> be deemed acceptable by marioroy

Yikes! I'm a rookie when it comes to C++ and simply here for the fun and learning.

> Am I missing something?

There's no reason to confirm to fixed length, IMO. I gutted the fixed-length code. It runs faster, completing in 0.490 seconds.

C++ Results:

# https://perlmonks.org/?node_id=11152156
$ ./rtoa-pgatram-fixed t1.txt t1.txt t1.txt t1.txt | cksum
read_input_files  : 15996000 items
read file time    :    0.356 secs
roman_to_dec time :    0.460 secs
output time       :    0.124 secs
total time        :    0.941 secs
737201628 75552000

# https://perlmonks.org/?node_id=11152177
$ NUM_THREADS=4 ./rtoa-pgatram-openmp t1.txt t1.txt t1.txt t1.txt | ck
+sum
use OpenMP
read_input_files  : 15996000 items
read file time    :    0.159 secs
roman_to_dec time :    0.469 secs
total time        :    0.628 secs
737201628 75552000

# https://perlmonks.org/?node_id=11152182
$ ./rtoa-pgatram-allinone t1.txt t1.txt t1.txt t1.txt | cksum
do_it_all time    :    0.637 secs
737201628 75552000

# https://perlmonks.org/?node_id=11152186
$ ./rtoa-pgatram-allinone2 t1.txt t1.txt t1.txt t1.txt | cksum
do_it_all time    :    0.515 secs  fast_io scan, line_get
do_it_all time    :    0.490 secs  fast_io memory mapping
737201628 75552000
[download]

Perl Results:

# https://perlmonks.org/?node_id=11152168 max_workers => 26
$ perl rtoa-pgatram-mce.pl t1.txt t1.txt t1.txt t1.txt | cksum
rtoa pgatram start
time 0.658 secs   Perl on Fedora Linux 38
time 0.574 secs   Perl on Clear Linux
737201628 75552000

# https://perlmonks.org/?node_id=11152168 max_workers => 32
$ perl rtoa-pgatram-mce.pl t1.txt t1.txt t1.txt t1.txt | cksum
rtoa pgatram start
time 0.548 secs   Perl on Fedora Linux 38
time 0.480 secs   Perl on Clear Linux
737201628 75552000
[download]

rtoa-pgatram-allinone2.cpp

Updated on May 19, 2023

Thanks so much for that! And for confirming it is not "cheating" in this performance race to remove all vectors from a solution and write to stdout directly.

Sadly, the documentation for fast_io seems to be non-existent and I always struggle to figure out how to use it. Hope someone writes a nice fast_io user manual.

I also get confused as to whether I've built with fast_io or not, so I've created a new version, rtoa-pgatram-allinone3.cpp below, based on your excellent rtoa-pgatram-allinone2.cpp, that prints whether it's using fastio or not (via a USE_FAST_IO_L define). When built to not use fastio this new version uses std C++11 I/O (which in my experience is significantly slower that std C I/O).

Here are the timings on my laptop.

$ 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
[download]

$ 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
[download]

Original rtoa-pgatram-allinone from this node using C std I/O for reading files and fast_io for writing to stdout:

$ 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
[download]

// rtoa-pgatram-allinone3.cpp. Crude allinone version.
//   based on rtoa-pgatram-allinone2.cpp https://perlmonks.org/?node_i
+d=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/inc
+lude 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 Roma
+n 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::fix
+ed);
   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;
}
[download]

> Sadly, the documentation for fast_io seems to be non-existent and I always struggle to figure out how to use it.

Same here. This took a team. I've been grepping for clues inside the fast_io benchmark and examples folders. To break 0.5 seconds, I updated the allinone2 demonstration with fast_io memory mapping.

