//===- BuildTree.cpp ------------------------------------------*- C++ -*-=====//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#include "clang/Tooling/Syntax/BuildTree.h"
#include "clang/AST/ASTFwd.h"
#include "clang/AST/Decl.h"
#include "clang/AST/DeclBase.h"
#include "clang/AST/DeclCXX.h"
#include "clang/AST/DeclarationName.h"
#include "clang/AST/RecursiveASTVisitor.h"
#include "clang/AST/Stmt.h"
#include "clang/AST/TypeLoc.h"
#include "clang/AST/TypeLocVisitor.h"
#include "clang/Basic/LLVM.h"
#include "clang/Basic/SourceLocation.h"
#include "clang/Basic/SourceManager.h"
#include "clang/Basic/Specifiers.h"
#include "clang/Basic/TokenKinds.h"
#include "clang/Lex/Lexer.h"
#include "clang/Tooling/Syntax/Nodes.h"
#include "clang/Tooling/Syntax/Tokens.h"
#include "clang/Tooling/Syntax/Tree.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/PointerUnion.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/ScopeExit.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/FormatVariadic.h"
#include "llvm/Support/MemoryBuffer.h"
#include "llvm/Support/raw_ostream.h"
#include <cstddef>
#include <map>

using namespace clang;

LLVM_ATTRIBUTE_UNUSED
static bool isImplicitExpr(clang::Expr *E) { return E->IgnoreImplicit() != E; }

namespace {
/// Get start location of the Declarator from the TypeLoc.
/// E.g.:
///   loc of `(` in `int (a)`
///   loc of `*` in `int *(a)`
///   loc of the first `(` in `int (*a)(int)`
///   loc of the `*` in `int *(a)(int)`
///   loc of the first `*` in `const int *const *volatile a;`
///
/// It is non-trivial to get the start location because TypeLocs are stored
/// inside out. In the example above `*volatile` is the TypeLoc returned
/// by `Decl.getTypeSourceInfo()`, and `*const` is what `.getPointeeLoc()`
/// returns.
struct GetStartLoc : TypeLocVisitor<GetStartLoc, SourceLocation> {
  SourceLocation VisitParenTypeLoc(ParenTypeLoc T) {
    auto L = Visit(T.getInnerLoc());
    if (L.isValid())
      return L;
    return T.getLParenLoc();
  }

  // Types spelled in the prefix part of the declarator.
  SourceLocation VisitPointerTypeLoc(PointerTypeLoc T) {
    return HandlePointer(T);
  }

  SourceLocation VisitMemberPointerTypeLoc(MemberPointerTypeLoc T) {
    return HandlePointer(T);
  }

  SourceLocation VisitBlockPointerTypeLoc(BlockPointerTypeLoc T) {
    return HandlePointer(T);
  }

  SourceLocation VisitReferenceTypeLoc(ReferenceTypeLoc T) {
    return HandlePointer(T);
  }

  SourceLocation VisitObjCObjectPointerTypeLoc(ObjCObjectPointerTypeLoc T) {
    return HandlePointer(T);
  }

  // All other cases are not important, as they are either part of declaration
  // specifiers (e.g. inheritors of TypeSpecTypeLoc) or introduce modifiers on
  // existing declarators (e.g. QualifiedTypeLoc). They cannot start the
  // declarator themselves, but their underlying type can.
  SourceLocation VisitTypeLoc(TypeLoc T) {
    auto N = T.getNextTypeLoc();
    if (!N)
      return SourceLocation();
    return Visit(N);
  }

  SourceLocation VisitFunctionProtoTypeLoc(FunctionProtoTypeLoc T) {
    if (T.getTypePtr()->hasTrailingReturn())
      return SourceLocation(); // avoid recursing into the suffix of declarator.
    return VisitTypeLoc(T);
  }

private:
  template <class PtrLoc> SourceLocation HandlePointer(PtrLoc T) {
    auto L = Visit(T.getPointeeLoc());
    if (L.isValid())
      return L;
    return T.getLocalSourceRange().getBegin();
  }
};
} // namespace

/// Gets the range of declarator as defined by the C++ grammar. E.g.
///     `int a;` -> range of `a`,
///     `int *a;` -> range of `*a`,
///     `int a[10];` -> range of `a[10]`,
///     `int a[1][2][3];` -> range of `a[1][2][3]`,
///     `int *a = nullptr` -> range of `*a = nullptr`.
/// FIMXE: \p Name must be a source range, e.g. for `operator+`.
static SourceRange getDeclaratorRange(const SourceManager &SM, TypeLoc T,
                                      SourceLocation Name,
                                      SourceRange Initializer) {
  SourceLocation Start = GetStartLoc().Visit(T);
  SourceLocation End = T.getSourceRange().getEnd();
  assert(End.isValid());
  if (Name.isValid()) {
    if (Start.isInvalid())
      Start = Name;
    if (SM.isBeforeInTranslationUnit(End, Name))
      End = Name;
  }
  if (Initializer.isValid()) {
    auto InitializerEnd = Initializer.getEnd();
    assert(SM.isBeforeInTranslationUnit(End, InitializerEnd) || End == InitializerEnd);
    End = InitializerEnd;
  }
  return SourceRange(Start, End);
}

