mfbt/RollingMean.h
author Petru Lingurar <petru.lingurar@softvision.ro>
Fri, 21 Dec 2018 08:56:47 +0000
changeset 501492 65621d0fe1262af0643cec37c23b2d9ec42588ad
parent 436847 b0e818e98bb51abf5336484a7cf47b10ddfb88fa
child 508163 6f3709b3878117466168c40affa7bca0b60cf75b
permissions -rw-r--r--
Bug 1513938 - Enforce a Bundle size limit and drop `privateSession` if exceeds it. r=JanH, a=jcristau The `privateSession` key would normally allow persisting the Private Browsing session across OOMs in Activity's Bundle. We need to do that to avoid storing private, sensible data on disk like we do with the normal browsing session. In some cases `privateSession` would contain a lot of data which, along with other possible concurrent transactions could overflow Binder's buffer which has a limited fixed size, currently 1Mb. To avoid this, we will drop `privateSession` from the Bundle if the resulting size is greater than a _speculative_ size of 300KBs which would mean that in the case of an OOM all Private Browsing state would be lost. Bug 1515592 is filed to investigate for a better solution. Differential Revision: https://phabricator.services.mozilla.com/D15067

/* -*- 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/. */

/* Calculate the rolling mean of a series of values. */

#ifndef mozilla_RollingMean_h_
#define mozilla_RollingMean_h_

#include "mozilla/Assertions.h"
#include "mozilla/TypeTraits.h"
#include "mozilla/Vector.h"

#include <stddef.h>

namespace mozilla {

/**
 * RollingMean<T> calculates a rolling mean of the values it is given. It
 * accumulates the total as values are added and removed. The second type
 * argument S specifies the type of the total. This may need to be a bigger
 * type in order to maintain that the sum of all values in the average doesn't
 * exceed the maximum input value.
 *
 * WARNING: Float types are not supported due to rounding errors.
 */
template<typename T, typename S>
class RollingMean
{
private:
  size_t mInsertIndex;
  size_t mMaxValues;
  Vector<T> mValues;
  S mTotal;

public:
  static_assert(!IsFloatingPoint<T>::value,
                "floating-point types are unsupported due to rounding "
                "errors");

  explicit RollingMean(size_t aMaxValues)
    : mInsertIndex(0),
      mMaxValues(aMaxValues),
      mTotal(0)
  {
    MOZ_ASSERT(aMaxValues > 0);
  }

  RollingMean& operator=(RollingMean&& aOther)
  {
    MOZ_ASSERT(this != &aOther, "self-assignment is forbidden");
    this->~RollingMean();
    new(this) RollingMean(aOther.mMaxValues);
    mInsertIndex = aOther.mInsertIndex;
    mTotal = aOther.mTotal;
    mValues.swap(aOther.mValues);
    return *this;
  }

  /**
   * Insert a value into the rolling mean.
   */
  bool insert(T aValue)
  {
    MOZ_ASSERT(mValues.length() <= mMaxValues);

    if (mValues.length() == mMaxValues) {
      mTotal = mTotal - mValues[mInsertIndex] + aValue;
      mValues[mInsertIndex] = aValue;
    } else {
      if (!mValues.append(aValue)) {
        return false;
      }
      mTotal = mTotal + aValue;
    }

    mInsertIndex = (mInsertIndex + 1) % mMaxValues;
    return true;
  }

  /**
   * Calculate the rolling mean.
   */
  T mean()
  {
    MOZ_ASSERT(!empty());
    return T(mTotal / int64_t(mValues.length()));
  }

  bool empty()
  {
    return mValues.empty();
  }

  /**
   * Remove all values from the rolling mean.
   */
  void clear()
  {
    mValues.clear();
    mInsertIndex = 0;
    mTotal = T(0);
  }

  size_t maxValues()
  {
    return mMaxValues;
  }
};

} // namespace mozilla

#endif // mozilla_RollingMean_h_