saturating_arithmetic.h

// Copyright 2025 The Pigweed Authors
//
// Licensed under the Apache License, Version 2.0 (the "License"); you may not
// use this file except in compliance with the License. You may obtain a copy of
// the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
// License for the specific language governing permissions and limitations under
// the License.
#pragma once
 
#if __has_include(<version>)
#include <version>
#endif  // __has_include(<version>)
 
#ifdef __cpp_lib_saturation_arithmetic
 
#include <numeric>
 
namespace pw {
 
 
using std::add_sat;
using std::mul_sat;
 
}  // namespace pw
 
#else
 
#include <limits>
#include <type_traits>
 
#include "pw_numeric/checked_arithmetic.h"
 
// Polyfills of C++26's saturating arithmetic operations in <numeric>.
 
namespace pw {
 
 
template <typename T>
constexpr T add_sat(T lhs, T rhs) noexcept {
  static_assert(std::is_integral_v<T>);
  if (T sum = 0; CheckedAdd(lhs, rhs, sum)) {
    return sum;
  }
  if constexpr (std::is_unsigned_v<T>) {
    return std::numeric_limits<T>::max();
  } else {  // since it overflowed, the operands had the same sign
    if (lhs < 0) {
      return std::numeric_limits<T>::min();
    }
    return std::numeric_limits<T>::max();
  }
}
 
template <typename T>
constexpr T mul_sat(T lhs, T rhs) noexcept {
  static_assert(std::is_integral_v<T>);
  if (T product = 0; CheckedMul(lhs, rhs, product)) {
    return product;
  }
  if constexpr (std::is_unsigned_v<T>) {
    return std::numeric_limits<T>::max();
  } else {  // determine sign of result by comparing signs of the factors
    if ((lhs < 0) == (rhs < 0)) {
      return std::numeric_limits<T>::max();
    }
    return std::numeric_limits<T>::min();
  }
}
 
}  // namespace pw
 
#endif  // __cpp_lib_saturation_arithmetic