namespace {
/// All AST hierarchy roots that can be represented as pointers.
using ASTPtr = llvm::PointerUnion<Stmt *, Decl *>;
/// Maintains a mapping from AST to syntax tree nodes. This class will get more
/// complicated as we support more kinds of AST nodes, e.g. TypeLocs.
/// FIXME: expose this as public API.
class ASTToSyntaxMapping {
public:
  void add(ASTPtr From, syntax::Tree *To) {
    assert(To != nullptr);
    assert(!From.isNull());

    bool Added = Nodes.insert({From, To}).second;
    (void)Added;
    assert(Added && "mapping added twice");
  }

  syntax::Tree *find(ASTPtr P) const { return Nodes.lookup(P); }

private:
  llvm::DenseMap<ASTPtr, syntax::Tree *> Nodes;
};
} // namespace

/// A helper class for constructing the syntax tree while traversing a clang
/// AST.
///
/// At each point of the traversal we maintain a list of pending nodes.
/// Initially all tokens are added as pending nodes. When processing a clang AST
/// node, the clients need to:
///   - create a corresponding syntax node,
///   - assign roles to all pending child nodes with 'markChild' and
///     'markChildToken',
///   - replace the child nodes with the new syntax node in the pending list
///     with 'foldNode'.
///
/// Note that all children are expected to be processed when building a node.
///
/// Call finalize() to finish building the tree and consume the root node.
class syntax::TreeBuilder {
public:
  TreeBuilder(syntax::Arena &Arena) : Arena(Arena), Pending(Arena) {
    for (const auto &T : Arena.tokenBuffer().expandedTokens())
      LocationToToken.insert({T.location().getRawEncoding(), &T});
  }

  llvm::BumpPtrAllocator &allocator() { return Arena.allocator(); }
  const SourceManager &sourceManager() const { return Arena.sourceManager(); }

  /// Populate children for \p New node, assuming it covers tokens from \p
  /// Range.
  void foldNode(llvm::ArrayRef<syntax::Token> Range, syntax::Tree *New,
                ASTPtr From) {
    assert(New);
    Pending.foldChildren(Arena, Range, New);
    if (From)
      Mapping.add(From, New);
  }
  void foldNode(llvm::ArrayRef<syntax::Token> Range, syntax::Tree *New,
                TypeLoc L) {
    // FIXME: add mapping for TypeLocs
    foldNode(Range, New, nullptr);
  }

  /// Notifies that we should not consume trailing semicolon when computing
  /// token range of \p D.
  void noticeDeclWithoutSemicolon(Decl *D);

  /// Mark the \p Child node with a corresponding \p Role. All marked children
  /// should be consumed by foldNode.
  /// When called on expressions (clang::Expr is derived from clang::Stmt),
  /// wraps expressions into expression statement.
  void markStmtChild(Stmt *Child, NodeRole Role);
  /// Should be called for expressions in non-statement position to avoid
  /// wrapping into expression statement.
  void markExprChild(Expr *Child, NodeRole Role);
  /// Set role for a token starting at \p Loc.
  void markChildToken(SourceLocation Loc, NodeRole R);
  /// Set role for \p T.
  void markChildToken(const syntax::Token *T, NodeRole R);

  /// Set role for \p N.
  void markChild(syntax::Node *N, NodeRole R);
  /// Set role for the syntax node matching \p N.
  void markChild(ASTPtr N, NodeRole R);

  /// Finish building the tree and consume the root node.
  syntax::TranslationUnit *finalize() && {
    auto Tokens = Arena.tokenBuffer().expandedTokens();
    assert(!Tokens.empty());
    assert(Tokens.back().kind() == tok::eof);

    // Build the root of the tree, consuming all the children.
    Pending.foldChildren(Arena, Tokens.drop_back(),
                         new (Arena.allocator()) syntax::TranslationUnit);

    auto *TU = cast<syntax::TranslationUnit>(std::move(Pending).finalize());
    TU->assertInvariantsRecursive();
    return TU;
  }

  /// Finds a token starting at \p L. The token must exist if \p L is valid.
  const syntax::Token *findToken(SourceLocation L) const;

  /// Finds the syntax tokens corresponding to the \p SourceRange.
  llvm::ArrayRef<syntax::Token> getRange(SourceRange Range) const {
    assert(Range.isValid());
    return getRange(Range.getBegin(), Range.getEnd());
  }

  /// Finds the syntax tokens corresponding to the passed source locations.
  /// \p First is the start position of the first token and \p Last is the start
  /// position of the last token.
  llvm::ArrayRef<syntax::Token> getRange(SourceLocation First,
                                         SourceLocation Last) const {
    assert(First.isValid());
    assert(Last.isValid());
    assert(First == Last ||
           Arena.sourceManager().isBeforeInTranslationUnit(First, Last));
    return llvm::makeArrayRef(findToken(First), std::next(findToken(Last)));
  }

  llvm::ArrayRef<syntax::Token>
  getTemplateRange(const ClassTemplateSpecializationDecl *D) const {
    auto Tokens = getRange(D->getSourceRange());
    return maybeAppendSemicolon(Tokens, D);
  }

