Index: clang-tools-extra/trunk/clangd/index/dex/Dex.h
===================================================================
--- clang-tools-extra/trunk/clangd/index/dex/Dex.h
+++ clang-tools-extra/trunk/clangd/index/dex/Dex.h
@@ -44,7 +44,7 @@
   // All symbols must outlive this index.
   template <typename Range>
   Dex(Range &&Symbols, llvm::ArrayRef<std::string> Schemes)
-      : URISchemes(Schemes) {
+      : Corpus(0), URISchemes(Schemes) {
     // If Schemes don't contain any items, fall back to SymbolCollector's
     // default URI schemes.
     if (URISchemes.empty()) {
@@ -101,6 +101,7 @@
   /// std. Inverted index is used to retrieve posting lists which are processed
   /// during the fuzzyFind process.
   llvm::DenseMap<Token, PostingList> InvertedIndex;
+  Corpus Corpus;
   std::shared_ptr<void> KeepAlive; // poor man's move-only std::any
   // Size of memory retained by KeepAlive.
   size_t BackingDataSize = 0;
Index: clang-tools-extra/trunk/clangd/index/dex/Dex.cpp
===================================================================
--- clang-tools-extra/trunk/clangd/index/dex/Dex.cpp
+++ clang-tools-extra/trunk/clangd/index/dex/Dex.cpp
@@ -56,7 +56,8 @@
 std::vector<std::unique_ptr<Iterator>> createFileProximityIterators(
     llvm::ArrayRef<std::string> ProximityPaths,
     llvm::ArrayRef<std::string> URISchemes,
-    const llvm::DenseMap<Token, PostingList> &InvertedIndex) {
+    const llvm::DenseMap<Token, PostingList> &InvertedIndex,
+    const Corpus &Corpus) {
   std::vector<std::unique_ptr<Iterator>> BoostingIterators;
   // Deduplicate parent URIs extracted from the ProximityPaths.
   llvm::StringSet<> ParentURIs;
@@ -91,8 +92,8 @@
     if (It != InvertedIndex.end()) {
       // FIXME(kbobyrev): Append LIMIT on top of every BOOST iterator.
       PathProximitySignals.SymbolURI = ParentURI;
-      BoostingIterators.push_back(createBoost(It->second.iterator(&It->first),
-                                              PathProximitySignals.evaluate()));
+      BoostingIterators.push_back(Corpus.boost(
+          It->second.iterator(&It->first), PathProximitySignals.evaluate()));
     }
   }
   return BoostingIterators;
@@ -101,6 +102,7 @@
 } // namespace
 
 void Dex::buildIndex() {
+  this->Corpus = dex::Corpus(Symbols.size());
   std::vector<std::pair<float, const Symbol *>> ScoredSymbols(Symbols.size());
 
   for (size_t I = 0; I < Symbols.size(); ++I) {
@@ -159,7 +161,7 @@
       TrigramIterators.push_back(It->second.iterator(&It->first));
   }
   if (!TrigramIterators.empty())
-    TopLevelChildren.push_back(createAnd(move(TrigramIterators)));
+    TopLevelChildren.push_back(Corpus.intersect(move(TrigramIterators)));
 
   // Generate scope tokens for search query.
   std::vector<std::unique_ptr<Iterator>> ScopeIterators;
@@ -170,22 +172,22 @@
       ScopeIterators.push_back(It->second.iterator(&It->first));
   }
   if (Req.AnyScope)
-    ScopeIterators.push_back(createBoost(createTrue(Symbols.size()),
-                                         ScopeIterators.empty() ? 1.0 : 0.2));
+    ScopeIterators.push_back(
+        Corpus.boost(Corpus.all(), ScopeIterators.empty() ? 1.0 : 0.2));
 
   // Add OR iterator for scopes if there are any Scope Iterators.
   if (!ScopeIterators.empty())
-    TopLevelChildren.push_back(createOr(move(ScopeIterators)));
+    TopLevelChildren.push_back(Corpus.unionOf(move(ScopeIterators)));
 
