mfbt/RollingMean.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 /* Calculate the rolling mean of a series of values. */
   8
   9 #ifndef mozilla_RollingMean_h_
  10 #define mozilla_RollingMean_h_
  11
  12 #include "mozilla/Assertions.h"
  13 #include "mozilla/Vector.h"
  14
  15 #include <stddef.h>
  16 #include <type_traits>
  17
  18 namespace mozilla {
  19
  20 /**
  21  * RollingMean<T> calculates a rolling mean of the values it is given. It
  22  * accumulates the total as values are added and removed. The second type
  23  * argument S specifies the type of the total. This may need to be a bigger
  24  * type in order to maintain that the sum of all values in the average doesn't
  25  * exceed the maximum input value.
  26  *
  27  * WARNING: Float types are not supported due to rounding errors.
  28  */
  29 template <typename T, typename S>
  30 class RollingMean {
  31  private:
  32   size_t mInsertIndex;
  33   size_t mMaxValues;
  34   Vector<T> mValues;
  35   S mTotal;
  36
  37  public:
  38   static_assert(!std::is_floating_point_v<T>,
  39                 "floating-point types are unsupported due to rounding "
  40                 "errors");
  41
  42   explicit RollingMean(size_t aMaxValues)
  43       : mInsertIndex(0), mMaxValues(aMaxValues), mTotal(0) {
  44     MOZ_ASSERT(aMaxValues > 0);
  45   }
  46
  47   RollingMean& operator=(RollingMean&& aOther) = default;
  48
  49   /**
  50    * Insert a value into the rolling mean.
  51    */
  52   bool insert(T aValue) {
  53     MOZ_ASSERT(mValues.length() <= mMaxValues);
  54
  55     if (mValues.length() == mMaxValues) {
  56       mTotal = mTotal - mValues[mInsertIndex] + aValue;
  57       mValues[mInsertIndex] = aValue;
  58     } else {
  59       if (!mValues.append(aValue)) {
  60         return false;
  61       }
  62       mTotal = mTotal + aValue;
  63     }
  64
  65     mInsertIndex = (mInsertIndex + 1) % mMaxValues;
  66     return true;
  67   }
  68
  69   /**
  70    * Calculate the rolling mean.
  71    */
  72   T mean() const {
  73     MOZ_ASSERT(!empty());
  74     return T(mTotal / int64_t(mValues.length()));
  75   }
  76
  77   bool empty() const { return mValues.empty(); }
  78
  79   /**
  80    * Remove all values from the rolling mean.
  81    */
  82   void clear() {
  83     mValues.clear();
  84     mInsertIndex = 0;
  85     mTotal = T(0);
  86   }
  87
  88   size_t maxValues() const { return mMaxValues; }
  89 };
  90
  91 }  // namespace mozilla
  92
  93 #endif  // mozilla_RollingMean_h_