LC_ALL=C sort -k1 big{1,2,3}.txt | awk -f tally-count.awk | LC_ALL=C sort -k2nr >out.txt

real time: 12.671 seconds

LC_ALL=C parsort -k1 big{1,2,3}.txt | awk -f tally-count.awk | LC_ALL=C parsort -k2nr >out.txt

real time:  6.561 seconds

LC_ALL=C parsort -k1 big{1,2,3}.txt | ./tally-count | LC_ALL=C parsort -k2nr >out.txt

real time:  3.615 seconds

##</code><code>##

LC_ALL=C parsort -k1 big{1,2,3}.txt | awk -f tally-count.awk | LC_ALL=C parsort -k2nr >out.txt

real time:  6.520 seconds

LC_ALL=C parsort -k1 big{1,2,3}.txt | ./tally-count | LC_ALL=C parsort -k2nr >out.txt

real time:  2.398 seconds

##</code><code>##

cat big{1,2,3}.txt | ./linux-q -tT "select c1, sum(c2) from - group by c1 order by sum(c2) desc, c1" >out.txt

real time: 84.027 seconds

./linux-q -tT "select c1, sum(c2) from big[1-3].txt group by c1 order by sum(c2) desc, c1" >out.txt

real time: 89.539 seconds

##</code><code>##

# Tally adjacent count fields of duplicate key names.

BEGIN {
   OFS = FS = "\t"
   key = ""
   sum = 0
   flg = 1
}

{
   if ( flg ) {
      flg = 0
      key = $1
   }
   if ( $1 == key ) {
      sum += $2
   } else {
      print key, sum
      key = $1
      sum = $2
   }
}

END {
   print key, sum
}

##</code><code>##

// ----------------------------------------------------------------------------
// tally-count.cpp
// Tally adjacent count fields of duplicate key names.
//
// Obtain the fast_io library (required dependency):
//    git clone --depth=1 https://github.com/cppfastio/fast_io
//
// clang++ compile on Linux:
//    clang++ -o tally-count -std=c++20 -Wall -O3 tally-count.cpp
//
// Example run:
//    LC_ALL=C parsort -k1 big{1,2,3}.txt | ./tally-count |\
//    LC_ALL=C parsort -k2nr > out.txt
// ----------------------------------------------------------------------------

#include <chrono>

// The fast_io header must come after chrono, else build error:
// "no member named 'concatln' in namespace 'fast_io'"
#include "fast_io/include/fast_io.h"

#include <cstdio>
#include <cstddef>
#include <cstdint>
#include <cstdlib>
#include <cstring>

#include <array>
#include <execution>

// fast_atoll64
// https://stackoverflow.com/questions/16826422/
// c-most-efficient-way-to-convert-string-to-int-faster-than-atoi

inline int64_t fast_atoll64( const char* str )
{
   int64_t val  = 0;
   int     sign = 0;
   if ( *str == '-' ) {
      sign = 1, ++str;
   }
   uint8_t digit;
   while ((digit = uint8_t(*str++ - '0')) <= 9) val = val * 10 + digit;
   return sign ? -val : val;
}

#define MAX_LINE_LEN_L 255

int main(int argc, char* argv[])
{
   std::array<char, MAX_LINE_LEN_L + 1> line, name;
   char* found;
   long long count, sum;
   int flag = 0;

   // obtain the first key-value pair delimited by tab
   while ( ::fgets( line.data(), static_cast<int>(MAX_LINE_LEN_L), ::stdin ) != NULL ) {
      if ( ( found = std::find( line.begin(), line.end(), '\t' ) ) == line.end() )
         continue;

      sum    = fast_atoll64( found + 1 );
      *found = '\0';  // key name
      ::memcpy( name.data(), line.data(), found - line.data() + 1 );
      flag = 1;

      break;
   }

   // process the rest of standard input
   while ( ::fgets( line.data(), static_cast<int>(MAX_LINE_LEN_L), ::stdin ) != NULL ) {
      if ( ( found = std::find( line.begin(), line.end(), '\t' ) ) == line.end() )
         continue;

      count  = fast_atoll64( found + 1 );
      *found = '\0';  // key name

      if ( ! ::strcmp( line.data(), name.data() ) ) {
         sum += count;
      } else {
         fast_io::io::println( fast_io::mnp::os_c_str(name.data()), "\t", sum );
         ::memcpy( name.data(), line.data(), found - line.data() + 1 );
         sum = count;
      }
   }

   if ( flag )
      fast_io::io::println( fast_io::mnp::os_c_str(name.data()), "\t", sum );

   return 0;
}