Coalesce geometries of features with the same attributes

README.md

@@ -51,8 +51,8 @@ in some places), so I need to figure out an equitable way to throw features away
 It also throws away any polygons that are too small to draw. I'm not sure yet
 if it is appropriate to do more than that.
 
-It should consolidate features in the same tile that share the same type and attributes,
+Features in the same tile that share the same type and attributes are coalesced
-to make the tile data smaller, but doesn't do that yet.
+together into a single geometry.
 
 Development
 -----------

tile.cc

@@ -446,6 +446,27 @@ struct coalesce {
 	int *meta;
 };
 
+int coalcmp(const void *v1, const void *v2) {
+	const struct coalesce *c1 = (const struct coalesce *) v1;
+	const struct coalesce *c2 = (const struct coalesce *) v2;
+
+	int cmp = c1->type - c2->type;
+	if (cmp != 0) {
+		return cmp;
+	}
+
+	int i;
+	for (i = 0; i < c1->nmeta && i < c2->nmeta; i++) {
+		cmp = c1->meta[i] - c2->meta[i];
+
+		if (cmp != 0) {
+			return cmp;
+		}
+	}
+
+	return c1->nmeta - c2->nmeta;
+}
+
 long long write_tile(struct index *start, struct index *end, char *metabase, unsigned *file_bbox, int z, unsigned tx, unsigned ty, int detail, int basezoom, struct pool *file_keys, char *layername, sqlite3 *outdb) {
 	GOOGLE_PROTOBUF_VERIFY_VERSION;
 
@@ -554,7 +575,28 @@ long long write_tile(struct index *start, struct index *end, char *metabase, uns
 		}
 	}
 
+	qsort(features, nfeatures, sizeof(struct coalesce), coalcmp);
 	int x;
+
+	int out = 0;
+	for (x = 0; x < nfeatures; x++) {
+		if (out > 0 && features[out - 1].ngeom + features[x].ngeom < 20000 && coalcmp(&features[x], &features[out - 1]) == 0) {
+			struct draw *tmp = (struct draw *) malloc((features[x].ngeom + features[out - 1].ngeom) * sizeof(struct draw));
+			memcpy(tmp, features[out - 1].geom, features[out - 1].ngeom * sizeof(struct draw));
+			memcpy(tmp + features[out - 1].ngeom, features[x].geom, features[x].ngeom * sizeof(struct draw));
+
+			free(features[x].geom);
+			free(features[out - 1].geom);
+			free(features[x].meta);
+
+			features[out - 1].ngeom += features[x].ngeom;
+			features[out - 1].geom = tmp;
+		} else {
+			features[out++] = features[x];
+		}
+	}
+	nfeatures = out;
+
 	for (x = 0; x < nfeatures; x++) {
 		mapnik::vector::tile_feature *feature = layer->add_features();