Add a flag to use an H3 index for the feature index

main.cpp

@@ -99,6 +99,7 @@ int retain_points_multiplier = 1;
 std::vector<std::string> unidecode_data;
 size_t maximum_string_attribute_length = 0;
 std::string accumulate_numeric;
+std::string use_h3_index;
 
 std::vector<order_field> order_by;
 bool order_reverse;
@@ -3128,6 +3129,7 @@ int main(int argc, char **argv) {
 		{"coalesce", no_argument, &additional[A_COALESCE], 1},
 		{"reverse", no_argument, &additional[A_REVERSE], 1},
 		{"hilbert", no_argument, &additional[A_HILBERT], 1},
+		{"use-h3-index", required_argument, 0, '~'},
 		{"order-by", required_argument, 0, '~'},
 		{"order-descending-by", required_argument, 0, '~'},
 		{"order-smallest-first", no_argument, 0, '~'},
@@ -3269,6 +3271,8 @@ int main(int argc, char **argv) {
 					fprintf(stderr, "%s: --extra-detail can be at most 30\n", argv[0]);
 					exit(EXIT_ARGS);
 				}
+			} else if (strcmp(opt, "use-h3-index") == 0) {
+				use_h3_index = optarg;
 			} else if (strcmp(opt, "order-by") == 0) {
 				order_by.push_back(order_field(optarg, false));
 			} else if (strcmp(opt, "order-descending-by") == 0) {

main.hpp

@@ -53,6 +53,7 @@ extern long long extend_zooms_max;
 extern int retain_points_multiplier;
 extern size_t maximum_string_attribute_length;
 extern std::string accumulate_numeric;
+extern std::string use_h3_index;
 extern unsigned long long preserve_multiplier_density_threshold;
 
 struct order_field {

serial.cpp

@@ -714,6 +714,19 @@ int serialize_feature(struct serialization_state *sst, serial_feature &sf, std::
 	}
 
 	bbox_index = encode_index(midx, midy);
+
+	if (use_h3_index.size() > 0) {
+		for (size_t i = 0; i < sf.full_keys.size(); i++) {
+			if (*(sf.full_keys[i]) == use_h3_index) {
+				unsigned long long h3_index = atoll(sf.full_values[i].s.c_str());
+				// the top 52 bits of the feature index are the H3 index;
+				// the bottom 12 bits are retained from what otherwise would have been the index.
+				bbox_index = ((h3_index & ((1LL << 52) - 1)) << 12) | (bbox_index & ((1 << 12) - 1));
+				break;
+			}
+		}
+	}
+
 	if (additional[A_CALCULATE_INDEX]) {
 		sf.full_keys.push_back(key_pool.pool("tippecanoe:index"));