Question on tmap2gmaps method

[DonPex]

Hi, I am trying to reproduce the ExpandedMBTilesTileProvider in Swift using GoogleMaps SDK for iOS.

I can't understand what this piece of code wants to achieve:

private int tms2gmaps(final int y, final int zoom) {
        final int ymax = 1 << zoom;
        return ymax - y - 1;
}