Improve tiling speed (#195)

* Add a way to run tippecanoe single-threaded for profiling

* Do less work when the tilestats sample values list is already full

* Save a copy when retrieving the attribute key

* Fewer atomic operations

* Move string hashing from mbtiles to text

* Only do approximate attribute deduplication when writing tiles

* Feature dropping tests are sensitive to exact tile size

* All tile creators now create a string pool for the tile

* Features clipped away to nothing should not participate in that tile

* Revert "Only do approximate attribute deduplication when writing tiles"

This reverts commit c42b34b498.

* Also revert the related test changes

* Revert "Revert "Only do approximate attribute deduplication when writing tiles""

This reverts commit 18509876c3.

* Be more specific about the string hash function

* Use fnv1a instead of std::hash for everything

* Reduce the chance of hash collisions

* Stick a hash search on the front of the tree search in addpool

* Eliminate repeated hashing of the same string

* Switch instead of ifs in json parsing

* A few more cases to populate the hash in addpool

* Store the hash in the tree instead of recalculating

* Add explanatory comment for mysterious argument

* Fewer copies in attribute stringification

* Clean up ancient weirdness in JSON attribute stringification

* More serial_val cleanup

* Pass a serial_feature to rewrite instead of many broken-down arguments

* Get rid of the multiple geometries within `partial`

* Revert "Pass a serial_feature to rewrite instead of many broken-down arguments"

This reverts commit 6f4ab9b725.

* Goodbye, struct coalesce

* Revert "Features clipped away to nothing should not participate in that tile"

This reverts commit 124462fbdc.

* Migrating fields from partial to serial_feature

* Name reconciliation between serial_feature and partial

* Replace struct partial with an augmented serial_feature

* Fix some overzealous search-and-replace renaming

* Don't say struct so often

* Remove more of the former partial construction

* Commenting and cleaning up

* Trying again to avoid all these arguments to rewrite

* I swear I did this same thing before and it didn't work.

* More rewrite cleanup

* Exile --detect-shared-borders to its own file

* Add missing headers

* More commenting and cleanup

* More comments

* Sprinkle consts around

* Emplacing and std::moving

* More cleanup

* That shouldn't have worked after a std::move

* Don't need to allocate memory to compare keys

* Reduce use of the global string pool in tiling

* Another avoidable mvt_value construction

* Further reduction to explicit string pool passing

* These reverses are no longer optimizations

* These layernames can all be references

* Don't drag an unused layername string around with every feature

* Heed a compiler warning about potential buffer overflow

* Fix my confusion about which feature's string pool is relevant

* Avoid some unnecessary allocations in attribute accumulation

* Maybe faster serialization?

* Eliminate a comparison

* Do the same here

* Save a couple of allocations when parsing numbers in JSON

* Immediately assign features to layers instead of subdividing later

* Maintain tilestats for tippecanoe:retain_points_multiplier_sequence

* Crunch out more duplicate attribute values when writing out the tile

* Do tilestats for tippecanoe:retain_points_multiplier_first too

* Shell filters need to be real threads, even if nothing else does

* Simplify tippecanoe_minzoom/maxzoom representation

* Update version and changelog

clip.cpp

@@ -785,7 +785,7 @@ struct tile_feature {
 	size_t seq = 0;
 };
 
-static void feature_out(std::vector<tile_feature> const &features, mvt_layer &outlayer, std::set<std::string> const &keep, std::unordered_map<std::string, attribute_op> const &attribute_accum) {
+static void feature_out(std::vector<tile_feature> const &features, mvt_layer &outlayer, std::set<std::string> const &keep, std::unordered_map<std::string, attribute_op> const &attribute_accum, std::shared_ptr<std::string> const &tile_stringpool) {
 	// Add geometry to output feature
 
 	mvt_feature outfeature;
@@ -849,7 +849,7 @@ static void feature_out(std::vector<tile_feature> const &features, mvt_layer &ou
 
 			for (size_t i = 0; i < full_keys.size(); i++) {
 				if (keep.size() == 0 || keep.find(full_keys[i]) != keep.end()) {
-					outlayer.tag(outfeature, full_keys[i], stringified_to_mvt_value(full_values[i].type, full_values[i].s.c_str()));
+					outlayer.tag(outfeature, full_keys[i], stringified_to_mvt_value(full_values[i].type, full_values[i].s.c_str(), tile_stringpool));
 				}
 			}
 		} else {
@@ -875,6 +875,7 @@ std::string overzoom(const mvt_tile &tile, int oz, int ox, int oy, int nz, int n
 		     std::vector<std::pair<unsigned, unsigned>> *next_overzoomed_tiles,
 		     bool demultiply, json_object *filter, bool preserve_input_order, std::unordered_map<std::string, attribute_op> const &attribute_accum) {
 	mvt_tile outtile;
+	std::shared_ptr<std::string> tile_stringpool = std::make_shared<std::string>();
 
 	for (auto const &layer : tile.layers) {
 		mvt_layer outlayer = mvt_layer();
@@ -917,7 +918,7 @@ std::string overzoom(const mvt_tile &tile, int oz, int ox, int oy, int nz, int n
 
 			if (flush_multiplier_cluster) {
 				if (pending_tile_features.size() > 0) {
-					feature_out(pending_tile_features, outlayer, keep, attribute_accum);
+					feature_out(pending_tile_features, outlayer, keep, attribute_accum, tile_stringpool);
 					pending_tile_features.clear();
 				}
 			}
@@ -1020,7 +1021,7 @@ std::string overzoom(const mvt_tile &tile, int oz, int ox, int oy, int nz, int n
 		}
 
 		if (pending_tile_features.size() > 0) {
-			feature_out(pending_tile_features, outlayer, keep, attribute_accum);
+			feature_out(pending_tile_features, outlayer, keep, attribute_accum, tile_stringpool);
 			pending_tile_features.clear();
 		}