Bug 1539322 - Add Vector::eraseIf and Vector::eraseIfEqual - r=froydnj

Convenience functions to erase elements based on a predicate, or by comparing to
a value. They are optimized to use the minimal amount of moves necessary.

Differential Revision: https://phabricator.services.mozilla.com/D25016

--HG--
extra : moz-landing-system : lando

mfbt/Vector.h

@@ -800,6 +800,20 @@ class MOZ_NON_PARAM Vector final : private AllocPolicy {
    */
   void erase(T* aBegin, T* aEnd);
 
+  /**
+   * Removes all elements that satisfy the predicate, shifting existing elements
+   * lower to fill erased gaps.
+   */
+  template <typename Pred>
+  void eraseIf(Pred aPred);
+
+  /**
+   * Removes all elements that compare equal to |aU|, shifting existing elements
+   * lower to fill erased gaps.
+   */
+  template <typename U>
+  void eraseIfEqual(const U& aU);
+
   /**
    * Measure the size of the vector's heap-allocated storage.
    */
@@ -1285,6 +1299,26 @@ inline void Vector<T, N, AP>::erase(T* aBegin, T* aEnd) {
   shrinkBy(aEnd - aBegin);
 }
 
+template <typename T, size_t N, class AP>
+template <typename Pred>
+void Vector<T, N, AP>::eraseIf(Pred aPred) {
+  // remove_if finds the first element to be erased, and then efficiently move-
+  // assigns elements to effectively overwrite elements that satisfy the
+  // predicate. It returns the new end pointer, after which there are only
+  // moved-from elements ready to be destroyed, so we just need to shrink the
+  // vector accordingly.
+  T* newEnd = std::remove_if(begin(), end(),
+                             [&aPred](const T& aT) { return aPred(aT); });
+  MOZ_ASSERT(newEnd <= end());
+  shrinkBy(end() - newEnd);
+}
+
+template <typename T, size_t N, class AP>
+template <typename U>
+void Vector<T, N, AP>::eraseIfEqual(const U& aU) {
+  return eraseIf([&aU](const T& aT) { return aT == aU; });
+}
+
 template <typename T, size_t N, class AP>
 template <typename U>
 MOZ_ALWAYS_INLINE bool Vector<T, N, AP>::append(const U* aInsBegin,

mfbt/tests/TestVector.cpp

@@ -22,6 +22,7 @@ struct mozilla::detail::VectorTesting {
   static void testExtractOrCopyRawBuffer();
   static void testReplaceRawBuffer();
   static void testInsert();
+  static void testErase();
   static void testPodResizeToFit();
 };
 
@@ -136,6 +137,8 @@ struct S {
     return *this;
   }
 
+  bool operator==(const S& rhs) const { return j == rhs.j && *k == *rhs.k; }
+
   S(const S&) = delete;
   S& operator=(const S&) = delete;
 };
@@ -437,6 +440,97 @@ void mozilla::detail::VectorTesting::testInsert() {
   MOZ_RELEASE_ASSERT(S::destructCount == 1);
 }
 
