Factoring out dot-dropping for unit testing

drop.hpp

@@ -0,0 +1,47 @@
+#ifndef DROP_HPP
+#define DROP_HPP
+
+// As features are read during the input phase, each one is represented by
+// an index entry giving its geometry type, its spatial index, and the location
+// in the geometry file of the rest of its data.
+//
+// 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?)
+
+struct index {
+	// first and last+1 byte of the feature in the geometry temp file
+	long long start = 0;
+	long long end = 0;
+
+	// z-index or hilbert index of the feature
+	unsigned long long ix = 0;
+
+	// which thread's geometry temp file this feature is in
+	short segment = 0;
+
+	// geometry type
+	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
+
+	index()
+	    : t(0),
+	      seq(0) {
+	}
+};
+
+struct drop_state {
+	double gap;
+	unsigned long long previndex;
+	double interval;
+	double seq;  // floating point because interval is
+};
+
+extern unsigned long long preserve_point_density_threshold;
+
+int calc_feature_minzoom(struct index *ix, struct drop_state *ds, int maxzoom, double gamma);
+
+#endif