/** Owns one backend startup and its quiescent teardown independently of windows. */

import { desktopErrorState } from './startup-error.ts'

/** Backend availability presented by the desktop window. */
export type DesktopBackendState = { readonly phase: 'starting' } | { readonly phase: 'ready' } | { readonly phase: 'error'; readonly message: string; readonly profileRecovery?: boolean }

/** Child lifecycle owned by the desktop backend controller. */
export interface DesktopBackendHost {
  /** @returns Readiness after the child accepts application requests. */
  start(): Promise<unknown>
  /** @returns Completion of child exit. */
  stop(): Promise<void>
}

interface Attempt<Host> {
  cancelled: boolean
  failure?: Error
  host?: Host
  cleanup?: Promise<void>
}

/** Serializes retries and prevents children from outliving a closed window. */
export class DesktopBackendController<Host extends DesktopBackendHost> {
  private current: DesktopBackendState = { phase: 'starting' }
  private attempt: Attempt<Host> | undefined
  private pending: Promise<void> | undefined
  private stopping: Promise<void> | undefined
  private closed = false

  /**
   * @param createHost - Allocate a child and route its fatal failures to the supplied callback.
   * @param publish - Receive availability changes until the controller closes.
   */
  constructor(
    private readonly createHost: (onFailure: (error: Error) => void) => Host,
    private readonly publish: (state: DesktopBackendState) => void,
  ) {}

  /** Current availability, including the last startup or child failure. */
  get state(): DesktopBackendState { return this.current }

  /** Child available to application requests; absent during startup and teardown. */
  get host(): Host | undefined { return !this.closed && !this.attempt?.cancelled && this.current.phase === 'ready' ? this.attempt?.host : undefined }

  /**
   * Prepare the profile and start one child; concurrent callers share the attempt.
   * @param prepare - Profile preparation that must finish before spawning.
   * @returns Completion of startup, rejecting on preparation, startup, or cleanup failure.
   */
  start(prepare: () => Promise<void>): Promise<void> {
    if (this.closed) return Promise.reject(new Error('desktop backend is closed'))
    if (this.stopping !== undefined) return Promise.reject(new Error('desktop backend is stopping'))
    if (this.pending !== undefined) return this.pending
    if (this.current.phase === 'ready') return Promise.resolve()
    const previous = this.attempt
    const attempt: Attempt<Host> = { cancelled: false, ...(previous?.cleanup === undefined ? {} : { cleanup: previous.cleanup }) }
    this.attempt = attempt
    this.update({ phase: 'starting' })
    const pending = Promise.resolve().then(async () => {
      try {
        await previous?.cleanup
        delete attempt.cleanup
        if (attempt.cancelled) return
        await prepare()
        // stop() can cancel this attempt while preparation is pending.
        // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
        if (attempt.cancelled) return
        const host = this.createHost((error) => { this.failed(attempt, error) })
        attempt.host = host
        await host.start()
        if (attempt.failure !== undefined) throw attempt.failure
        // stop() can cancel this attempt while the child starts.
        // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
        if (!attempt.cancelled) this.update({ phase: 'ready' })
      } catch (error) {
        const cancelled = attempt.cancelled
        attempt.cancelled = true
        let failure = error
        try { await this.cleanup(attempt) } catch (cleanupError) {
          if (cleanupError !== error) failure = new AggregateError([error, cleanupError], 'desktop backend startup and cleanup failed')
        }
        if (!cancelled) this.update(desktopErrorState(failure))
        throw failure
      }
    }).finally(() => { if (this.pending === pending) this.pending = undefined })
    this.pending = pending
    return pending
  }

  /**
   * Stop pending preparation and the child before allowing another start.
   * @returns Completion of pending work and child exit; rejects if cleanup fails.
   */
  stop(): Promise<void> {
    if (this.stopping !== undefined) return this.stopping
    const attempt = this.attempt
    if (attempt !== undefined) attempt.cancelled = true
    if (!this.closed) this.update({ phase: 'starting' })
    const pending = this.pending
    const stopping = Promise.allSettled([
      attempt === undefined ? Promise.resolve() : this.cleanup(attempt),
      pending,
    ]).then((results) => {
      const cleanup = results[0]
      if (cleanup.status === 'rejected') throw cleanup.reason
      if (this.attempt === attempt) this.attempt = undefined
    }).finally(() => { if (this.stopping === stopping) this.stopping = undefined })
    this.stopping = stopping
    return stopping
  }

  /**
   * Permanently prevent startup and suppress further availability notifications.
   * @returns Completion of pending work and child exit; rejects if cleanup fails.
   */
  close(): Promise<void> {
    this.closed = true
    return this.stop()
  }

  private cleanup(attempt: Attempt<Host>): Promise<void> {
    if (attempt.cleanup === undefined) {
      attempt.cleanup = Promise.resolve().then(async () => { await attempt.host?.stop() })
    }
    return attempt.cleanup
  }

  private failed(attempt: Attempt<Host>, error: Error): void {
    if (this.attempt !== attempt || attempt.cancelled) return
    attempt.failure = error
    if (this.current.phase !== 'ready') return
    attempt.cancelled = true
    const cleanup = this.cleanup(attempt)
    this.update(desktopErrorState(error))
    void cleanup.catch((cleanupError: unknown) => {
      if (this.attempt === attempt) this.update(desktopErrorState(new AggregateError([error, cleanupError], 'Desktop backend failed and could not stop')))
    })
  }

  private update(state: DesktopBackendState): void {
    if (this.closed) return
    this.current = state
    try { this.publish(state) } catch (error) {
      console.error('desktop backend state listener failed', error)
    }
  }
}