   // Add proximity paths boosting.
   auto BoostingIterators = createFileProximityIterators(
-      Req.ProximityPaths, URISchemes, InvertedIndex);
+      Req.ProximityPaths, URISchemes, InvertedIndex, Corpus);
   // Boosting iterators do not actually filter symbols. In order to preserve
   // the validity of resulting query, TRUE iterator should be added along
   // BOOSTs.
   if (!BoostingIterators.empty()) {
-    BoostingIterators.push_back(createTrue(Symbols.size()));
-    TopLevelChildren.push_back(createOr(move(BoostingIterators)));
+    BoostingIterators.push_back(Corpus.all());
+    TopLevelChildren.push_back(Corpus.unionOf(move(BoostingIterators)));
   }
 
   if (Req.RestrictForCodeCompletion)
@@ -196,14 +198,14 @@
   // Use TRUE iterator if both trigrams and scopes from the query are not
   // present in the symbol index.
   auto QueryIterator = TopLevelChildren.empty()
-                           ? createTrue(Symbols.size())
-                           : createAnd(move(TopLevelChildren));
+                           ? Corpus.all()
+                           : Corpus.intersect(move(TopLevelChildren));
   // Retrieve more items than it was requested: some of  the items with high
   // final score might not be retrieved otherwise.
   // FIXME(kbobyrev): Pre-scoring retrieval threshold should be adjusted as
   // using 100x of the requested number might not be good in practice, e.g.
   // when the requested number of items is small.
-  auto Root = Req.Limit ? createLimit(move(QueryIterator), *Req.Limit * 100)
+  auto Root = Req.Limit ? Corpus.limit(move(QueryIterator), *Req.Limit * 100)
                         : move(QueryIterator);
   SPAN_ATTACH(Tracer, "query", llvm::to_string(*Root));
   vlog("Dex query tree: {0}", *Root);
Index: clang-tools-extra/trunk/clangd/index/dex/Iterator.h
===================================================================
--- clang-tools-extra/trunk/clangd/index/dex/Iterator.h
+++ clang-tools-extra/trunk/clangd/index/dex/Iterator.h
@@ -101,8 +101,6 @@
     return Iterator.dump(OS);
   }
 
-  constexpr static float DEFAULT_BOOST_SCORE = 1;
-
 private:
   virtual llvm::raw_ostream &dump(llvm::raw_ostream &OS) const = 0;
 };
@@ -117,69 +115,74 @@
 /// to acquire preliminary scores of requested items.
 std::vector<std::pair<DocID, float>> consume(Iterator &It);
 
-/// Returns AND Iterator which performs the intersection of the PostingLists of
-/// its children.
-///
-/// consume(): AND Iterator returns the product of Childrens' boosting scores
-/// when not exhausted and DEFAULT_BOOST_SCORE otherwise.
-std::unique_ptr<Iterator>
-createAnd(std::vector<std::unique_ptr<Iterator>> Children);
-
-/// Returns OR Iterator which performs the union of the PostingLists of its
-/// children.
-///
-/// consume(): OR Iterator returns the highest boost value among children
-/// pointing to requested item when not exhausted and DEFAULT_BOOST_SCORE
-/// otherwise.
-std::unique_ptr<Iterator>
-createOr(std::vector<std::unique_ptr<Iterator>> Children);
-
-/// Returns TRUE Iterator which iterates over "virtual" PostingList containing
-/// all items in range [0, Size) in an efficient manner.
-///
-/// TRUE returns DEFAULT_BOOST_SCORE for each processed item.
-std::unique_ptr<Iterator> createTrue(DocID Size);
-
-/// Returns BOOST iterator which multiplies the score of each item by given
-/// factor. Boosting can be used as a computationally inexpensive filtering.
-/// Users can return significantly more items using consumeAndBoost() and then
-/// trim Top K using retrieval score.
-std::unique_ptr<Iterator> createBoost(std::unique_ptr<Iterator> Child,
-                                      float Factor);
-
-/// Returns LIMIT iterator, which yields up to N elements of its child iterator.
-/// Elements only count towards the limit if they are part of the final result
-/// set. Therefore the following iterator (AND (2) (LIMIT (1 2) 1)) yields (2),
-/// not ().
-std::unique_ptr<Iterator> createLimit(std::unique_ptr<Iterator> Child,
-                                      size_t Limit);
-
-/// This allows createAnd(create(...), create(...)) syntax.
-template <typename... Args> std::unique_ptr<Iterator> createAnd(Args... args) {
-  std::vector<std::unique_ptr<Iterator>> Children;
-  populateChildren(Children, args...);
-  return createAnd(move(Children));
-}
-
-/// This allows createOr(create(...), create(...)) syntax.
-template <typename... Args> std::unique_ptr<Iterator> createOr(Args... args) {
-  std::vector<std::unique_ptr<Iterator>> Children;
-  populateChildren(Children, args...);
-  return createOr(move(Children));
-}
-
-template <typename HeadT, typename... TailT>
+namespace detail {
+// Variadic template machinery.
+inline void populateChildren(std::vector<std::unique_ptr<Iterator>> &) {}
+template <typename... TailT>
 void populateChildren(std::vector<std::unique_ptr<Iterator>> &Children,
-                      HeadT &Head, TailT &... Tail) {
+                      std::unique_ptr<Iterator> Head, TailT... Tail) {
   Children.push_back(move(Head));
-  populateChildren(Children, Tail...);
+  populateChildren(Children, move(Tail)...);
 }
+} // namespace detail
 
