[libcamera-devel] [PATCH v2 1/4] libcamera: utils: Add helper class for std::chrono::duration

[Naushir Patuck]

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
+
 } /* 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
+ * 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