gecko-dev/mfbt/SmallPointerArray.h

216 строки
5.7 KiB
C++

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
/* A vector of pointers space-optimized for a small number of elements. */
#ifndef mozilla_SmallPointerArray_h
#define mozilla_SmallPointerArray_h
#include "mozilla/Assertions.h"
#include <iterator>
#include <vector>
namespace mozilla {
// Array class for situations where a small number of elements (<= 2) is
// expected, a large number of elements must be accomodated if necessary,
// and the size of the class must be minimal. Typical vector implementations
// will fulfill the first two requirements by simply adding inline storage
// alongside the rest of their member variables. While this strategy works,
// it brings unnecessary storage overhead for vectors with an expected small
// number of elements. This class is intended to deal with that problem.
//
// This class is similar in performance to a vector class. Accessing its
// elements when it has not grown over a size of 2 does not require an extra
// level of indirection and will therefore be faster.
//
// The minimum (inline) size is 2 * sizeof(void*).
//
// Any modification of the array invalidates any outstanding iterators.
template<typename T>
class SmallPointerArray
{
public:
SmallPointerArray()
{
mInlineElements[0] = mInlineElements[1] = nullptr;
static_assert(sizeof(SmallPointerArray<T>) == (2 * sizeof(void*)),
"SmallPointerArray must compile to the size of 2 pointers");
static_assert(offsetof(SmallPointerArray<T>, mArray) ==
offsetof(SmallPointerArray<T>, mInlineElements) + sizeof(T*),
"mArray and mInlineElements[1] are expected to overlap in memory");
static_assert(offsetof(SmallPointerArray<T>, mPadding) ==
offsetof(SmallPointerArray<T>, mInlineElements),
"mPadding and mInlineElements[0] are expected to overlap in memory");
}
~SmallPointerArray()
{
if (!mInlineElements[0] && mArray) {
delete mArray;
}
}
void Clear() {
if (!mInlineElements[0] && mArray) {
delete mArray;
mArray = nullptr;
return;
}
mInlineElements[0] = mInlineElements[1] = nullptr;
}
void AppendElement(T* aElement) {
// Storing nullptr as an element is not permitted, but we do check for it
// to avoid corruption issues in non-debug builds.
// In addition to this we assert in debug builds to point out mistakes to
// users of the class.
MOZ_ASSERT(aElement != nullptr);
if (!mInlineElements[0]) {
if (!mArray) {
mInlineElements[0] = aElement;
// Harmless if aElement == nullptr;
return;
}
if (!aElement) {
return;
}
mArray->push_back(aElement);
return;
}
if (!aElement) {
return;
}
if (!mInlineElements[1]) {
mInlineElements[1] = aElement;
return;
}
mArray = new std::vector<T*>({ mInlineElements[0], mInlineElements[1], aElement });
mInlineElements[0] = nullptr;
}
void RemoveElement(T* aElement) {
MOZ_ASSERT(aElement != nullptr);
if (aElement == nullptr) {
return;
}
if (mInlineElements[0] == aElement) {
// Expectected case.
mInlineElements[0] = mInlineElements[1];
mInlineElements[1] = nullptr;
return;
}
if (mInlineElements[0]) {
if (mInlineElements[1] == aElement) {
mInlineElements[1] = nullptr;
}
return;
}
if (mArray) {
for (auto iter = mArray->begin(); iter != mArray->end(); iter++) {
if (*iter == aElement) {
mArray->erase(iter);
return;
}
}
}
}
size_t Length() const
{
if (mInlineElements[0]) {
if (!mInlineElements[1]) {
return 1;
}
return 2;
}
if (mArray) {
return mArray->size();
}
return 0;
}
T* ElementAt(size_t aIndex) const {
MOZ_ASSERT(aIndex < Length());
if (mInlineElements[0]) {
return mInlineElements[aIndex];
}
return (*mArray)[aIndex];
}
T* operator[](size_t aIndex) const
{
return ElementAt(aIndex);
}
typedef T** iterator;
typedef const T** const_iterator;
// Methods for range-based for loops. Manipulation invalidates these.
iterator begin() {
return beginInternal();
}
const_iterator begin() const {
return beginInternal();
}
const_iterator cbegin() const { return begin(); }
iterator end() {
return beginInternal() + Length();
}
const_iterator end() const {
return beginInternal() + Length();
}
const_iterator cend() const { return end(); }
private:
T** beginInternal() const {
if (mInlineElements[0] || !mArray) {
return const_cast<T**>(&mInlineElements[0]);
}
if (mArray->empty()) {
return nullptr;
}
return &(*mArray)[0];
}
// mArray and mInlineElements[1] share the same area in memory.
//
// When !mInlineElements[0] && !mInlineElements[1] the array is empty.
//
// When mInlineElements[0] && !mInlineElements[1], mInlineElements[0]
// contains the first element. The array is of size 1.
//
// When mInlineElements[0] && mInlineElements[1], mInlineElements[0]
// contains the first element and mInlineElements[1] the second. The
// array is of size 2.
//
// When !mInlineElements[0] && mArray, mArray contains the full contents
// of the array and is of arbitrary size.
union {
T* mInlineElements[2];
struct {
void* mPadding;
std::vector<T*>* mArray;
};
};
};
} // namespace mozilla
#endif // mozilla_SmallPointerArray_h