Google App Engine Geohashing

Question

I am writing a web application using GWT and App Engine. My application will need to post and query items based on their latitude, longitude.

As a result of google\

Answer 1

I was also in need of a Java version of GeoModel. I was working with Geohash before, which allowed me to fetch locations in a given bounding box. But there are considerable limitations to this when it comes to sorting: in order to get BigTable to accept a filter like geohash > '" + bottomLeft + "' && geohash < '" + topRight + "'", you have to order the list by geohash as well, which makes it impossible to sort it by other criteria (especially if you want to use pagination). At the same time, I just can't think of a solution to sort the results by distance (from a given user-position, i.e. the center of the bounding box), other than in Java-code. Again, this will not work if you need to have pagination.

Because of these problems I had to use a different approach, and GeoModel/Geoboxes seemed to be the way. So, I ported the Python-code to Java and it's just working fine! Here is the result:

public class Geobox {

    private static double roundSlicedown(double coord, double slice) {
        double remainder = coord % slice;
        if (remainder == Double.NaN) {
            return coord;
        }
        if (coord > 0) {
            return coord - remainder + slice;
        } else {
            return coord - remainder;
        }
    }

    private static double[] computeTuple(double lat, double lng,
            int resolution, double slice) {
        slice = slice * Math.pow(10, -resolution);
        double adjustedLat = roundSlicedown(lat, slice);
        double adjustedLng = roundSlicedown(lng, slice);
        return new double[] { adjustedLat, adjustedLng - slice,
                adjustedLat - slice, adjustedLng };
    }

    private static String formatTuple(double[] values, int resolution) {
        StringBuffer s = new StringBuffer();
        String format = String.format("%%.%df", resolution);
        for (int i = 0; i < values.length; i++) {
            s.append(String.format(format, values[i]).replace(',','.'));
            if (i < values.length - 1) {
                s.append("|");
            }
        }
        return s.toString();
    }

    public static String compute(double lat, double lng, int resolution,
            int slice) {
        return formatTuple(computeTuple(lat, lng, resolution, slice),
                resolution);
    }

    public static List<String> computeSet(double lat, double lng,
            int resolution, double slice) {
        double[] primaryBox = computeTuple(lat, lng, resolution, slice);
        slice = slice * Math.pow(10, -resolution);
        List<String> set = new ArrayList<String>();
        for (int i = -1; i < 2; i++) {
            double latDelta = slice * i;
            for (int j = -1; j < 2; j++) {
                double lngDelta = slice * j;
                double[] adjustedBox = new double[] { primaryBox[0] + latDelta,
                        primaryBox[1] + lngDelta, primaryBox[2] + latDelta,
                        primaryBox[3] + lngDelta };
                set.add(formatTuple(adjustedBox, resolution));
            }
        }
        return set;
    }
}

Answer 2

I'm working on a GWT/GAE project and had the same problem. My solution was to use a Geohash class that I modified slightly to be GWT-friendly. It's great for my needs of proximity searches.

If you've never seen Geohashes in action, check out Dave Troy's JS demo page.