[core] coveredByChildren is false if at least one child is uncovered

2 files changed, 55 insertions, 5 deletions

diff --git a/src/mbgl/algorithm/covered_by_children.hpp b/src/mbgl/algorithm/covered_by_children.hpp
index ad2f1dd5dd..fe5af3f3db 100644
--- a/src/mbgl/algorithm/covered_by_children.hpp
+++ b/src/mbgl/algorithm/covered_by_children.hpp
@@ -8,15 +8,23 @@ namespace algorithm {
 template <typename Iterator>
 bool coveredByChildren(const UnwrappedTileID& id, Iterator it, const Iterator& end) {
     for (const auto& child : id.children()) {
-        it = std::lower_bound(it, end, child, [](auto& a, auto& b) { return std::get<0>(a) < b; });
+        it = std::lower_bound(it, end, child, [](const auto& a, const auto& b) { return std::get<0>(a) < b; });
+
+        // Child is not present, neither its grandchildren.
         if (it == end) {
             return false;
-        } else if (std::get<0>(*it) != child) {
-            return coveredByChildren(child, it, end);
+        }
+
+        // Child is not present, but its grandchildren are.
+        if (std::get<0>(*it) != child) {
+            // This child is not covered by its grandchildren.
+            if (!coveredByChildren(child, it, end)) {
+                return false;
+            }
         }
     }
 
-    // We looked at all four immediate children and verified that they're covered.
+    // We looked at all four children (recursively) and verified that they're covered.
     return true;
 }
 
diff --git a/test/algorithm/covered_by_children.test.cpp b/test/algorithm/covered_by_children.test.cpp
index b2a219bf29..84f5aa6a21 100644
--- a/test/algorithm/covered_by_children.test.cpp
+++ b/test/algorithm/covered_by_children.test.cpp
@@ -8,6 +8,48 @@ using namespace mbgl;
 
 using List = std::map<UnwrappedTileID, bool>;
 
+TEST(CoveredByChildren, GrandChildren) {
+    const List list {
+        { UnwrappedTileID{ 0, 0, 0 }, true },
+        // These grandchildren covers 1/0/0, but 1/0/0 only covers a quarter of
+        // 0/0/0.
+        { UnwrappedTileID{ 2, 0, 0 }, true },
+        { UnwrappedTileID{ 2, 0, 1 }, true },
+        { UnwrappedTileID{ 2, 1, 0 }, true },
+        { UnwrappedTileID{ 2, 1, 1 }, true },
+    };
+    EXPECT_FALSE(algorithm::coveredByChildren(UnwrappedTileID{ 0, 0, 0 }, list));
+
+    const List list2 {
+        { UnwrappedTileID{ 0, 0, 0 }, true },
+
+        // Children of 1/0/0
+        { UnwrappedTileID{ 2, 0, 0 }, true },
+        { UnwrappedTileID{ 2, 0, 1 }, true },
+        { UnwrappedTileID{ 2, 1, 0 }, true },
+        { UnwrappedTileID{ 2, 1, 1 }, true },
+
+        // Children of 1/0/1
+        { UnwrappedTileID{ 2, 0, 2 }, true },
+        { UnwrappedTileID{ 2, 0, 3 }, true },
+        { UnwrappedTileID{ 2, 1, 2 }, true },
+        { UnwrappedTileID{ 2, 1, 3 }, true },
+
+        // Children of 1/1/0
+        { UnwrappedTileID{ 2, 2, 0 }, true },
+        { UnwrappedTileID{ 2, 2, 1 }, true },
+        { UnwrappedTileID{ 2, 3, 0 }, true },
+        { UnwrappedTileID{ 2, 3, 1 }, true },
+
+        // Children of 1/0/1
+        { UnwrappedTileID{ 2, 2, 2 }, true },
+        { UnwrappedTileID{ 2, 2, 3 }, true },
+        { UnwrappedTileID{ 2, 3, 2 }, true },
+        { UnwrappedTileID{ 2, 3, 3 }, true },
+    };
+    EXPECT_TRUE(algorithm::coveredByChildren(UnwrappedTileID{ 0, 0, 0 }, list2));
+}
+
 TEST(CoveredByChildren, NotCovered) {
     const List list1;
     EXPECT_FALSE(algorithm::coveredByChildren(UnwrappedTileID{ 0, 0, 0 }, list1));
@@ -24,7 +66,7 @@ TEST(CoveredByChildren, NotCovered) {
 
     const List list3{
         { UnwrappedTileID{ 0, 0, 0 }, true },
-        // all four child tiles, with with a different wrap index
+        // all four child tiles, with a different wrap index
         { UnwrappedTileID{ 1, { 1, 0, 0 } }, true },
         { UnwrappedTileID{ 1, { 1, 0, 1 } }, true },
         { UnwrappedTileID{ 1, { 1, 1, 0 } }, true },