Limit decay into polygon dust to reasonably compact polygons

geometry.cpp

@@ -176,6 +176,18 @@ void check_polygon(drawvec &geom) {
 	}
 }
 
+double get_perimeter(const drawvec &geom, size_t i, size_t j) {
+	double perimeter = 0;
+
+	for (size_t k = i; k + 1 < j; k++) {
+		double dx = geom[k].x - geom[k + 1].x;
+		double dy = geom[k].y - geom[k + 1].y;
+		perimeter += sqrt(dx * dx + dy * dy);
+	}
+
+	return perimeter;
+}
+
 drawvec reduce_tiny_poly(drawvec &geom, int z, int detail, bool *still_needs_simplification, bool *reduced_away, double *accum_area, serial_feature *this_feature, serial_feature *tiny_feature) {
 	drawvec out;
 	const double pixel = (1LL << (32 - detail - z)) * (double) tiny_polygon_size;
@@ -196,6 +208,17 @@ drawvec reduce_tiny_poly(drawvec &geom, int z, int detail, bool *still_needs_sim
 			}
 
 			double area = get_area(geom, i, j);
+			double schwartzberg = 1;
+
+			if (area != 0) {
+				// polygon compactness measure
+				// https://fisherzachary.github.io/public/r-output.html
+				// Schwartzberg, Joseph E. 1965. “Reapportionment, gerrymanders, and the notion of compactness”.
+				// In: Minn. L. Rev. 50, 443.
+
+				double perimeter = get_perimeter(geom, i, j);
+				schwartzberg = 1 / (perimeter / (2 * M_PI * sqrt(std::fabs(area) / M_PI)));
+			}
 
 			// XXX There is an ambiguity here: If the area of a ring is 0 and it is followed by holes,
 			// we don't know whether the area-0 ring was a hole too or whether it was the outer ring
@@ -213,7 +236,8 @@ drawvec reduce_tiny_poly(drawvec &geom, int z, int detail, bool *still_needs_sim
 				// OR it is an inner ring and we haven't output an outer ring for it to be
 				// cut out of, so we are just subtracting its area from the tiny polygon
 				// rather than trying to deal with it geometrically
-				if ((area > 0 && area <= pixel * pixel) || (area < 0 && !included_last_outer)) {
+				if ((area > 0 && area <= pixel * pixel && schwartzberg > 0.3) ||
+				    (area < 0 && !included_last_outer)) {
 					*accum_area += area;
 					*reduced_away = true;