$ time ./rtoa-pgatram-allinone3 t1.txt t1.txt t1.txt t1.txt >f4.tmp
not using fast_io
do_it_all time    :    1.936 secs
real    0m1.953s
user    0m1.832s
sys     0m0.121s
$ cmp f4.tmp fixed4.tmp

##</code><code>##

$ time ./rtoa-pgatram-allinone3 t1.txt t1.txt t1.txt t1.txt >f4.tmp
using fast_io
do_it_all time    :    0.875 secs

real    0m0.894s
user    0m0.803s
sys     0m0.091s
$ cmp f4.tmp fixed4.tmp

##</code><code>##

$ time ./rtoa-pgatram-allinone t1.txt t1.txt t1.txt t1.txt >f4.tmp
do_it_all time    :    1.034 secs

real    0m1.049s
user    0m0.988s
sys     0m0.061s

##</code><code>##

// rtoa-pgatram-allinone3.cpp. Crude allinone version.
//   based on rtoa-pgatram-allinone2.cpp https://perlmonks.org/?node_id=11152186
//
// Obtain the fast_io library (required dependency):
//   git clone --depth=1 https://github.com/cppfastio/fast_io
//
// Compile with g++ or clang++:
//   g++ -o rtoa-pgatram-allinone3 -std=c++20 -Wall -O3 -I fast_io/include rtoa-pgatram-allinone3.cpp

// Uncomment to use the fast_io C++ library
#define USE_FAST_IO_L 1

#include <cctype>
#include <cstring>
#include <string>
#include <numeric>

#include <chrono>
#include <thread>

#include <iostream>
#include <iomanip>

#ifdef USE_FAST_IO_L
// See [id://11149504] for more info on the fast_io C++ library
#include <fast_io.h>
#include <fast_io_legacy.h>
#else
#include <fstream>
#include <sstream>
#endif

// ---------------------------------------------------------------

typedef std::chrono::high_resolution_clock high_resolution_clock;
typedef std::chrono::high_resolution_clock::time_point time_point;
typedef std::chrono::milliseconds milliseconds;

double elaspe_time(
   time_point cend,
   time_point cstart)
{
   return double (
      std::chrono::duration_cast<milliseconds>(cend - cstart).count()
   ) * 1e-3;
}

// ---------------------------------------------------------------
// Though there are less than 256 initializers in this ascii table,
// the others are guaranteed by ANSI C to be initialized to zero.
static const int romtab[256] = {
    0,0,0,0,0,0,   0,   0,   0,   0,  //  00- 09
    0,0,0,0,0,0,   0,   0,   0,   0,  //  10- 19
    0,0,0,0,0,0,   0,   0,   0,   0,  //  20- 29
    0,0,0,0,0,0,   0,   0,   0,   0,  //  30- 39
    0,0,0,0,0,0,   0,   0,   0,   0,  //  40- 49
    0,0,0,0,0,0,   0,   0,   0,   0,  //  50- 59
    0,0,0,0,0,0,   0, 100, 500,   0,  //  60- 69
    0,0,0,1,0,0,  50,1000,   0,   0,  //  70- 79
    0,0,0,0,0,0,   5,   0,  10,   0,  //  80- 89
    0,0,0,0,0,0,   0,   0,   0, 100,  //  90- 99
  500,0,0,0,0,1,   0,   0,  50,1000,  // 100-109
    0,0,0,0,0,0,   0,   0,   5,   0,  // 110-119
   10,0,0,0,0,0,   0,   0,   0,   0   // 120-129
};

// Return the arabic number for a roman letter c.
// Return zero if the roman letter c is invalid.
inline int urtoa(int c) { return romtab[c]; }

inline int accfn(int t, char c) {
   return t + urtoa(c) - t % urtoa(c) * 2;
}

inline int roman_to_dec(std::string_view st) {
   return std::accumulate(st.begin(), st.end(), 0, accfn);
}

#ifdef USE_FAST_IO_L

// Read an input file of Roman Numerals and do it all (fast_io version)
static void do_it_all(
   std::string_view fname       //  in: file name containing a list of Roman Numerals
)
{
   try {
      fast_io::filebuf_file fbf(fname, fast_io::open_mode::in);
      for (std::string line; fast_io::io::scan<true>(fbf, fast_io::mnp::line_get(line)); ) {
         fast_io::io::println(roman_to_dec(line));
      }
   }
   catch (fast_io::error e) {
      fast_io::io::perrln("Error opening '", fname, "' : ", e);
   };
}

#else

// Read an input file of Roman Numerals and do it all (std C++ version)
static void do_it_all(
   const char* fname       //  in: file name containing a list of Roman Numerals
)
{
   std::ifstream infile(fname);
   if (!infile.is_open()) {
      std::cerr << "Error opening '" << fname << "'\n";
      return;
   }
   for (std::string line; std::getline(infile, line); ) {
      std::cout << roman_to_dec(line) << "\n";
   }
   infile.close();
}

#endif

int main(int argc, char* argv[])
{
   if (argc < 2) {
      if (argc > 0)
         std::cerr << "usage: rtoa-pgatram-allinone3 file... >out.txt\n";
      return 1;
   }

#ifdef USE_FAST_IO_L
   std::cerr << "using fast_io\n";
#else
   std::cerr << "not using fast_io\n";
#endif

   // Get the list of input files from the command line
   int    nfiles = argc - 1;
   char** fname  = &argv[1];

   std::cerr << std::setprecision(3) << std::setiosflags(std::ios::fixed);
   time_point cstartall, cendall;

   cstartall = high_resolution_clock::now();
   for (int i = 0; i < nfiles; ++i)
      do_it_all( fname[i] );
   cendall = high_resolution_clock::now();

   double ctakenall = elaspe_time(cendall, cstartall);
   std::cerr << "do_it_all time    : " << std::setw(8) << ctakenall << " secs\n";

   return 0;
}