Revert "Revert "Add a flag to retain multiplier features by minimum distance""

This reverts commit 776da4a1b8.

main.cpp

@@ -93,6 +93,7 @@ size_t limit_tile_feature_count_at_maxzoom = 0;
 unsigned int drop_denser = 0;
 std::map<std::string, serial_val> set_attributes;
 unsigned long long preserve_point_density_threshold = 0;
+unsigned long long preserve_multiplier_density_threshold = 0;
 long long extend_zooms_max = 0;
 int retain_points_multiplier = 1;
 std::vector<std::string> unidecode_data;
@@ -3109,6 +3110,7 @@ int main(int argc, char **argv) {
 		{"cluster-distance", required_argument, 0, 'K'},
 		{"cluster-maxzoom", required_argument, 0, 'k'},
 		{"preserve-point-density-threshold", required_argument, 0, '~'},
+		{"preserve-multiplier-density-threshold", required_argument, 0, '~'},
 
 		{"Dropping or merging a fraction of features to keep under tile size limits", 0, 0, 0},
 		{"drop-densest-as-needed", no_argument, &additional[A_DROP_DENSEST_AS_NEEDED], 1},
@@ -3320,6 +3322,8 @@ int main(int argc, char **argv) {
 				}
 			} else if (strcmp(opt, "preserve-point-density-threshold") == 0) {
 				preserve_point_density_threshold = atoll_require(optarg, "Preserve point density threshold");
+			} else if (strcmp(opt, "preserve-multiplier-density-threshold") == 0) {
+				preserve_multiplier_density_threshold = atoll_require(optarg, "Preserve multiplier density threshold");
 			} else if (strcmp(opt, "extend-zooms-if-still-dropping-maximum") == 0) {
 				extend_zooms_max = atoll_require(optarg, "Maximum number by which to extend zooms");
 			} else if (strcmp(opt, "retain-points-multiplier") == 0) {

main.hpp

@@ -65,6 +65,7 @@ extern long long extend_zooms_max;
 extern int retain_points_multiplier;
 extern size_t maximum_string_attribute_length;
 extern std::string accumulate_numeric;
+extern unsigned long long preserve_multiplier_density_threshold;
 
 struct order_field {
 	std::string name;

serial.hpp

@@ -94,6 +94,7 @@ struct serial_feature {
 
 #define FEATURE_DROPPED -1
 #define FEATURE_KEPT 0
+#define FEATURE_NO_REALLY_KEEP_IT_AROUND INT_MAX
 	// <0: dropped
 	//  0: kept
 	// >0: sequence number of additional feature kept by retain-points-multiplier

tile.cpp

@@ -1237,6 +1237,9 @@ static serial_feature next_feature(decompressor *geoms, std::atomic<long long> *
 			} else if (z + extra_multiplier_zooms >= feature_minzoom && count->second + 1 < retain_points_multiplier) {
 				count->second++;
 				sf.dropped = count->second;
+			} else if (preserve_multiplier_density_threshold > 0 &&
+				   sf.gap > ((1LL << (32 - z)) / preserve_multiplier_density_threshold) * ((1LL << (32 - z)) / preserve_multiplier_density_threshold)) {
+				sf.dropped = FEATURE_NO_REALLY_KEEP_IT_AROUND;
 			} else {
 				sf.dropped = FEATURE_DROPPED;
 			}
@@ -1890,8 +1893,11 @@ long long write_tile(decompressor *geoms, std::atomic<long long> *geompos_in, ch
 			} else {
 				can_stop_early = false;
 
-				if (sf.dropped != FEATURE_DROPPED) {
+				if (sf.dropped != FEATURE_DROPPED && sf.dropped != FEATURE_NO_REALLY_KEEP_IT_AROUND) {
 					// Does the current multiplier cluster already have too many features?
+					// (Because we are dropping dynamically, and we have already filled the
+					// cluster with features that were dynamically dropped from being
+					// primary features)
 					// If so, we have to drop this one, even if it would potentially qualify
 					// as a secondary feature to be exposed by filtering
 					if (layer.multiplier_cluster_size >= (size_t) retain_points_multiplier) {
@@ -2134,6 +2140,8 @@ long long write_tile(decompressor *geoms, std::atomic<long long> *geompos_in, ch
 					if (sf.dropped == FEATURE_KEPT) {
 						layer.multiplier_cluster_size = 1;
 						lead_features_count++;
+					} else if (sf.dropped == FEATURE_NO_REALLY_KEEP_IT_AROUND) {
+						other_multiplier_cluster_features_count++;
 					} else {
 						layer.multiplier_cluster_size++;
 						other_multiplier_cluster_features_count++;