  /// Returns true if \p D is the last declarator in a chain and is thus
  /// reponsible for creating SimpleDeclaration for the whole chain.
  template <class T>
  bool isResponsibleForCreatingDeclaration(const T *D) const {
    static_assert((std::is_base_of<DeclaratorDecl, T>::value ||
                   std::is_base_of<TypedefNameDecl, T>::value),
                  "only DeclaratorDecl and TypedefNameDecl are supported.");

    const Decl *Next = D->getNextDeclInContext();

    // There's no next sibling, this one is responsible.
    if (Next == nullptr) {
      return true;
    }
    const auto *NextT = llvm::dyn_cast<T>(Next);

    // Next sibling is not the same type, this one is responsible.
    if (NextT == nullptr) {
      return true;
    }
    // Next sibling doesn't begin at the same loc, it must be a different
    // declaration, so this declarator is responsible.
    if (NextT->getBeginLoc() != D->getBeginLoc()) {
      return true;
    }

    // NextT is a member of the same declaration, and we need the last member to
    // create declaration. This one is not responsible.
    return false;
  }

  llvm::ArrayRef<syntax::Token> getDeclarationRange(Decl *D) {
    llvm::ArrayRef<clang::syntax::Token> Tokens;
    // We want to drop the template parameters for specializations.
    if (const auto *S = llvm::dyn_cast<TagDecl>(D))
      Tokens = getRange(S->TypeDecl::getBeginLoc(), S->getEndLoc());
    else
      Tokens = getRange(D->getSourceRange());
    return maybeAppendSemicolon(Tokens, D);
  }

  llvm::ArrayRef<syntax::Token> getExprRange(const Expr *E) const {
    return getRange(E->getSourceRange());
  }

  /// Find the adjusted range for the statement, consuming the trailing
  /// semicolon when needed.
  llvm::ArrayRef<syntax::Token> getStmtRange(const Stmt *S) const {
    auto Tokens = getRange(S->getSourceRange());
    if (isa<CompoundStmt>(S))
      return Tokens;

    // Some statements miss a trailing semicolon, e.g. 'return', 'continue' and
    // all statements that end with those. Consume this semicolon here.
    if (Tokens.back().kind() == tok::semi)
      return Tokens;
    return withTrailingSemicolon(Tokens);
  }

private:
  llvm::ArrayRef<syntax::Token>
  maybeAppendSemicolon(llvm::ArrayRef<syntax::Token> Tokens,
                       const Decl *D) const {
    if (llvm::isa<NamespaceDecl>(D))
      return Tokens;
    if (DeclsWithoutSemicolons.count(D))
      return Tokens;
    // FIXME: do not consume trailing semicolon on function definitions.
    // Most declarations own a semicolon in syntax trees, but not in clang AST.
    return withTrailingSemicolon(Tokens);
  }

  llvm::ArrayRef<syntax::Token>
  withTrailingSemicolon(llvm::ArrayRef<syntax::Token> Tokens) const {
    assert(!Tokens.empty());
    assert(Tokens.back().kind() != tok::eof);
    // We never consume 'eof', so looking at the next token is ok.
    if (Tokens.back().kind() != tok::semi && Tokens.end()->kind() == tok::semi)
      return llvm::makeArrayRef(Tokens.begin(), Tokens.end() + 1);
    return Tokens;
  }

  void setRole(syntax::Node *N, NodeRole R) {
    assert(N->role() == NodeRole::Detached);
    N->setRole(R);
  }

  /// A collection of trees covering the input tokens.
  /// When created, each tree corresponds to a single token in the file.
  /// Clients call 'foldChildren' to attach one or more subtrees to a parent
  /// node and update the list of trees accordingly.
  ///
  /// Ensures that added nodes properly nest and cover the whole token stream.
  struct Forest {
    Forest(syntax::Arena &A) {
      assert(!A.tokenBuffer().expandedTokens().empty());
      assert(A.tokenBuffer().expandedTokens().back().kind() == tok::eof);
      // Create all leaf nodes.
      // Note that we do not have 'eof' in the tree.
      for (auto &T : A.tokenBuffer().expandedTokens().drop_back()) {
        auto *L = new (A.allocator()) syntax::Leaf(&T);
        L->Original = true;
        L->CanModify = A.tokenBuffer().spelledForExpanded(T).hasValue();
        Trees.insert(Trees.end(), {&T, L});
      }
    }

    void assignRole(llvm::ArrayRef<syntax::Token> Range,
                    syntax::NodeRole Role) {
      assert(!Range.empty());
      auto It = Trees.lower_bound(Range.begin());
      assert(It != Trees.end() && "no node found");
      assert(It->first == Range.begin() && "no child with the specified range");
      assert((std::next(It) == Trees.end() ||
              std::next(It)->first == Range.end()) &&
             "no child with the specified range");
      assert(It->second->role() == NodeRole::Detached &&
             "re-assigning role for a child");
      It->second->setRole(Role);
    }

