percentile.js

import isEmpty from 'underscore/modules/isEmpty';
import sortBy from 'underscore/modules/sortBy.js';

// Return the percentile value of the numeric elements from the collection
//Ex : 50th,75th,99th etc.
//https://en.wikipedia.org/wiki/Percentile
export default function percentile(collection, percentile) {
  if (isEmpty(collection)) return 0;
  if (typeof percentile !== 'number') throw new TypeError('Percentile must be a number between 0 - 100');
  if (percentile <= 0) return collection[0];
  if (percentile >= 100) return collection[collection.length - 1];

  collection = sortBy(collection);
  var index = (percentile/100) * (collection.length - 1),
    lowerIndex = Math.floor(index),
    upperIndex = lowerIndex + 1,
    weight = index % 1;

  if (upperIndex >= collection.length) return collection[lowerIndex];
  return collection[lowerIndex] * (1 - weight) + collection[upperIndex] * weight;
}