-template <typename HeadT>
-void populateChildren(std::vector<std::unique_ptr<Iterator>> &Children,
-                      HeadT &Head) {
-  Children.push_back(move(Head));
-}
+// A corpus is a set of documents, and a factory for iterators over them.
+class Corpus {
+  DocID Size;
+
+public:
+  explicit Corpus(DocID Size) : Size(Size) {}
+
+  /// Returns AND Iterator which performs the intersection of the PostingLists
+  /// of its children.
+  ///
+  /// consume(): AND Iterator returns the product of Childrens' boosting
+  /// scores.
+  std::unique_ptr<Iterator>
+  intersect(std::vector<std::unique_ptr<Iterator>> Children) const;
+
+  /// Returns OR Iterator which performs the union of the PostingLists of its
+  /// children.
+  ///
+  /// consume(): OR Iterator returns the highest boost value among children
+  /// containing the requested item.
+  std::unique_ptr<Iterator>
+  unionOf(std::vector<std::unique_ptr<Iterator>> Children) const;
+
+  /// Returns TRUE Iterator which iterates over "virtual" PostingList
+  /// containing all items in range [0, Size) in an efficient manner.
+  std::unique_ptr<Iterator> all() const;
+
+  /// Returns BOOST iterator which multiplies the score of each item by given
+  /// factor. Boosting can be used as a computationally inexpensive filtering.
+  /// Users can return significantly more items using consumeAndBoost() and
+  /// then trim Top K using retrieval score.
+  std::unique_ptr<Iterator> boost(std::unique_ptr<Iterator> Child,
+                                  float Factor) const;
+
+  /// Returns LIMIT iterator, which yields up to N elements of its child
+  /// iterator. Elements only count towards the limit if they are part of the
+  /// final result set. Therefore the following iterator (AND (2) (LIMIT (1 2)
+  /// 1)) yields (2), not ().
+  std::unique_ptr<Iterator> limit(std::unique_ptr<Iterator> Child,
+                                  size_t Limit) const;
+
+  /// This allows intersect(create(...), create(...)) syntax.
+  template <typename... Args>
+  std::unique_ptr<Iterator> intersect(Args... args) const {
+    std::vector<std::unique_ptr<Iterator>> Children;
+    detail::populateChildren(Children, std::forward<Args>(args)...);
+    return intersect(move(Children));
+  }
+
+  /// This allows unionOf(create(...), create(...)) syntax.
+  template <typename... Args>
+  std::unique_ptr<Iterator> unionOf(Args... args) const {
+    std::vector<std::unique_ptr<Iterator>> Children;
+    detail::populateChildren(Children, std::forward<Args>(args)...);
+    return unionOf(move(Children));
+  }
+};
 
