mfbt/Saturate.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 /* Provides saturation arithmetics for scalar types. */
   8
   9 #ifndef mozilla_Saturate_h
  10 #define mozilla_Saturate_h
  11
  12 #include "mozilla/Attributes.h"
  13 #include "mozilla/Move.h"
  14 #include "mozilla/TypeTraits.h"
  15
  16 #include <limits>
  17
  18 namespace mozilla {
  19 namespace detail {
  20
  21 /**
  22  * |SaturateOp<T>| wraps scalar values for saturation arithmetics. Usage:
  23  *
  24  *    uint32_t value = 1;
  25  *
  26  *    ++SaturateOp<uint32_t>(value); // value is 2
  27  *    --SaturateOp<uint32_t>(value); // value is 1
  28  *    --SaturateOp<uint32_t>(value); // value is 0
  29  *    --SaturateOp<uint32_t>(value); // value is still 0
  30  *
  31  * Please add new operators when required.
  32  *
  33  * |SaturateOp<T>| will saturate at the minimum and maximum values of
  34  * type T. If you need other bounds, implement a clamped-type class and
  35  * specialize the type traits accordingly.
  36  */
  37 template <typename T>
  38 class SaturateOp {
  39  public:
  40   explicit SaturateOp(T& aValue) : mValue(aValue) {
  41     // We should actually check for |std::is_scalar<T>::value| to be
  42     // true, but this type trait is not available everywhere. Relax
  43     // this assertion if you want to use floating point values as well.
  44     static_assert(IsIntegral<T>::value,
  45                   "Integral type required in instantiation");
  46   }
  47
  48   // Add and subtract operators
  49
  50   T operator+(const T& aRhs) const { return T(mValue) += aRhs; }
  51
  52   T operator-(const T& aRhs) const { return T(mValue) -= aRhs; }
  53
  54   // Compound operators
  55
  56   const T& operator+=(const T& aRhs) const {
  57     const T min = std::numeric_limits<T>::min();
  58     const T max = std::numeric_limits<T>::max();
  59
  60     if (aRhs > static_cast<T>(0)) {
  61       mValue = (max - aRhs) < mValue ? max : mValue + aRhs;
  62     } else {
  63       mValue = (min - aRhs) > mValue ? min : mValue + aRhs;
  64     }
  65     return mValue;
  66   }
  67
  68   const T& operator-=(const T& aRhs) const {
  69     const T min = std::numeric_limits<T>::min();
  70     const T max = std::numeric_limits<T>::max();
  71
  72     if (aRhs > static_cast<T>(0)) {
  73       mValue = (min + aRhs) > mValue ? min : mValue - aRhs;
  74     } else {
  75       mValue = (max + aRhs) < mValue ? max : mValue - aRhs;
  76     }
  77     return mValue;
  78   }
  79
  80   // Increment and decrement operators
  81
  82   const T& operator++() const  // prefix
  83   {
  84     return operator+=(static_cast<T>(1));
  85   }
  86
  87   T operator++(int) const  // postfix
  88   {
  89     const T value(mValue);
  90     operator++();
  91     return value;
  92   }
  93
  94   const T& operator--() const  // prefix
  95   {
  96     return operator-=(static_cast<T>(1));
  97   }
  98
  99   T operator--(int) const  // postfix
 100   {
 101     const T value(mValue);
 102     operator--();
 103     return value;
 104   }
 105
 106  private:
 107   SaturateOp(const SaturateOp<T>&) = delete;
 108   SaturateOp(SaturateOp<T>&&) = delete;
 109   SaturateOp& operator=(const SaturateOp<T>&) = delete;
 110   SaturateOp& operator=(SaturateOp<T>&&) = delete;
 111
 112   T& mValue;
 113 };
 114
 115 /**
 116  * |Saturate<T>| is a value type for saturation arithmetics. It's
 117  * built on top of |SaturateOp<T>|.
