gecko-dev/xpcom/threads/Queue.h

189 строки
4.8 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/. */
#ifndef mozilla_Queue_h
#define mozilla_Queue_h
namespace mozilla {
// A queue implements a singly linked list of pages, each of which contains some
// number of elements. Since the queue needs to store a "next" pointer, the
// actual number of elements per page won't be quite as many as were requested.
//
// This class should only be used if it's valid to construct T elements from all
// zeroes. The class also fails to call the destructor on items. However, it
// will only destroy items after it has moved out their contents. The queue is
// required to be empty when it is destroyed.
template<class T, size_t RequestedItemsPerPage = 256>
class Queue
{
public:
Queue() {}
~Queue()
{
MOZ_RELEASE_ASSERT(IsEmpty());
if (mHead) {
free(mHead);
}
}
T& Push(T&& aElement)
{
if (!mHead) {
mHead = NewPage();
MOZ_ASSERT(mHead);
mTail = mHead;
mOffsetHead = 0;
mOffsetTail = 0;
} else if (mOffsetTail == ItemsPerPage) {
Page* page = NewPage();
MOZ_ASSERT(page);
mTail->mNext = page;
mTail = page;
mOffsetTail = 0;
}
T& eltLocation = mTail->mEvents[mOffsetTail];
eltLocation = Move(aElement);
++mOffsetTail;
return eltLocation;
}
bool IsEmpty() const
{
return !mHead || (mHead == mTail && mOffsetHead == mOffsetTail);
}
T Pop()
{
MOZ_ASSERT(!IsEmpty());
MOZ_ASSERT(mOffsetHead < ItemsPerPage);
MOZ_ASSERT_IF(mHead == mTail, mOffsetHead <= mOffsetTail);
T result = Move(mHead->mEvents[mOffsetHead++]);
MOZ_ASSERT(mOffsetHead <= ItemsPerPage);
// Check if mHead points to empty Page
if (mOffsetHead == ItemsPerPage) {
Page* dead = mHead;
mHead = mHead->mNext;
free(dead);
mOffsetHead = 0;
}
return result;
}
void FirstElementAssertions() const
{
MOZ_ASSERT(!IsEmpty());
MOZ_ASSERT(mOffsetHead < ItemsPerPage);
MOZ_ASSERT_IF(mHead == mTail, mOffsetHead <= mOffsetTail);
}
T& FirstElement()
{
FirstElementAssertions();
return mHead->mEvents[mOffsetHead];
}
const T& FirstElement() const
{
FirstElementAssertions();
return mHead->mEvents[mOffsetHead];
}
void LastElementAssertions() const
{
MOZ_ASSERT(!IsEmpty());
MOZ_ASSERT(mOffsetTail > 0);
MOZ_ASSERT(mOffsetTail <= ItemsPerPage);
MOZ_ASSERT_IF(mHead == mTail, mOffsetHead <= mOffsetTail);
}
T& LastElement()
{
LastElementAssertions();
return mTail->mEvents[mOffsetTail - 1];
}
const T& LastElement() const
{
LastElementAssertions();
return mTail->mEvents[mOffsetTail - 1];
}
size_t Count() const
{
// It is obvious count is 0 when the queue is empty.
if (!mHead) {
return 0;
}
/* How we count the number of events in the queue:
* 1. Let pageCount(x, y) denote the number of pages excluding the tail page
* where x is the index of head page and y is the index of the tail page.
* 2. Then we have pageCount(x, y) = y - x.
*
* Ex: pageCount(0, 0) = 0 where both head and tail pages point to page 0.
* pageCount(0, 1) = 1 where head points to page 0 and tail points page 1.
*
* 3. number of events = (ItemsPerPage * pageCount(x, y))
* - (empty slots in head page) + (non-empty slots in tail page)
* = (ItemsPerPage * pageCount(x, y)) - mOffsetHead + mOffsetTail
*/
int count = -mOffsetHead;
// Compute (ItemsPerPage * pageCount(x, y))
for (Page* page = mHead; page != mTail; page = page->mNext) {
count += ItemsPerPage;
}
count += mOffsetTail;
MOZ_ASSERT(count >= 0);
return count;
}
private:
static_assert((RequestedItemsPerPage & (RequestedItemsPerPage - 1)) == 0,
"RequestedItemsPerPage should be a power of two to avoid heap slop.");
// Since a Page must also contain a "next" pointer, we use one of the items to
// store this pointer. If sizeof(T) > sizeof(Page*), then some space will be
// wasted. So be it.
static const size_t ItemsPerPage = RequestedItemsPerPage - 1;
// Page objects are linked together to form a simple deque.
struct Page
{
struct Page* mNext;
T mEvents[ItemsPerPage];
};
static Page* NewPage()
{
return static_cast<Page*>(moz_xcalloc(1, sizeof(Page)));
}
Page* mHead = nullptr;
Page* mTail = nullptr;
uint16_t mOffsetHead = 0; // offset into mHead where next item is removed
uint16_t mOffsetTail = 0; // offset into mTail where next item is added
};
} // namespace mozilla
#endif // mozilla_Queue_h