added side-by-side comparison system/high, C++ (vc++)

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//Microsoft (R) C/C++ Optimizing Compiler Version 19.00.23506 for x86

#include <iostream>
#include <sstream>
#include <chrono>
#include <string>
#include <iomanip>
#include <cassert>
#include <thread>
#include <vector>

namespace g3 {

typedef std::chrono::time_point<std::chrono::system_clock>  system_time_point;
   typedef std::chrono::time_point<std::chrono::high_resolution_clock> high_resolution_time_point;
   typedef std::chrono::milliseconds milliseconds;
   typedef std::chrono::microseconds microseconds;

namespace internal {
      const std::string date_formatted = "%Y/%m/%d";
      const std::string time_formatted = "%H:%M:%S %f6";

enum class Fractional {Millisecond, Microsecond, Nanosecond, NanosecondDefault};
      Fractional getFractional(const std::string& format_buffer, size_t pos);
      std::string to_string(const g3::system_time_point& ts, Fractional fractional);
      std::string localtime_formatted_fractions(const g3::system_time_point& ts, std::string format_buffer);
      // %f: fractions of seconds (%f is nanoseconds)
      // %f3: milliseconds, 3 digits: 001
      // %6: microseconds: 6 digits: 000001  --- default for the time_format
      // %f9, %f: nanoseconds, 9 digits: 000000001
    
      const std::string kFractionalIdentier   = "%f";
      const size_t kFractionalIdentierSize = 2;

Fractional getFractional(const std::string& format_buffer, size_t pos) {
         char  ch  = (format_buffer.size() > pos + kFractionalIdentierSize ? format_buffer.at(pos + kFractionalIdentierSize) : '\0');
         Fractional type = Fractional::NanosecondDefault;
         switch (ch) {
            case '3': type = Fractional::Millisecond; break;
            case '6': type = Fractional::Microsecond; break;
            case '9': type = Fractional::Nanosecond; break;
            default: type = Fractional::NanosecondDefault; break;
         }
         return type;
      }

// Returns the fractional as a string with padded zeroes
      // 1 ms --> 001
      // 1 us --> 000001
      // 1 ns --> 000000001
      std::string to_string(const g3::system_time_point& ts, Fractional fractional) {
         auto duration = ts.time_since_epoch();
         auto sec_duration = std::chrono::duration_cast<std::chrono::seconds>(duration);
         duration -= sec_duration;
         auto ns = std::chrono::duration_cast<std::chrono::nanoseconds>(duration).count();

auto zeroes = 9; // default ns
         auto digitsToCut = 1; // default ns, divide by 1 makes no change
         switch (fractional) {
            case Fractional::Millisecond : {
               zeroes = 3;
               digitsToCut = 1000000;
               break;
            }
            case Fractional::Microsecond : {
               zeroes = 6;
               digitsToCut = 1000;
               break;
            }
            case Fractional::Nanosecond :
            case Fractional::NanosecondDefault:
            default:
               zeroes = 9;
               digitsToCut = 1;

}

ns /= digitsToCut;
         auto value = std::string(std::to_string(ns));
         return std::string(zeroes - value.size(), '0') + value;
      }

std::string localtime_formatted_fractions(const g3::system_time_point& ts, std::string format_buffer) {
         // iterating through every "%f" instance in the format string
         auto identifierExtraSize = 0;
         for (size_t pos = 0;
               (pos = format_buffer.find(g3::internal::kFractionalIdentier, pos)) != std::string::npos;
               pos += g3::internal::kFractionalIdentierSize + identifierExtraSize) {
            // figuring out whether this is nano, micro or milli identifier
            auto type = g3::internal::getFractional(format_buffer, pos);
            auto value = g3::internal::to_string(ts, type);
            auto padding = 0;
            if (type != g3::internal::Fractional::NanosecondDefault) {
               padding = 1;
            }

// replacing "%f[3|6|9]" with sec fractional part value
            format_buffer.replace(pos, g3::internal::kFractionalIdentier.size() + padding, value);
         }
         return format_buffer;
      }
     } // internal

inline system_time_point to_system_time(const high_resolution_time_point& ts)
   {
	   using namespace std::chrono;
	   return time_point_cast<system_clock::duration>(system_clock::now() + (ts - high_resolution_clock::now()));
   }
    
    
   // This mimics the original "std::put_time(const std::tm* tmb, const charT* fmt)"
   // This is needed since latest version (at time of writing) of gcc4.7 does not implement this library function yet.
   // return value is SIMPLIFIED to only return a std::string
   std::string put_time(const struct tm* tmb, const char* c_time_format) {
#if (defined(WIN32) || defined(_WIN32) || defined(__WIN32__)) && !defined(__MINGW32__)
      std::ostringstream oss;
      oss.fill('0');
      // BOGUS hack done for VS2012: C++11 non-conformant since it SHOULD take a "const struct tm*  "
      oss << std::put_time(const_cast<struct tm*> (tmb), c_time_format);
      return oss.str();
#else    // LINUX
      const size_t size = 1024;
      char buffer[size]; // IMPORTANT: check now and then for when gcc will implement std::put_time.
      //                    ... also ... This is way more buffer space then we need

auto success = std::strftime(buffer, size, c_time_format, tmb);
      // In DEBUG the assert will trigger a process exit. Once inside the if-statement
      // the 'always true' expression will be displayed as reason for the exit
      //
      // In Production mode
      // the assert will do nothing but the format string will instead be returned
      if (0 == success) {
         assert((0 != success) && "strftime fails with illegal formatting");
         return c_time_format;
      }
      return buffer;
#endif
   }

