Doing the scan line through the segments

polygon.cpp

@@ -1,3 +1,4 @@
+#include <stdio.h>
 #include "geometry.hpp"
 
 struct point {
@@ -11,6 +12,10 @@ struct point {
 
 typedef std::pair<point, point> segment;
 
+bool intersect(std::vector<segment> &segs, size_t s1, size_t s2) {
+	return false;
+}
+
 struct scan_transition {
 	double y;
 	size_t segment;
@@ -38,21 +43,63 @@ std::vector<segment> snap_round(std::vector<segment> segs) {
 		// to find the pairs that intersect
 		// while not looking at pairs that can't possibly intersect
 
+		// index by rounded y coordinates, since we will be
+		// intersecting with rounded coordinates
+
 		std::vector<scan_transition> tops;
 		std::vector<scan_transition> bottoms;
 
 		for (size_t i = 0; i < segs.size(); i++) {
-			if (segs[i].first.y < segs[i].second.y) {
-				tops.emplace_back(segs[i].first.y, i);
-				bottoms.emplace_back(segs[i].second.y, i);
+			if (std::round(segs[i].first.y) < std::round(segs[i].second.y)) {
+				tops.emplace_back(std::round(segs[i].first.y), i);
+				bottoms.emplace_back(std::round(segs[i].second.y), i);
 			} else {
-				tops.emplace_back(segs[i].second.y, i);
-				bottoms.emplace_back(segs[i].first.y, i);
+				tops.emplace_back(std::round(segs[i].second.y), i);
+				bottoms.emplace_back(std::round(segs[i].first.y), i);
 			}
 		}
 
 		std::sort(tops.begin(), tops.end());
 		std::sort(bottoms.begin(), bottoms.end());
+
+		// do the scan
+
+		std::set<size_t> active;
+		size_t bottom = 0;
+		for (size_t i = 0; i < tops.size(); i++) {
+			// activate anything that is coming into view
+
+			active.insert(tops[i].segment);
+			if (i + 1 < tops.size() && tops[i + 1].y == tops[i].y) {
+				continue;
+			}
+
+			// look at the active segments
+
+			for (size_t s1 : active) {
+				for (size_t s2 : active) {
+					if (s1 < s2) {
+						if (intersect(segs, s1, s2)) {
+							// if the segments intersected,
+							// we need to do another scan,
+							// because introducing a new node
+							// may have caused new intersections
+							again = true;
+						}
+					}
+				}
+			}
+
+			// deactivate anything that is going out of view
+
+			if (i + 1 < tops.size()) {
+				while (bottom < bottoms.size() && bottoms[bottom].y < tops[i + 1].y) {
+					auto found = active.find(bottoms[bottom].segment);
+					active.erase(found);
+					bottom++;
+				}
+			}
+		}
 	}
 
 	return segs;
@@ -70,7 +117,7 @@ drawvec clean_polygon(drawvec const &geom, int z, int detail) {
 			size_t j;
 
 			for (j = i + 1; j < geom.size(); j++) {
-				if (geom[i].op != VT_LINETO) {
+				if (geom[j].op != VT_LINETO) {
 					break;
 				}
 			}

polygon.hpp

@@ -0,0 +1 @@
+drawvec clean_polygon(drawvec const &geom, int z, int detail);

tile.cpp

@@ -43,6 +43,7 @@
 #include "errors.hpp"
 #include "compression.hpp"
 #include "protozero/varint.hpp"
+#include "polygon.hpp"
 
 extern "C" {
 #include "jsonpull/jsonpull.h"
@@ -612,6 +613,11 @@ void *partial_feature_worker(void *v) {
 		int out_detail = (*partials)[i].extra_detail;
 
 		drawvec geom = (*partials)[i].geoms[0];
+
+		if (t == VT_POLYGON) {
+			clean_polygon(geom, z, out_detail);
+		}
+
 		to_tile_scale(geom, z, out_detail);
 
 		if (t == VT_POLYGON) {