Factoring out dot-dropping for unit testing

drop.cpp

@@ -0,0 +1,58 @@
+#include "drop.hpp"
+#include "options.hpp"
+#include "geometry.hpp"
+
+unsigned long long preserve_point_density_threshold = 0;
+
+int calc_feature_minzoom(struct index *ix, struct drop_state *ds, int maxzoom, double gamma) {
+	int feature_minzoom = 0;
+
+	if (gamma >= 0 && (ix->t == VT_POINT ||
+			   (additional[A_LINE_DROP] && ix->t == VT_LINE) ||
+			   (additional[A_POLYGON_DROP] && ix->t == VT_POLYGON))) {
+		for (ssize_t i = maxzoom; i >= 0; i--) {
+			ds[i].seq++;
+		}
+		ssize_t chosen = maxzoom + 1;
+		for (ssize_t i = maxzoom; i >= 0; i--) {
+			if (ds[i].seq < 0) {
+				feature_minzoom = i + 1;
+
+				// The feature we are pushing out
+				// appears in zooms i + 1 through maxzoom,
+				// so track where that was so we can make sure
+				// not to cluster something else that is *too*
+				// far away into it.
+				for (ssize_t j = i + 1; j <= maxzoom; j++) {
+					ds[j].previndex = ix->ix;
+				}
+
+				chosen = i + 1;
+				break;
+			} else {
+				ds[i].seq -= ds[i].interval;
+			}
+		}
+
+		// If this feature has been chosen only for a high zoom level,
+		// check whether at a low zoom level it is nevertheless too far
+		// from the last feature chosen for that low zoom, in which case
+		// we will go ahead and push it out.
+
+		if (preserve_point_density_threshold > 0) {
+			for (ssize_t i = 0; i < chosen && i < maxzoom; i++) {
+				if (ix->ix - ds[i].previndex > ((1LL << (32 - i)) / preserve_point_density_threshold) * ((1LL << (32 - i)) / preserve_point_density_threshold)) {
+					feature_minzoom = i;
+
+					for (ssize_t j = i; j <= maxzoom; j++) {
+						ds[j].previndex = ix->ix;
+					}
+
+					break;
+				}
+			}
+		}
+	}
+
+	return feature_minzoom;
+}