    /// Add \p Node to the forest and attach child nodes based on \p Tokens.
    void foldChildren(const syntax::Arena &A,
                      llvm::ArrayRef<syntax::Token> Tokens,
                      syntax::Tree *Node) {
      // Attach children to `Node`.
      assert(Node->firstChild() == nullptr && "node already has children");

      auto *FirstToken = Tokens.begin();
      auto BeginChildren = Trees.lower_bound(FirstToken);

      assert((BeginChildren == Trees.end() ||
              BeginChildren->first == FirstToken) &&
             "fold crosses boundaries of existing subtrees");
      auto EndChildren = Trees.lower_bound(Tokens.end());
      assert(
          (EndChildren == Trees.end() || EndChildren->first == Tokens.end()) &&
          "fold crosses boundaries of existing subtrees");

      // We need to go in reverse order, because we can only prepend.
      for (auto It = EndChildren; It != BeginChildren; --It) {
        auto *C = std::prev(It)->second;
        if (C->role() == NodeRole::Detached)
          C->setRole(NodeRole::Unknown);
        Node->prependChildLowLevel(C);
      }

      // Mark that this node came from the AST and is backed by the source code.
      Node->Original = true;
      Node->CanModify = A.tokenBuffer().spelledForExpanded(Tokens).hasValue();

      Trees.erase(BeginChildren, EndChildren);
      Trees.insert({FirstToken, Node});
    }

    // EXPECTS: all tokens were consumed and are owned by a single root node.
    syntax::Node *finalize() && {
      assert(Trees.size() == 1);
      auto *Root = Trees.begin()->second;
      Trees = {};
      return Root;
    }

    std::string str(const syntax::Arena &A) const {
      std::string R;
      for (auto It = Trees.begin(); It != Trees.end(); ++It) {
        unsigned CoveredTokens =
            It != Trees.end()
                ? (std::next(It)->first - It->first)
                : A.tokenBuffer().expandedTokens().end() - It->first;

        R += std::string(llvm::formatv(
            "- '{0}' covers '{1}'+{2} tokens\n", It->second->kind(),
            It->first->text(A.sourceManager()), CoveredTokens));
        R += It->second->dump(A);
      }
      return R;
    }

  private:
    /// Maps from the start token to a subtree starting at that token.
    /// Keys in the map are pointers into the array of expanded tokens, so
    /// pointer order corresponds to the order of preprocessor tokens.
    std::map<const syntax::Token *, syntax::Node *> Trees;
  };

  /// For debugging purposes.
  std::string str() { return Pending.str(Arena); }

  syntax::Arena &Arena;
  /// To quickly find tokens by their start location.
  llvm::DenseMap</*SourceLocation*/ unsigned, const syntax::Token *>
      LocationToToken;
  Forest Pending;
  llvm::DenseSet<Decl *> DeclsWithoutSemicolons;
  ASTToSyntaxMapping Mapping;
};

namespace {
class BuildTreeVisitor : public RecursiveASTVisitor<BuildTreeVisitor> {
public:
  explicit BuildTreeVisitor(ASTContext &Ctx, syntax::TreeBuilder &Builder)
      : Builder(Builder), LangOpts(Ctx.getLangOpts()) {}

  bool shouldTraversePostOrder() const { return true; }

  bool WalkUpFromDeclaratorDecl(DeclaratorDecl *DD) {
    return processDeclaratorAndDeclaration(DD);
  }

  bool WalkUpFromTypedefNameDecl(TypedefNameDecl *TD) {
    return processDeclaratorAndDeclaration(TD);
  }

  bool VisitDecl(Decl *D) {
    assert(!D->isImplicit());
    Builder.foldNode(Builder.getDeclarationRange(D),
                     new (allocator()) syntax::UnknownDeclaration(), D);
    return true;
  }

  // RAV does not call WalkUpFrom* on explicit instantiations, so we have to
  // override Traverse.
  // FIXME: make RAV call WalkUpFrom* instead.
  bool
  TraverseClassTemplateSpecializationDecl(ClassTemplateSpecializationDecl *C) {
    if (!RecursiveASTVisitor::TraverseClassTemplateSpecializationDecl(C))
      return false;
    if (C->isExplicitSpecialization())
      return true; // we are only interested in explicit instantiations.
    auto *Declaration =
        cast<syntax::SimpleDeclaration>(handleFreeStandingTagDecl(C));
    foldExplicitTemplateInstantiation(
        Builder.getTemplateRange(C), Builder.findToken(C->getExternLoc()),
        Builder.findToken(C->getTemplateKeywordLoc()), Declaration, C);
    return true;
  }

  bool WalkUpFromTemplateDecl(TemplateDecl *S) {
    foldTemplateDeclaration(
        Builder.getDeclarationRange(S),
        Builder.findToken(S->getTemplateParameters()->getTemplateLoc()),
        Builder.getDeclarationRange(S->getTemplatedDecl()), S);
    return true;
  }

  bool WalkUpFromTagDecl(TagDecl *C) {
    // FIXME: build the ClassSpecifier node.
    if (!C->isFreeStanding()) {
      assert(C->getNumTemplateParameterLists() == 0);
      return true;
    }
    handleFreeStandingTagDecl(C);
    return true;
  }

  syntax::Declaration *handleFreeStandingTagDecl(TagDecl *C) {
    assert(C->isFreeStanding());
    // Class is a declaration specifier and needs a spanning declaration node.
    auto DeclarationRange = Builder.getDeclarationRange(C);
    syntax::Declaration *Result = new (allocator()) syntax::SimpleDeclaration;
    Builder.foldNode(DeclarationRange, Result, nullptr);

