gecko-dev/xpcom/io/nsSegmentedBuffer.h

/* -*- 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 nsSegmentedBuffer_h__
#define nsSegmentedBuffer_h__

#include <stddef.h>
#include <functional>

#include "nsCOMPtr.h"
#include "nsDebug.h"
#include "nsError.h"
#include "nsTArray.h"
#include "mozilla/DataMutex.h"

class nsIEventTarget;

class nsSegmentedBuffer {
 public:
  nsSegmentedBuffer()
      : mSegmentSize(0),
        mSegmentArray(nullptr),
        mSegmentArrayCount(0),
        mFirstSegmentIndex(0),
        mLastSegmentIndex(0) {}

  ~nsSegmentedBuffer() { Empty(); }

  nsresult Init(uint32_t aSegmentSize);

  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 char* GetSegment(uint32_t aIndex) {
    NS_ASSERTION(aIndex < GetSegmentCount(), "index out of bounds");
    int32_t i = ModSegArraySize(mFirstSegmentIndex + (int32_t)aIndex);
    return mSegmentArray[i];
  }

 protected:
  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;
  }

 protected:
  uint32_t mSegmentSize;
  char** mSegmentArray;
  uint32_t mSegmentArrayCount;
  int32_t mFirstSegmentIndex;
  int32_t mLastSegmentIndex;

 private:
  class FreeOMTPointers {
    NS_INLINE_DECL_THREADSAFE_REFCOUNTING(FreeOMTPointers)

   public:
    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();
      tasks->AppendElement(std::move(aTask));
      return tasks->Length();
    }

   private:
    ~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.
#define NS_SEGMENTARRAY_INITIAL_COUNT 32

#endif  // nsSegmentedBuffer_h__