type ResolveReject = [(value: void | PromiseLike<void>) => void, (reason?: any) => void]

/**
 * Represents a class that can be used to create a singleton instance of a class that is created asynchronously.
 */
export default class AsyncSingleton<T, R> {
  private readonly factory: (r: R) => Promise<T>
  private readonly checker: (value: T) => Promise<boolean>

  private queue: ResolveReject[] = []
  private instantiating: boolean = false

  private value?: T

  /**
   * Creates a new instance of the AsyncSingleton class.
   * @param factory The factory function that creates the instance.
   * @param checker The function that checks if the instance is valid.
   */
  constructor(
    factory: (r: R) => Promise<T>,
    checker: (value: T) => Promise<boolean> = async () => true
  ) {
    this.factory = factory
    this.checker = checker
  }

  /**
   * Gets the instance of the class.
   * @param r The configuration object for creating the instance.
   */
  public async instance(r: R): Promise<T> {
    if (this.value && (await this.checker(this.value))) return this.value

    if (this.instantiating) {
      return await this.awaitOtherInit()
    }

    return await this.initHere(r)
  }

  /**
   * Gets the current value, may be undefined or invalid.
   */
  public getValue(): T | undefined {
    return this.value
  }

  /**
   * Clears the current value and rejects all pending promises.
   */
  public clear() {
    this.value = undefined
    for (const [, rej] of this.queue) {
      rej()
    }
    this.queue = []
  }

  private async initHere(r: R): Promise<T> {
    try {
      this.instantiating = true
      this.value = await this.factory(r)

      for (const [res] of this.queue) {
        res()
      }

      this.queue = []

      return this.value
    } finally {
      for (const [, rej] of this.queue) {
        rej()
      }
      this.instantiating = false
    }
  }

  private async awaitOtherInit(): Promise<T> {
    await new Promise<void>((res, rej) => {
      this.queue.push([res, rej])
    })
    return this.value!
  }
}