website/static/workers/util.js

function decodeNumberArr(str, b, shift, length) {
  const result = [];
  for (let j = 0; j < str.length; j += length) {
    const token = str.slice(j, j + length);
    result.push(decodeNumber(token, b, shift));
  }
  return result;
}

function decodeNumber(str, b, shift) {
  let x = 0;
  let p = 1;
  for (let i = str.length; i--;) {
    x += (str.charCodeAt(i) - shift) * p;
    p *= b;
  }
  return x;
}

/**
 * https://github.com/mapbox/polyline
 *
 * Decodes to a [longitude, latitude] coordinates array.
 *
 * This is adapted from the implementation in Project-OSRM.
 *
 * @param {String} str
 * @param {Number} precision
 * @returns {Array}
 *
 * @see https://github.com/Project-OSRM/osrm-frontend/blob/master/WebContent/routing/OSRM.RoutingGeometry.js
 */
function decodePolyline(str, precision) {
  let index = 0,
    lat = 0,
    lng = 0,
    coordinates = [],
    shift = 0,
    result = 0,
    byte = null,
    latitude_change,
    longitude_change,
    factor = Math.pow(10, precision || 5);

    // Coordinates have variable length when encoded, so just keep
    // track of whether we've hit the end of the string. In each
    // loop iteration, a single coordinate is decoded.
  while (index < str.length) {

        // Reset shift, result, and byte
    byte = null;
    shift = 0;
    result = 0;

    do {
      byte = str.charCodeAt(index++) - 63;
      result |= (byte & 0x1f) << shift;
      shift += 5;
    } while (byte >= 0x20);

    latitude_change = ((result & 1) ? ~(result >> 1) : (result >> 1));

    shift = result = 0;

    do {
      byte = str.charCodeAt(index++) - 63;
      result |= (byte & 0x1f) << shift;
      shift += 5;
    } while (byte >= 0x20);

    longitude_change = ((result & 1) ? ~(result >> 1) : (result >> 1));

    lat += latitude_change;
    lng += longitude_change;

    coordinates.push([lng / factor, lat / factor]);
  }

  return coordinates;
}