const {Readable, Transform} = require('stream');

// SplitArrayStream can be used two ways:
// 
//   - An intermediary stream that receives arrays and emits its items
//   - A source stream that takes a function as an argument, that is expected to
//     return an array each time it is called
// 
// In both cases, an array is only asked for when the destination stream is
// ready.
class SplitArrayStream extends Transform {
  constructor(getArrayFn) {
    super({objectMode: true})

    if (typeof getArrayFn === 'function') {
      this.getArrayFn = getArrayFn
      this._read = this._readFromFn.bind(this)
    }
  }

  async _readFromFn() {
    let consumerStreamReady = true

    const array = await this.getArrayFn()

    while (consumerStreamReady && array.length > 0) {
      consumerStreamReady = this.push(array.shift())
    }

    if (consumerStreamReady && array.length > 0) {
      setImmediate(() => this._read(array))
    }
  }

  _transform(array, enc, next) {
    let consumerStreamReady = true

    while (consumerStreamReady && array.length > 0) {
      consumerStreamReady = this.push(array.shift())
    }

    if (consumerStreamReady && array.length === 0) {
      next()
    } else {
      setImmediate(() => this._transform(array, enc, next))
    }
  }
}

// For testing purposes, only returns 5 pages of results
// (2 buckets per page, 10 buckets total)
let numPagesReturned = 0
async function makeGoogleApiRequest() {
  const simulatedApiResponse = {
    kind: 'storage#listBuckets',
    items: [
      {
        kind: 'storage#bucket',
        id: `bucket-a-from-page-${numPagesReturned + 1}`,
      },
      {
        kind: 'storage#bucket',
        id: `bucket-b-from-page-${numPagesReturned + 1}`,
      }
    ],
    nextPageToken: '...',
  }

  if (numPagesReturned > 3) {
    delete simulatedApiResponse.nextPageToken
  }

  numPagesReturned++
  return Promise.resolve(simulatedApiResponse)
}

// Simulated "storage.getBuckets()" method
async function getBuckets(options) {
  const reqOpts = {
    method: 'GET',
    uri: 'https://www.googleapis.com/storage/v1/buckets/list',
  }

  if (options.pageToken) {
    reqOpts.qs = {pageToken: options.pageToken}
  }

  const apiResponse = await makeGoogleApiRequest(reqOpts)

  const simulatedBucketObjects = apiResponse.items.map(bucket => {
    return {
      name: bucket.id,
      metadata: bucket,
    }
  })

  return Promise.resolve([simulatedBucketObjects, apiResponse])
}

function getBucketsStreamOptionA() {
  let nextPageToken

  const readStream = new Readable({
    objectMode: true,
    read: async function() {
      const options = {autoPaginate: false}

      if (nextPageToken) {
        options.pageToken = nextPageToken
      }

      const [buckets, apiResponse] = await getBuckets(options)
      nextPageToken = apiResponse.nextPageToken

      this.push(buckets)

      if (!nextPageToken) {
        this.push(null)
      }
    }
  })

  return readStream.pipe(new SplitArrayStream())
}

function getBucketsStreamOptionB() {
  let nextPageToken

  const getBucketsAsStream = async () => {
    const options = {autoPaginate: false}

    if (nextPageToken) {
      options.pageToken = nextPageToken
    }

    const [buckets, apiResponse] = await getBuckets(options)

    nextPageToken = apiResponse.nextPageToken

    if (!nextPageToken) {
      buckets.push(null)
    }

    return Promise.resolve(buckets)
  }

  return new SplitArrayStream(getBucketsAsStream)
}

getBucketsStreamOptionA()
  .on('data', bucket => {
    console.log('Bucket name:', bucket.name)
  })

getBucketsStreamOptionB()
  .on('data', bucket => {
    console.log('Bucket name:', bucket.name)
  })