[PATCH] libcamera: utils: Add a helper to convert floating-point to fixed-point

[Paul Elder]

Add a helper function to convert floating-point numbers to fixed-point
numbers.

Signed-off-by: Paul Elder <paul.elder at ideasonboard.com>
---
I originally needed this for rkisp1 crosstalk but then I realized it's
more efficient to just have the tuning tool output fixed-point and have
the IPA simply copy those values instead of calculating them on-the-fly.
Still, I made this helper so if people think it's useful...
---
 include/libcamera/base/utils.h | 27 +++++++++++++++++++++++++++
 src/libcamera/base/utils.cpp   | 10 ++++++++++
 2 files changed, 37 insertions(+)

diff --git a/include/libcamera/base/utils.h b/include/libcamera/base/utils.h
index 37d9af60..ab421cc7 100644
--- a/include/libcamera/base/utils.h
+++ b/include/libcamera/base/utils.h
@@ -369,6 +369,33 @@ decltype(auto) abs_diff(const T &a, const T &b)
 
 double strtod(const char *__restrict nptr, char **__restrict endptr);
 
+#ifndef __DOXYGEN__
+template<unsigned int I, unsigned int F, typename R, typename T,
+	 std::enable_if_t<std::is_integral_v<R> &&
+			  std::is_floating_point_v<T>> * = nullptr>
+#else
+template<unsigned int I, unsigned int F, typename R, typename T>
+#endif
+constexpr R floatingToFixedPoint(T number)
+{
+	static_assert(I + F <= sizeof(R) * 8);
+
+	R integer = static_cast<R>(number);
+	T fractional = number - integer;
+
+	R mask = (1 << I) - 1;
+	R whole = (integer >= 0 ? integer : ~integer + 1) & mask;
+
+	R frac = 0;
+	for (unsigned int i = 0; i < F; i++) {
+		fractional *= 2;
+		frac <<= 1;
+		frac |= (static_cast<R>(fractional) & 0x1);
+	}
+
+	return (whole << F) | frac;
+}
+
 } /* namespace utils */
 
 #ifndef __DOXYGEN__
diff --git a/src/libcamera/base/utils.cpp b/src/libcamera/base/utils.cpp
index 3b73b442..38bee317 100644
--- a/src/libcamera/base/utils.cpp
+++ b/src/libcamera/base/utils.cpp
@@ -521,6 +521,16 @@ double strtod(const char *__restrict nptr, char **__restrict endptr)
 #endif
 }
 
+/**
+ * \fn R floatingToFixedPoint(T number)
+ * \brief Convert a floating point number to a fixed-point representation
+ * \tparam I Bit width of the integer part of the fixed-point
+ * \tparam F Bit width of the fractional part of the fixed-point
+ * \tparam R Return type of the fixed-point representation
+ * \tparam T Input type of the floating point representation
+ * \return The converted value
+ */
+
 } /* namespace utils */
 
 #ifndef __DOXYGEN__
-- 
2.39.2