# https://perlmonks.org/?node_id=11152186
$ ./rtoa-pgatram-allinone2 t1.txt t1.txt t1.txt t1.txt | cksum
do_it_all time    :    0.515 secs  fast_io scan, line_get
do_it_all time    :    0.490 secs  fast_io memory mapping
737201628 75552000
[download]

Memory mapping update:

// Read an input file of Roman Numerals and do it all
static void do_it_all(
   std::string_view fname       //  in: file name containing a list of
+ Roman Numerals
)
{
   try {
   #if 1
      // Load entire file to memory through memory mapping.
      using file_loader_type = fast_io::native_file_loader;
      file_loader_type loader(fname, fast_io::open_mode::in | fast_io:
+:open_mode::follow);
      // Loop through contiguous container of the file.
      for (char const *first{loader.data()}, *last{loader.data()+loade
+r.size()}; first!=last; ) {
         auto start_ptr{first}; first = fast_io::find_lf(first, last);
+ auto end_ptr{first};
         if (start_ptr == end_ptr) continue;
         int dec = roman_to_dec(std::string_view(start_ptr, end_ptr - 
+start_ptr));
         fast_io::io::println(dec);
         ++first;
      }
   #else
      fast_io::filebuf_file fbf(fname, fast_io::open_mode::in | fast_i
+o::open_mode::follow);
      for (std::string line; fast_io::io::scan<true>(fbf, fast_io::mnp
+::line_get(line)); ) {
         fast_io::io::println(roman_to_dec(line));
      }
   #endif
   }
   catch (fast_io::error e) {
      fast_io::io::perrln("Error opening '", fname, "' : ", e);
   };
}
[download]

It requires running Perl on Clear Linux to keep up :)

# https://perlmonks.org/?node_id=11152186
$ time ./rtoa-pgatram-allinone2 t1.txt t1.txt t1.txt t1.txt | cksum
do_it_all time    :    0.490 secs  fast_io memory mapping
737201628 75552000

real    0m0.492s
user    0m0.471s
sys     0m0.037s

# https://perlmonks.org/?node_id=11152168 max_workers => 32
$ time perl rtoa-pgatram-mce.pl t1.txt t1.txt t1.txt t1.txt | cksum
rtoa pgatram start
time 0.480 secs
737201628 75552000

real    0m0.504s
user    0m13.887s
sys     0m0.231s

# https://perlmonks.org/?node_id=11152168 max_workers => 64
$ time perl rtoa-pgatram-mce.pl t1.txt t1.txt t1.txt t1.txt | cksum
rtoa pgatram start
time 0.425 secs
737201628 75552000

real    0m0.449s
user    0m21.884s
sys     0m0.592s
[download]

Perl MCE is mind-boggling to me. Because, the UNIX time includes all of it; the time to launch Perl, load modules, spawn workers, notify workers per each input file / run, and finally reap workers. The overhead is 0.075 and 0.093 seconds for 32 and 64 workers, respectively. The overhead also includes MCE::Relay where workers must wait their turn orderly by chunk_id value, behind the scene.

I commented out a few lines of code to measure the overhead time running MCE including MCE::Relay.

      user_func   => sub {
         my ( $mce, $slurp_ref, $chunk_id, $output ) = ( @_, '' );
       # open my $fh, '<', $slurp_ref;
       # while ( <$fh> ) {
       #    chomp;
       #    my $n = 0;
       #    $n += $_ - $n % $_ * 2 for @rtoa[ unpack 'c*', $_ ];
       #    $output .= "$n\n";
       # }
       # close $fh;
         # output orderly
         MCE::relay {
       #    print $output;
         };
      }
[download]

How much time does Perl MCE chunking four input files including MCE::Relay take, factoring out compute and output? Notice how doubling up on workers only adds less than 2 hundreds of a second to the UNIX real time. But notice the user and sys times. Chunking an input file and involving relay occur simultaneously behind the scene. The relay block runs serially.

# max_workers => 32
$ time perl rtoa-pgatram-mce.pl t1.txt t1.txt t1.txt t1.txt
rtoa pgatram start
time 0.052 secs

real    0m0.075s
user    0m0.122s
sys     0m0.170s

# max_workers => 64
$ time perl rtoa-pgatram-mce.pl t1.txt t1.txt t1.txt t1.txt
rtoa pgatram start
time 0.070 secs

real    0m0.093s
user    0m0.195s
sys     0m0.441s
[download]