[core] add algorithm for detecting whether an ordered map contains covering children

src/mbgl/algorithm/covered_by_children.hpp

@@ -0,0 +1,34 @@
+#ifndef MBGL_ALGORITHM_COVERED_BY_CHILDREN
+#define MBGL_ALGORITHM_COVERED_BY_CHILDREN
+
+#include <mbgl/tile/tile_id.hpp>
+
+namespace mbgl {
+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; });
+        if (it == end) {
+            return false;
+        } else if (std::get<0>(*it) != child) {
+            return coveredByChildren(child, it, end);
+        }
+    }
+
+    // We looked at all four immediate children and verified that they're covered.
+    return true;
+}
+
+template <typename Container>
+bool coveredByChildren(const UnwrappedTileID& id, const Container& container) {
+    return coveredByChildren(
+        id, container.upper_bound(id),
+        container.lower_bound(UnwrappedTileID{ static_cast<int16_t>(id.wrap + 1), { 0, 0, 0 } }));
+}
+
+} // namespace algorithm
+} // namespace mbgl
+
+#endif