 } // namespace dex
 } // namespace clangd
Index: clang-tools-extra/trunk/clangd/index/dex/Iterator.cpp
===================================================================
--- clang-tools-extra/trunk/clangd/index/dex/Iterator.cpp
+++ clang-tools-extra/trunk/clangd/index/dex/Iterator.cpp
@@ -64,7 +64,7 @@
 
   float consume() override {
     assert(!reachedEnd() && "AND iterator can't consume() at the end.");
-    float Boost = DEFAULT_BOOST_SCORE;
+    float Boost = 1;
     for (const auto &Child : Children)
       Boost *= Child->consume();
     return Boost;
@@ -175,12 +175,12 @@
     return Result;
   }
 
-  // Returns the maximum boosting score among all Children when iterator is not
-  // exhausted and points to the given ID, DEFAULT_BOOST_SCORE otherwise.
+  // Returns the maximum boosting score among all Children when iterator
+  // points to the current ID.
   float consume() override {
     assert(!reachedEnd() && "OR iterator can't consume() at the end.");
     const DocID ID = peek();
-    float Boost = DEFAULT_BOOST_SCORE;
+    float Boost = 1;
     for (const auto &Child : Children)
       if (!Child->reachedEnd() && Child->peek() == ID)
         Boost = std::max(Boost, Child->consume());
@@ -236,7 +236,7 @@
 
   float consume() override {
     assert(!reachedEnd() && "TRUE iterator can't consume() at the end.");
-    return DEFAULT_BOOST_SCORE;
+    return 1;
   }
 
   size_t estimateSize() const override { return Size; }
@@ -330,30 +330,30 @@
 }
 
 std::unique_ptr<Iterator>
-createAnd(std::vector<std::unique_ptr<Iterator>> Children) {
+Corpus::intersect(std::vector<std::unique_ptr<Iterator>> Children) const {
   // If there is exactly one child, pull it one level up: AND(Child) -> Child.
   return Children.size() == 1 ? std::move(Children.front())
                               : llvm::make_unique<AndIterator>(move(Children));
 }
 
 std::unique_ptr<Iterator>
-createOr(std::vector<std::unique_ptr<Iterator>> Children) {
+Corpus::unionOf(std::vector<std::unique_ptr<Iterator>> Children) const {
   // If there is exactly one child, pull it one level up: OR(Child) -> Child.
   return Children.size() == 1 ? std::move(Children.front())
                               : llvm::make_unique<OrIterator>(move(Children));
 }
 
-std::unique_ptr<Iterator> createTrue(DocID Size) {
+std::unique_ptr<Iterator> Corpus::all() const {
   return llvm::make_unique<TrueIterator>(Size);
 }
 
-std::unique_ptr<Iterator> createBoost(std::unique_ptr<Iterator> Child,
-                                      float Factor) {
+std::unique_ptr<Iterator> Corpus::boost(std::unique_ptr<Iterator> Child,
+                                        float Factor) const {
   return llvm::make_unique<BoostIterator>(move(Child), Factor);
 }
 
-std::unique_ptr<Iterator> createLimit(std::unique_ptr<Iterator> Child,
-                                      size_t Limit) {
+std::unique_ptr<Iterator> Corpus::limit(std::unique_ptr<Iterator> Child,
+                                        size_t Limit) const {
   return llvm::make_unique<LimitIterator>(move(Child), Limit);
 }
 
Index: clang-tools-extra/trunk/clangd/index/dex/PostingList.cpp
===================================================================
--- clang-tools-extra/trunk/clangd/index/dex/PostingList.cpp
+++ clang-tools-extra/trunk/clangd/index/dex/PostingList.cpp
@@ -63,7 +63,7 @@
   float consume() override {
     assert(!reachedEnd() &&
            "Posting List iterator can't consume() at the end.");
-    return DEFAULT_BOOST_SCORE;
+    return 1;
   }
 
   size_t estimateSize() const override {
Index: clang-tools-extra/trunk/unittests/clangd/DexTests.cpp
===================================================================
--- clang-tools-extra/trunk/unittests/clangd/DexTests.cpp
+++ clang-tools-extra/trunk/unittests/clangd/DexTests.cpp
@@ -70,15 +70,16 @@
 }
 
 TEST(DexIterators, AndTwoLists) {
+  Corpus C{10000};
   const PostingList L0({0, 5, 7, 10, 42, 320, 9000});
   const PostingList L1({0, 4, 7, 10, 30, 60, 320, 9000});
 
