From 035c1a75099ad32f85c28d8ab6c985694ca43cb1 Mon Sep 17 00:00:00 2001
From: Wouter van Kleunen <wouter.van@kleunen.nl>
Date: Sat, 1 May 2021 11:19:27 +0000
Subject: [PATCH] Prune polygon to area within tile bounding box

---
 src/output_object.cpp | 80 +++++++++++++++++++++++++++++++++++--------
 1 file changed, 65 insertions(+), 15 deletions(-)

diff --git a/src/output_object.cpp b/src/output_object.cpp
index 2e7669ea..a9962e12 100644
--- a/src/output_object.cpp
+++ b/src/output_object.cpp
@@ -80,9 +80,9 @@ Geometry buildWayGeometry(OSMStore &osmStore, OutputObject const &oo, const Tile
 
 		case OutputGeometryType::LINESTRING:
 		{
+			MultiLinestring out;
 			auto const &ls = osmStore.retrieve<mmap::linestring_t>(oo.handle);
 
-			MultiLinestring out;
 			if(ls.empty()) 
 				return out;
 
@@ -95,6 +95,7 @@ Geometry buildWayGeometry(OSMStore &osmStore, OutputObject const &oo, const Tile
 						out.push_back(std::move(current_ls));
 					current_ls.clear();
 				}
+
 				geom::append(current_ls, ls[i]);
 			}
 
@@ -109,23 +110,72 @@ Geometry buildWayGeometry(OSMStore &osmStore, OutputObject const &oo, const Tile
 			auto const &mp = osmStore.retrieve<mmap::multi_polygon_t>(oo.handle);
 
 			Polygon clippingPolygon;
-
 			geom::convert(bbox.clippingBox, clippingPolygon);
-			if (!geom::intersects(mp, clippingPolygon)) { return MultiPolygon(); }
-			if (geom::within(mp, clippingPolygon)) { 
-				MultiPolygon out;
-				boost::geometry::assign(out, mp);
-				return out; 
-			}
 
-			try {
-				MultiPolygon out;
-				geom::intersection(mp, clippingPolygon, out);
-				return out;
-			} catch (geom::overlay_invalid_input_exception &err) {
-				std::cout << "Couldn't clip polygon (self-intersection)" << std::endl;
-				return MultiPolygon(); // blank
+			MultiPolygon out;
+			for(auto const &p: mp) {
+				Polygon newp;
+				if(geom::within(p.outer, clippingPolygon)) {
+					boost::geometry::assign(newp, p);
+					out.push_back(newp);
+					continue;
+				}
+
+				for(auto const &inner: p.inners) {
+					if(!inner.empty()) {
+						Ring ring;
+						geom::append(ring, inner[0]);
+						for(std::size_t i = 1; i < inner.size(); ++i) {
+							if(geom::within(inner[i], bbox.clippingBox)) {
+								geom::append(ring, inner[i]);
+								continue;
+							}
+
+							std::size_t next_i = (i + 1) % inner.size();
+							Linestring ls({ ring.back(), inner[next_i] });
+
+							if(!geom::within(ls, inner) || geom::intersects(ls, bbox.clippingBox)) {
+								geom::append(ring, inner[i]);
+							}
+						}
+
+						if(ring.size() >= 3) {
+							newp.inners().resize(newp.inners().size() + 1);
+							boost::geometry::assign(newp.inners().back(), ring);
+						}
+					}
+				}
+			
+				Ring outer_ring;
+				geom::append(outer_ring, p.outer[0]);
+				for(std::size_t i = 1; i < p.outer.size(); ++i) {
+					if(geom::within(p.outer[i], bbox.clippingBox)) {
+						geom::append(outer_ring, p.outer[i]);
+						continue;
+					}
+
+					std::size_t next_i = (i + 1) % p.outer.size();
+					Linestring ls({ outer_ring.back(), p.outer[next_i] });
+					if(!geom::within(ls, p.outer)) {
+						geom::append(outer_ring, p.outer[i]);
+						continue;
+					}
+
+					bool intersects = geom::intersects(ls, bbox.clippingBox);
+					for(auto const &inner: newp.inners()) {
+						intersects |= geom::intersects(ls, inner);
+					}
+
+					if(intersects) {
+						geom::append(outer_ring, p.outer[i]);
+					}
+				}
+
+				boost::geometry::assign(newp.outer(), outer_ring);
+				out.push_back(newp);
 			}
+
+			return out;
 		}
 
 		default: