const BUCKETS = 120 // multiple of 3 for correct split on 3-variant experiments
const seed = '8fc40cab' // random seed, change to reallocate users

function hashFNV(s, h = 0x811c9dc5) {
  for (let i = 0; i < s.length; i++) {
    h ^= s.charCodeAt(i);
    h += (h << 1) + (h << 4) + (h << 7) + (h << 8) + (h << 24);
  }
  return h >>> 0;
}


function getBucket(key) {
  let n = hashFNV(key);
  return n % BUCKETS
}

function trackImpression(userId) {
  // ...
}

function getVariant(userId, experiment, trafficPercentage = 100, variants = 2) {
  const key = `${userId}-${experiment}-${seed}`
  const bucket = getBucket(key)
  for (let i = 0; i < variants; i++) {
    let threshold = (i + 1) * (trafficPercentage / variants / 100 * BUCKETS)
    if (bucket < threshold) {
      trackImpression(userId)
      return i
    }
  }
  return undefined // not in experiment
}