I'm not sure how this ever worked before

drop.hpp

@@ -8,7 +8,7 @@
 // Note that the fields are in a specific order so that `segment` and `t` will
 // packed together with `seq` so that the total structure size will be only 32 bytes
 // instead of 40. (Could we save a few more, perhaps, by tracking `len` instead of
-// `end` and limiting the size of individual features to 32 bits?)
+// `end` and limiting the size of individual features to 2^32 bytes?)
 
 struct index {
 	// first and last+1 byte of the feature in the geometry temp file
@@ -25,7 +25,7 @@ struct index {
 	unsigned short t : 2;
 
 	// sequence number (sometimes with gaps in numbering) of the feature in the original input file
-	unsigned long long seq : (64 - 18);  // pack with segment and t to stay in 32 bytes
+	unsigned long long seq : (64 - 16 - 2);	 // pack with segment and t to stay in 32 bytes
 
 	index()
 	    : t(0),
@@ -33,15 +33,33 @@ struct index {
 	}
 };
 
+// Each zoom level has a drop_state that is used to account for the fraction of
+// point features that are supposed to be dropped in that zoom level. As it goes
+// through the spatially-sorted features, it is basically doing a diffusion dither
+// to keep the density of features in each vicinity at each zoom level
+// approximately correct by including or excluding individual features
+// to maintain the balance.
+
 struct drop_state {
-	double gap;
+	// the z-index or hilbert index of the last feature that was placed in this zoom level
 	unsigned long long previndex;
+
+	// the preservation rate (1 or more) for features in this zoom level.
+	// 1 would be to keep all the features; 2 would drop every other feature;
+	// 4 every fourth feature, and so on.
 	double interval;
-	double seq;  // floating point because interval is
+
+	// the current accumulated error in this zoom level:
+	// positive if too many features have been dropped;
+	// negative if not enough features have been dropped.
+	//
+	// this is floating-point because the interval is.
+	double error;
 };
 
 extern unsigned long long preserve_point_density_threshold;
 
-int calc_feature_minzoom(struct index *ix, struct drop_state *ds, int maxzoom, double gamma);
+int calc_feature_minzoom(struct index *ix, struct drop_state ds[], int maxzoom, double gamma);
+void prep_drop_states(struct drop_state ds[], int maxzoom, int basezoom, double droprate);
 
 #endif