diff --git a/include/Compiler/Clang.h b/include/Compiler/Clang.h
new file mode 100644
index 00000000..8e742837
--- /dev/null
+++ b/include/Compiler/Clang.h
@@ -0,0 +1,8 @@
+#pragma once
+
+#include <clang/Lex/Preprocessor.h>
+#include <clang/AST/ASTTypeTraits.h>
+#include <clang/AST/RecursiveASTVisitor.h>
+#include <clang/Frontend/CompilerInstance.h>
+#include <clang/Frontend/FrontendActions.h>
+#include <clang/Tooling/Syntax/Tokens.h>
diff --git a/include/Compiler/Compiler.h b/include/Compiler/Compiler.h
index 300bf58a..fcf0b3be 100644
--- a/include/Compiler/Compiler.h
+++ b/include/Compiler/Compiler.h
@@ -1,21 +1,9 @@
 #pragma once
 
-#include <Support/ADT.h>
-#include <clang/Frontend/CompilerInstance.h>
+#include <Compiler/Clang.h>
 
 namespace clice {
 
-// TODO:
-
-class Preamble;
-
-// std::unique_ptr<clang::CompilerInvocation> createInvocation(StringRef filename,
-//                                                             StringRef content,
-//                                                             llvm::ArrayRef<const char*> args,
-//                                                             Preamble* preamble = nullptr);
-//
-// std::unique_ptr<clang::CompilerInstance> createInstance(std::shared_ptr<clang::CompilerInvocation> invocation);
-
 class Compiler {
 public:
     Compiler(llvm::StringRef filepath,
@@ -34,7 +22,7 @@ public:
 
     bool applyPCM(llvm::StringRef filepath, llvm::StringRef name);
 
-    /// build AST.
+    /// Build AST.
     void buildAST();
 
     /// Generate the PCH(PreCompiledHeader) to output path. Generally execute `clang::GeneratePCHAction`.
@@ -45,6 +33,7 @@ public:
     /// `clang::GenerateReducedModuleInterfaceAction`.
     void generatePCM(llvm::StringRef outpath);
 
+    /// Run code complete in given file and location.
     void codeCompletion(llvm::StringRef filepath,
                         std::uint32_t line,
                         std::uint32_t column,
@@ -54,6 +43,10 @@ public:
         return instance->getSema();
     }
 
+    clang::FileManager& fileMgr() {
+        return instance->getFileManager();
+    }
+
     clang::SourceManager& srcMgr() {
         return instance->getSourceManager();
     }
@@ -66,6 +59,10 @@ public:
         return instance->getASTContext().getTranslationUnitDecl();
     }
 
+    clang::syntax::TokenBuffer& tokBuf() {
+        return *buffer;
+    }
+
 private:
     void ExecuteAction();
 
diff --git a/include/Compiler/Dependency.h b/include/Compiler/Dependency.h
index 925036e5..db196a7b 100644
--- a/include/Compiler/Dependency.h
+++ b/include/Compiler/Dependency.h
@@ -1,10 +1,10 @@
 #pragma once
 
-#include <Support/ADT.h>
+#include <Compiler/Clang.h>
 
 namespace clice::dependencies {
 
-void load(ArrayRef<StringRef> dirs);
+void load(llvm::ArrayRef<llvm::StringRef> dirs);
 
 /// Record the include graph of the translation unit.
 struct IncludeGraph {};
diff --git a/include/Compiler/Diagnostic.h b/include/Compiler/Diagnostic.h
index 6d41a2b4..b42c1e5b 100644
--- a/include/Compiler/Diagnostic.h
+++ b/include/Compiler/Diagnostic.h
@@ -1,19 +1,10 @@
 #pragma once
 