    // Build TemplateDeclaration nodes if we had template parameters.
    auto ConsumeTemplateParameters = [&](const TemplateParameterList &L) {
      const auto *TemplateKW = Builder.findToken(L.getTemplateLoc());
      auto R = llvm::makeArrayRef(TemplateKW, DeclarationRange.end());
      Result =
          foldTemplateDeclaration(R, TemplateKW, DeclarationRange, nullptr);
      DeclarationRange = R;
    };
    if (auto *S = llvm::dyn_cast<ClassTemplatePartialSpecializationDecl>(C))
      ConsumeTemplateParameters(*S->getTemplateParameters());
    for (unsigned I = C->getNumTemplateParameterLists(); 0 < I; --I)
      ConsumeTemplateParameters(*C->getTemplateParameterList(I - 1));
    return Result;
  }

  bool WalkUpFromTranslationUnitDecl(TranslationUnitDecl *TU) {
    // We do not want to call VisitDecl(), the declaration for translation
    // unit is built by finalize().
    return true;
  }

  bool WalkUpFromCompoundStmt(CompoundStmt *S) {
    using NodeRole = syntax::NodeRole;

    Builder.markChildToken(S->getLBracLoc(), NodeRole::OpenParen);
    for (auto *Child : S->body())
      Builder.markStmtChild(Child, NodeRole::CompoundStatement_statement);
    Builder.markChildToken(S->getRBracLoc(), NodeRole::CloseParen);

    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::CompoundStatement, S);
    return true;
  }

  // Some statements are not yet handled by syntax trees.
  bool WalkUpFromStmt(Stmt *S) {
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::UnknownStatement, S);
    return true;
  }

  bool TraverseCXXForRangeStmt(CXXForRangeStmt *S) {
    // We override to traverse range initializer as VarDecl.
    // RAV traverses it as a statement, we produce invalid node kinds in that
    // case.
    // FIXME: should do this in RAV instead?
    if (S->getInit() && !TraverseStmt(S->getInit()))
      return false;
    if (S->getLoopVariable() && !TraverseDecl(S->getLoopVariable()))
      return false;
    if (S->getRangeInit() && !TraverseStmt(S->getRangeInit()))
      return false;
    if (S->getBody() && !TraverseStmt(S->getBody()))
      return false;
    return true;
  }

  bool TraverseStmt(Stmt *S) {
    if (auto *DS = llvm::dyn_cast_or_null<DeclStmt>(S)) {
      // We want to consume the semicolon, make sure SimpleDeclaration does not.
      for (auto *D : DS->decls())
        Builder.noticeDeclWithoutSemicolon(D);
    } else if (auto *E = llvm::dyn_cast_or_null<Expr>(S)) {
      // Do not recurse into subexpressions.
      // We do not have syntax trees for expressions yet, so we only want to see
      // the first top-level expression.
      return WalkUpFromExpr(E->IgnoreImplicit());
    }
    return RecursiveASTVisitor::TraverseStmt(S);
  }

  // Some expressions are not yet handled by syntax trees.
  bool WalkUpFromExpr(Expr *E) {
    assert(!isImplicitExpr(E) && "should be handled by TraverseStmt");
    Builder.foldNode(Builder.getExprRange(E),
                     new (allocator()) syntax::UnknownExpression, E);
    return true;
  }

  bool WalkUpFromNamespaceDecl(NamespaceDecl *S) {
    auto Tokens = Builder.getDeclarationRange(S);
    if (Tokens.front().kind() == tok::coloncolon) {
      // Handle nested namespace definitions. Those start at '::' token, e.g.
      // namespace a^::b {}
      // FIXME: build corresponding nodes for the name of this namespace.
      return true;
    }
    Builder.foldNode(Tokens, new (allocator()) syntax::NamespaceDefinition, S);
    return true;
  }

  bool TraverseParenTypeLoc(ParenTypeLoc L) {
    // We reverse order of traversal to get the proper syntax structure.
    if (!WalkUpFromParenTypeLoc(L))
      return false;
    return TraverseTypeLoc(L.getInnerLoc());
  }

  bool WalkUpFromParenTypeLoc(ParenTypeLoc L) {
    Builder.markChildToken(L.getLParenLoc(), syntax::NodeRole::OpenParen);
    Builder.markChildToken(L.getRParenLoc(), syntax::NodeRole::CloseParen);
    Builder.foldNode(Builder.getRange(L.getLParenLoc(), L.getRParenLoc()),
                     new (allocator()) syntax::ParenDeclarator, L);
    return true;
  }

