ES6: Prepare the cache so that the same elements are not extracted

randomPickFromCache.js

/*
 * # usage
 * const pool = randomSampleInCache(array, 5);
 * const elem = pool.pick();
 *
 * TODO: clamp cacheNum in [0 ... list.length]
 */

import { difference, sample, range } from "lodash"

export default (list, cacheNum = 1) => {
  if (list.length === 1) cacheNum = 0
  const cached = new Array(cacheNum)
  const indexes = range(list.length)
  function pick() {
    const diffIndexes = difference(indexes, cached)
    const pickIndex = sample(diffIndexes)
    if (cacheNum > 0) {
      cached.push(pickIndex)
      if (cached.length > cacheNum) cached.shift()
    }
    return list[pickIndex]
  }
  return {
    list,
    pick,
  }
}