Bug 1194721: Add |Saturate| template for saturation arithmetics, r=nf…

…royd

|Saturate<T>| implements saturation arithmetics for arbitrary basic
types. Operations on its value won't over- or underflow the type's
range.

mfbt/Saturate.h

@@ -0,0 +1,287 @@
+/* -*- 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/. */
+
+/* Provides saturation arithmetics for scalar types. */
+
+#ifndef mozilla_Saturate_h
+#define mozilla_Saturate_h
+
+#include "mozilla/Attributes.h"
+#include "mozilla/Move.h"
+#include "mozilla/NumericLimits.h"
+#include "mozilla/TypeTraits.h"
+
+namespace mozilla {
+namespace detail {
+
+/**
+ * |SaturateOp<T>| wraps scalar values for saturation arithmetics. Usage:
+ *
+ *    uint32_t value = 1;
+ *
+ *    ++SaturateOp<uint32_t>(value); // value is 2
+ *    --SaturateOp<uint32_t>(value); // value is 1
+ *    --SaturateOp<uint32_t>(value); // value is 0
+ *    --SaturateOp<uint32_t>(value); // value is still 0
+ *
+ * Please add new operators when required.
+ *
+ * |SaturateOp<T>| will saturate at the minimum and maximum values of
+ * type T. If you need other bounds, implement a clamped-type class and
+ * specialize the type traits accordingly.
+ */
+template <typename T>
+class SaturateOp
+{
+public:
+  explicit SaturateOp(T& aValue)
+    : mValue(aValue)
+  {
+    // We should actually check for |std::is_scalar<T>::value| to be
+    // true, but this type trait is not available everywhere. Relax
+    // this assertion if you want to use floating point values as well.
+    static_assert(IsIntegral<T>::value,
+                  "Integral type required in instantiation");
+  }
+
+  // Add and subtract operators
+
+  T operator+(const T& aRhs) const
+  {
+    return T(mValue) += aRhs;
+  }
+
+  T operator-(const T& aRhs) const
+  {
+    return T(mValue) -= aRhs;
+  }
+
+  // Compound operators
+
+  const T& operator+=(const T& aRhs) const
+  {
+    const T min = NumericLimits<T>::min();
+    const T max = NumericLimits<T>::max();
+
+    if (aRhs > static_cast<T>(0)) {
+      mValue = (max - aRhs) < mValue ? max : mValue + aRhs;
+    } else {
+      mValue = (min - aRhs) > mValue ? min : mValue + aRhs;
+    }
+    return mValue;
+  }
+
+  const T& operator-=(const T& aRhs) const
+  {
+    const T min = NumericLimits<T>::min();
+    const T max = NumericLimits<T>::max();
+
+    if (aRhs > static_cast<T>(0)) {
+      mValue = (min + aRhs) > mValue ? min : mValue - aRhs;
+    } else {
+      mValue = (max + aRhs) < mValue ? max : mValue - aRhs;
+    }
+    return mValue;
+  }
+
+  // Increment and decrement operators
+
+  const T& operator++() const // prefix
+  {
+    return operator+=(static_cast<T>(1));
+  }
+
+  T operator++(int) const // postfix
+  {
+    const T value(mValue);
+    operator++();
+    return value;
+  }
+
+  const T& operator--() const // prefix
+  {
+    return operator-=(static_cast<T>(1));
+  }
+
+  T operator--(int) const // postfix
+  {
+    const T value(mValue);
+    operator--();
+    return value;
+  }
+
+private:
+  SaturateOp(const SaturateOp<T>&) = delete;
+  SaturateOp(SaturateOp<T>&&) = delete;
+  SaturateOp& operator=(const SaturateOp<T>&) = delete;
+  SaturateOp& operator=(SaturateOp<T>&&) = delete;
+
+  T& mValue;
+};
+
+/**
+ * |Saturate<T>| is a value type for saturation arithmetics. It's
+ * build on top of |SaturateOp<T>|.