-#include <clang/AST/RecursiveASTVisitor.h>
-#include <clang/Basic/Diagnostic.h>
-#include <clang/Frontend/CompilerInstance.h>
-#include <clang/Frontend/FrontendActions.h>
-#include <clang/Frontend/TextDiagnosticPrinter.h>
-#include <clang/Sema/Sema.h>
-#include <clang/Tooling/CompilationDatabase.h>
-#include <clang/Tooling/Syntax/Tokens.h>
-#include <clang/Lex/PPCallbacks.h>
-#include "clang/Sema/TemplateDeduction.h"
+#include <Compiler/Clang.h>
 
 namespace clice {
 
-class Diagnostic : public clang::DiagnosticConsumer {
+class DiagnosticCollector : public clang::DiagnosticConsumer {
 public:
     void BeginSourceFile(const clang::LangOptions& Opts, const clang::Preprocessor* PP) override;
 
diff --git a/include/Compiler/Directive.h b/include/Compiler/Directive.h
index 7d5a3bb8..ad75a3cd 100644
--- a/include/Compiler/Directive.h
+++ b/include/Compiler/Directive.h
@@ -1,4 +1,6 @@
-#include "Diagnostic.h"
+#pragma once
+
+#include <Compiler/Clang.h>
 
 namespace clice {
 
diff --git a/include/Compiler/ParsedAST.h b/include/Compiler/ParsedAST.h
index ba800256..e8ada415 100644
--- a/include/Compiler/ParsedAST.h
+++ b/include/Compiler/ParsedAST.h
@@ -1,6 +1,6 @@
 #pragma once
 
-#include "Preamble.h"
+#include <Compiler/Clang.h>
 
 namespace clice {
 
@@ -11,12 +11,10 @@ struct ParsedAST {
     clang::FileManager& fileManager;
     clang::SourceManager& sourceManager;
     clang::syntax::TokenBuffer tokenBuffer;
-    std::unique_ptr<Directives> directive;
+    // std::unique_ptr<Directives> directive;
     std::unique_ptr<clang::FrontendAction> action;
     std::unique_ptr<clang::CompilerInstance> instance;
 
-
-
     clang::FileID getFileID(llvm::StringRef filename) const {
         auto entry = fileManager.getFileRef(filename);
         if(!entry) {
diff --git a/include/Compiler/Preamble.h b/include/Compiler/Preamble.h
index 6504fb0b..f1a63953 100644
--- a/include/Compiler/Preamble.h
+++ b/include/Compiler/Preamble.h
@@ -1,6 +1,6 @@
 #pragma once
 
-#include "Directive.h"
+#include <Compiler/Clang.h>
 
 namespace clice {
 
diff --git a/include/Compiler/Resolver.h b/include/Compiler/Resolver.h
index dcc79489..9b026b30 100644
--- a/include/Compiler/Resolver.h
+++ b/include/Compiler/Resolver.h
@@ -1,9 +1,6 @@
 #pragma once
 
-#include "ParsedAST.h"
-#include <clang/Sema/Lookup.h>
-#include <clang/Sema/Template.h>
-#include <stack>
+#include <Compiler/Clang.h>
 
 namespace clice {
 
@@ -21,6 +18,7 @@ public:
 
     // TODO:
     // use a relative clear way to resolve `UnresolvedLookupExpr`.
+
 private:
     clang::Sema& sema;
 };
diff --git a/include/Compiler/Selection.h b/include/Compiler/Selection.h
index 2352b963..caad0d8d 100644
--- a/include/Compiler/Selection.h
+++ b/include/Compiler/Selection.h
@@ -1,5 +1,6 @@
-#include "ParsedAST.h"
-#include <clang/AST/ASTTypeTraits.h>
+#pragma once
+
+#include <Compiler/Clang.h>
 
 namespace clice {
 
diff --git a/include/Index/CSIF.h b/include/Index/CSIF.h
index 89db31a3..c76602f0 100644
--- a/include/Index/CSIF.h
+++ b/include/Index/CSIF.h
@@ -19,28 +19,28 @@ struct Occurrence;
 /// [SemanticDB](https://scalameta.org/docs/semanticdb/specification.html).
 struct CSIF {
     /// The version of the CSIF format.
-    StringRef version;
+    llvm::StringRef version;
     /// The language of the source code, currently only supports "c" and "c++".
-    StringRef language;
+    llvm::StringRef language;
     /// The URI of the source file.
-    StringRef uri;
+    llvm::StringRef uri;
     /// The context of the source file, used to check whether need to re-index the source file.
-    StringRef content;
+    llvm::StringRef content;
     /// The commands used to compile the source file.
-    ArrayRef<StringRef> commands;
+    llvm::ArrayRef<llvm::StringRef> commands;
 
