Decoding paths in here maps

In my panic to move my website off Google Maps, I took some shortcuts. One of those was to continue to use the helper function google.maps.geometry.encoding.decodePath. I primarily use this for decoding the paths of Strava segments, since these are encoded using the same algorithm as Google. But loading up the Google Maps API script for a single function is rather ridiculous, so I’ve converted the path decoder from the rather marvellous (and seemingly abandoned) Strava.NET library.

For your pleasure, here is some TypeScript that will create a here maps LineString from an encoded path. Other map providers are available and adjusting it for them should be fairly straightforward.

function decodePath(polylinechars: string): H.geo.LineString {
  var poly = new H.geo.LineString();

  if (polylinechars == null || polylinechars === "") {
    return poly;
  }

  var index = 0;

  var currentLat = 0;
  var currentLng = 0;

  while (index < polylinechars.length) {
    // calculate next latitude
    var sum = 0;
    var shifter = 0;
    var next5bits;
    do {
      next5bits = polylinechars.charCodeAt(index++) - 63;
      sum |= (next5bits & 31) << shifter;
      shifter += 5;
    } while (next5bits >= 32 && index < polylinechars.length);

    if (index >= polylinechars.length)
      break;

    currentLat += (sum & 1) == 1 ? ~(sum >> 1) : (sum >> 1);

    // calculate next longitude
    sum = 0;
    shifter = 0;
    do {
      next5bits = polylinechars.charCodeAt(index++) - 63;
      sum |= (next5bits & 31) << shifter;
      shifter += 5;
    } while (next5bits >= 32 && index < polylinechars.length);

    if (index >= polylinechars.length && next5bits >= 32)
      break;

    currentLng += (sum & 1) == 1 ? ~(sum >> 1) : (sum >> 1);

    poly.pushPoint(new H.geo.Point(currentLat / 100000.0, currentLng / 100000.0));
  }

  return poly;
}