  // Declarator chunks, they are produced by type locs and some clang::Decls.
  bool WalkUpFromArrayTypeLoc(ArrayTypeLoc L) {
    Builder.markChildToken(L.getLBracketLoc(), syntax::NodeRole::OpenParen);
    Builder.markExprChild(L.getSizeExpr(),
                          syntax::NodeRole::ArraySubscript_sizeExpression);
    Builder.markChildToken(L.getRBracketLoc(), syntax::NodeRole::CloseParen);
    Builder.foldNode(Builder.getRange(L.getLBracketLoc(), L.getRBracketLoc()),
                     new (allocator()) syntax::ArraySubscript, L);
    return true;
  }

  bool WalkUpFromFunctionTypeLoc(FunctionTypeLoc L) {
    Builder.markChildToken(L.getLParenLoc(), syntax::NodeRole::OpenParen);
    for (auto *P : L.getParams()) {
      Builder.markChild(P, syntax::NodeRole::ParametersAndQualifiers_parameter);
    }
    Builder.markChildToken(L.getRParenLoc(), syntax::NodeRole::CloseParen);
    Builder.foldNode(Builder.getRange(L.getLParenLoc(), L.getEndLoc()),
                     new (allocator()) syntax::ParametersAndQualifiers, L);
    return true;
  }

  bool WalkUpFromFunctionProtoTypeLoc(FunctionProtoTypeLoc L) {
    if (!L.getTypePtr()->hasTrailingReturn())
      return WalkUpFromFunctionTypeLoc(L);

    auto *TrailingReturnTokens = BuildTrailingReturn(L);
    // Finish building the node for parameters.
    Builder.markChild(TrailingReturnTokens,
                      syntax::NodeRole::ParametersAndQualifiers_trailingReturn);
    return WalkUpFromFunctionTypeLoc(L);
  }

  bool WalkUpFromMemberPointerTypeLoc(MemberPointerTypeLoc L) {
    auto SR = L.getLocalSourceRange();
    Builder.foldNode(Builder.getRange(SR),
                     new (allocator()) syntax::MemberPointer, L);
    return true;
  }

  // The code below is very regular, it could even be generated with some
  // preprocessor magic. We merely assign roles to the corresponding children
  // and fold resulting nodes.
  bool WalkUpFromDeclStmt(DeclStmt *S) {
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::DeclarationStatement, S);
    return true;
  }

  bool WalkUpFromNullStmt(NullStmt *S) {
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::EmptyStatement, S);
    return true;
  }

  bool WalkUpFromSwitchStmt(SwitchStmt *S) {
    Builder.markChildToken(S->getSwitchLoc(),
                           syntax::NodeRole::IntroducerKeyword);
    Builder.markStmtChild(S->getBody(), syntax::NodeRole::BodyStatement);
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::SwitchStatement, S);
    return true;
  }

  bool WalkUpFromCaseStmt(CaseStmt *S) {
    Builder.markChildToken(S->getKeywordLoc(),
                           syntax::NodeRole::IntroducerKeyword);
    Builder.markExprChild(S->getLHS(), syntax::NodeRole::CaseStatement_value);
    Builder.markStmtChild(S->getSubStmt(), syntax::NodeRole::BodyStatement);
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::CaseStatement, S);
    return true;
  }

  bool WalkUpFromDefaultStmt(DefaultStmt *S) {
    Builder.markChildToken(S->getKeywordLoc(),
                           syntax::NodeRole::IntroducerKeyword);
    Builder.markStmtChild(S->getSubStmt(), syntax::NodeRole::BodyStatement);
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::DefaultStatement, S);
    return true;
  }

  bool WalkUpFromIfStmt(IfStmt *S) {
    Builder.markChildToken(S->getIfLoc(), syntax::NodeRole::IntroducerKeyword);
    Builder.markStmtChild(S->getThen(),
                          syntax::NodeRole::IfStatement_thenStatement);
    Builder.markChildToken(S->getElseLoc(),
                           syntax::NodeRole::IfStatement_elseKeyword);
    Builder.markStmtChild(S->getElse(),
                          syntax::NodeRole::IfStatement_elseStatement);
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::IfStatement, S);
    return true;
  }

  bool WalkUpFromForStmt(ForStmt *S) {
    Builder.markChildToken(S->getForLoc(), syntax::NodeRole::IntroducerKeyword);
    Builder.markStmtChild(S->getBody(), syntax::NodeRole::BodyStatement);
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::ForStatement, S);
    return true;
  }

  bool WalkUpFromWhileStmt(WhileStmt *S) {
    Builder.markChildToken(S->getWhileLoc(),
                           syntax::NodeRole::IntroducerKeyword);
    Builder.markStmtChild(S->getBody(), syntax::NodeRole::BodyStatement);
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::WhileStatement, S);
    return true;
  }

  bool WalkUpFromContinueStmt(ContinueStmt *S) {
    Builder.markChildToken(S->getContinueLoc(),
                           syntax::NodeRole::IntroducerKeyword);
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::ContinueStatement, S);
    return true;
  }

  bool WalkUpFromBreakStmt(BreakStmt *S) {
    Builder.markChildToken(S->getBreakLoc(),
                           syntax::NodeRole::IntroducerKeyword);
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::BreakStatement, S);
    return true;
  }

  bool WalkUpFromReturnStmt(ReturnStmt *S) {
    Builder.markChildToken(S->getReturnLoc(),
                           syntax::NodeRole::IntroducerKeyword);
    Builder.markExprChild(S->getRetValue(),
                          syntax::NodeRole::ReturnStatement_value);
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::ReturnStatement, S);
    return true;
  }

  bool WalkUpFromCXXForRangeStmt(CXXForRangeStmt *S) {
    Builder.markChildToken(S->getForLoc(), syntax::NodeRole::IntroducerKeyword);
    Builder.markStmtChild(S->getBody(), syntax::NodeRole::BodyStatement);
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::RangeBasedForStatement, S);
    return true;
  }