     /// The symbols in the source file.
-    ArrayRef<Symbol> symbols;
+    llvm::ArrayRef<Symbol> symbols;
     /// The occurrences in the source file.
-    ArrayRef<Occurrence> occurrences;
+    llvm::ArrayRef<Occurrence> occurrences;
     /// The semantic tokens in the source file.
-    ArrayRef<std::uint32_t> semanticTokens;
+    llvm::ArrayRef<std::uint32_t> semanticTokens;
 
     // FIXME:
     /// The diagnostics in the source file.
-    // ArrayRef<Diagnostic> diagnostics;
+    // llvm::ArrayRef<Diagnostic> diagnostics;
     /// The inlay hints in the source file.
-    // ArrayRef<InlayHint> inlayHints;
+    // llvm::ArrayRef<InlayHint> inlayHints;
 };
 
 enum Role {
@@ -69,12 +69,12 @@ struct Symbol {
     /// The ID of the symbol.
     SymbolID id;
     /// display when hover.
-    StringRef document;
+    llvm::StringRef document;
 
     // TODO: append more useful information.
 
     /// The relations of the symbol.
-    ArrayRef<Relation> relations;
+    llvm::ArrayRef<Relation> relations;
 };
 
 struct Occurrence {
diff --git a/include/Support/ADT.h b/include/Support/ADT.h
index b966f542..ec7ae074 100644
--- a/include/Support/ADT.h
+++ b/include/Support/ADT.h
@@ -9,9 +9,7 @@
 #include <llvm/ADT/SmallVector.h>
 #include <llvm/ADT/SmallString.h>
 
+#include <llvm/ADT/StringExtras.h>
+
 namespace clice {
-using llvm::ArrayRef;
-using llvm::StringRef;
-using llvm::SmallVector;
-using llvm::SmallString;
 }  // namespace clice
diff --git a/src/Compiler/Compiler.cpp b/src/Compiler/Compiler.cpp
index eef5c8cd..88a5636a 100644
--- a/src/Compiler/Compiler.cpp
+++ b/src/Compiler/Compiler.cpp
@@ -2,6 +2,7 @@
 #include <Compiler/Preamble.h>
 
 #include <clang/Lex/PreprocessorOptions.h>
+#include <clang/Frontend/TextDiagnosticPrinter.h>
 
 namespace clice {
 
@@ -9,13 +10,15 @@ static void setInvocation(clang::CompilerInvocation& invocation) {
     clang::LangOptions& langOpts = invocation.getLangOpts();
     langOpts.CommentOpts.ParseAllComments = true;
     langOpts.RetainCommentsFromSystemHeaders = true;
+
+    // FIXME: add more.
 }
 
 Compiler::Compiler(llvm::StringRef filepath,
                    llvm::StringRef content,
                    llvm::ArrayRef<const char*> args,
                    llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> vfs) : filepath(filepath), content(content) {
-    // TODO: figure out should we use createInvocation?
+    // FIXME: figure out should we use createInvocation?
     clang::CreateInvocationOptions options;
     auto invocation = clang::createInvocation(args, options);
 
@@ -23,6 +26,7 @@ Compiler::Compiler(llvm::StringRef filepath,
 
     instance->setInvocation(std::move(invocation));
 
+    // FIXME: customize DiagnosticConsumer
     instance->createDiagnostics(new clang::TextDiagnosticPrinter(llvm::outs(), new clang::DiagnosticOptions()), true);
 
     if(!instance->createTarget()) {
@@ -32,7 +36,7 @@ Compiler::Compiler(llvm::StringRef filepath,
 }
 
 bool Compiler::applyPCH(llvm::StringRef filepath, std::uint32_t bound, bool endAtStart) {
-    // TODO: check reuseable?
+    // FIXME: check reuseable?
     auto& preproc = instance->getPreprocessorOpts();
     preproc.UsePredefines = false;
     preproc.ImplicitPCHInclude = filepath;
@@ -43,7 +47,7 @@ bool Compiler::applyPCH(llvm::StringRef filepath, std::uint32_t bound, bool endA
 }
 
 bool Compiler::applyPCM(llvm::StringRef filepath, llvm::StringRef name) {
-    // TODO: check reuseable?
+    // FIXME: check reuseable?
     instance->getHeaderSearchOpts().PrebuiltModuleFiles.try_emplace(name.str(), filepath);
     return true;
 }
@@ -89,10 +93,24 @@ void Compiler::ExecuteAction() {
         std::terminate();
     }
 