 118  */
 119 template <typename T>
 120 class Saturate {
 121  public:
 122   Saturate() = default;
 123   MOZ_IMPLICIT Saturate(const Saturate<T>&) = default;
 124
 125   MOZ_IMPLICIT Saturate(Saturate<T>&& aValue) {
 126     mValue = std::move(aValue.mValue);
 127   }
 128
 129   explicit Saturate(const T& aValue) : mValue(aValue) {}
 130
 131   const T& value() const { return mValue; }
 132
 133   // Compare operators
 134
 135   bool operator==(const Saturate<T>& aRhs) const {
 136     return mValue == aRhs.mValue;
 137   }
 138
 139   bool operator!=(const Saturate<T>& aRhs) const { return !operator==(aRhs); }
 140
 141   bool operator==(const T& aRhs) const { return mValue == aRhs; }
 142
 143   bool operator!=(const T& aRhs) const { return !operator==(aRhs); }
 144
 145   // Assignment operators
 146
 147   Saturate<T>& operator=(const Saturate<T>&) = default;
 148
 149   Saturate<T>& operator=(Saturate<T>&& aRhs) {
 150     mValue = std::move(aRhs.mValue);
 151     return *this;
 152   }
 153
 154   // Add and subtract operators
 155
 156   Saturate<T> operator+(const Saturate<T>& aRhs) const {
 157     Saturate<T> lhs(mValue);
 158     return lhs += aRhs.mValue;
 159   }
 160
 161   Saturate<T> operator+(const T& aRhs) const {
 162     Saturate<T> lhs(mValue);
 163     return lhs += aRhs;
 164   }
 165
 166   Saturate<T> operator-(const Saturate<T>& aRhs) const {
 167     Saturate<T> lhs(mValue);
 168     return lhs -= aRhs.mValue;
 169   }
 170
 171   Saturate<T> operator-(const T& aRhs) const {
 172     Saturate<T> lhs(mValue);
 173     return lhs -= aRhs;
 174   }
 175
 176   // Compound operators
 177
 178   Saturate<T>& operator+=(const Saturate<T>& aRhs) {
 179     SaturateOp<T>(mValue) += aRhs.mValue;
 180     return *this;
 181   }
 182
 183   Saturate<T>& operator+=(const T& aRhs) {
 184     SaturateOp<T>(mValue) += aRhs;
 185     return *this;
 186   }
 187
 188   Saturate<T>& operator-=(const Saturate<T>& aRhs) {
 189     SaturateOp<T>(mValue) -= aRhs.mValue;
 190     return *this;
 191   }
 192
 193   Saturate<T>& operator-=(const T& aRhs) {
 194     SaturateOp<T>(mValue) -= aRhs;
 195     return *this;
 196   }
 197
 198   // Increment and decrement operators
 199
 200   Saturate<T>& operator++()  // prefix
 201   {
 202     ++SaturateOp<T>(mValue);
 203     return *this;
 204   }
 205
 206   Saturate<T> operator++(int)  // postfix
 207   {
 208     return Saturate<T>(SaturateOp<T>(mValue)++);
 209   }
 210
 211   Saturate<T>& operator--()  // prefix
 212   {
 213     --SaturateOp<T>(mValue);
 214     return *this;
 215   }
 216
 217   Saturate<T> operator--(int)  // postfix
 218   {
 219     return Saturate<T>(SaturateOp<T>(mValue)--);
 220   }
 221
 222  private:
 223   T mValue;
 224 };
 225
 226 }  // namespace detail
 227
 228 typedef detail::Saturate<int8_t> SaturateInt8;
 229 typedef detail::Saturate<int16_t> SaturateInt16;
 230 typedef detail::Saturate<int32_t> SaturateInt32;
 231 typedef detail::Saturate<uint8_t> SaturateUint8;
 232 typedef detail::Saturate<uint16_t> SaturateUint16;
 233 typedef detail::Saturate<uint32_t> SaturateUint32;
 234
 235 }  // namespace mozilla
 236
 237 template <typename LhsT, typename RhsT>
 238 bool operator==(LhsT aLhs, const mozilla::detail::Saturate<RhsT>& aRhs) {
 239   return aRhs.operator==(static_cast<RhsT>(aLhs));
 240 }
 241
 242 template <typename LhsT, typename RhsT>
 243 bool operator!=(LhsT aLhs, const mozilla::detail::Saturate<RhsT>& aRhs) {
 244   return !(aLhs == aRhs);
 245 }
 246
 247 #endif  // mozilla_Saturate_h