How to decode google polyline, when bitwise OR is destructive?

Question

I'm looking to decode an encoded google polyline:

`~oia@

However to reverse one of the steps requires reversing the bitwise OR operation, which is destructive.

I see it's done here: How to decode Google's Polyline Algorithm? but I can't see how to do that in Javascript.

Here is what I have so far:

const partialDecodedPolyline = "`~oia@".split('').map(char => (char.codePointAt()-63).toString(2))

console.log(partialDecodedPolyline)

The next step is to reverse the bitwise OR... how is that possible?

Answer 1

There is a libriary for that https://github.com/mapbox/polyline/blob/master/src/polyline.js

/*
  https://github.com/mapbox/polyline/blob/master/src/polyline.js
*/
const decode = function(str, precision) {
    var index = 0,
        lat = 0,
        lng = 0,
        coordinates = [],
        shift = 0,
        result = 0,
        byte = null,
        latitude_change,
        longitude_change,
        factor = Math.pow(10, Number.isInteger(precision) ? 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([lat / factor, lng / factor]);
    }

    return coordinates;
};

console.log(decode("`~oia@"));