chromium/base/bits.h

// Copyright 2013 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// This file defines some bit utilities.

#ifndef BASE_BITS_H_
#define BASE_BITS_H_

#include <stddef.h>
#include <stdint.h>

#include <bit>
#include <concepts>

#include "base/check.h"

namespace base::bits {

// Bit functions in <bit> are restricted to a specific set of types of unsigned
// integer; restrict functions in this file that are related to those in that
// header to match for consistency.
UnsignedInteger;

// We want to migrate all users of these functions to use the unsigned type
// versions of the functions, but until they are all moved over, create a
// concept that captures all the types that must be supported for compatibility
// but that we want to remove.
//
// TODO(crbug.com/40256225): Switch uses to supported functions and
// remove.
SignedIntegerDeprecatedDoNotUse;

// Returns true iff |value| is a power of 2. DEPRECATED; use
// std::has_single_bit() instead.
//
// TODO(crbug.com/40256225): Switch uses and remove.
template <typename T>
  requires SignedIntegerDeprecatedDoNotUse<T>
constexpr bool IsPowerOfTwoDeprecatedDoNotUse(T value) { … }

// Round down |size| to a multiple of alignment, which must be a power of two.
template <typename T>
  requires UnsignedInteger<T>
inline constexpr T AlignDown(T size, T alignment) { … }

// Round down |size| to a multiple of alignment, which must be a power of two.
// DEPRECATED; use the UnsignedInteger version.
//
// TODO(crbug.com/40256225): Switch uses and remove.
template <typename T>
  requires SignedIntegerDeprecatedDoNotUse<T>
inline constexpr T AlignDownDeprecatedDoNotUse(T size, T alignment) { … }

// Move |ptr| back to the previous multiple of alignment, which must be a power
// of two. Defined for types where sizeof(T) is one byte.
template <typename T>
  requires(sizeof(T) == 1)
inline T* AlignDown(T* ptr, uintptr_t alignment) { … }

// Round up |size| to a multiple of alignment, which must be a power of two.
template <typename T>
  requires UnsignedInteger<T>
inline constexpr T AlignUp(T size, T alignment) { … }

// Round up |size| to a multiple of alignment, which must be a power of two.
// DEPRECATED; use the UnsignedInteger version.
//
// TODO(crbug.com/40256225): Switch uses and remove.
template <typename T>
  requires SignedIntegerDeprecatedDoNotUse<T>
inline constexpr T AlignUpDeprecatedDoNotUse(T size, T alignment) { … }

// Advance |ptr| to the next multiple of alignment, which must be a power of
// two. Defined for types where sizeof(T) is one byte.
template <typename T>
  requires(sizeof(T) == 1)
inline T* AlignUp(T* ptr, uintptr_t alignment) { … }

// Returns the integer i such as 2^i <= n < 2^(i+1).
//
// A common use for this function is to measure the number of bits required to
// contain a value; for that case use std::bit_width().
//
// A common use for this function is to take its result and use it to left-shift
// a bit; instead of doing so, use std::bit_floor().
constexpr int Log2Floor(uint32_t n) { … }

// Returns the integer i such as 2^(i-1) < n <= 2^i.
//
// A common use for this function is to measure the number of bits required to
// contain a value; for that case use std::bit_width().
//
// A common use for this function is to take its result and use it to left-shift
// a bit; instead of doing so, use std::bit_ceil().
constexpr int Log2Ceiling(uint32_t n) { … }

// Returns a value of type T with a single bit set in the left-most position.
// Can be used instead of manually shifting a 1 to the left. Unlike the other
// functions in this file, usable for any integral type.
template <typename T>
  requires std::integral<T>
constexpr T LeftmostBit() { … }

}  // namespace base::bits

#endif  // BASE_BITS_H_