Add an option to convert double-precision attributes to single

Makefile

@@ -351,7 +351,7 @@ layer-json-test:
 prep-test: $(TESTS)
 
 tests/%.json: Makefile tippecanoe tippecanoe-decode
-	./tippecanoe -aE -q -a@ -f -o $@.check.mbtiles $(subst @,:,$(subst %,/,$(subst _, ,$(patsubst %.json,%,$(word 4,$(subst /, ,$@)))))) $(foreach suffix,$(suffixes),$(sort $(wildcard $(subst $(SPACE),/,$(wordlist 1,2,$(subst /, ,$@)))/*.$(suffix))))
+	./tippecanoe -q -a@ -f -o $@.check.mbtiles $(subst @,:,$(subst %,/,$(subst _, ,$(patsubst %.json,%,$(word 4,$(subst /, ,$@)))))) $(foreach suffix,$(suffixes),$(sort $(wildcard $(subst $(SPACE),/,$(wordlist 1,2,$(subst /, ,$@)))/*.$(suffix))))
 	./tippecanoe-decode -x generator $@.check.mbtiles > $@
 	cmp $(patsubst %.check,%,$@) $@
 	rm $@.check.mbtiles

README.md

@@ -411,6 +411,7 @@ be reduced to the maximum that can be used with the specified _maxzoom_.
  * `-pe` or `--empty-csv-columns-are-null`: Treat empty CSV columns as nulls rather than as empty strings.
  * `-aI` or `--convert-stringified-ids-to-numbers`: If a feature ID is the string representation of a number, convert it to a plain number to use as the feature ID.
  * `--use-attribute-for-id=`*name*: Use the attribute with the specified *name* as if it were specified as the feature ID. (If this attribute is a stringified number, you must also use `-aI` to convert it to a number.)
+ * `-pN` or `--single-precision`: Write double-precision numeric attribute values to tiles as single-precision to reduce tile size.
 
 ### Filtering features by attributes
 

main.cpp

@@ -2667,6 +2667,7 @@ int main(int argc, char **argv) {
 		{"empty-csv-columns-are-null", no_argument, &prevent[P_EMPTY_CSV_COLUMNS], 1},
 		{"convert-stringified-ids-to-numbers", no_argument, &additional[A_CONVERT_NUMERIC_IDS], 1},
 		{"use-attribute-for-id", required_argument, 0, '~'},
+		{"single-precision", required_argument, &prevent[P_SINGLE_PRECISION], 1},
 
 		{"Filtering features by attributes", 0, 0, 0},
 		{"feature-filter-file", required_argument, 0, 'J'},

man/tippecanoe.1

@@ -513,6 +513,8 @@ to specify how the named \fIattribute\fP is accumulated onto the attribute of th
 \fB\fC\-aI\fR or \fB\fC\-\-convert\-stringified\-ids\-to\-numbers\fR: If a feature ID is the string representation of a number, convert it to a plain number to use as the feature ID.
 .IP \(bu 2
 \fB\fC\-\-use\-attribute\-for\-id=\fR\fIname\fP: Use the attribute with the specified \fIname\fP as if it were specified as the feature ID. (If this attribute is a stringified number, you must also use \fB\fC\-aI\fR to convert it to a number.)
+.IP \(bu 2
+\fB\fC\-pN\fR or \fB\fC\-\-single\-precision\fR: Write double\-precision numeric attribute values to tiles as single\-precision to reduce tile size.
 .RE
 .SS Filtering features by attributes
 .RS

options.hpp

@@ -30,6 +30,7 @@
 #define P_SIMPLIFY ((int) 's')
 #define P_SIMPLIFY_LOW ((int) 'S')
 #define P_SIMPLIFY_SHARED_NODES ((int) 'n')
+#define P_SINGLE_PRECISION ((int) 'N')
 #define P_FEATURE_LIMIT ((int) 'f')
 #define P_KILOBYTE_LIMIT ((int) 'k')
 #define P_DYNAMIC_DROP ((int) 'd')

serial.cpp

@@ -596,6 +596,15 @@ int serialize_feature(struct serialization_state *sst, serial_feature &sf) {
 	for (ssize_t i = (ssize_t) sf.full_keys.size() - 1; i >= 0; i--) {
 		coerce_value(sf.full_keys[i], sf.full_values[i].type, sf.full_values[i].s, sst->attribute_types);
 
+		if (prevent[P_SINGLE_PRECISION]) {
+			if (sf.full_values[i].type == mvt_double) {
+				// don't coerce integers to floats, since that is counterproductive
+				if (sf.full_values[i].s.find('.') != std::string::npos) {
+					sf.full_values[i].s = milo::dtoa_milo((float) atof(sf.full_values[i].s.c_str()));
+				}
+			}
+		}
+
 		if (sf.full_keys[i] == attribute_for_id) {
 			if (sf.full_values[i].type != mvt_double && !additional[A_CONVERT_NUMERIC_IDS]) {
 				static bool warned = false;