[GitHub] [nifi-minifi-cpp] martinzink commented on a change in pull request #1225: MINIFICPP-1688: When storing time durations we should use std::chrono…

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martinzink commented on a change in pull request #1225:
URL: https://github.com/apache/nifi-minifi-cpp/pull/1225#discussion_r781363451



##########
File path: libminifi/include/utils/TimeUtil.h
##########
@@ -197,6 +193,91 @@ inline bool getDateTimeStr(int64_t unix_timestamp, std::string& date_time_str) {
   return true;
 }
 
+namespace details {
+
+template<class Duration>
+bool unit_matches(const std::string&) {
+  return false;
+}
+
+template<>
+inline bool unit_matches<std::chrono::nanoseconds>(const std::string& unit) {
+  return unit == "ns" || unit == "nano" || unit == "nanos" || unit == "nanoseconds";
+}
+
+template<>
+inline bool unit_matches<std::chrono::microseconds>(const std::string& unit) {
+  return unit == "us" || unit == "micro" || unit == "micros" || unit == "microseconds" || unit == "microsecond";
+}
+
+template<>
+inline bool unit_matches<std::chrono::milliseconds>(const std::string& unit) {
+  return unit == "msec" || unit == "ms" || unit == "millisecond" || unit == "milliseconds" || unit == "msecs" || unit == "millis" || unit == "milli";
+}
+
+template<>
+inline bool unit_matches<std::chrono::seconds>(const std::string& unit) {
+  return unit == "sec" || unit == "s" || unit == "second" || unit == "seconds" || unit == "secs";
+}
+
+template<>
+inline bool unit_matches<std::chrono::minutes>(const std::string& unit) {
+  return unit == "min" || unit == "m" || unit == "mins" || unit == "minute" || unit == "minutes";
+}
+
+template<>
+inline bool unit_matches<std::chrono::hours>(const std::string& unit) {
+  return unit == "h" || unit == "hr" || unit == "hour" || unit == "hrs" || unit == "hours";
+}
+
+template<>
+inline bool unit_matches<std::chrono::days>(const std::string& unit) {
+  return unit == "d" || unit == "day" || unit == "days";
+}
+
+
+template<class TargetDuration, class SourceDuration>
+std::optional<TargetDuration> cast_if_unit_matches(const std::string& unit, const int64_t value) {
+  if (unit_matches<SourceDuration>(unit)) {
+    return std::chrono::duration_cast<TargetDuration>(SourceDuration(value));
+  } else {
+    return std::nullopt;
+  }
+}
+
+template<class TargetDuration, typename... T>
+std::optional<TargetDuration> cast_to_matching_unit(std::string& unit, const int64_t value) {
+  std::optional<TargetDuration> result;
+  ((result = cast_if_unit_matches<TargetDuration, T>(unit, value)) || ...);
+  return result;
+}
+
+inline bool get_unit_and_value(const std::string& input, std::string& unit, int64_t& value) {
+  std::stringstream input_stream(input);
+  input_stream >> value >> unit;
+  std::transform(unit.begin(), unit.end(), unit.begin(), ::tolower);
+  return !input_stream.fail();
+}
+
+}  // namespace details
+
+template<class TargetDuration>
+std::optional<TargetDuration> StringToDuration(const std::string& input) {
+  std::string unit;
+  int64_t value;
+  if (!details::get_unit_and_value(input, unit, value))
+    return std::nullopt;
+
+  return details::cast_to_matching_unit<TargetDuration,
+    std::chrono::nanoseconds,
+    std::chrono::microseconds,
+    std::chrono::milliseconds,
+    std::chrono::seconds,
+    std::chrono::minutes,
+    std::chrono::hours,
+    std::chrono::days>(unit, value);
+}

Review comment:
       Good idea, however I didnt find any good constexpr way to cast a string to int (std::stoi, std::from_chars are not constexpr).
   
   Of course we could implement our own ([related stackoverflow question)](https://stackoverflow.com/questions/25195176/how-do-i-convert-a-c-string-to-a-int-at-compile-time) but I am not sure if it worth it.




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