  bool WalkUpFromEmptyDecl(EmptyDecl *S) {
    Builder.foldNode(Builder.getDeclarationRange(S),
                     new (allocator()) syntax::EmptyDeclaration, S);
    return true;
  }

  bool WalkUpFromStaticAssertDecl(StaticAssertDecl *S) {
    Builder.markExprChild(S->getAssertExpr(),
                          syntax::NodeRole::StaticAssertDeclaration_condition);
    Builder.markExprChild(S->getMessage(),
                          syntax::NodeRole::StaticAssertDeclaration_message);
    Builder.foldNode(Builder.getDeclarationRange(S),
                     new (allocator()) syntax::StaticAssertDeclaration, S);
    return true;
  }

  bool WalkUpFromLinkageSpecDecl(LinkageSpecDecl *S) {
    Builder.foldNode(Builder.getDeclarationRange(S),
                     new (allocator()) syntax::LinkageSpecificationDeclaration,
                     S);
    return true;
  }

  bool WalkUpFromNamespaceAliasDecl(NamespaceAliasDecl *S) {
    Builder.foldNode(Builder.getDeclarationRange(S),
                     new (allocator()) syntax::NamespaceAliasDefinition, S);
    return true;
  }

  bool WalkUpFromUsingDirectiveDecl(UsingDirectiveDecl *S) {
    Builder.foldNode(Builder.getDeclarationRange(S),
                     new (allocator()) syntax::UsingNamespaceDirective, S);
    return true;
  }

  bool WalkUpFromUsingDecl(UsingDecl *S) {
    Builder.foldNode(Builder.getDeclarationRange(S),
                     new (allocator()) syntax::UsingDeclaration, S);
    return true;
  }

  bool WalkUpFromUnresolvedUsingValueDecl(UnresolvedUsingValueDecl *S) {
    Builder.foldNode(Builder.getDeclarationRange(S),
                     new (allocator()) syntax::UsingDeclaration, S);
    return true;
  }

  bool WalkUpFromUnresolvedUsingTypenameDecl(UnresolvedUsingTypenameDecl *S) {
    Builder.foldNode(Builder.getDeclarationRange(S),
                     new (allocator()) syntax::UsingDeclaration, S);
    return true;
  }

  bool WalkUpFromTypeAliasDecl(TypeAliasDecl *S) {
    Builder.foldNode(Builder.getDeclarationRange(S),
                     new (allocator()) syntax::TypeAliasDeclaration, S);
    return true;
  }

private:
  template <class T> SourceLocation getQualifiedNameStart(T *D) {
    static_assert((std::is_base_of<DeclaratorDecl, T>::value ||
                   std::is_base_of<TypedefNameDecl, T>::value),
                  "only DeclaratorDecl and TypedefNameDecl are supported.");

    auto DN = D->getDeclName();
    bool IsAnonymous = DN.isIdentifier() && !DN.getAsIdentifierInfo();
    if (IsAnonymous)
      return SourceLocation();

    if (const auto *DD = llvm::dyn_cast<DeclaratorDecl>(D)) {
      if (DD->getQualifierLoc()) {
        return DD->getQualifierLoc().getBeginLoc();
      }
    }

    return D->getLocation();
  }

  SourceRange getInitializerRange(Decl *D) {
    if (auto *V = llvm::dyn_cast<VarDecl>(D)) {
      auto *I = V->getInit();
      // Initializers in range-based-for are not part of the declarator
      if (I && !V->isCXXForRangeDecl())
        return I->getSourceRange();
    }

    return SourceRange();
  }

  /// Folds SimpleDeclarator node (if present) and in case this is the last
  /// declarator in the chain it also folds SimpleDeclaration node.
  template <class T> bool processDeclaratorAndDeclaration(T *D) {
    SourceRange Initializer = getInitializerRange(D);
    auto Range = getDeclaratorRange(Builder.sourceManager(),
                                    D->getTypeSourceInfo()->getTypeLoc(),
                                    getQualifiedNameStart(D), Initializer);

    // There doesn't have to be a declarator (e.g. `void foo(int)` only has
    // declaration, but no declarator).
    if (Range.getBegin().isValid()) {
      auto *N = new (allocator()) syntax::SimpleDeclarator;
      Builder.foldNode(Builder.getRange(Range), N, nullptr);
      Builder.markChild(N, syntax::NodeRole::SimpleDeclaration_declarator);
    }

    if (Builder.isResponsibleForCreatingDeclaration(D)) {
      Builder.foldNode(Builder.getDeclarationRange(D),
                       new (allocator()) syntax::SimpleDeclaration, D);
    }
    return true;
  }

  /// Returns the range of the built node.
  syntax::TrailingReturnType *BuildTrailingReturn(FunctionProtoTypeLoc L) {
    assert(L.getTypePtr()->hasTrailingReturn());

    auto ReturnedType = L.getReturnLoc();
    // Build node for the declarator, if any.
    auto ReturnDeclaratorRange =
        getDeclaratorRange(this->Builder.sourceManager(), ReturnedType,
                           /*Name=*/SourceLocation(),
                           /*Initializer=*/SourceLocation());
    syntax::SimpleDeclarator *ReturnDeclarator = nullptr;
    if (ReturnDeclaratorRange.isValid()) {
      ReturnDeclarator = new (allocator()) syntax::SimpleDeclarator;
      Builder.foldNode(Builder.getRange(ReturnDeclaratorRange),
                       ReturnDeclarator, nullptr);
    }

