[RFC,v3,05/22] libcamera: base: cxx20: Add `has_single_bit()`

24908 diff mbox series

`has_single_bit()` checks if the given unsigned integer has a single
bit set, that is, whether the number is a power of two or not.

Not all requirements of the C++20 standard are implemented.

See https://en.cppreference.com/w/cpp/numeric/has_single_bit

Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
---
 include/libcamera/base/internal/cxx20.h | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

Quoting Barnabás Pőcze (2025-10-30 16:57:59)
> `has_single_bit()` checks if the given unsigned integer has a single
> bit set, that is, whether the number is a power of two or not.
> 
> Not all requirements of the C++20 standard are implemented.
> 
> See https://en.cppreference.com/w/cpp/numeric/has_single_bit
> 
> Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
> Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
> ---
>  include/libcamera/base/internal/cxx20.h | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/include/libcamera/base/internal/cxx20.h b/include/libcamera/base/internal/cxx20.h
> index 1f4caf56f8..d47168a70a 100644
> --- a/include/libcamera/base/internal/cxx20.h
> +++ b/include/libcamera/base/internal/cxx20.h
> @@ -7,6 +7,8 @@
>  
>  #pragma once
>  
> +#include <type_traits>
> +
>  /**
>   * \internal
>   * \file cxx20.h
> @@ -39,4 +41,18 @@ template<typename T> struct type_identity {
>   */
>  template<typename T> using type_identity_t = typename type_identity<T>::type;
>  
> +/**
> + * \internal
> + * \brief std::has_single_bit()
> + *
> + * Implementation of std::has_single_bit() for C++17.
> + */
> +template<typename T>
> +constexpr bool has_single_bit(T x) noexcept
> +{
> +       static_assert(std::is_unsigned_v<T>);

Can't this be forced to unsigned with an enable_if or such in the
template? But I guess compile time static assert is just the same and
perhaps gives a nicer error so:

Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

> +
> +       return x != 0 && (x & (x - 1)) == 0;
> +}
> +
>  } /* namespace libcamera::internal::cxx20 */
> -- 
> 2.51.1
>

2025. 10. 31. 12:22 keltezéssel, Kieran Bingham írta:
> Quoting Barnabás Pőcze (2025-10-30 16:57:59)
>> `has_single_bit()` checks if the given unsigned integer has a single
>> bit set, that is, whether the number is a power of two or not.
>>
>> Not all requirements of the C++20 standard are implemented.
>>
>> See https://en.cppreference.com/w/cpp/numeric/has_single_bit
>>
>> Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
>> Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
>> ---
>>   include/libcamera/base/internal/cxx20.h | 16 ++++++++++++++++
>>   1 file changed, 16 insertions(+)
>>
>> diff --git a/include/libcamera/base/internal/cxx20.h b/include/libcamera/base/internal/cxx20.h
>> index 1f4caf56f8..d47168a70a 100644
>> --- a/include/libcamera/base/internal/cxx20.h
>> +++ b/include/libcamera/base/internal/cxx20.h
>> @@ -7,6 +7,8 @@
>>
>>   #pragma once
>>
>> +#include <type_traits>
>> +
>>   /**
>>    * \internal
>>    * \file cxx20.h
>> @@ -39,4 +41,18 @@ template<typename T> struct type_identity {
>>    */
>>   template<typename T> using type_identity_t = typename type_identity<T>::type;
>>
>> +/**
>> + * \internal
>> + * \brief std::has_single_bit()
>> + *
>> + * Implementation of std::has_single_bit() for C++17.
>> + */
>> +template<typename T>
>> +constexpr bool has_single_bit(T x) noexcept
>> +{
>> +       static_assert(std::is_unsigned_v<T>);
> 
> Can't this be forced to unsigned with an enable_if or such in the
> template? But I guess compile time static assert is just the same and
> perhaps gives a nicer error so:

Yes. But if there are no overloads, then I think a static_assert provides
better error messages.


> 
> Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
> 
>> +
>> +       return x != 0 && (x & (x - 1)) == 0;
>> +}
>> +
>>   } /* namespace libcamera::internal::cxx20 */
>> --
>> 2.51.1
>>

Quoting Barnabás Pőcze (2025-10-31 11:31:57)
> 2025. 10. 31. 12:22 keltezéssel, Kieran Bingham írta:
> > Quoting Barnabás Pőcze (2025-10-30 16:57:59)
> >> `has_single_bit()` checks if the given unsigned integer has a single
> >> bit set, that is, whether the number is a power of two or not.
> >>
> >> Not all requirements of the C++20 standard are implemented.
> >>
> >> See https://en.cppreference.com/w/cpp/numeric/has_single_bit
> >>
> >> Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
> >> Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
> >> ---
> >>   include/libcamera/base/internal/cxx20.h | 16 ++++++++++++++++
> >>   1 file changed, 16 insertions(+)
> >>
> >> diff --git a/include/libcamera/base/internal/cxx20.h b/include/libcamera/base/internal/cxx20.h
> >> index 1f4caf56f8..d47168a70a 100644
> >> --- a/include/libcamera/base/internal/cxx20.h
> >> +++ b/include/libcamera/base/internal/cxx20.h
> >> @@ -7,6 +7,8 @@
> >>
> >>   #pragma once
> >>
> >> +#include <type_traits>
> >> +
> >>   /**
> >>    * \internal
> >>    * \file cxx20.h
> >> @@ -39,4 +41,18 @@ template<typename T> struct type_identity {
> >>    */
> >>   template<typename T> using type_identity_t = typename type_identity<T>::type;
> >>
> >> +/**
> >> + * \internal
> >> + * \brief std::has_single_bit()
> >> + *
> >> + * Implementation of std::has_single_bit() for C++17.
> >> + */
> >> +template<typename T>
> >> +constexpr bool has_single_bit(T x) noexcept
> >> +{
> >> +       static_assert(std::is_unsigned_v<T>);
> > 
> > Can't this be forced to unsigned with an enable_if or such in the
> > template? But I guess compile time static assert is just the same and
> > perhaps gives a nicer error so:
> 
> Yes. But if there are no overloads, then I think a static_assert provides
> better error messages.
> 

Agreed, this looks good to me.

> 
> > 
> > Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
> > 
> >> +
> >> +       return x != 0 && (x & (x - 1)) == 0;
> >> +}
> >> +
> >>   } /* namespace libcamera::internal::cxx20 */
> >> --
> >> 2.51.1
> >>
>