fix a polygon clipping edge case

Port of mapbox/geojson-vt@2ba995a

Author: mourner

include/mapbox/geojsonvt/clip.hpp

@@ -127,15 +127,15 @@ class clipper {
 
                     slice = newSlice(line);
 
-                } else if (bk >= k1) { // ---|-->  |
+                } else if (bk > k1) { // ---|-->  |
                     t = calc_progress<I>(a, b, k1);
                     slice.push_back(intersect<I>(a, b, k1, t));
                     if (lineMetrics) slice.segStart = lineLen + segLen * t;
 
                     if (i == len - 2)
                         slice.push_back(b); // last point
                 }
-            } else if (ak >= k2) {
+            } else if (ak > k2) {
                 if (bk < k1) { // <--|-----|---
                     t = calc_progress<I>(a, b, k2);
                     slice.push_back(intersect<I>(a, b, k2, t));
@@ -202,7 +202,7 @@ class clipper {
             const double bk = get<I>(b);
 
             if (ak < k1) {
-                if (bk >= k1) {
+                if (bk > k1) {
                     // ---|-->  |
                     slice.push_back(intersect<I>(a, b, k1, calc_progress<I>(a, b, k1)));
                     if (bk > k2)
@@ -211,7 +211,7 @@ class clipper {
                     else if (i == len - 2)
                         slice.push_back(b); // last point
                 }
-            } else if (ak >= k2) {
+            } else if (ak > k2) {
                 if (bk < k2) { // |  <--|---
                     slice.push_back(intersect<I>(a, b, k2, calc_progress<I>(a, b, k2)));
                     if (bk < k1) // <--|-----|---

test/fixtures/polygon-bug.json

@@ -0,0 +1,10 @@
+{
+    "type": "Polygon",
+    "coordinates": [[
+        [42.1875, 57.32652122521708],
+        [47.8125, 57.32652122521708],
+        [47.8125, 54.16243396806781],
+        [42.1875, 54.16243396806781],
+        [42.1875, 57.32652122521708]
+    ]]
+}

test/test.cpp

@@ -243,6 +243,27 @@ TEST(GetTile, AntimeridianTriangle) {
     }
 }
 
+TEST(GetTile, PolygonClippingBug) {
+    const auto geojson = mapbox::geojson::parse(loadFile("test/fixtures/polygon-bug.json"));
+
+    Options options;
+    options.buffer = 1024;
+
+    GeoJSONVT index{ geojson, options };
+
+    auto tile = index.getTile(5, 19, 9);
+    ASSERT_EQ(tile.features.size(), 1);
+    ASSERT_EQ(tile.num_points, 5);
+
+    const mapbox::geometry::polygon<int16_t> expected{
+        { {3072, 3072}, {5120, 3072}, {5120, 5120}, {3072, 5120}, {3072, 3072} }
+    };
+
+    const auto actual = tile.features[0].geometry.get<mapbox::geometry::polygon<int16_t>>();
+
+    ASSERT_EQ(actual, expected);
+}
+
 TEST(GetTile, Projection) {
     const auto geojson = mapbox::geojson::parse(loadFile("test/fixtures/linestring.json"));