diff --git a/clang-tools-extra/clangd/Headers.h b/clang-tools-extra/clangd/Headers.h
--- a/clang-tools-extra/clangd/Headers.h
+++ b/clang-tools-extra/clangd/Headers.h
@@ -62,7 +62,7 @@
 llvm::raw_ostream &operator<<(llvm::raw_ostream &, const Inclusion &);
 bool operator==(const Inclusion &LHS, const Inclusion &RHS);
 
-// Contains information about one file in the build grpah and its direct
+// Contains information about one file in the build graph and its direct
 // dependencies. Doesn't own the strings it references (IncludeGraph is
 // self-contained).
 struct IncludeGraphNode {
@@ -112,6 +112,22 @@
 // in any non-preamble inclusions.
 class IncludeStructure {
 public:
+  using File = unsigned;
+
+  llvm::Optional<File> getFile(StringRef Name) const {
+    auto It = NameToIndex.find(Name);
+    if (It == NameToIndex.end() || Name.empty())
+      return llvm::None;
+    return It->second;
+  }
+
+  llvm::ArrayRef<File> getIncludedFiles(File Index) const {
+    auto It = IncludeChildren.find(Index);
+    if (It == IncludeChildren.end())
+      return {};
+    return It->second;
+  }
+
   std::vector<Inclusion> MainFileIncludes;
 
   // Return all transitively reachable files.
@@ -122,24 +138,27 @@
   // Root --> 0, #included file --> 1, etc.
   // Root is clang's name for a file, which may not be absolute.
   // Usually it should be SM.getFileEntryForID(SM.getMainFileID())->getName().
-  llvm::StringMap<unsigned> includeDepth(llvm::StringRef Root) const;
+  llvm::StringMap<unsigned> includeDepth(StringRef Name) const;
 
   // This updates IncludeDepth(), but not MainFileIncludes.
   void recordInclude(llvm::StringRef IncludingName,
                      llvm::StringRef IncludedName,
                      llvm::StringRef IncludedRealName);
 
+  using AbstractIncludeGraph = llvm::DenseMap<File, SmallVector<File>>;
+
 private:
+  File getOrCreatefileIndex(llvm::StringRef Name);
+
   // Identifying files in a way that persists from preamble build to subsequent
   // builds is surprisingly hard. FileID is unavailable in InclusionDirective(),
   // and RealPathName and UniqueID are not preserved in the preamble.
   // We use the FileEntry::Name, which is stable, interned into a "file index".
   // The paths we want to expose are the RealPathName, so store those too.
   std::vector<std::string> RealPathNames; // In file index order.
-  unsigned fileIndex(llvm::StringRef Name);
-  llvm::StringMap<unsigned> NameToIndex; // Values are file indexes.
+  llvm::StringMap<File> NameToIndex;      // Values are file indexes.
   // Maps a file's index to that of the files it includes.
-  llvm::DenseMap<unsigned, llvm::SmallVector<unsigned>> IncludeChildren;
+  AbstractIncludeGraph IncludeChildren;
 };
 
 /// Returns a PPCallback that visits all inclusions in the main file.
diff --git a/clang-tools-extra/clangd/Headers.cpp b/clang-tools-extra/clangd/Headers.cpp
--- a/clang-tools-extra/clangd/Headers.cpp
+++ b/clang-tools-extra/clangd/Headers.cpp
@@ -157,31 +157,33 @@
 void IncludeStructure::recordInclude(llvm::StringRef IncludingName,
                                      llvm::StringRef IncludedName,
                                      llvm::StringRef IncludedRealName) {
-  auto Child = fileIndex(IncludedName);
+  auto Child = getOrCreatefileIndex(IncludedName);
   if (!IncludedRealName.empty() && RealPathNames[Child].empty())
     RealPathNames[Child] = std::string(IncludedRealName);
-  auto Parent = fileIndex(IncludingName);
+  auto Parent = getOrCreatefileIndex(IncludingName);
   IncludeChildren[Parent].push_back(Child);
 }
 
-unsigned IncludeStructure::fileIndex(llvm::StringRef Name) {
+IncludeStructure::File
+IncludeStructure::getOrCreatefileIndex(llvm::StringRef Name) {
   auto R = NameToIndex.try_emplace(Name, RealPathNames.size());
   if (R.second)
     RealPathNames.emplace_back();
   return R.first->getValue();
 }
 
-llvm::StringMap<unsigned>
-IncludeStructure::includeDepth(llvm::StringRef Root) const {
+llvm::StringMap<unsigned> IncludeStructure::includeDepth(StringRef Name) const {
   // Include depth 0 is the main file only.
   llvm::StringMap<unsigned> Result;
-  Result[Root] = 0;
+  Result[Name] = 0;
   std::vector<unsigned> CurrentLevel;
-  llvm::DenseSet<unsigned> Seen;
-  auto It = NameToIndex.find(Root);
-  if (It != NameToIndex.end()) {
-    CurrentLevel.push_back(It->second);
-    Seen.insert(It->second);
+  llvm::DenseSet<IncludeStructure::File> Seen;
+  auto Root = getFile(Name);
+  if (Root) {
+    CurrentLevel.push_back(*Root);
+    Seen.insert(*Root);
+  } else {
+    elog("Can not find the root ({0}) for includeDepth.", Name);
   }
 
   // Each round of BFS traversal finds the next depth level.