Externalizing polygon shard detection (#146)

* Start of externalizing polygon shard detection

* Completely untested external quicksort

* Add unit test for external quicksort

* Remember to clean up temporary files

* Sort and scan the vertices

* Bring over more vertex logic

* Make nodes from vertices

* Checkpoint on switching over to global shared nodes

* Do the thing

* Revert unintended change to coalesced linestring behavior

* Take shared nodes into account in early simplification

* Let it do more sorting in memory

* Fix overnoding of collinear linestrings

* Remove duplicate nodes, since only duplicate vertices now matter

* Remember to delete temporary files

* Fix out of bounds memory access below, apparently

* Fix the actual undefined behavior

* Still running out of memory in one case. Find out where.

* Forgot the conditional

* Try again to make it not run out of memory

* Update version and changelog

geometry.cpp

@@ -485,7 +485,7 @@ drawvec impose_tile_boundaries(drawvec &geom, long long extent) {
 	return out;
 }
 
-drawvec simplify_lines(drawvec &geom, int z, int detail, bool mark_tile_bounds, double simplification, size_t retain, drawvec const &shared_nodes) {
+drawvec simplify_lines(drawvec &geom, int z, int tx, int ty, int detail, bool mark_tile_bounds, double simplification, size_t retain, drawvec const &shared_nodes, struct node *shared_nodes_map, size_t nodepos) {
 	int res = 1 << (32 - detail - z);
 	long long area = 1LL << (32 - z);
 
@@ -501,10 +501,35 @@ drawvec simplify_lines(drawvec &geom, int z, int detail, bool mark_tile_bounds,
 		}
 
 		if (prevent[P_SIMPLIFY_SHARED_NODES]) {
+			// This is kind of weird, because we have two lists of shared nodes to look through:
+			// * the drawvec, which is nodes that were introduced during clipping to the tile edge,
+			//   and which are in local tile coordinates
+			// * the shared_nodes_map, which was made globally before tiling began, and which
+			//   is in global quadkey coordinates.
+			// To look through the latter, we need to offset and encode the coordinates
+			// of the feature we are simplifying.
+
 			auto pt = std::lower_bound(shared_nodes.begin(), shared_nodes.end(), geom[i]);
 			if (pt != shared_nodes.end() && *pt == geom[i]) {
 				geom[i].necessary = true;
 			}
+
+			if (nodepos > 0) {
+				// offset to global
+				draw d = geom[i];
+				if (z != 0) {
+					d.x += tx * (1LL << (32 - z));
+					d.y += ty * (1LL << (32 - z));
+				}
+
+				// to quadkey
+				struct node n;
+				n.index = encode_quadkey((unsigned) d.x, (unsigned) d.y);
+
+				if (bsearch(&n, shared_nodes_map, nodepos / sizeof(node), sizeof(node), nodecmp) != NULL) {
+					geom[i].necessary = true;
+				}
+			}
 		}
 	}
 
@@ -661,6 +686,9 @@ drawvec fix_polygon(drawvec &geom) {
 				ring = tmp;
 			}
 
+			// Now we are rotating the ring to make the first/last point
+			// one that would be unlikely to be simplified away.
+
 			// calculate centroid
 			// a + 1 < size() because point 0 is duplicated at the end
 			long long xtotal = 0;