    // Build node for trailing return type.
    auto Return = Builder.getRange(ReturnedType.getSourceRange());
    const auto *Arrow = Return.begin() - 1;
    assert(Arrow->kind() == tok::arrow);
    auto Tokens = llvm::makeArrayRef(Arrow, Return.end());
    Builder.markChildToken(Arrow, syntax::NodeRole::TrailingReturnType_arrow);
    if (ReturnDeclarator)
      Builder.markChild(ReturnDeclarator,
                        syntax::NodeRole::TrailingReturnType_declarator);
    auto *R = new (allocator()) syntax::TrailingReturnType;
    Builder.foldNode(Tokens, R, L);
    return R;
  }

  void foldExplicitTemplateInstantiation(
      ArrayRef<syntax::Token> Range, const syntax::Token *ExternKW,
      const syntax::Token *TemplateKW,
      syntax::SimpleDeclaration *InnerDeclaration, Decl *From) {
    assert(!ExternKW || ExternKW->kind() == tok::kw_extern);
    assert(TemplateKW && TemplateKW->kind() == tok::kw_template);
    Builder.markChildToken(
        ExternKW,
        syntax::NodeRole::ExplicitTemplateInstantiation_externKeyword);
    Builder.markChildToken(TemplateKW, syntax::NodeRole::IntroducerKeyword);
    Builder.markChild(
        InnerDeclaration,
        syntax::NodeRole::ExplicitTemplateInstantiation_declaration);
    Builder.foldNode(
        Range, new (allocator()) syntax::ExplicitTemplateInstantiation, From);
  }

  syntax::TemplateDeclaration *foldTemplateDeclaration(
      ArrayRef<syntax::Token> Range, const syntax::Token *TemplateKW,
      ArrayRef<syntax::Token> TemplatedDeclaration, Decl *From) {
    assert(TemplateKW && TemplateKW->kind() == tok::kw_template);
    Builder.markChildToken(TemplateKW, syntax::NodeRole::IntroducerKeyword);

    auto *N = new (allocator()) syntax::TemplateDeclaration;
    Builder.foldNode(Range, N, From);
    Builder.markChild(N, syntax::NodeRole::TemplateDeclaration_declaration);
    return N;
  }

  /// A small helper to save some typing.
  llvm::BumpPtrAllocator &allocator() { return Builder.allocator(); }

  syntax::TreeBuilder &Builder;
  const LangOptions &LangOpts;
};
} // namespace

void syntax::TreeBuilder::noticeDeclWithoutSemicolon(Decl *D) {
  DeclsWithoutSemicolons.insert(D);
}

void syntax::TreeBuilder::markChildToken(SourceLocation Loc, NodeRole Role) {
  if (Loc.isInvalid())
    return;
  Pending.assignRole(*findToken(Loc), Role);
}

void syntax::TreeBuilder::markChildToken(const syntax::Token *T, NodeRole R) {
  if (!T)
    return;
  Pending.assignRole(*T, R);
}

void syntax::TreeBuilder::markChild(syntax::Node *N, NodeRole R) {
  assert(N);
  setRole(N, R);
}

void syntax::TreeBuilder::markChild(ASTPtr N, NodeRole R) {
  auto *SN = Mapping.find(N);
  assert(SN != nullptr);
  setRole(SN, R);
}

void syntax::TreeBuilder::markStmtChild(Stmt *Child, NodeRole Role) {
  if (!Child)
    return;

  syntax::Tree *ChildNode = Mapping.find(Child);
  assert(ChildNode != nullptr);

  // This is an expression in a statement position, consume the trailing
  // semicolon and form an 'ExpressionStatement' node.
  if (isa<Expr>(Child)) {
    setRole(ChildNode, NodeRole::ExpressionStatement_expression);
    ChildNode = new (allocator()) syntax::ExpressionStatement;
    // (!) 'getStmtRange()' ensures this covers a trailing semicolon.
    Pending.foldChildren(Arena, getStmtRange(Child), ChildNode);
  }
  setRole(ChildNode, Role);
}

void syntax::TreeBuilder::markExprChild(Expr *Child, NodeRole Role) {
  if (!Child)
    return;
  Child = Child->IgnoreImplicit();

  syntax::Tree *ChildNode = Mapping.find(Child);
  assert(ChildNode != nullptr);
  setRole(ChildNode, Role);
}

const syntax::Token *syntax::TreeBuilder::findToken(SourceLocation L) const {
  if (L.isInvalid())
    return nullptr;
  auto It = LocationToToken.find(L.getRawEncoding());
  assert(It != LocationToToken.end());
  return It->second;
}

syntax::TranslationUnit *
syntax::buildSyntaxTree(Arena &A, const TranslationUnitDecl &TU) {
  TreeBuilder Builder(A);
  BuildTreeVisitor(TU.getASTContext(), Builder).TraverseAST(TU.getASTContext());
  return std::move(Builder).finalize();
}