Track zoom truncation in the strategies list in the tileset metadata

mbtiles.cpp

@@ -444,6 +444,14 @@ std::string stringify_strategies(std::vector<strategy> const &strategies) {
 			any = true;
 		}
 
+		if (strategies[i].truncated_zooms > 0) {
+			state.nospace = true;
+			state.json_write_string("truncated_zooms");
+			state.nospace = true;
+			state.json_write_number(strategies[i].truncated_zooms);
+			any = true;
+		}
+
 		state.nospace = true;
 		state.json_end_hash();
 	}

tests/ne_110m_populated_places/out/-z20_-r1_--truncate-zooms_--limit-tile-feature-count_50.json

@@ -9,6 +9,7 @@
 "maxzoom": "20",
 "minzoom": "0",
 "name": "tests/ne_110m_populated_places/out/-z20_-r1_--truncate-zooms_--limit-tile-feature-count_50.json.check.mbtiles",
+"strategies": "[{},{\"truncated_zooms\":2},{\"truncated_zooms\":2},{\"truncated_zooms\":8},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{}]",
 "tippecanoe_decisions": "{\"basezoom\":20,\"droprate\":1,\"retain_points_multiplier\":1}",
 "type": "overlay",
 "version": "2"

tests/tl_2022_11_tract/out/-z14_-Z12_--coalesce-densest-as-needed_--truncate-zooms_-M25000.json

@@ -9,7 +9,7 @@
 "maxzoom": "14",
 "minzoom": "12",
 "name": "tests/tl_2022_11_tract/out/-z14_-Z12_--coalesce-densest-as-needed_--truncate-zooms_-M25000.json.check.mbtiles",
-"strategies": "[{},{},{},{},{},{},{},{},{},{},{},{},{\"coalesced_as_needed\":87,\"tile_size_desired\":39190},{},{}]",
+"strategies": "[{},{},{},{},{},{},{},{},{},{},{},{},{\"coalesced_as_needed\":87,\"tile_size_desired\":39190,\"truncated_zooms\":3},{\"truncated_zooms\":18},{}]",
 "type": "overlay",
 "version": "2"
 }, "features": [

tile-join.cpp

@@ -877,6 +877,8 @@ void handle_strategies(const unsigned char *s, std::vector<strategy> *st) {
 							(*st)[i].dropped_as_needed += v->value.number.number;
 						} else if (strcmp(k->value.string.string, "coalesced_as_needed") == 0) {
 							(*st)[i].coalesced_as_needed += v->value.number.number;
+						} else if (strcmp(k->value.string.string, "truncated_zooms") == 0) {
+							(*st)[i].truncated_zooms += v->value.number.number;
 						} else if (strcmp(k->value.string.string, "detail_reduced") == 0) {
 							(*st)[i].detail_reduced += v->value.number.number;
 						} else if (strcmp(k->value.string.string, "tiny_polygons") == 0) {

tile.cpp

@@ -2697,6 +2697,7 @@ long long write_tile(decompressor *geoms, std::atomic<long long> *geompos_in, ch
 					// We succeeded in stopping early.
 					// Prune the child tiles.
 
+					strategy.truncated_zooms++;
 					skip_children_out.insert(zxy(z, tx, ty));
 				}
 
@@ -3206,4 +3207,5 @@ void atomic_strategy::add_from(struct strategy const &src) {
 	coalesced_as_needed += src.coalesced_as_needed;
 	detail_reduced += src.detail_reduced;
 	tiny_polygons += src.tiny_polygons;
+	truncated_zooms += src.truncated_zooms;
 }

tile.hpp

@@ -18,6 +18,7 @@ struct atomic_strategy {
 	std::atomic<size_t> coalesced_as_needed;
 	std::atomic<size_t> detail_reduced;
 	std::atomic<size_t> tiny_polygons;
+	std::atomic<size_t> truncated_zooms;
 
 	atomic_strategy()
 	    : dropped_by_rate(0),
@@ -25,7 +26,8 @@ struct atomic_strategy {
 	      dropped_as_needed(0),
 	      coalesced_as_needed(0),
 	      detail_reduced(0),
-	      tiny_polygons(0) {
+	      tiny_polygons(0),
+	      truncated_zooms(0) {
 	}
 
 	void add_from(struct strategy const &src);
@@ -38,6 +40,7 @@ struct strategy {
 	size_t coalesced_as_needed = 0;
 	size_t detail_reduced = 0;
 	size_t tiny_polygons = 0;
+	size_t truncated_zooms = 0;
 
 	size_t tile_size = 0;
 	size_t feature_count = 0;
@@ -51,6 +54,7 @@ struct strategy {
 		tile_size = ts;
 		feature_count = fc;
 		tiny_polygons = s.tiny_polygons;
+		truncated_zooms = s.truncated_zooms;
 	}
 
 	strategy() = default;