[libcamera-devel] [PATCH v3 1/4] libcamera: utils: Add helper class for std::chrono::duration

[David Plowman]

Hi Naush

Thanks for the updated patch!

On Fri, 21 May 2021 at 09:05, Naushir Patuck <naush at raspberrypi.com> wrote:
>
> A new utils::Duration class is defined to represent a
> std::chrono::duration type with double precision nanosecond timebase.
> Using a double minimises the loss of precision when converting timebases.
> This helper class may be used by IPAs to represent variables such as frame
> durations and exposure times.
>
> An operator << overload is define to help with displaying
> utils::Duration value in stream objects. Currently, this will display
> the duration value in microseconds.
>
> Signed-off-by: Naushir Patuck <naush at raspberrypi.com>
> ---
>  include/libcamera/internal/utils.h | 42 ++++++++++++++++++++++++++++++
>  src/libcamera/utils.cpp            | 36 +++++++++++++++++++++++++
>  2 files changed, 78 insertions(+)
>
> diff --git a/include/libcamera/internal/utils.h b/include/libcamera/internal/utils.h
> index 83dada7cc16c..a377d4ea07ab 100644
> --- a/include/libcamera/internal/utils.h
> +++ b/include/libcamera/internal/utils.h
> @@ -316,6 +316,48 @@ auto enumerate(T (&iterable)[N]) -> details::enumerate_adapter<T *>
>  }
>  #endif
>
> +using BaseDuration = std::chrono::duration<double, std::nano>;
> +
> +class Duration : public BaseDuration
> +{
> +public:
> +       Duration() = default;
> +
> +       template<typename Rep, typename Period>
> +       constexpr Duration(const std::chrono::duration<Rep, Period> &d)
> +               : BaseDuration(d)
> +       {
> +       }
> +
> +       template<typename Period>
> +       double get() const
> +       {
> +               auto const c = std::chrono::duration_cast<std::chrono::duration<double, Period>>(*this);
> +               return c.count();
> +       }
> +
> +       explicit constexpr operator bool() const
> +       {
> +               return *this != BaseDuration::zero();
> +       }
> +};
> +
> +#ifndef __DOXYGEN__
> +template<class CharT, class Traits>
> +static inline std::basic_ostream<CharT, Traits> &operator<<(std::basic_ostream<CharT, Traits> &os,
> +                                                           const Duration &d)
> +{
> +       std::basic_ostringstream<CharT, Traits> s;
> +
> +       s.flags(os.flags());
> +       s.imbue(os.getloc());
> +       s.setf(std::ios_base::fixed, std::ios_base::floatfield);
> +       s.precision(2);
> +       s << d.get<std::micro>() << "us";
> +       return os << s.str();
> +}
> +#endif

One small thing I wonder about is that many classes have a toString
method. Do we want one here? It could even take an optional argument
to allow you to format the output differently... but I think that's
overkill and best left for now!

> +
>  } /* namespace utils */
>
>  } /* namespace libcamera */
> diff --git a/src/libcamera/utils.cpp b/src/libcamera/utils.cpp
> index 826144d3c837..3131aa2d6666 100644
> --- a/src/libcamera/utils.cpp
> +++ b/src/libcamera/utils.cpp
> @@ -506,6 +506,42 @@ std::string libcameraSourcePath()
>   * loop, iterates over an indexed view of the \a iterable
>   */
>
> +/**
> + * \class Duration
> + * \brief Helper for std::chrono::duration types. Derived from \a BaseDuration
> + */
> +
> +/**
> + * \fn Duration::Duration(const std::chrono::duration<Rep, Period> &d)
> + * \brief Copy constructor from a \a std::chrono::duration type.
> + * \param[in] d The std::chrono::duration object to convert from.
> + *
> + * Constructs a \a Duration object from a \a std::chrono::duration object,
> + * converting the representation to \a BaseDuration type.
> + */
> +
> +/**
> + * \fn Duration::get<Period>()
> + * \brief Retrieve the tick count, coverted to the timebase provided by the

s/coverted/converted/

With this tiny fix:

Reviewed-by: David Plowman <david.plowman at raspberrypi.com>

Thanks!
David

> + * template argument Period of type \a std::ratio.
> + *
> + * A typical usage example is given below:
> + *
> + * \code{.cpp}
> + * utils::Duration d = 5s;
> + * double d_in_ms = d.get<std::milli>();
> + * \endcode
> + *
> + * \return Tick count
> + */
> +
> +/**
> + * \fn Duration::operator bool()
> + * \brief Boolean operator to test if a \a Duration holds a non-zero time value.
> + *
> + * \return True if \a Duration is a non-zero time value, False otherwise.
> + */
> +
>  } /* namespace utils */
>
>  } /* namespace libcamera */
> --
> 2.25.1
>