const poolSize = Math.min(navigator.hardwareConcurrency || 4, 12)
const activeJobs = new Map()
const queue = []
const workers = []

function initWorker() {
  const worker = new Worker(new URL('./worker.js', import.meta.url), { type: 'module' })

  worker.id = workers.length
  worker.isAvailable = true
  worker.onmessage = handleMessage
  worker.onerror = handleError

  workers.push(worker)

  return worker
}

function getAvailableWorker() {
  let worker = workers.find((worker) => worker.isAvailable)

  if (worker) return worker

  if (workers.length < poolSize) return initWorker()
}

function assignJob(worker, job) {
  worker.isAvailable = false
  worker.postMessage(job.data)
  activeJobs.set(worker.id, job)
}

function handleMessage(message) {
  const job = activeJobs.get(this.id)

  activeJobs.delete(this.id)

  job.resolve(message.data)

  this.isAvailable = true

  processQueue()
}

function processQueue() {
  const nextJob = queue.unshift()

  if (!nextJob) return

  const worker = getAvailableWorker()

  if (!worker) return

  assignJob(worker, job)
}

function handleError(error) {
  const job = activeJobs.get(this.id)

  delete activeJobs.get(this.id)

  job.reject(error)
}

export default {
  execute(data) {
    return new Promise((resolve, reject) => {
      const worker = getAvailableWorker()

      const job = {
        data,
        resolve,
        reject,
      }

      if (worker) {
        assignJob(worker, job)
      } else {
        queue.push(job)
      }
    })
  },

  workerCount() {
    return workers.length
  },
}