IProcessOptions<TStatePayload, TProducerValue> {
  producer: () => Observable<TProducerValue>;
  reducer: (currentState: TStatePayload, value: TProducerValue) => TStatePayload;
  autoPause?: TAutoPause;
  autoStart?: boolean;
}

export interface IScheduledActionOptions<TStatePayload, TActionPayload> {
  action: StateAction<TStatePayload, TActionPayload>;
  payload: TActionPayload;
  intervalMs: number;
  autoPause?: TAutoPause;
}

/**
 * creates an Observable<boolean> from the Page Visibility API.
 * emits true when the page is visible, false when hidden.
 * in Node.js (no document), returns an always-true observable.
 */
function createVisibilityObservable(): Observable<boolean> {
  if (typeof document === 'undefined') {
    return of(true);
  }
  return new Observable<boolean>((subscriber) => {
    subscriber.next(!document.hidden);
    const handler = () => subscriber.next(!document.hidden);
    document.addEventListener('visibilitychange', handler);
    return () => document.removeEventListener('visibilitychange', handler);
  });
}

/**
 * a managed, pausable process that ties an observable producer to state updates.
 * supports lifecycle management (start/pause/resume/dispose) and auto-pause signals.
 */
export class StateProcess<TStatePartName, TStatePayload, TProducerValue> {
  private readonly statePartRef: StatePart<TStatePartName, TStatePayload>;
  private readonly producerFn: () => Observable<TProducerValue>;
  private readonly reducer?: (currentState: TStatePayload, value: TProducerValue) => TStatePayload;
  private readonly sideEffect?: (value: TProducerValue) => Promise<void> | void;
  private readonly autoPauseOption: TAutoPause;

  private statusSubject = new BehaviorSubject<TProcessStatus>('idle');
  private producerSubscription: Subscription | null = null;
  private autoPauseSubscription: Subscription | null = null;
  private processingQueue: Promise<void> = Promise.resolve();

  constructor(
    statePartRef: StatePart<TStatePartName, TStatePayload>,
    options: {
      producer: () => Observable<TProducerValue>;
      reducer?: (currentState: TStatePayload, value: TProducerValue) => TStatePayload;
      sideEffect?: (value: TProducerValue) => Promise<void> | void;
      autoPause?: TAutoPause;
    }
  ) {
    this.statePartRef = statePartRef;
    this.producerFn = options.producer;
    this.reducer = options.reducer;
    this.sideEffect = options.sideEffect;
    this.autoPauseOption = options.autoPause ?? false;
  }

  public get status(): TProcessStatus {
    return this.statusSubject.getValue();
  }

  public get status$(): Observable<TProcessStatus> {
    return this.statusSubject.asObservable();
  }

  public start(): void {
    if (this.status === 'disposed') {
      throw new Error('Cannot start a disposed process');
    }
    if (this.status === 'running') return;
    this.statusSubject.next('running');
    this.subscribeProducer();
    this.setupAutoPause();
  }

  public pause(): void {
    if (this.status === 'disposed') {
      throw new Error('Cannot pause a disposed process');
    }
    if (this.status !== 'running') return;
    this.statusSubject.next('paused');
    this.unsubscribeProducer();
  }

  public resume(): void {
    if (this.status === 'disposed') {
      throw new Error('Cannot resume a disposed process');
    }
    if (this.status !== 'paused') return;
    this.statusSubject.next('running');
    this.subscribeProducer();
  }

  public dispose(): void {
    if (this.status === 'disposed') return;
    this.unsubscribeProducer();
    this.teardownAutoPause();
    this.statusSubject.next('disposed');
    this.statusSubject.complete();
    this.statePartRef._removeProcess(this);
  }

  private subscribeProducer(): void {
    this.unsubscribeProducer();
    const source = this.producerFn();
    this.producerSubscription = source.subscribe({
      next: (value) => {
        // Queue value processing to ensure each reads fresh state after the previous completes
        this.processingQueue = this.processingQueue.then(async () => {
          try {
            if (this.sideEffect) {
              await this.sideEffect(value);
            } else if (this.reducer) {
              const currentState = this.statePartRef.getState();
              if (currentState !== undefined) {
                await this.statePartRef.setState(this.reducer(currentState, value));
              }
            }
          } catch (err) {
            console.error('StateProcess value handling error:', err);
          }
        });
      },
      error: (err) => {
        console.error('StateProcess producer error:', err);
        if (this.status === 'running') {
          this.statusSubject.next('paused');
          this.unsubscribeProducer();
        }
      },
    });
  }

  private unsubscribeProducer(): void {
    if (this.producerSubscription) {
      this.producerSubscription.unsubscribe();
      this.producerSubscription = null;
    }
  }

  private setupAutoPause(): void {
    this.teardownAutoPause();
    if (!this.autoPauseOption) return;

    const signal$ = this.autoPauseOption === 'visibility'
      ? createVisibilityObservable()
      : this.autoPauseOption;

    this.autoPauseSubscription = signal$.subscribe((active) => {
      if (!active && this.status === 'running') {
        this.pause();
      } else if (active && this.status === 'paused') {
        this.resume();
      }
    });
  }

  private teardownAutoPause(): void {
    if (this.autoPauseSubscription) {
      this.autoPauseSubscription.unsubscribe();
      this.autoPauseSubscription = null;
    }
  }
}