[libcamera-devel] libcamera: utils: Add reverse adapter for range-based loop

10749 diff mbox series

Add a utils::reverse() function that creates an adapter wrapping in
iterable, such that range-based iteration over the adapter iterates over
the iterable in reverse order.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
---
 include/libcamera/internal/utils.h | 27 +++++++++++++++++++++++++++
 src/libcamera/utils.cpp            |  8 ++++++++
 2 files changed, 35 insertions(+)

This will be used in an upcoming patch series, I'm already sending it
out to get the idea reviewed.

Hi Laurent,

I like template magic :-)

On 2020-12-27 12:03:31 +0200, Laurent Pinchart wrote:
> Add a utils::reverse() function that creates an adapter wrapping in
> iterable, such that range-based iteration over the adapter iterates over
> the iterable in reverse order.
> 
> Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> ---
>  include/libcamera/internal/utils.h | 27 +++++++++++++++++++++++++++
>  src/libcamera/utils.cpp            |  8 ++++++++
>  2 files changed, 35 insertions(+)
> 
> This will be used in an upcoming patch series, I'm already sending it
> out to get the idea reviewed.

Looking forward do see what you have in store ;-)

Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

> 
> diff --git a/include/libcamera/internal/utils.h b/include/libcamera/internal/utils.h
> index f08134afb5ba..d0146b71727d 100644
> --- a/include/libcamera/internal/utils.h
> +++ b/include/libcamera/internal/utils.h
> @@ -203,6 +203,33 @@ constexpr unsigned int alignUp(unsigned int value, unsigned int alignment)
>  	return (value + alignment - 1) / alignment * alignment;
>  }
>  
> +namespace details {
> +
> +template<typename T>
> +struct reverse_adapter {
> +	T &iterable;
> +};
> +
> +template<typename T>
> +auto begin(reverse_adapter<T> r)
> +{
> +	return std::rbegin(r.iterable);
> +}
> +
> +template<typename T>
> +auto end(reverse_adapter<T> r)
> +{
> +	return std::rend(r.iterable);
> +}
> +
> +} /* namespace details */
> +
> +template<typename T>
> +details::reverse_adapter<T> reverse(T &&iterable)
> +{
> +	return { iterable };
> +}
> +
>  } /* namespace utils */
>  
>  } /* namespace libcamera */
> diff --git a/src/libcamera/utils.cpp b/src/libcamera/utils.cpp
> index e90375ae115c..c4098a74e0ab 100644
> --- a/src/libcamera/utils.cpp
> +++ b/src/libcamera/utils.cpp
> @@ -464,6 +464,14 @@ std::string libcameraSourcePath()
>   * \return The value rounded up to the nearest multiple of \a alignment
>   */
>  
> +/**
> + * \fn reverse(T &&iterable)
> + * \brief Wrap an iterable to reverse iteration in a range-based loop
> + * \param[in] iterable The iterable
> + * \return A value of unspecified type that, when used in a range-based for
> + * loop, will cause the loop to iterate over the \a iterable in reverse order
> + */
> +
>  } /* namespace utils */
>  
>  } /* namespace libcamera */
> -- 
> Regards,
> 
> Laurent Pinchart
> 
> _______________________________________________
> libcamera-devel mailing list
> libcamera-devel@lists.libcamera.org
> https://lists.libcamera.org/listinfo/libcamera-devel

Hi Laurent,

On Sun, Dec 27, 2020 at 12:03:31PM +0200, Laurent Pinchart wrote:
> Add a utils::reverse() function that creates an adapter wrapping in
> iterable, such that range-based iteration over the adapter iterates over
> the iterable in reverse order.
>
> Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> ---
>  include/libcamera/internal/utils.h | 27 +++++++++++++++++++++++++++
>  src/libcamera/utils.cpp            |  8 ++++++++
>  2 files changed, 35 insertions(+)
>
> This will be used in an upcoming patch series, I'm already sending it
> out to get the idea reviewed.
>
> diff --git a/include/libcamera/internal/utils.h b/include/libcamera/internal/utils.h
> index f08134afb5ba..d0146b71727d 100644
> --- a/include/libcamera/internal/utils.h
> +++ b/include/libcamera/internal/utils.h
> @@ -203,6 +203,33 @@ constexpr unsigned int alignUp(unsigned int value, unsigned int alignment)
>  	return (value + alignment - 1) / alignment * alignment;
>  }
>
> +namespace details {
> +
> +template<typename T>
> +struct reverse_adapter {
> +	T &iterable;
> +};
> +
> +template<typename T>
> +auto begin(reverse_adapter<T> r)
> +{
> +	return std::rbegin(r.iterable);
> +}
> +
> +template<typename T>
> +auto end(reverse_adapter<T> r)
> +{
> +	return std::rend(r.iterable);
> +}
> +
> +} /* namespace details */
> +
> +template<typename T>
> +details::reverse_adapter<T> reverse(T &&iterable)
> +{
> +	return { iterable };
> +}
> +

Clever!!

I thought we could get away with reverse_iterator, but mixin it
range-based loops is cumbersome. This feels very natural on the
contrary

Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

Thanks
   j

>  } /* namespace utils */
>
>  } /* namespace libcamera */
> diff --git a/src/libcamera/utils.cpp b/src/libcamera/utils.cpp
> index e90375ae115c..c4098a74e0ab 100644
> --- a/src/libcamera/utils.cpp
> +++ b/src/libcamera/utils.cpp
> @@ -464,6 +464,14 @@ std::string libcameraSourcePath()
>   * \return The value rounded up to the nearest multiple of \a alignment
>   */
>
> +/**
> + * \fn reverse(T &&iterable)
> + * \brief Wrap an iterable to reverse iteration in a range-based loop
> + * \param[in] iterable The iterable
> + * \return A value of unspecified type that, when used in a range-based for
> + * loop, will cause the loop to iterate over the \a iterable in reverse order
> + */
> +
>  } /* namespace utils */
>
>  } /* namespace libcamera */
> --
> Regards,
>
> Laurent Pinchart
>
> _______________________________________________
> libcamera-devel mailing list
> libcamera-devel@lists.libcamera.org
> https://lists.libcamera.org/listinfo/libcamera-devel