[traffxml] Use std::chrono:utc_clock for IsoTime, improve parsing

Signed-off-by: mvglasow <michael -at- vonglasow.com>

traffxml/traff_model.cpp

@@ -79,19 +79,17 @@ const std::map<EventType, uint16_t> kEventDelayMap{
 std::optional<IsoTime> IsoTime::ParseIsoTime(std::string timeString)
 {
   /*
-   * TODO this is ugly because we need to work around some compiler deficiencies.
+   * We cannot use `std::chrono::from_stream` because it requires GCC 14+, and as of mid-2025, the
+   * supported development platform (Ubuntu 24.04) has GCC 13.2. Clang still does not support it.
    *
-   * Ideally, we would be using `std::chrono::time_point<std::chrono::utc_clock>` and parse the
-   * string using `std::chrono::from_stream`, using `%FT%T%z` for the format string.
-   * This works in GCC 14+ and is pleasantly liberal about the time zone format (all of +01, +0100
-   * and +01:00 are parsed correctly). Alas, Ubuntu 24.04 (currently the default dev platform) comes
-   * with GCC 13.2, which lacks this support. Clang, as of mid-2025, doesn’t support it at all.
+   * As a reasonably portable workaround, we first parse the time string into its constituent values
+   * using a regex, then build a `sys_seconds` instance from the values and use `clock_cast` to
+   * convert it to a `std::chrono::time_point<std::chrono::utc_clock>` instance, which is the data
+   * type we use internally.
    *
-   * The workaround is therefore to use `std::chrono::time_point<std::chrono::system_clock>`, which
-   * exposes the same API as its `utc_clock` counterpart, making transition at a later point easy.
-   * In addition, however, it can be constructed from `std::time_t`, which we can generate from
-   * `std::tm`. Still not the prettiest way (as it relies on legacy C functions which are not
-   * thread-safe), but the best we can get until we have proper compiler support for `from_stream`.
+   * Once we have proper support for `std::chrono::from_stream` in all toolchains we support, this
+   * function can be rewritten accordingly. In GCC 14+, using `%FT%T%z` for the format string will
+   * work with all known UTC offset formats (+01, +0100 and +01:00), just like the regex does.
    */
   /*
    * Regex for ISO 8601 time, with some tolerance for time zone offset. If matched, the matcher
@@ -121,17 +119,17 @@ std::optional<IsoTime> IsoTime::ParseIsoTime(std::string timeString)
     if (offset_h < 0)
       offset_m *= -1;
 
-    std::tm tm = {};
-    tm.tm_year = std::stoi(iso8601Matcher[1]) - 1900;
-    tm.tm_mon = std::stoi(iso8601Matcher[2]) - 1;
-    tm.tm_mday = std::stoi(iso8601Matcher[3]);
-    tm.tm_hour = std::stoi(iso8601Matcher[4]) - offset_h;
-    tm.tm_min = std::stoi(iso8601Matcher[5]) - offset_m;
-    tm.tm_sec = std::stof(iso8601Matcher[6]) + 0.5f;
+    const auto y = static_cast<std::chrono::year>(std::stoi(iso8601Matcher[1]));
+    const auto mo = static_cast<std::chrono::month>(std::stoi(iso8601Matcher[2]));
+    const auto d = static_cast<std::chrono::day>(std::stoi(iso8601Matcher[3]));
+    const std::chrono::hours h{std::stoi(iso8601Matcher[4]) - offset_h};
+    const std::chrono::minutes min{std::stoi(iso8601Matcher[5]) - offset_m};
+    const std::chrono::seconds s{static_cast<uint8_t>(std::stof(iso8601Matcher[6]) + 0.5f)};
 
-    std::time_t tt = timegm(&tm);
+    std::chrono::sys_seconds sys_s = std::chrono::sys_days{y/mo/d};
+    sys_s = sys_s + h + min + s;
 
-    std::chrono::time_point<std::chrono::system_clock> tp = std::chrono::system_clock::from_time_t(tt);
+    std::chrono::time_point<std::chrono::utc_clock> tp = std::chrono::clock_cast<std::chrono::utc_clock>(sys_s);
 
     IsoTime result(tp);
     return result;
@@ -145,16 +143,16 @@ std::optional<IsoTime> IsoTime::ParseIsoTime(std::string timeString)
 
 IsoTime IsoTime::Now()
 {
-  return IsoTime(std::chrono::system_clock::now());
+  return IsoTime(std::chrono::utc_clock::now());
 }
 
-IsoTime::IsoTime(std::chrono::time_point<std::chrono::system_clock> tp)
+IsoTime::IsoTime(std::chrono::time_point<std::chrono::utc_clock> tp)
   : m_tp(tp)
 {}
 
 bool IsoTime::IsPast()
 {
-  return m_tp < std::chrono::system_clock::now();
+  return m_tp < std::chrono::utc_clock::now();
 }
 
 bool IsoTime::operator< (IsoTime & rhs)

traffxml/traff_model.hpp

@@ -77,9 +77,9 @@ public:
 private:
   friend std::string DebugPrint(IsoTime time);
 
-  IsoTime(std::chrono::time_point<std::chrono::system_clock> tp);
+  IsoTime(std::chrono::time_point<std::chrono::utc_clock> tp);
 
-  std::chrono::time_point<std::chrono::system_clock> m_tp;
+  std::chrono::time_point<std::chrono::utc_clock> m_tp;
 };
 
 // TODO enum urgency