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// Copyright 2020 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef LIB_STDCOMPAT_INTERNAL_BIT_H_
#define LIB_STDCOMPAT_INTERNAL_BIT_H_
#include <limits>
#include <type_traits>
#include "../type_traits.h"
namespace cpp20 {
namespace internal {
// Helper for exclusing specific char types when char is considered unsigned by the compiler.
template <typename T>
struct is_bit_type
: std::integral_constant<
bool, std::is_unsigned<T>::value && !std::is_same<bool, T>::value &&
!std::is_same<T, char>::value && !std::is_same<T, char16_t>::value &&
!std::is_same<T, char32_t>::value && !std::is_same<T, wchar_t>::value> {};
// Rotation implementation.
// Only internal for usage in implementation of certain methods.
template <class T>
[[gnu::warn_unused_result]] constexpr std::enable_if_t<is_bit_type<T>::value, T> rotl(
T x, int s) noexcept {
const auto digits = std::numeric_limits<T>::digits;
const auto rotate_by = s % digits;
if (rotate_by > 0) {
return static_cast<T>((x << rotate_by) | (x >> (digits - rotate_by)));
}
if (rotate_by < 0) {
return static_cast<T>((x >> -rotate_by) | (x << (digits + rotate_by)));
}
return x;
}
template <class T>
[[gnu::warn_unused_result]] constexpr std::enable_if_t<is_bit_type<T>::value, T> rotr(
T x, int s) noexcept {
int digits = std::numeric_limits<T>::digits;
int rotate_by = s % digits;
if (rotate_by > 0) {
return static_cast<T>((x >> rotate_by) | (x << (digits - rotate_by)));
}
if (rotate_by < 0) {
return rotl(x, -rotate_by);
}
return x;
}
// Overloads for intrinsics.
// Precondition: |value| != 0.
template <typename T>
constexpr std::enable_if_t<is_bit_type<T>::value && sizeof(T) <= sizeof(unsigned), int>
count_zeros_from_right(T value) noexcept {
return __builtin_ctz(static_cast<unsigned>(value));
}
template <typename T>
constexpr std::enable_if_t<is_bit_type<T>::value && sizeof(unsigned) < sizeof(T) &&
sizeof(T) <= sizeof(unsigned long),
int>
count_zeros_from_right(T value) noexcept {
return __builtin_ctzl(static_cast<unsigned long>(value));
}
template <typename T>
constexpr std::enable_if_t<is_bit_type<T>::value && sizeof(unsigned long) < sizeof(T) &&
sizeof(T) <= sizeof(unsigned long long),
int>
count_zeros_from_right(T value) noexcept {
return __builtin_ctzll(static_cast<unsigned long long>(value));
}
template <typename T>
constexpr std::enable_if_t<is_bit_type<T>::value && sizeof(unsigned long long) < sizeof(T), int>
count_zeros_from_right(T value) noexcept {
int count = 0;
int iter_count = 0;
const unsigned long long max_digits = std::numeric_limits<unsigned long long>::digits;
for (int slot = 0; slot * max_digits < std::numeric_limits<T>::digits; ++slot) {
const unsigned long long chunk =
static_cast<unsigned long long>(internal::rotr(value, (slot)*max_digits));
iter_count = (chunk == 0) ? static_cast<int>(max_digits) : count_zeros_from_right(chunk);
count += iter_count;
if (iter_count != max_digits) {
break;
}
}
return count - static_cast<int>(std::numeric_limits<T>::digits % max_digits);
}
template <typename T, typename U,
typename std::enable_if<sizeof(T) >= sizeof(U), bool>::type = true>
struct digit_diff
: std::integral_constant<T, std::numeric_limits<T>::digits - std::numeric_limits<U>::digits> {};
template <typename T>
constexpr std::enable_if_t<is_bit_type<T>::value && sizeof(T) <= sizeof(unsigned), int>
count_zeros_from_left(T value) noexcept {
return __builtin_clz(static_cast<unsigned>(value)) -
static_cast<int>(digit_diff<unsigned, T>::value);
}
template <typename T>
constexpr std::enable_if_t<is_bit_type<T>::value && sizeof(unsigned) < sizeof(T) &&
sizeof(T) <= sizeof(unsigned long),
int>
count_zeros_from_left(T value) noexcept {
