splitster/src/records/test/utils/setWinningVariant.js

Javascript AB testing tool

import * as R from "ramda";

import seedRandom from "seedrandom";

const getSeedNumber = key => seedRandom(key)();

export const getWinningVariant = (variants, defaultVariant, seedNumber) => {
  const ratioSum = R.sum(
    R.map(([variantId, variant]) => variant.ratio, variants)
  );

  // Seed number (from interval [0, 1]) is interpolated to interval [0-ratio sum]
  let floater = seedNumber * ratioSum;

  const winningVariant = R.find(([variantId, variant]) => {
    floater -= variant.ratio;
    return floater <= 0;
  }, variants);
  return winningVariant ? winningVariant[0] : defaultVariant;
};

const setWinningVariant = (userId, { override = {}, testSeed }) => ([
  testId,
  test
]) => {
  const key = `${testId}_${test.version}`;
  if (override[key] && test.variants[override[key]]) {
    const variant = override[key];
    return [
      testId,
      {
        ...test,
        disabled: false,
        disabledReason: null,
        winningVariant: variant
      }
    ];
    return [testId, R.assoc("winningVariant", variant, test)];
  }

  if (test.disabled) {
    return [testId, R.assoc("winningVariant", test.defaultVariant, test)];
  }

  const seedNumber = R.isNil(testSeed)
    ? getSeedNumber(`${testId}_${test.version}:${userId}`)
    : testSeed;

  const winningVariant = getWinningVariant(
    R.toPairs(test.variants),
    test.defaultVariant,
    seedNumber
  );
  return [testId, R.assoc("winningVariant", winningVariant, test)];
};

export default setWinningVariant;