assignExperiment.ts

import { crc32 } from 'common/components/Features/Experiment/crc32';
import { Config } from 'common/api/models/Config';
const sortBy = require('lodash.sortby');

export interface Experiments {
  [name: string]: Variant[];
}

export interface Variant {
  name: string;
  weight: string;
}

/** Returns selected variants of given experiments */
export function getVariants(
  { config: { experiments = {} } }: Config,
  userId: string
) {
  return Object.keys(experiments).reduce(
    (acc, name) => ({
      ...acc,
      [name]: assignExperimentVariant(experiments[name], userId),
    }),
    {}
  );
}

/** Select a variant of an experiment (deterministic if given userId) */
export function assignExperimentVariant(variants: Variant[], userId: string) {
  /*
  Choosing a weighted variant:
    For C, A, B with weights 2, 4, 8
    variants = A, B, C
    weights = 4, 8, 2
    weightSum = 14
    weightedIndex = 9
    AAAABBBBBBBBCC
    ========^
    Select B
*/
  const sortedVariants = sortBy(variants, ({ name }: Variant) => name);
  const weights = sortedVariants.map(({ weight }: Variant) =>
    parseInt(weight, 10)
  );
  const weightSum = weights.reduce((acc: number, weight: number) => {
    return acc + weight;
  }, 0);

  // A random number between 0 and weightSum
  let weightedIndex = Math.abs(crc32(userId) % weightSum);

  // Iterate through the sorted weights, and deduct each from the weightedIndex.
  // If weightedIndex drops < 0, select the variant. If weightedIndex does not
  // drop < 0, default to the last variant in the array that is initially assigned.
  let selectedVariant = sortedVariants[sortedVariants.length - 1].name;
  for (let index = 0; index < weights.length; index++) {
    weightedIndex -= weights[index];
    if (weightedIndex < 0) {
      selectedVariant = sortedVariants[index].name;
      break;
    }
  }

  return selectedVariant;
}