GeoHash implementation in Objective-C for iOS and Mac OS X

See http://en.wikipedia.org/wiki/Geohash

• (NSString *) encodeGeohash:(CLLocationCoordinate2D) coordinate;
• (NSString *) encodeGeohash:(CLLocationCoordinate2D) coordinate withPrecision:(NSUInteger) precision;
• (NSString ) encodeGeohashFromLocation:(CLLocation) location;
• (NSString ) encodeGeohashFromLocation:(CLLocation) location withPrecision:(NSUInteger) precision;

These methods will return a geohash for the given coordinate/location. The methods with the precision parameter allow you to define the precision of the resulting geocode, the ones without that parameter use a precision of 12.

• (GHLocationRect) decodeGeohash:(NSString *) geohash;
• (CLRegion *) decodeGeohashAsRegion:(NSString *) geohash;

Since a geohash does not denote a point but a square region, these methods do not return coordinates. The first method returns a GHLocationRect, which is a struct containing to CLLocationCoordinate2D entries, one for the north-eastern coordinate and one for the south-western coordinate. The method return a CLRegion transforms the GHLocationRect into a circular region of about the same size as the rectangle.

• (NSArray *) adjacentGeohashes:(NSString *) geohash includeSelf:(BOOL) include;

The resulting array will contain the geohashes which are adjacent to the given geohash. These will be 8 geohashes for north-east, north, north-west, west, south-west, south, south-east and east. If inculde is YES, the given geohosh will be included in the resulting array.

In addition to the latitude and longitude values you should store the correspondin geohashes of the entries. The client can then compute the geohash for a location, find the adjacent geohashes and perform a search for those hashes on the database. The radius of the search can be controlled by the precision of the geohash.

This implementation is based on https://github.com/davetroy/geohash-js by Dave Troy (https://github.com/davetroy)