$ for i in $(seq 1 210); do cat large.txt; done >/tmp/big.txt
$ ls -lh /tmp/big.txt 
-rw-r--r-- 1 mario mario 5.0G May 19 11:19 /tmp/big.txt

##</code><code>##

$ time grep -c "[aA].*[eE].*[iI].*[oO].*[uU]" /tmp/big.txt 
2195760

real    0m8.721s
user    0m8.317s
sys     0m0.404s

##</code><code>##

$ time ./egrep.pl --max-workers=2 -c "[aA].*[eE].*[iI].*[oO].*[uU]" /tmp/big.txt 
2195760

N-thds  2           5           10          20          30
real    0m39.068s   0m16.088s   0m8.146s    0m4.229s    0m3.084s
user    1m17.614s   1m19.624s   1m20.441s   1m21.823s   1m25.296s
sys     0m0.419s    0m0.464s    0m0.434s    0m0.526s    0m0.645s

##</code><code>##

$ time NUM_THREADS=2 ./grep-count-pcre2 "[aA].*[eE].*[iI].*[oO].*[uU]" /tmp/big.txt
parallel (2) Total Lines: 105000000, Matching Lines: 2195760

N-thds  2           5           10          20          30
real    0m9.256s    0m4.382s    0m2.748s    0m1.929s    0m1.689s
user    0m15.858s   0m16.563s   0m17.267s   0m18.391s   0m19.497s
sys     0m1.678s    0m1.339s    0m1.186s    0m1.225s    0m1.260s

##</code><code>##

$ time NUM_THREADS=2 ./grep-count-pmap "[aA].*[eE].*[iI].*[oO].*[uU]" /tmp/big.txt
parallel (2) Total Lines: 105000000, Matching Lines: 2195760

N-thds  2           5           10          20          30
real    0m8.113s    0m3.249s    0m1.675s    0m0.958s    0m0.707s
user    0m15.333s   0m15.646s   0m15.809s   0m16.775s   0m18.110s
sys     0m0.840s    0m0.266s    0m0.343s    0m0.489s    0m0.506s

##</code><code>##

$ time NUM_THREADS=2 ./grep-count-chunk "[aA].*[eE].*[iI].*[oO].*[uU]" /tmp/big.txt
parallel (2) Total Lines: 105000000, Matching Lines: 2195760

N-thds  2           5           10          20          30
real    0m8.017s    0m3.257s    0m1.537s    0m0.801s    0m0.618s
user    0m14.524s   0m14.997s   0m14.906s   0m15.397s   0m17.519s
sys     0m1.010s    0m1.019s    0m0.412s    0m0.515s    0m0.649s

##</code><code>##

// Grep PCRE2 and Chunking C++ OpenMP demonstration for website:
//    https://cpufun.substack.com/p/processing-a-file-with-openmp
//
// By Mario Roy - May 29, 2023
//    License: GNU General Public License.
//    See http://www.gnu.org/licenses/ for license information.
//    SPDX-License-Identifier: GPL-3.0-or-later
// Based on rtoa-pgatram-allinone2c.cpp:
//    https://perlmonks.org/?node_id=11152306
//
// Install the C++ wrapper for PCRE2 Library (required dependency).
//    https://github.com/jpcre2/jpcre2
//    sudo dnf install jpcre2-devel for RPM-based systems
//
// Compile with g++ or clang++: (Select the library pcre2-8, pcre2-16, or pcre2-32)
//    g++ -o grep-count-chunk -std=c++20 -fopenmp -O3 -msse4.1 -Wall grep-count-chunk.cc -lpcre2-8
//
// On macOS, you need Homebrew (https://brew.sh).
// 1. Install gcc@12 and pcre2.
//    brew install gcc@12 pcre2
// 2. Install C++ wrapper for the PCRE2 Library.
//    export CPATH="/usr/local/include"
//    export LIBRARY_PATH="/usr/local/lib"
//    export CC="gcc-12"
//    export CXX="g++-12"
//    git clone --depth=1 https://github.com/jpcre2/jpcre2
//    cd jpcre2 && ./configure && make install
// 3. Specify the -msse4.1 option on Intel Mac.
//    g++-12 -o grep-count-chunk -std=c++20 -fopenmp -O3 -Wall grep-count-chunk.cc -lpcre2-8
//    g++-12 -o grep-count-chunk -std=c++20 -fopenmp -O3 -msse4.1 -Wall grep-count-chunk.cc -lpcre2-8
//
// Running:
//    Usage: grep-count-chunk regular-expression [ file... ]
//    $ time ./grep-count-chunk "[aA].*[eE].*[iI].*[oO].*[uU]" < large.txt
//    $ time ./grep-count-chunk "[aA].*[eE].*[iI].*[oO].*[uU]" large.txt
//
// OpenMP Little Book:
//    https://nanxiao.gitbooks.io/openmp-little-book/content/

#include <iostream>
#include <fstream>
#include <cstring>
#include <string>
#include <string_view>
#include <semaphore>
#include <thread>