/** return time representing POD struct (ref ctime + wchar) that is normally
   * retrieved with std::localtime. g3::localtime is threadsafe which std::localtime is not.
   * g3::localtime is probably used together with @ref g3::systemtime_now */
   tm localtime(const std::time_t& ts) {
      struct tm tm_snapshot;
#if (defined(WIN32) || defined(_WIN32) || defined(__WIN32__) && !defined(__GNUC__))
      localtime_s(&tm_snapshot, &ts); // windsows
#else
      localtime_r(&ts, &tm_snapshot); // POSIX
#endif
      return tm_snapshot;
   }
    
    
   /** format string must conform to std::put_time's demands.
   * WARNING: At time of writing there is only so-so compiler support for
   * std::put_time. A possible fix if your c++11 library is not updated is to
   * modify this to use std::strftime instead */
   std::string localtime_formatted(const g3::system_time_point& ts, const std::string& time_format) {
	  auto format_buffer = internal::localtime_formatted_fractions(ts, time_format);
      auto time_point = std::chrono::system_clock::to_time_t(ts);
      std::tm t = localtime(time_point);
      return g3::put_time(&t, format_buffer.c_str()); // format example: //"%Y/%m/%d %H:%M:%S");
   }
}

int main()
{
    std::chrono::system_clock::period rsc; 
    std::cout << "Sys:     " << rsc.num/double(rsc.den) << " s" << std::endl;
    std::chrono::high_resolution_clock::period rhr; 
    std::cout << "Highres: " << rsc.num/double(rhr.den) << " s" << std::endl;
    std::chrono::steady_clock::period rst; 
    std::cout << "Steady:  " << rsc.num/double(rst.den) << " s" << std::endl;
        
    // test
    auto now = std::chrono::high_resolution_clock::now();
    
    std::ostringstream entry;
    entry << "g3log test timestamp: %a %b %d %H:%M:%S %Y: " << g3::localtime_formatted(g3::to_system_time(now) , "%a %b %d %H:%M:%S %Y") << std::endl;
    std::cout << entry.str() << std::endl;    
    
    auto now_formatted = g3::localtime_formatted(g3::to_system_time(now), {g3::internal::date_formatted + " " + g3::internal::time_formatted});
    std::cout << "date timestamp formatted: (now) " << now_formatted << std::endl << std::endl;
    
    double a = 1.2;
    
    std::vector<g3::high_resolution_time_point> high_times(10);
    std::vector<g3::system_time_point> system_times(10);
    for(size_t i = 0; i < 10; ++i)
    {
        system_times[i] = std::chrono::system_clock::now();
        high_times[i] = std::chrono::high_resolution_clock::now();
    }

std::vector<std::string> high_formatted(10);
    std::vector<std::string> system_formatted(10);
    
    for(size_t i = 0; i < 10; ++i)
    {
        auto high_t = high_times[i];
        auto system_t = system_times[i];
        
        auto timeformatted = g3::localtime_formatted(g3::to_system_time(high_t), {g3::internal::date_formatted + " " + g3::internal::time_formatted});
        std::cout << "High date timestamp formatted:  " << timeformatted << std::endl;
        std::cout << "\tms: " << g3::localtime_formatted(g3::to_system_time(high_t), "%f3") 
            << "\n\tus: " << g3::localtime_formatted(g3::to_system_time(high_t), "%f6") 
            << "\n\tns: " << g3::localtime_formatted(g3::to_system_time(high_t), "%f") << std::endl;
        high_formatted[i] = g3::localtime_formatted(g3::to_system_time(high_t), "%f");
        
        std::cout << "System date timestamp formatted:  " << timeformatted << std::endl;
        std::cout << "\tms: " << g3::localtime_formatted(system_t, "%f3") 
            << "\n\tus: " << g3::localtime_formatted(system_t, "%f6") 
            << "\n\tns: " << g3::localtime_formatted(system_t, "%f") << std::endl;
        system_formatted[i] = g3::localtime_formatted(system_t, "%f");
        std::cout << "-----------------\n" << std::endl;
    }
    
      std::cout << "High\t\t\tSystem" << std::endl;
     for(size_t i = 0; i < 10; ++i)
    {
        std::cout << high_formatted[i] << "\t\t\t" << system_formatted[i] << std::endl;  
    }
    
}

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Compilation time: 2,76 sec, absolute running time: 0,44 sec, absolute service time: 3,22 sec