+ */
+template <typename T>
+class Saturate
+{
+public:
+  Saturate() = default;
+  MOZ_IMPLICIT Saturate(const Saturate<T>&) = default;
+
+  MOZ_IMPLICIT Saturate(Saturate<T>&& aValue)
+  {
+    mValue = Move(aValue.mValue);
+  }
+
+  explicit Saturate(const T& aValue)
+    : mValue(aValue)
+  { }
+
+  const T& value() const
+  {
+    return mValue;
+  }
+
+  // Compare operators
+
+  bool operator==(const Saturate<T>& aRhs) const
+  {
+    return mValue == aRhs.mValue;
+  }
+
+  bool operator!=(const Saturate<T>& aRhs) const
+  {
+    return !operator==(aRhs);
+  }
+
+  bool operator==(const T& aRhs) const
+  {
+    return mValue == aRhs;
+  }
+
+  bool operator!=(const T& aRhs) const
+  {
+    return !operator==(aRhs);
+  }
+
+  // Assignment operators
+
+  Saturate<T>& operator=(const Saturate<T>&) = default;
+
+  Saturate<T>& operator=(Saturate<T>&& aRhs)
+  {
+    mValue = Move(aRhs.mValue);
+    return *this;
+  }
+
+  // Add and subtract operators
+
+  Saturate<T> operator+(const Saturate<T>& aRhs) const
+  {
+    Saturate<T> lhs(mValue);
+    return lhs += aRhs.mValue;
+  }
+
+  Saturate<T> operator+(const T& aRhs) const
+  {
+    Saturate<T> lhs(mValue);
+    return lhs += aRhs;
+  }
+
+  Saturate<T> operator-(const Saturate<T>& aRhs) const
+  {
+    Saturate<T> lhs(mValue);
+    return lhs -= aRhs.mValue;
+  }
+
+  Saturate<T> operator-(const T& aRhs) const
+  {
+    Saturate<T> lhs(mValue);
+    return lhs -= aRhs;
+  }
+
+  // Compound operators
+
+  Saturate<T>& operator+=(const Saturate<T>& aRhs)
+  {
+    SaturateOp<T>(mValue) += aRhs.mValue;
+    return *this;
+  }
+
+  Saturate<T>& operator+=(const T& aRhs)
+  {
+    SaturateOp<T>(mValue) += aRhs;
+    return *this;
+  }
+
+  Saturate<T>& operator-=(const Saturate<T>& aRhs)
+  {
+    SaturateOp<T>(mValue) -= aRhs.mValue;
+    return *this;
+  }
+
+  Saturate<T>& operator-=(const T& aRhs)
+  {
+    SaturateOp<T>(mValue) -= aRhs;
+    return *this;
+  }
+
+  // Increment and decrement operators
+
+  Saturate<T>& operator++() // prefix
+  {
+    ++SaturateOp<T>(mValue);
+    return *this;
+  }
+
+  Saturate<T> operator++(int) // postfix
+  {
+    return Saturate<T>(SaturateOp<T>(mValue)++);
+  }
+
+  Saturate<T>& operator--() // prefix
+  {
+    --SaturateOp<T>(mValue);
+    return *this;
+  }
+
+  Saturate<T> operator--(int) // postfix
+  {
+    return Saturate<T>(SaturateOp<T>(mValue)--);
+  }
+
+private:
+  T mValue;
+};
+
+} // namespace detail
+
+typedef detail::Saturate<int8_t> SaturateInt8;
+typedef detail::Saturate<int16_t> SaturateInt16;
+typedef detail::Saturate<int32_t> SaturateInt32;
+typedef detail::Saturate<uint8_t> SaturateUint8;
+typedef detail::Saturate<uint16_t> SaturateUint16;
+typedef detail::Saturate<uint32_t> SaturateUint32;
+
+} // namespace mozilla
+
+template<typename LhsT, typename RhsT>
+bool
+operator==(LhsT aLhs, const mozilla::detail::Saturate<RhsT>& aRhs)
+{
+  return aRhs.operator==(static_cast<RhsT>(aLhs));
+}
+
+template<typename LhsT, typename RhsT>
+bool
+operator!=(LhsT aLhs, const mozilla::detail::Saturate<RhsT>& aRhs)
+{
+  return !(aLhs == aRhs);
+}
+
+#endif // mozilla_Saturate_h

mfbt/moz.build

@@ -73,6 +73,7 @@ EXPORTS.mozilla = [
     'RefPtr.h',
     'ReverseIterator.h',
     'RollingMean.h',
+    'Saturate.h',
     'Scoped.h',
     'ScopeExit.h',
     'SegmentedVector.h',