Difference between revisions of "cpp/numeric/bit ceil"
From cppreference.com
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(→Possible implementation: adopt the impl in libc++) |
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===Possible implementation=== | ===Possible implementation=== | ||
See possible implementations in [https://github.com/gcc-mirror/gcc/blob/62c25d7adb1a5664982449dda0e7f9ca63cf4735/libstdc%2B%2B-v3/include/std/bit#L217-L248 libstdc++ (gcc)] and [https://github.com/llvm-mirror/libcxx/blob/78d6a7767ed57b50122a161b91f59f19c9bd0d19/include/bit#L439-L456 libc++ (clang)] | See possible implementations in [https://github.com/gcc-mirror/gcc/blob/62c25d7adb1a5664982449dda0e7f9ca63cf4735/libstdc%2B%2B-v3/include/std/bit#L217-L248 libstdc++ (gcc)] and [https://github.com/llvm-mirror/libcxx/blob/78d6a7767ed57b50122a161b91f59f19c9bd0d19/include/bit#L439-L456 libc++ (clang)] | ||
| − | + | {{eq fun|1= | |
| + | template <std::unsigned_integral T> | ||
| + | requires !std::same_as<T, bool> && !std::same_as<T, char> && | ||
| + | !std::same_as<T, char8_t> && !std::same_as<T, char16_t> && | ||
| + | !std::same_as<T, char32_t> && !std::same_as<T, wchar_t> | ||
| + | constexpr T bit_ceil(T x) noexcept | ||
| + | { | ||
| + | if (x <= 1u) | ||
| + | return T(1); | ||
| + | if constexpr (sizeof(+x) == sizeof(x)) | ||
| + | return T(1) << std::bit_width(T(x - 1)); | ||
| + | else { // for types subject to integral promotion | ||
| + | constexpr int offset_for_ub = | ||
| + | std::numeric_limits<unsigned>::digit - std::numeric_limits<T>::digit; | ||
| + | return T(1u << (std::bit_width(T(x - 1)) + offset_for_ub) >> offset_for_ub); | ||
| + | } | ||
| + | } | ||
| + | }} | ||
=== Example === | === Example === | ||
Revision as of 02:51, 11 April 2020
| Defined in header <bit>
|
||
| template< class T > constexpr T bit_ceil(T x); |
(since C++20) | |
Calculates the smallest integral power of two that is not smaller than x. If that value is not representable in T, the behavior is undefined.
This function is constexpr only if the result is representable in T.
This overload participates in overload resolution only if T is an unsigned integer type (that is, unsigned char, unsigned short, unsigned int, unsigned long, unsigned long long, or an extended unsigned integer type).
Contents |
Parameters
| x | - | values of unsigned integer type |
Return value
The smallest integral power of two that is not smaller than x.
Exceptions
(none)
Possible implementation
See possible implementations in libstdc++ (gcc) and libc++ (clang)
template <std::unsigned_integral T> requires !std::same_as<T, bool> && !std::same_as<T, char> && !std::same_as<T, char8_t> && !std::same_as<T, char16_t> && !std::same_as<T, char32_t> && !std::same_as<T, wchar_t> constexpr T bit_ceil(T x) noexcept { if (x <= 1u) return T(1); if constexpr (sizeof(+x) == sizeof(x)) return T(1) << std::bit_width(T(x - 1)); else { // for types subject to integral promotion constexpr int offset_for_ub = std::numeric_limits<unsigned>::digit - std::numeric_limits<T>::digit; return T(1u << (std::bit_width(T(x - 1)) + offset_for_ub) >> offset_for_ub); } } |
Example
Run this code
#include <bit> #include <bitset> #include <iostream> auto main() -> int { using bin = std::bitset<8>; for (unsigned x{0}; x != 10; ++x) { auto const z = std::bit_ceil(x); // `ceil2` before P1956R1 std::cout << "bit_ceil(" << bin(x) << ") = " << bin(z) << '\n'; } }
Output:
bit_ceil(00000000) = 00000001 bit_ceil(00000001) = 00000001 bit_ceil(00000010) = 00000010 bit_ceil(00000011) = 00000100 bit_ceil(00000100) = 00000100 bit_ceil(00000101) = 00001000 bit_ceil(00000110) = 00001000 bit_ceil(00000111) = 00001000 bit_ceil(00001000) = 00001000 bit_ceil(00001001) = 00010000
See also
| (C++20) |
finds the largest integral power of two not greater than the given value (function template) |