-  auto And = createAnd(L1.iterator(), L0.iterator());
+  auto And = C.intersect(L1.iterator(), L0.iterator());
 
   EXPECT_FALSE(And->reachedEnd());
   EXPECT_THAT(consumeIDs(*And), ElementsAre(0U, 7U, 10U, 320U, 9000U));
 
-  And = createAnd(L0.iterator(), L1.iterator());
+  And = C.intersect(L0.iterator(), L1.iterator());
 
   And->advanceTo(0);
   EXPECT_EQ(And->peek(), 0U);
@@ -94,11 +95,12 @@
 }
 
 TEST(DexIterators, AndThreeLists) {
+  Corpus C{10000};
   const PostingList L0({0, 5, 7, 10, 42, 320, 9000});
   const PostingList L1({0, 4, 7, 10, 30, 60, 320, 9000});
   const PostingList L2({1, 4, 7, 11, 30, 60, 320, 9000});
 
-  auto And = createAnd(L0.iterator(), L1.iterator(), L2.iterator());
+  auto And = C.intersect(L0.iterator(), L1.iterator(), L2.iterator());
   EXPECT_EQ(And->peek(), 7U);
   And->advanceTo(300);
   EXPECT_EQ(And->peek(), 320U);
@@ -108,10 +110,11 @@
 }
 
 TEST(DexIterators, OrTwoLists) {
+  Corpus C{10000};
   const PostingList L0({0, 5, 7, 10, 42, 320, 9000});
   const PostingList L1({0, 4, 7, 10, 30, 60, 320, 9000});
 
-  auto Or = createOr(L0.iterator(), L1.iterator());
+  auto Or = C.unionOf(L0.iterator(), L1.iterator());
 
   EXPECT_FALSE(Or->reachedEnd());
   EXPECT_EQ(Or->peek(), 0U);
@@ -134,18 +137,19 @@
   Or->advanceTo(9001);
   EXPECT_TRUE(Or->reachedEnd());
 
-  Or = createOr(L0.iterator(), L1.iterator());
+  Or = C.unionOf(L0.iterator(), L1.iterator());
 
   EXPECT_THAT(consumeIDs(*Or),
               ElementsAre(0U, 4U, 5U, 7U, 10U, 30U, 42U, 60U, 320U, 9000U));
 }
 
 TEST(DexIterators, OrThreeLists) {
+  Corpus C{10000};
   const PostingList L0({0, 5, 7, 10, 42, 320, 9000});
   const PostingList L1({0, 4, 7, 10, 30, 60, 320, 9000});
   const PostingList L2({1, 4, 7, 11, 30, 60, 320, 9000});
 
-  auto Or = createOr(L0.iterator(), L1.iterator(), L2.iterator());
+  auto Or = C.unionOf(L0.iterator(), L1.iterator(), L2.iterator());
 
   EXPECT_FALSE(Or->reachedEnd());
   EXPECT_EQ(Or->peek(), 0U);
@@ -194,17 +198,18 @@
   //                  |1, 5, 7, 9|                |1, 5|    |0, 3, 5|
   //                  +----------+                +----+    +-------+
   //
+  Corpus C{10};
   const PostingList L0({1, 3, 5, 8, 9});
   const PostingList L1({1, 5, 7, 9});
   const PostingList L2({1, 5});
   const PostingList L3({0, 3, 5});
 
   // Root of the query tree: [1, 5]
-  auto Root = createAnd(
+  auto Root = C.intersect(
       // Lower And Iterator: [1, 5, 9]
-      createAnd(L0.iterator(), createBoost(L1.iterator(), 2U)),
+      C.intersect(L0.iterator(), C.boost(L1.iterator(), 2U)),
       // Lower Or Iterator: [0, 1, 5]
-      createOr(createBoost(L2.iterator(), 3U), createBoost(L3.iterator(), 4U)));
+      C.unionOf(C.boost(L2.iterator(), 3U), C.boost(L3.iterator(), 4U)));
 
   EXPECT_FALSE(Root->reachedEnd());
   EXPECT_EQ(Root->peek(), 1U);
@@ -226,6 +231,7 @@
 }
 
