Add point retention multiplier (#179)

* Add an option to retain N times as many points as usual at each zoom

* Tests for point multipler with specified and guessed maxzooms

* Work in progress on inverse spatial ordering

* Fix inverse spatial feature order

* --reorder was depending on a feature index that wasn't being preserved

* Separate ordering by feature_minzoom from ordering inverse-spatially

* Add a test for the inverse spatial ordering

* Store the basezoom/droprate/multiplier decisions in tileset metadata

* Progress on adding filters to tippecanoe-overzoom

* Type promotion for comparison

* Look up the attribute value for ordering

* Add test of thinning and ordering features

* Plumb tippecanoe_decisions metadata through pmtiles

* Be careful not to put infinities in JSON

* Fix accidental dropping in what is meant to preserve sparse points

* Start distinguishing true, false, and null in expressions

* Most of the type conversions

* Add boolean conversions

* Literals and conjunctions

* Add filtering to tippecanoe-overzoom

* Add a test of filtering in overzoom

* Fix boolean conjunctions

* Handle the combination of cluster size and filtering

* Rework dot dropping to reconcile density threshold and multiplier

* Revert "Rework dot dropping to reconcile density threshold and multiplier"

This reverts commit f253a66382.

* Retain points by multiplier within each tile, not in global probability

* Test that intends to verify that the multiplier is reversible

* Get the test to detect the discrepancy

* Mark the start of multiplier clusters with a magic attribute

* Add string-contains

* Add in and ni operators

* Revert "Look up the attribute value for ordering"

This reverts commit 56bc73e49a.

* Revert "Type promotion for comparison"

This reverts commit 6f3256f5af.

* Make number formatting in tippecanoe_decisions consistent

* Revert "Add a test for the inverse spatial ordering"

This reverts commit c8047de9ab.

* Revert "Separate ordering by feature_minzoom from ordering inverse-spatially"

This reverts commit 35b19a223c.

* Revert "Fix inverse spatial feature order"

This reverts commit 5978ecdb44.

* Revert "Work in progress on inverse spatial ordering"

This reverts commit fdf230f632.

* Somehow missed the tests associated with that last revert

* Round-robin assign attributes to partials from across the multiplier

* Count the multiplier separately in each layer

* Fix distribution of accumulated attribute across multiplier features

* Update changelog, version, and docs

* Add "is null" and "isnt null" expressions

* Update interpretation of FSL expressions to pass the tests

* Test to assert that polygons are unaffected by the multiplier

* Clean up and comment

* Remove accidental unused case

pmtiles_file.cpp

@@ -103,6 +103,13 @@ std::string metadata_to_pmtiles_json(metadata m) {
 		state.json_write_string("strategies");
 		state.json_write_json(m.strategies_json);
 	}
+
+	if (m.decisions_json.size() > 0) {
+		state.json_comma_newline();
+		state.json_write_string("tippecanoe_decisions");
+		state.json_write_json(m.decisions_json);
+	}
+
 	out(state, "generator", m.generator);
 	out(state, "generator_options", m.generator_options);
 
@@ -428,6 +435,12 @@ sqlite3 *pmtilesmeta2tmp(const char *fname, const char *pmtiles_map) {
 				fprintf(stderr, "set %s in metadata: %s\n", key, err);
 			}
 			sqlite3_free(sql);
+		} else if (strcmp(key, "tippecanoe_decisions") == 0 && o->value.object.values[i]->type == JSON_HASH) {
+			sql = sqlite3_mprintf("INSERT INTO metadata (name, value) VALUES ('tippecanoe_decisions', %Q);", json_stringify(o->value.object.values[i]));
+			if (sqlite3_exec(db, sql, NULL, NULL, &err) != SQLITE_OK) {
+				fprintf(stderr, "set %s in metadata: %s\n", key, err);
+			}
+			sqlite3_free(sql);
 		} else if (o->value.object.keys[i]->type != JSON_STRING || o->value.object.values[i]->type != JSON_STRING) {
 			fprintf(stderr, "%s\n", key);
 			fprintf(stderr, "%s: non-string in metadata\n", fname);