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/* -*- 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 */
#ifndef nsSegmentedBuffer_h__
#define nsSegmentedBuffer_h__
#include <stddef.h>
#include "nsCOMPtr.h"
#include "nsDebug.h"
#include "nsError.h"
#include "mozilla/DataMutex.h"
class nsIEventTarget;
class nsSegmentedBuffer {
: mSegmentSize(0),
mLastSegmentIndex(0) {}
~nsSegmentedBuffer() { Empty(); }
nsresult Init(uint32_t aSegmentSize, uint32_t aMaxSize);
char* AppendNewSegment(); // pushes at end
// returns true if no more segments remain:
bool DeleteFirstSegment(); // pops from beginning
// returns true if no more segments remain:
bool DeleteLastSegment(); // pops from beginning
// Call Realloc() on last segment. This is used to reduce memory
// consumption when data is not an exact multiple of segment size.
bool ReallocLastSegment(size_t aNewSize);
void Empty(); // frees all segments
inline uint32_t GetSegmentCount() {
if (mFirstSegmentIndex <= mLastSegmentIndex) {
return mLastSegmentIndex - mFirstSegmentIndex;
} else {
return mSegmentArrayCount + mLastSegmentIndex - mFirstSegmentIndex;
inline uint32_t GetSegmentSize() { return mSegmentSize; }
inline uint32_t GetMaxSize() { return mMaxSize; }
inline uint32_t GetSize() { return GetSegmentCount() * mSegmentSize; }
inline char* GetSegment(uint32_t aIndex) {
NS_ASSERTION(aIndex < GetSegmentCount(), "index out of bounds");
int32_t i = ModSegArraySize(mFirstSegmentIndex + (int32_t)aIndex);
return mSegmentArray[i];
inline int32_t ModSegArraySize(int32_t aIndex) {
uint32_t result = aIndex & (mSegmentArrayCount - 1);
NS_ASSERTION(result == aIndex % mSegmentArrayCount,
"non-power-of-2 mSegmentArrayCount");
return result;
inline bool IsFull() {
return ModSegArraySize(mLastSegmentIndex + 1) == mFirstSegmentIndex;
uint32_t mSegmentSize;
uint32_t mMaxSize;
char** mSegmentArray;
uint32_t mSegmentArrayCount;
int32_t mFirstSegmentIndex;
int32_t mLastSegmentIndex;
class FreeOMTPointers {
FreeOMTPointers() : mTasks("nsSegmentedBuffer::FreeOMTPointers") {}
void FreeAll();
// Adds a task to the array. Returns the size of the array.
size_t AddTask(std::function<void()>&& aTask) {
auto tasks = mTasks.Lock();
return tasks->Length();
~FreeOMTPointers() = default;
mozilla::DataMutex<nsTArray<std::function<void()>>> mTasks;
void FreeOMT(void* aPtr);
void FreeOMT(std::function<void()>&& aTask);
nsCOMPtr<nsIEventTarget> mIOThread;
// This object is created the first time we need to dispatch to another thread
// to free segments. It is only freed when the nsSegmentedBufer is destroyed
// or when the runnable is finally handled and its refcount goes to 0.
RefPtr<FreeOMTPointers> mFreeOMT;
// NS_SEGMENTARRAY_INITIAL_SIZE: This number needs to start out as a
// power of 2 given how it gets used. We double the segment array
// when we overflow it, and use that fact that it's a power of 2
// to compute a fast modulus operation in IsFull.
// 32 segment array entries can accommodate 128k of data if segments
// are 4k in size. That seems like a reasonable amount that will avoid
// needing to grow the segment array.
#endif // nsSegmentedBuffer_h__