+    auto& preproc = instance->getPreprocessor();
+
+    // FIXME: add PPCallbacks to collect information.
+
+    // Beacuse CompilerInstance may create new Preprocessor in `BeginSourceFile`,
+    // So we must need to create TokenCollector here.
+    clang::syntax::TokenCollector collector{preproc};
+
+    // FIXME: clang-tidy, include-fixer, etc?
+
     if(auto error = action->Execute()) {
         llvm::errs() << "Failed to execute action: " << error << "\n";
         std::terminate();
     }
+
+    // Build TokenBuffer and index expanded tokens for improving performance.
+    buffer = std::make_unique<clang::syntax::TokenBuffer>(std::move(collector).consume());
+    buffer->indexExpandedTokens();
 }
 
 Compiler::~Compiler() {
diff --git a/src/Compiler/Dependency.cpp b/src/Compiler/Dependency.cpp
index 84146e79..0b063d0e 100644
--- a/src/Compiler/Dependency.cpp
+++ b/src/Compiler/Dependency.cpp
@@ -7,7 +7,7 @@ namespace clice::dependencies {
 namespace {
 
 /// module name -> file path.
-llvm::StringMap<StringRef> moduleMap;
+llvm::StringMap<llvm::StringRef> moduleMap;
 llvm::BumpPtrAllocator allocator;
 
 void scan() {
@@ -23,7 +23,7 @@ void scan() {
 
 }  // namespace
 
-void load(ArrayRef<StringRef> dirs) {
+void load(llvm::ArrayRef<llvm::StringRef> dirs) {
     for(auto dir: dirs) {
 
         std::string message;
diff --git a/src/Compiler/Diagnostic.cpp b/src/Compiler/Diagnostic.cpp
index 9a48a95c..bd5067a4 100644
--- a/src/Compiler/Diagnostic.cpp
+++ b/src/Compiler/Diagnostic.cpp
@@ -2,18 +2,18 @@
 
 namespace clice {
 
-void Diagnostic::BeginSourceFile(const clang::LangOptions& Opts, const clang::Preprocessor* PP) {
+void DiagnosticCollector::BeginSourceFile(const clang::LangOptions& Opts, const clang::Preprocessor* PP) {
 
 };
 
-void Diagnostic::HandleDiagnostic(clang::DiagnosticsEngine::Level level, const clang::Diagnostic& diagnostic) {
+void DiagnosticCollector::HandleDiagnostic(clang::DiagnosticsEngine::Level level, const clang::Diagnostic& diagnostic) {
     llvm::SmallString<128> message;
     diagnostic.FormatDiagnostic(message);
     // diagnostic.getLocation();
     llvm::outs() << "Diagnostic: " << message << "\n";
 };
 
-void Diagnostic::EndSourceFile() {
+void DiagnosticCollector::EndSourceFile() {
 
 };
 
diff --git a/src/Compiler/Resolver.cpp b/src/Compiler/Resolver.cpp
index 2744bc98..e4bfb666 100644
--- a/src/Compiler/Resolver.cpp
+++ b/src/Compiler/Resolver.cpp
@@ -1,5 +1,7 @@
 #include <Compiler/Resolver.h>
+#include <clang/Sema/Template.h>
 #include <clang/Sema/TreeTransform.h>
+#include <clang/Sema/TemplateDeduction.h>
 
 namespace clice {
 
diff --git a/src/Index/Pack.cpp b/src/Index/Pack.cpp
index 0fc8b187..7f34f747 100644
--- a/src/Index/Pack.cpp
+++ b/src/Index/Pack.cpp
@@ -25,16 +25,16 @@ struct Metadata {
 
