a JavaScript implementation of A Practical Guide to Building Recommender Systems Epsilon-based Dithering

epsilon-recommendation-dithering-in-javascript.js

/**
 * The following is a JavaScript implementation of the Spark+Scala code
 * snippets for performing recommendation dithering via an epsilon-based
 * post-processing step using normally distributed random noise.
 *
 * Source:
 *   Hristakeva, M. (2015, November 12). Dithering.
 *     A Practical Guide to Building Recommender Systems.
 *     https://buildingrecommenders.wordpress.com/2015/11/11/dithering/
 */

const gaussian = require('gaussian');

function ditherRecsForUser (recommendations, epsilon) {
  let variance = (epsilon > 1.0) ? Math.log(epsilon) : 1e-10;
  let distribution = gaussian(0, variance);

  /* Sort recommendations by score descending */
  recommendations.sort((a, b) => (a.score > b.score) ? -1 : 1)

  /* Calculate dither score by rank */
  for (let ii in recommendations) {
    let recommendation = recommendations[ii];
    let rank = ii;

    recommendation.ditherScore = (Math.log(rank + 1) + distribution.ppf(Math.random()));
  }

  /* Sort recommendations by ditherScore ascending */
  recommendations.sort((a, b) => (a.ditherScore > b.ditherScore) ? 1 : -1)

  return recommendations;
}

// Testing

function createNormalizedRecommendations (recommendationCount) {
  const recommendations = [];
  let sum = 0;
  for (let ii = 1; ii <= recommendationCount; ++ii) {
    let score = (recommendationCount - ii);
    sum += score;
    recommendations.push({ id: ii, score: score });
  }
  for (let ii in recommendations) {
    recommendations[ii].score = (recommendations[ii].score / sum);
  }
  return recommendations;
}

let recommendations = createNormalizedRecommendations(30);
let epsilons = [1.0, 1.25, 1.5, 2.0, 2.5, 3.0, 5.0];
let table = [];
for (let ii in epsilons) {
  let eps = epsilons[ii];
  table.push(ditherRecsForUser(JSON.parse(JSON.stringify(recommendations)), eps));
}

let rows = [];
for (let ii in epsilons) {
  epsilons[ii] = 'eps=' + epsilons[ii];
}
rows.push(epsilons);
for (let xx in table[0]) {
  let row = [];
  for (let yy in table) {
    row.push(table[yy][xx].id);
  }
  rows.push(row);
}
console.table(rows);