better output formatting

doc/source/doxygen-docs/changelog.md

@@ -6,6 +6,8 @@
     - mrpt::maps::CSimpleMap changes:
       - Added an optional twist field.
       - Simplified API for preferred usage with structured binding tuples.
+  - \ref mrpt_system_grp
+    - More readable results in mrpt::system::unitsFormat() for the special case of exactly `0`.
 - BUG FIXES:
   - Fix filtering of NANs input point clouds in mrpt::maps::CPointsMap::insertAnotherMap().
 

libs/system/include/mrpt/system/string_utils.h

@@ -95,7 +95,15 @@ bool decodeBase64(const std::string& inString, std::vector<uint8_t>& outData);
 
 /** This function implements formatting with the appropriate SI metric unit
  * prefix: 1e-12->'p', 1e-9->'n', 1e-6->'u', 1e-3->'m', 1->'', 1e3->'K',
- * 1e6->'M', 1e9->'G', 1e12->'T' \sa intervalFormat */
+ * 1e6->'M', 1e9->'G', 1e12->'T'
+ * If the input is exactly 0, it will return the string `"0"`, so the overall
+ * result looks like that (e.g. using meter units on the caller side):
+ * \code
+ *  13445.0  => 13.44 km
+ *  0.0      =>     0 m
+ * \endcode
+ * \sa
+ * intervalFormat */
 std::string unitsFormat(
 	const double val, int nDecimalDigits = 2, bool middle_space = true);
 

libs/system/src/string_utils.cpp

@@ -138,6 +138,9 @@ std::string mrpt::system::unitsFormat(
 	double mult;
 	const double aVal = std::abs(val);
 
+	// special case for 0:
+	if (val == 0) return middle_space ? "0 " : "0";
+
 	if (aVal >= 1e12)
 	{
 		mult = 1e-12;