Experiment assignment for Cloudfront

assignment-FNV-multi-variant.js

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
}

assignment.js

const BUCKETS = 100

function toInt32(buf) {
    return Math.abs(new Uint8Array(buf).slice(0,4).reduce((p,c) => p << 8 | c, 0))
}

async function hash(input){
  const data = new TextEncoder().encode(input);
  const buf = await crypto.subtle.digest("SHA-256", data);
  return toInt32(buf)
}

async function getBucket(key) {
  let n = await hash(key);
  return n % BUCKETS
}

const seed = '8fc40cab' // change will reallocate users

async function getVariant (userID, exp) {
  const key = `${userID}-${exp}-${seed}`
  const bucket = await getBucket(key)
  return bucket < (BUCKETS/2) ? 0 : 1
}

traffic-ramp-up-node.js

const seed = '8fc40cab' // random seed, change to force reallocation of 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;
}

/* simplified version that works with a traffic % from 0 to 100 */
function isFeatureEnabled (userID, featureName, trafficPercentage = 50) {
  const key = `${userID}-${featureName}-${seed}`
  const bucket = hashFNV(key) % 100
  return bucket < trafficPercentage
}

https://codepen.io/ricardobeat/pen/oNVvyPX

test

const N_USERS = 1000000
const N = 1_000_000
const userIds = new Array(N_USERS).fill(1).map(n => (Math.random() * 1000000 | 0).toString())
const count = {}

console.time('benchmark')

for (let i = 0; i < N; i++) {
  const treatment = getVariant(userIds[i % N_USERS], 'my_experiment', 60, 5)
  count[treatment] = (count[treatment] || 0) + 1
}

console.timeEnd('benchmark')

console.log(count)
for (let key in count) {
  console.log(key, count[key] / N)
}