Bug 1431122 - Part 1: Add UnorderedRemoveElement[s]At to nsTArray, r=froydnj

MozReview-Commit-ID: H0oqG7H7299

xpcom/ds/nsTArray-inl.h

@@ -273,6 +273,61 @@ nsTArray_base<Alloc, Copy>::ShiftData(index_type aStart,
   }
 }
 
+template<class Alloc, class Copy>
+template<typename ActualAlloc>
+void
+nsTArray_base<Alloc, Copy>::SwapFromEnd(index_type aStart,
+                                        size_type aCount,
+                                        size_type aElemSize,
+                                        size_t aElemAlign)
+{
+  // This method is part of the implementation of
+  // nsTArray::SwapRemoveElement{s,}At. For more information, read the
+  // documentation on that method.
+  if (aCount == 0) {
+    return;
+  }
+
+  // We are going to be removing aCount elements. Update our length to point to
+  // the new end of the array.
+  size_type oldLength = mHdr->mLength;
+  mHdr->mLength -= aCount;
+
+  if (mHdr->mLength == 0) {
+    // If we have no elements remaining in the array, we can free our buffer.
+    ShrinkCapacity(aElemSize, aElemAlign);
+    return;
+  }
+
+  // Determine how many elements we need to move from the end of the array into
+  // the now-removed section. This will either be the number of elements which
+  // were removed (if there are more elements in the tail of the array), or the
+  // entire tail of the array, whichever is smaller.
+  size_type relocCount = std::min(aCount, mHdr->mLength - aStart);
+  if (relocCount == 0) {
+    return;
+  }
+
+  // Move the elements which are now stranded after the end of the array back
+  // into the now-vacated memory.
+  index_type sourceBytes = (oldLength - relocCount) * aElemSize;
+  index_type destBytes = aStart * aElemSize;
+
+  // Perform the final copy. This is guaranteed to be a non-overlapping copy
+  // as our source contains only still-valid entries, and the destination
+  // contains only invalid entries which need to be overwritten.
+  MOZ_ASSERT(sourceBytes >= destBytes,
+             "The source should be after the destination.");
+  MOZ_ASSERT(sourceBytes - destBytes >= relocCount * aElemSize,
+             "The range should be nonoverlapping");
+
+  char* baseAddr = reinterpret_cast<char*>(mHdr + 1);
+  Copy::MoveNonOverlappingRegion(baseAddr + destBytes,
+                                 baseAddr + sourceBytes,
+                                 relocCount,
+                                 aElemSize);
+}
+
 template<class Alloc, class Copy>
 template<typename ActualAlloc>
 bool

xpcom/ds/nsTArray.h

@@ -410,6 +410,17 @@ protected:
   void ShiftData(index_type aStart, size_type aOldLen, size_type aNewLen,
                  size_type aElemSize, size_t aElemAlign);
 
+  // This method may be called to swap elements from the end of the array to
+  // fill a "gap" in the array. If the resulting array has zero elements, then
+  // the array's memory is free'd.
+  // @param aStart     The starting index of the gap.
+  // @param aCount     The length of the gap.
+  // @param aElemSize  The size of an array element.
+  // @param aElemAlign The alignment in bytes of an array element.
+  template<typename ActualAlloc>
+  void SwapFromEnd(index_type aStart, size_type aCount,
+                   size_type aElemSize, size_t aElemAlign);
+
   // This method increments the length member of the array's header.
   // Note that mHdr may actually be sEmptyHdr in the case where a
   // zero-length array is inserted into our array. But then aNum should
@@ -1717,6 +1728,58 @@ public:
   // A variation on the RemoveElementsAt method defined above.
   void RemoveElementAt(index_type aIndex) { RemoveElementsAt(aIndex, 1); }
 
+  // This method performs index-based removals from an array without preserving
+  // the order of the array. This is useful if you are using the array as a
+  // set-like data structure.
+  //
+  // These removals are efficient, as they move as few elements as possible. At
+  // most N elements, where N is the number of removed elements, will have to
+  // be relocated.
+  //
+  // ## Examples
+  //
+  // When removing an element from the end of the array, it can be removed in
+  // place, by destroying it and decrementing the length.
+  //
+  // [ 1, 2, 3 ] => [ 1, 2 ]
+  //         ^
+  //
+  // When removing any other single element, it is removed by swapping it with
+  // the last element, and then decrementing the length as before.
+  //
+  // [ 1, 2, 3, 4, 5, 6 ]  => [ 1, 6, 3, 4, 5 ]
+  //      ^
+  //
+  // This method also supports efficiently removing a range of elements. If they
+  // are at the end, then they can all be removed like in the one element case.
+  //
+  // [ 1, 2, 3, 4, 5, 6 ] => [ 1, 2 ]
+  //         ^--------^
+  //
+  // If more elements are removed than exist after the removed section, the
+  // remaining elements will be shifted down like in a normal removal.
+  //
+  // [ 1, 2, 3, 4, 5, 6, 7, 8 ] => [ 1, 2, 7, 8 ]
+  //         ^--------^
+  //
+  // And if fewer elements are removed than exist after the removed section,
+  // elements will be moved from the end of the array to fill the vacated space.
+  //
+  // [ 1, 2, 3, 4, 5, 6, 7, 8 ] => [ 1, 7, 8, 4, 5, 6 ]
+  //      ^--^
+  //
+  // @param aStart The starting index of the elements to remove. @param aCount
+  // The number of elements to remove.
+  void UnorderedRemoveElementsAt(index_type aStart, size_type aCount);
+
+  // A variation on the UnorderedRemoveElementsAt method defined above to remove
+  // a single element. This operation is sometimes called `SwapRemove`.
+  //
+  // This method is O(1), but does not preserve the order of the elements.
+  void UnorderedRemoveElementAt(index_type aIndex) {
+    UnorderedRemoveElementsAt(aIndex, 1);
+  }
+
   void Clear() {
     ClearAndRetainStorage();
     Compact();
@@ -2053,6 +2116,28 @@ nsTArray_Impl<E, Alloc>::RemoveElementsAt(index_type aStart, size_type aCount)
                                             MOZ_ALIGNOF(elem_type));
 }
 
+template<typename E, class Alloc>
+void
+nsTArray_Impl<E, Alloc>::UnorderedRemoveElementsAt(index_type aStart, size_type aCount)
+{
+  MOZ_ASSERT(aCount == 0 || aStart < Length(), "Invalid aStart index");
+
+  mozilla::CheckedInt<index_type> rangeEnd = aStart;
+  rangeEnd += aCount;
+
+  if (MOZ_UNLIKELY(!rangeEnd.isValid() || rangeEnd.value() > Length())) {
+    InvalidArrayIndex_CRASH(aStart, Length());
+  }
+
+  // Destroy the elements which are being removed, and then swap elements in to
+  // replace them from the end. See the docs on the declaration of this
+  // function.
+  DestructRange(aStart, aCount);
+  this->template SwapFromEnd<InfallibleAlloc>(aStart, aCount,
+                                              sizeof(elem_type),
+                                              MOZ_ALIGNOF(elem_type));
+}
+
 template<typename E, class Alloc>
 template<typename Predicate>
 void