Compare absolute value of polygon ring area

src/mbgl/tile/geometry_tile_data.cpp

@@ -127,7 +127,7 @@ void limitHoles(GeometryCollection& polygon, uint32_t maxHoles) {
                          polygon.begin() + 1 + maxHoles,
                          polygon.end(),
                          [] (const auto& a, const auto& b) {
-                             return signedArea(a) > signedArea(b);
+                             return std::fabs(signedArea(a)) > std::fabs(signedArea(b));
                          });
         polygon.resize(1 + maxHoles);
     }

test/tile/geometry_tile_data.test.cpp

@@ -3,6 +3,18 @@
 
 using namespace mbgl;
 
+static double _signedArea(const GeometryCoordinates& ring) {
+    double sum = 0;
+
+    for (std::size_t i = 0, len = ring.size(), j = len - 1; i < len; j = i++) {
+        const GeometryCoordinate& p1 = ring[i];
+        const GeometryCoordinate& p2 = ring[j];
+        sum += (p2.x - p1.x) * (p1.y + p2.y);
+    }
+
+    return sum;
+}
+
 TEST(GeometryTileData, classifyRings1) {
     std::vector<GeometryCollection> polygons = classifyRings({
       { {0, 0}, {0, 40}, {40, 40}, {40, 0}, {0, 0} }
@@ -59,3 +71,37 @@ TEST(GeometryTileData, limitHoles2) {
     ASSERT_EQ(polygon[0][0].x, 0);
     ASSERT_EQ(polygon[1][0].x, 10);
 }
+
+TEST(GeometryTileData, limitHoles3) {
+    // real world polygon with interior rings with negative areas
+    // that need to be sorted in `limitHoles` by comparing absolute
+    // area not signed
+    GeometryCollection polygon = {
+     { {7336,-248},{7304,-248},{7272,-168},{7176,-200},{7080,-136},{7048,-56},{7128,-8},{7176,-56},{7288,-56},{7316,0},{6918,0},{6904,-40},{6984,-72},{6952,-88},{6952,-168},{6888,-88},{6856,-88},{6856,-8},{6872,0},{6170,0},{6184,-40},{6136,-72},{6104,-56},{6132,0},{6028,0},{6104,-152},{6184,-200},{6206,-256},{6272,-256},{6264,-248},{6248,-120},{6280,-136},{6280,-232},{6288,-256},{6790,-256},{6792,-248},{6800,-256},{7058,-256},{7064,-248},{7096,-256},{7338,-256},{7336,-248} },
+     { {6344,-104},{6264,-8},{6392,-72},{6360,-200},{6344,-104} },
+     { {6744,-24},{6760,-72},{6728,-104},{6744,-24} },
+     { {6616,-104},{6648,-88},{6632,-72},{6664,-56},{6664,-120},{6616,-104} }
+    };
+
+    // make a copy for later testing
+    GeometryCollection original(polygon);
+
+    ASSERT_EQ(polygon.size(), 4u);
+    ASSERT_EQ(_signedArea(polygon.at(0)), 515360); // exterior
+    ASSERT_EQ(_signedArea(polygon.at(1)), -12288); // biggest interior ring
+    ASSERT_EQ(_signedArea(polygon.at(2)), -2048);  // smallest interior ring
+    ASSERT_EQ(_signedArea(polygon.at(3)), -3072);  // second largest interior ring
+
+    limitHoles(polygon, 2);
+
+    // output: polygon 1 has 1 exterior, 2 interior
+    ASSERT_EQ(polygon.size(), 3u);
+
+    // ensure we've kept the exterior ring
+    ASSERT_EQ(original.at(0), polygon.at(0));
+
+    // ensure we've kept the two largest interior rings
+    ASSERT_EQ(original.at(1), polygon.at(1));
+    ASSERT_EQ(original.at(3), polygon.at(2));
+
+}