+void mozilla::detail::VectorTesting::testErase() {
+  S::resetCounts();
+
+  Vector<S, 8> vec;
+  MOZ_RELEASE_ASSERT(vec.reserve(8));
+  for (size_t i = 0; i < 7; i++) {
+    vec.infallibleEmplaceBack(i, i * i);
+  }
+
+  // vec: [0, 1, 2, 3, 4, 5, 6]
+  MOZ_RELEASE_ASSERT(vec.length() == 7);
+  MOZ_ASSERT(vec.reserved() == 8);
+  MOZ_RELEASE_ASSERT(S::constructCount == 7);
+  MOZ_RELEASE_ASSERT(S::moveCount == 0);
+  MOZ_RELEASE_ASSERT(S::destructCount == 0);
+  S::resetCounts();
+
+  vec.erase(&vec[4]);
+  // vec: [0, 1, 2, 3, 5, 6]
+  MOZ_RELEASE_ASSERT(vec.length() == 6);
+  MOZ_ASSERT(vec.reserved() == 8);
+  MOZ_RELEASE_ASSERT(S::constructCount == 0);
+  // 5 and 6 should have been moved into 4 and 5.
+  MOZ_RELEASE_ASSERT(S::moveCount == 2);
+  MOZ_RELEASE_ASSERT(S::destructCount == 1);
+  MOZ_RELEASE_ASSERT(vec[4] == S(5, 5 * 5));
+  MOZ_RELEASE_ASSERT(vec[5] == S(6, 6 * 6));
+  S::resetCounts();
+
+  vec.erase(&vec[3], &vec[5]);
+  // vec: [0, 1, 2, 6]
+  MOZ_RELEASE_ASSERT(vec.length() == 4);
+  MOZ_ASSERT(vec.reserved() == 8);
+  MOZ_RELEASE_ASSERT(S::constructCount == 0);
+  // 6 should have been moved into 3.
+  MOZ_RELEASE_ASSERT(S::moveCount == 1);
+  MOZ_RELEASE_ASSERT(S::destructCount == 2);
+  MOZ_RELEASE_ASSERT(vec[3] == S(6, 6 * 6));
+
+  S s2(2, 2 * 2);
+  S::resetCounts();
+
+  vec.eraseIfEqual(s2);
+  // vec: [0, 1, 6]
+  MOZ_RELEASE_ASSERT(vec.length() == 3);
+  MOZ_ASSERT(vec.reserved() == 8);
+  MOZ_RELEASE_ASSERT(S::constructCount == 0);
+  // 6 should have been moved into 2.
+  MOZ_RELEASE_ASSERT(S::moveCount == 1);
+  MOZ_RELEASE_ASSERT(S::destructCount == 1);
+  MOZ_RELEASE_ASSERT(vec[2] == S(6, 6 * 6));
+  S::resetCounts();
+
+  // Predicate to find one element.
+  vec.eraseIf([](const S& s) { return s.j == 1; });
+  // vec: [0, 6]
+  MOZ_RELEASE_ASSERT(vec.length() == 2);
+  MOZ_ASSERT(vec.reserved() == 8);
+  MOZ_RELEASE_ASSERT(S::constructCount == 0);
+  // 6 should have been moved into 1.
+  MOZ_RELEASE_ASSERT(S::moveCount == 1);
+  MOZ_RELEASE_ASSERT(S::destructCount == 1);
+  MOZ_RELEASE_ASSERT(vec[1] == S(6, 6 * 6));
+  S::resetCounts();
+
+  // Generic predicate that flags everything.
+  vec.eraseIf([](auto&&) { return true; });
+  // vec: []
+  MOZ_RELEASE_ASSERT(vec.length() == 0);
+  MOZ_ASSERT(vec.reserved() == 8);
+  MOZ_RELEASE_ASSERT(S::constructCount == 0);
+  MOZ_RELEASE_ASSERT(S::moveCount == 0);
+  MOZ_RELEASE_ASSERT(S::destructCount == 2);
+
+  for (size_t i = 0; i < 7; i++) {
+    vec.infallibleEmplaceBack(i, i * i);
+  }
+  // vec: [0, 1, 2, 3, 4, 5, 6]
+  MOZ_RELEASE_ASSERT(vec.length() == 7);
+  S::resetCounts();
+
+  // Predicate that flags all even numbers.
+  vec.eraseIf([](const S& s) { return s.j % 2 == 0; });
+  // vec: [1 (was 0), 3 (was 1), 5 (was 2)]
+  MOZ_RELEASE_ASSERT(vec.length() == 3);
+  MOZ_ASSERT(vec.reserved() == 8);
+  MOZ_RELEASE_ASSERT(S::constructCount == 0);
+  MOZ_RELEASE_ASSERT(S::moveCount == 3);
+  MOZ_RELEASE_ASSERT(S::destructCount == 4);
+}
+
 void mozilla::detail::VectorTesting::testPodResizeToFit() {
   // Vectors not using inline storage realloc capacity to exact length.
   Vector<int, 0> v1;
@@ -576,6 +670,7 @@ int main() {
   VectorTesting::testExtractOrCopyRawBuffer();
   VectorTesting::testReplaceRawBuffer();
   VectorTesting::testInsert();
+  VectorTesting::testErase();
   VectorTesting::testPodResizeToFit();
   TestVectorBeginNonNull();
 }