return __builtin_clzl(static_cast<unsigned long>(value)) -
static_cast<int>(digit_diff<unsigned long, T>::value);
}
template <typename T>
constexpr std::enable_if_t<is_bit_type<T>::value && sizeof(unsigned long) < sizeof(T) &&
sizeof(T) <= sizeof(unsigned long long),
int>
count_zeros_from_left(T value) noexcept {
return __builtin_clzll(static_cast<unsigned long long>(value)) -
static_cast<int>(digit_diff<unsigned long long, T>::value);
}
template <typename T>
constexpr std::enable_if_t<is_bit_type<T>::value && sizeof(unsigned long long) < sizeof(T), int>
count_zeros_from_left(T value) noexcept {
int count = 0;
int iter_count = 0;
const unsigned int max_digits = std::numeric_limits<unsigned long long>::digits;
for (int slot = 0; slot * max_digits < std::numeric_limits<T>::digits; ++slot) {
const unsigned long long chunk =
static_cast<unsigned long long>(internal::rotl(value, (slot + 1) * max_digits));
iter_count = (chunk == 0) ? static_cast<int>(max_digits) : count_zeros_from_left(chunk);
count += iter_count;
if (iter_count != max_digits) {
break;
}
}
return count - static_cast<int>(std::numeric_limits<T>::digits % max_digits);
}
template <typename T>
constexpr std::enable_if_t<is_bit_type<T>::value && sizeof(T) <= sizeof(unsigned), int> popcount(
T value) noexcept {
return __builtin_popcount(static_cast<unsigned>(value));
}
template <typename T>
constexpr std::enable_if_t<is_bit_type<T>::value && sizeof(unsigned) < sizeof(T) &&
sizeof(T) <= sizeof(unsigned long),
int>
popcount(T value) noexcept {
return __builtin_popcountl(static_cast<unsigned long>(value));
}
template <typename T>
constexpr std::enable_if_t<is_bit_type<T>::value && sizeof(unsigned long) < sizeof(T) &&
sizeof(T) <= sizeof(unsigned long long),
int>
popcount(T value) noexcept {
return __builtin_popcountll(static_cast<unsigned long long>(value));
}
template <typename T>
constexpr std::enable_if_t<is_bit_type<T>::value && sizeof(unsigned long long) < sizeof(T), int>
popcount(T value) noexcept {
int accumulated_count = 0;
while (value != 0) {
accumulated_count += popcount(static_cast<unsigned long long>(value));
value >>= std::numeric_limits<unsigned long long>::digits;
}
return accumulated_count;
}
template <typename T>
constexpr std::enable_if_t<is_bit_type<T>::value, int> bit_width(T value) {
const int zeros_left = (value == 0) ? std::numeric_limits<T>::digits
: static_cast<int>(count_zeros_from_left(value));
return std::numeric_limits<T>::digits - zeros_left;
}
template <typename T>
constexpr std::enable_if_t<is_bit_type<T>::value && !std::is_same<T, decltype(+T())>::value, T>
bit_ceil(T value) {
unsigned ub_offset = std::numeric_limits<unsigned>::digits - std::numeric_limits<T>::digits;
return static_cast<T>(1 << (bit_width(static_cast<T>(value - 1)) + ub_offset) >> ub_offset);
}
// When there is no integer promotion.
template <typename T>
constexpr std::enable_if_t<is_bit_type<T>::value && std::is_same<T, decltype(+T())>::value, T>
bit_ceil(T value) {
auto width = bit_width(value - 1);
if (!cpp20::is_constant_evaluated() && width == std::numeric_limits<T>::digits) {
__builtin_abort();
}
return static_cast<T>(1) << width;
}
template <typename T>
constexpr std::enable_if_t<is_bit_type<T>::value, T> bit_floor(T value) {
return value == 0 ? 0 : static_cast<T>(T(1) << (static_cast<T>(bit_width(value)) - T(1)));
}
template <unsigned little, unsigned big>
constexpr unsigned native_endianess() {
auto curr = __BYTE_ORDER__;
if (curr == __ORDER_LITTLE_ENDIAN__) {
return little;
}
if (curr == __ORDER_BIG_ENDIAN__) {
return big;
}
return 0x404;
}
} // namespace internal
} // namespace cpp20
#endif // LIB_STDCOMPAT_INTERNAL_BIT_H_
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