1 // Copyright 2023 The Chromium Authors
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
4
5 #ifndef BASE_NUMERICS_WRAPPING_MATH_H_
6 #define BASE_NUMERICS_WRAPPING_MATH_H_
7
8 #include <type_traits>
9
10 namespace base {
11
12 // Returns `a + b` with overflow defined to wrap around, i.e. modulo 2^N where N
13 // is the bit width of `T`.
14 template <typename T>
WrappingAdd(T a,T b)15 inline constexpr T WrappingAdd(T a, T b) {
16 static_assert(std::is_integral_v<T>);
17 // Unsigned arithmetic wraps, so convert to the corresponding unsigned type.
18 // Note that, if `T` is smaller than `int`, e.g. `int16_t`, the values are
19 // promoted to `int`, which brings us back to undefined overflow. This is fine
20 // here because the sum of any two `int16_t`s fits in `int`, but `WrappingMul`
21 // will need a more complex implementation.
22 using Unsigned = std::make_unsigned_t<T>;
23 return static_cast<T>(static_cast<Unsigned>(a) + static_cast<Unsigned>(b));
24 }
25
26 // Returns `a - b` with overflow defined to wrap around, i.e. modulo 2^N where N
27 // is the bit width of `T`.
28 template <typename T>
WrappingSub(T a,T b)29 inline constexpr T WrappingSub(T a, T b) {
30 static_assert(std::is_integral_v<T>);
31 // Unsigned arithmetic wraps, so convert to the corresponding unsigned type.
32 // Note that, if `T` is smaller than `int`, e.g. `int16_t`, the values are
33 // promoted to `int`, which brings us back to undefined overflow. This is fine
34 // here because the difference of any two `int16_t`s fits in `int`, but
35 // `WrappingMul` will need a more complex implementation.
36 using Unsigned = std::make_unsigned_t<T>;
37 return static_cast<T>(static_cast<Unsigned>(a) - static_cast<Unsigned>(b));
38 }
39
40 } // namespace base
41
42 #endif // BASE_NUMERICS_WRAPPING_MATH_H_
43