Use circular mean to determine how categorical each attribute is

mbtiles.cpp

@@ -276,6 +276,12 @@ void tilestats(std::map<std::string, layermap_entry> const &layermap1, size_t el
 			state.json_write_string("attribute");
 			state.json_write_string(attribute.first);
 
+			double xd = attribute.second.xsum / attribute.second.count;
+			double yd = attribute.second.ysum / attribute.second.count;
+			double d = sqrt(xd * xd + yd * yd);
+
+			printf("%s %0.6f\n", attribute.first.c_str(), d);
+
 			size_t val_count = attribute.second.sample_values.size();
 			if (val_count > max_tilestats_sample_values) {
 				val_count = max_tilestats_sample_values;
@@ -848,6 +854,22 @@ void add_to_file_keys(std::map<std::string, type_and_string_stats> &file_keys, s
 		if (fka->second.sample_values.size() > max_tilestats_sample_values) {
 			fka->second.sample_values.pop_back();
 		}
+
+		unsigned hash = 0;
+		for (size_t i = 0; i < val.string.size(); i++) {
+			// https://en.wikipedia.org/wiki/Hash_function#Fibonacci_hashing
+			hash = hash * 2654435769 + val.string[i];
+		}
+		// extra multiply so even single-byte values are distributed
+		// around the circle rather than clumped together on one side.
+		hash = hash * 2654435769;
+		double angle = ((double) hash) / UINT_MAX * 2 * M_PI;
+		double x = cos(angle);
+		double y = sin(angle);
+
+		fka->second.xsum += x;
+		fka->second.ysum += y;
+		fka->second.count++;
 	}
 
 	fka->second.type |= (1 << val.type);

mbtiles.hpp

@@ -22,6 +22,11 @@ struct type_and_string_stats {
 	double min = INFINITY;
 	double max = -INFINITY;
 	int type = 0;
+
+	// for circular mean of value hashes
+	double xsum = 0;
+	double ysum = 0;
+	size_t count = 0;
 };
 
 struct layermap_entry {