@antora/assembler/lib/util/promise-queue.js

An extension library for Antora that assembles content from multiple pages into a single AsciiDoc file to converted and publish.

'use strict'

class PromiseQueue {
  #scheduler
  #started
  #pending
  #rejection
  #startTask
  #trapRejection

  constructor ({ concurrency = Infinity } = {}) {
    this.concurrency = concurrency
    this.#scheduler = Promise.resolve()
    this.#started = []
    this.#pending = []
    this.#startTask = (task) => {
      if (this.#rejection) return
      if (this.concurrency === Infinity) {
        this.#started.push(task().catch(this.#trapRejection))
      } else {
        let current
        this.#pending.push(
          (current = task()
            .catch(this.#trapRejection)
            .finally(() => this.#pending.splice(this.#pending.indexOf(current), 1)))
        )
        this.#started.push(current)
      }
    }
    this.#trapRejection = (err) => (this.#rejection = err || new Error()) && undefined
  }

  add (tasks) {
    if (!Array.isArray(tasks)) tasks = [tasks]
    for (const task of tasks) {
      if (this.#pending.length < this.concurrency) {
        this.#startTask(task)
      } else {
        this.#scheduler = this.#scheduler.then(
          () => this.#pending.length && Promise.race(this.#pending).then(() => this.#startTask(task))
        )
      }
    }
    return this
  }

  async toPromise () {
    // what about promiseAll(), all(), or promise()?
    await this.#scheduler
    return Promise.all(this.#started).then((returnValues) => {
      if (this.#rejection) throw this.#rejection
      return returnValues
    })
  }
}

module.exports = PromiseQueue