 TEST(DexIterators, StringRepresentation) {
+  Corpus C{10};
   const PostingList L1({1, 3, 5});
   const PostingList L2({1, 7, 9});
 
@@ -238,56 +244,60 @@
   auto I2 = L1.iterator(&Tok);
   EXPECT_EQ(llvm::to_string(*I2), "T=L2");
 
-  auto Tree = createLimit(createAnd(move(I1), move(I2)), 10);
+  auto Tree = C.limit(C.intersect(move(I1), move(I2)), 10);
   EXPECT_EQ(llvm::to_string(*Tree), "(LIMIT 10 (& [1 3 5] T=L2))");
 }
 
 TEST(DexIterators, Limit) {
+  Corpus C{10000};
   const PostingList L0({3, 6, 7, 20, 42, 100});
   const PostingList L1({1, 3, 5, 6, 7, 30, 100});
   const PostingList L2({0, 3, 5, 7, 8, 100});
 
-  auto DocIterator = createLimit(L0.iterator(), 42);
+  auto DocIterator = C.limit(L0.iterator(), 42);
   EXPECT_THAT(consumeIDs(*DocIterator), ElementsAre(3, 6, 7, 20, 42, 100));
 
-  DocIterator = createLimit(L0.iterator(), 3);
+  DocIterator = C.limit(L0.iterator(), 3);
   EXPECT_THAT(consumeIDs(*DocIterator), ElementsAre(3, 6, 7));
 
-  DocIterator = createLimit(L0.iterator(), 0);
+  DocIterator = C.limit(L0.iterator(), 0);
   EXPECT_THAT(consumeIDs(*DocIterator), ElementsAre());
 
-  auto AndIterator = createAnd(
-      createLimit(createTrue(9000), 343), createLimit(L0.iterator(), 2),
-      createLimit(L1.iterator(), 3), createLimit(L2.iterator(), 42));
+  auto AndIterator =
+      C.intersect(C.limit(C.all(), 343), C.limit(L0.iterator(), 2),
+                  C.limit(L1.iterator(), 3), C.limit(L2.iterator(), 42));
   EXPECT_THAT(consumeIDs(*AndIterator), ElementsAre(3, 7));
 }
 
 TEST(DexIterators, True) {
-  auto TrueIterator = createTrue(0U);
+  Corpus C{0};
+  auto TrueIterator = C.all();
   EXPECT_TRUE(TrueIterator->reachedEnd());
   EXPECT_THAT(consumeIDs(*TrueIterator), ElementsAre());
 
+  C = Corpus{7};
   const PostingList L0({1, 2, 5, 7});
-  TrueIterator = createTrue(7U);
+  TrueIterator = C.all();
   EXPECT_THAT(TrueIterator->peek(), 0);
-  auto AndIterator = createAnd(L0.iterator(), move(TrueIterator));
+  auto AndIterator = C.intersect(L0.iterator(), move(TrueIterator));
   EXPECT_FALSE(AndIterator->reachedEnd());
   EXPECT_THAT(consumeIDs(*AndIterator), ElementsAre(1, 2, 5));
 }
 
 TEST(DexIterators, Boost) {
-  auto BoostIterator = createBoost(createTrue(5U), 42U);
+  Corpus C{5};
+  auto BoostIterator = C.boost(C.all(), 42U);
   EXPECT_FALSE(BoostIterator->reachedEnd());
   auto ElementBoost = BoostIterator->consume();
   EXPECT_THAT(ElementBoost, 42U);
 
   const PostingList L0({2, 4});
   const PostingList L1({1, 4});
-  auto Root = createOr(createTrue(5U), createBoost(L0.iterator(), 2U),
-                       createBoost(L1.iterator(), 3U));
+  auto Root = C.unionOf(C.all(), C.boost(L0.iterator(), 2U),
+                        C.boost(L1.iterator(), 3U));
 
   ElementBoost = Root->consume();
-  EXPECT_THAT(ElementBoost, Iterator::DEFAULT_BOOST_SCORE);
+  EXPECT_THAT(ElementBoost, 1);
   Root->advance();
   EXPECT_THAT(Root->peek(), 1U);
   ElementBoost = Root->consume();