 class Encoder {
 public:
-    void encode(StringRef& string) {
+    void encode(llvm::StringRef& string) {
         std::size_t offset = stringData.size();
         stringData.insert(stringData.end(), string.begin(), string.end());
         stringData.push_back('\0');
         // modify pointer to offset
-        string = StringRef(reinterpret_cast<char*>(offset), string.size());
+        string = llvm::StringRef(reinterpret_cast<char*>(offset), string.size());
     }
 
     template <typename T>
-    void encode(ArrayRef<T>& array) {
+    void encode(llvm::ArrayRef<T>& array) {
         std::size_t offset = arrayData.size();
         arrayData.reserve(arrayData.size() + array.size() * sizeof(T));
         for(auto elem: array) {
@@ -44,7 +44,7 @@ public:
             arrayData.insert(arrayData.end(), begin, begin + sizeof(T));
         }
         // modify pointer to offset
-        array = ArrayRef<T>(reinterpret_cast<T*>(offset), array.size());
+        array = llvm::ArrayRef<T>(reinterpret_cast<T*>(offset), array.size());
     }
 
     template <typename T>
@@ -91,13 +91,13 @@ private:
 
 class Decoder {
 public:
-    void decode(StringRef& string) {
-        string = StringRef(data + stringOffset, string.size());
+    void decode(llvm::StringRef& string) {
+        string = llvm::StringRef(data + stringOffset, string.size());
     }
 
     template <typename T>
-    void decode(ArrayRef<T>& array) {
-        array = ArrayRef<T>(reinterpret_cast<T*>(data + arrayOffset), array.size());
+    void decode(llvm::ArrayRef<T>& array) {
+        array = llvm::ArrayRef<T>(reinterpret_cast<T*>(data + arrayOffset), array.size());
         for(auto& elem: array) {
             decodeMemberRef(const_cast<T&>(elem));
         }
diff --git a/src/Support/URI.cpp b/src/Support/URI.cpp
index 9e6bde84..47fa09f5 100644
--- a/src/Support/URI.cpp
+++ b/src/Support/URI.cpp
@@ -1,6 +1,6 @@
 #include <Support/URI.h>
 #include <Support/FileSystem.h>
-
+#include <Support/ADT.h>
 namespace clice {
 
 /// returns true if the scheme is valid according to RFC 3986.
diff --git a/tests/AST/Selection.cpp b/tests/AST/Selection.cpp
index f253dbbf..a433c75a 100644
--- a/tests/AST/Selection.cpp
+++ b/tests/AST/Selection.cpp
@@ -1,5 +1,6 @@
 #include "../Test.h"
 #include <Compiler/Selection.h>
+#include <Compiler/Compiler.h>
 
 namespace {
 
@@ -15,13 +16,22 @@ using namespace clice;
 
 TEST(clice, SelectionTree) {
     foreachFile("SelectionTree", [](std::string file, llvm::StringRef content) {
-        // auto compiler = Compiler("main.cpp", content, compileArgs);
-        // auto AST = ParsedAST::build("main.cpp", content, compileArgs);
-        // auto id = AST->getFileID("main.cpp");
-        // auto& sm = AST->context.getSourceManager();
-        // auto begin = sm.translateLineCol(id, 7, 17);
-        // auto end = sm.translateLineCol(id, 7, 17);
-        // SelectionTree tree(sm.getFileOffset(begin), sm.getFileOffset(end), AST->context, AST->tokenBuffer);
+        auto compiler = Compiler("main.cpp", content, compileArgs);
+        compiler.buildAST();
+        auto& fileMgr = compiler.fileMgr();
+        auto entry = fileMgr.getFileRef("main.cpp");
+        if(!entry) {
+            llvm::outs() << "Failed to get file id\n";
+            std::terminate();
+        }
+        auto& srcMgr = compiler.srcMgr();
+        auto id = srcMgr.translateFile(*entry);
+        auto begin = srcMgr.translateLineCol(id, 7, 17);
+        auto end = srcMgr.translateLineCol(id, 7, 17);
+        SelectionTree tree(srcMgr.getFileOffset(begin),
+                           srcMgr.getFileOffset(end),
+                           compiler.context(),
+                           compiler.tokBuf());
     });
 }