[clangd] Implement TRUE Iterator

This patch introduces TRUE Iterator which efficiently handles posting
lists containing all items within `[0, Size)` range.

Reviewed by: ioeric

Differential Revision: https://reviews.llvm.org/D50955

llvm-svn: 340155

clang-tools-extra/clangd/index/dex/Iterator.cpp

@@ -225,6 +225,41 @@ private:
   std::vector<std::unique_ptr<Iterator>> Children;
 };
 
+/// TrueIterator handles PostingLists which contain all items of the index. It
+/// stores size of the virtual posting list, and all operations are performed
+/// in O(1).
+class TrueIterator : public Iterator {
+public:
+  TrueIterator(DocID Size) : Size(Size) {}
+
+  bool reachedEnd() const override { return Index >= Size; }
+
+  void advance() override {
+    assert(!reachedEnd() && "Can't advance iterator after it reached the end.");
+    ++Index;
+  }
+
+  void advanceTo(DocID ID) override {
+    assert(!reachedEnd() && "Can't advance iterator after it reached the end.");
+    Index = std::min(ID, Size);
+  }
+
+  DocID peek() const override {
+    assert(!reachedEnd() && "TrueIterator can't call peek() at the end.");
+    return Index;
+  }
+
+private:
+  llvm::raw_ostream &dump(llvm::raw_ostream &OS) const override {
+    OS << "(TRUE {" << Index << "} out of " << Size << ")";
+    return OS;
+  }
+
+  DocID Index = 0;
+  /// Size of the underlying virtual PostingList.
+  DocID Size;
+};
+
 } // end namespace
 
 std::vector<DocID> consume(Iterator &It, size_t Limit) {
@@ -249,6 +284,10 @@ createOr(std::vector<std::unique_ptr<Iterator>> Children) {
   return llvm::make_unique<OrIterator>(move(Children));
 }
 
+std::unique_ptr<Iterator> createTrue(DocID Size) {
+  return llvm::make_unique<TrueIterator>(Size);
+}
+
 } // namespace dex
 } // namespace clangd
 } // namespace clang

clang-tools-extra/clangd/index/dex/Iterator.h

@@ -129,6 +129,10 @@ createAnd(std::vector<std::unique_ptr<Iterator>> Children);
 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.
+std::unique_ptr<Iterator> createTrue(DocID Size);
+
 /// This allows createAnd(create(...), create(...)) syntax.
 template <typename... Args> std::unique_ptr<Iterator> createAnd(Args... args) {
   std::vector<std::unique_ptr<Iterator>> Children;

clang-tools-extra/unittests/clangd/DexIndexTests.cpp

@@ -262,6 +262,19 @@ TEST(DexIndexIterators, Limit) {
   EXPECT_THAT(consume(*DocIterator, 0), ElementsAre());
 }
 
+TEST(DexIndexIterators, True) {
+  auto TrueIterator = createTrue(0U);
+  EXPECT_THAT(TrueIterator->reachedEnd(), true);
+  EXPECT_THAT(consume(*TrueIterator), ElementsAre());
+
+  PostingList L0 = {1, 2, 5, 7};
+  TrueIterator = createTrue(7U);
+  EXPECT_THAT(TrueIterator->peek(), 0);
+  auto AndIterator = createAnd(create(L0), move(TrueIterator));
+  EXPECT_THAT(AndIterator->reachedEnd(), false);
+  EXPECT_THAT(consume(*AndIterator), ElementsAre(1, 2, 5));
+}
+
 testing::Matcher<std::vector<Token>>
 trigramsAre(std::initializer_list<std::string> Trigrams) {
   std::vector<Token> Tokens;