From 6b70e2aebd354d7d360253ecacb633f3f33df66f Mon Sep 17 00:00:00 2001
From: Eric Fischer <enf@pobox.com>
Date: Wed, 25 Apr 2018 14:59:41 +0200
Subject: [PATCH] Tag all LineString crossings of tile boundaries

---
 geometry.cpp | 59 ++++++++++++++++++++++++++++++++++++++++++++++++++--
 geometry.hpp |  2 +-
 tile.cpp     |  2 +-
 3 files changed, 59 insertions(+), 4 deletions(-)

diff --git a/geometry.cpp b/geometry.cpp
index acc1576..1b1ea6f 100644
--- a/geometry.cpp
+++ b/geometry.cpp
@@ -790,11 +790,66 @@ bool point_within_tile(long long x, long long y, int z) {
 	return x >= 0 && y >= 0 && x < area && y < area;
 }
 
-drawvec clip_lines(drawvec &geom, int z, long long buffer) {
+drawvec mark_tile_edges(drawvec geom, long long width, long long *pointid) {
+	drawvec out;
+
+	for (size_t i = 0; i < geom.size(); i++) {
+		if (geom[i].op == VT_LINETO) {
+			if (geom[i - 1].x / width != geom[i].x / width) {
+				long long min, max;
+
+				if (geom[i - 1].x < geom[i].x) {
+					for (long long xx = geom[i - 1].x / width * width + width; xx <= geom[i].x / width * width; xx += width) {
+						long long yy = (xx - geom[i - 1].x) * (geom[i].y - geom[i - 1].y) / (geom[i].x - geom[i - 1].x) + geom[i - 1].y;
+						out.push_back(draw(VT_LINETO, xx, yy, ++*pointid));
+					}
+				} else {
+					for (long long xx = geom[i].x / width * width + width; xx <= geom[i - 1].x / width * width; xx += width) {
+						long long yy = (xx - geom[i].x) * (geom[i - 1].y - geom[i].y) / (geom[i - 1].x - geom[i].x) + geom[i].y;
+						out.push_back(draw(VT_LINETO, xx, yy, ++*pointid));
+					}
+				}
+			}
+		}
+
+		out.push_back(geom[i]);
+	}
+
+	geom = out;
+
+	for (size_t i = 0; i < geom.size(); i++) {
+		if (geom[i].op == VT_LINETO) {
+			if (geom[i - 1].y / width != geom[i].y / width) {
+				long long min, may;
+
+				if (geom[i - 1].y < geom[i].y) {
+					for (long long yy = geom[i - 1].y / width * width + width; yy <= geom[i].y / width * width; yy += width) {
+						long long xx = (yy - geom[i - 1].y) * (geom[i].x - geom[i - 1].x) / (geom[i].y - geom[i - 1].y) + geom[i - 1].x;
+						out.push_back(draw(VT_LINETO, xx, yy, ++*pointid));
+					}
+				} else {
+					for (long long yy = geom[i].y / width * width + width; yy <= geom[i - 1].y / width * width; yy += width) {
+						long long xx = (yy - geom[i].y) * (geom[i - 1].x - geom[i].x) / (geom[i - 1].y - geom[i].y) + geom[i].x;
+						out.push_back(draw(VT_LINETO, xx, yy, ++*pointid));
+					}
+				}
+			}
+		}
+
+		out.push_back(geom[i]);
+	}
+
+	return out;
+}
+
+drawvec clip_lines(drawvec geom, int z, long long buffer, long long *pointid) {
 	drawvec out;
 
 	long long min = 0;
 	long long area = 1LL << (32 - z);
+
+	geom = mark_tile_edges(geom, area, pointid);
+
 	min -= buffer * area / 256;
 	area += buffer * area / 256;
 
@@ -974,7 +1029,7 @@ drawvec simplify_lines(drawvec &geom, int z, int detail, bool mark_tile_bounds,
 	}
 
 	if (mark_tile_bounds) {
-		geom = impose_tile_boundaries(geom, area, pointid);
+		// geom = impose_tile_boundaries(geom, area, pointid);
 	}
 
 	for (size_t i = 0; i < geom.size(); i++) {
diff --git a/geometry.hpp b/geometry.hpp
index d3856ad..362367e 100644
--- a/geometry.hpp
+++ b/geometry.hpp
@@ -74,7 +74,7 @@ drawvec clean_or_clip_poly(drawvec &geom, int z, int buffer, bool clip);
 drawvec simple_clip_poly(drawvec &geom, int z, int buffer);
 drawvec close_poly(drawvec &geom);
 drawvec reduce_tiny_poly(drawvec &geom, int z, int detail, bool *reduced, double *accum_area);
-drawvec clip_lines(drawvec &geom, int z, long long buffer);
+drawvec clip_lines(drawvec geom, int z, long long buffer, long long *pointid);
 drawvec stairstep(drawvec &geom, int z, int detail);
 bool point_within_tile(long long x, long long y, int z);
 int quick_check(long long *bbox, int z, long long buffer);
diff --git a/tile.cpp b/tile.cpp
index fd94ce7..f0183dc 100644
--- a/tile.cpp
+++ b/tile.cpp
@@ -531,7 +531,7 @@ void *partial_feature_worker(void *v) {
 		if (t == VT_LINE) {
 			for (size_t g = 0; g < geoms.size(); g++) {
 				from_tile_scale(geoms[g], z, line_detail);
-				geoms[g] = clip_lines(geoms[g], z, (*partials)[i].buffer);
+				geoms[g] = clip_lines(geoms[g], z, (*partials)[i].buffer, (*partials)[i].pointid);
 				to_tile_scale(geoms[g], z, line_detail);
 			}
 		}