// C++ wrapper for PCRE2 Library
#include <jpcre2.hpp>

// Select the basic data type (char, wchar_t, char16_t or char32_t)
typedef jpcre2::select<char> jp;

#ifdef _OPENMP
#include <omp.h>
#endif

inline constexpr auto CHUNK_SIZE  = 2097152;
inline constexpr auto LINE_LENGTH = 255;

// Helper function to count character occurrences using SSE4.1.
// https://stackoverflow.com/questions/54541129/how-to-count-character-occurrences-using-simd
// https://gist.github.com/powturbo/456edcae788a61ebe2fc
//
#if defined(__SSE4_1__)
#include <smmintrin.h>

size_t memcount_simd(const char *s, int c, size_t n)
{    
   // https://gist.github.com/powturbo/456edcae788a61ebe2fc
   // build source with -msse4.1, -mavx, -mavx2, or -march=native
   //
   __m128i cv = _mm_set1_epi8(c), sum = _mm_setzero_si128(), acr0, acr1, acr2, acr3;
   const char *p, *pe;

   for (p = s; p != (char *)s + (n- (n % (252*16))); ) { 
      for (acr0 = acr1 = acr2 = acr3 = _mm_setzero_si128(), pe = p+252*16; p != pe; p += 64) { 
         acr0 = _mm_add_epi8(acr0, _mm_cmpeq_epi8(cv, _mm_loadu_si128((const __m128i *)p))); 
         acr1 = _mm_add_epi8(acr1, _mm_cmpeq_epi8(cv, _mm_loadu_si128((const __m128i *)(p+16)))); 
         acr2 = _mm_add_epi8(acr2, _mm_cmpeq_epi8(cv, _mm_loadu_si128((const __m128i *)(p+32)))); 
         acr3 = _mm_add_epi8(acr3, _mm_cmpeq_epi8(cv, _mm_loadu_si128((const __m128i *)(p+48)))); __builtin_prefetch(p+1024);
      }
      sum = _mm_add_epi64(sum, _mm_sad_epu8(_mm_sub_epi8(_mm_setzero_si128(), acr0), _mm_setzero_si128()));
      sum = _mm_add_epi64(sum, _mm_sad_epu8(_mm_sub_epi8(_mm_setzero_si128(), acr1), _mm_setzero_si128()));
      sum = _mm_add_epi64(sum, _mm_sad_epu8(_mm_sub_epi8(_mm_setzero_si128(), acr2), _mm_setzero_si128()));
      sum = _mm_add_epi64(sum, _mm_sad_epu8(_mm_sub_epi8(_mm_setzero_si128(), acr3), _mm_setzero_si128()));
   }

   size_t count = _mm_extract_epi64(sum, 0) + _mm_extract_epi64(sum, 1);   
   while (p != (char *)s + n) count += *p++ == c;

   return count;
}
#endif

// Helper function to find '\n'.
inline constexpr char const* find_lf(char const* first, char const* last)
{
   while (first != last) {
      if (*first == '\n') break;
      ++first;
   }
   return first;
}

// Grep file, count matching lines
static void grep_file(
   std::string_view fname,      // in: file name
   auto&            fin,        // in: file input stream
   jp::Regex& re,               // in: regular expression
   int nthds                    // in: number of threads
)
{
   std::string mode = (nthds > 1) ? "parallel" : "serial";
   size_t lines = 0, matched_lines = 0, next_chunk_id = 0;
   std::binary_semaphore *sem[nthds];

   // Create semaphores for orderly output.
   for (int i = 0; i < nthds; ++i)
      sem[i] = new std::binary_semaphore{0};

   #pragma omp parallel reduction(+:lines, matched_lines)
   {
      std::string buf; buf.resize(CHUNK_SIZE + LINE_LENGTH + 1, '\0');
      const char *first, *last;
      size_t chunk_id, len;

      while (fin.good()) {
         len = 0;

         // Read the next chunk serially.
         //
         #pragma omp critical
         {
            fin.read(&buf[0], CHUNK_SIZE);
            if ((len = fin.gcount()) > 0) {
               chunk_id = ++next_chunk_id;
               if (buf[len - 1] != '\n' && fin.getline(&buf[len], LINE_LENGTH)) {
                  // Getline discards the newline char and appends null char.
                  // Therefore, change '\0' to '\n'.
                  len += fin.gcount();
                  buf[len - 1] = '\n';
               }
            }
         }

         if (!len) break;

         buf[len] = '\0';
         first    = &buf[0];
         last     = &buf[len];

         // Process max Nthreads chunks concurrently.
#if 1
         // Match all and tally the match count - faster than matching per line.
         // This requires anchoring in the event a line has multiple matches.
         // E.g. "^.*" prepended to pattern during regex construction.
         matched_lines += re.match(std::basic_string(first, last - first + 1), "g");

         // Count the number of lines in this chunk.
         #if defined(__SSE4_1__)
         lines += memcount_simd(&buf[0], '\n', last - first + 1);
         #else
         for (size_t i = 0; i < len; i++) {
            lines += (buf[i] == '\n');
         }
         #endif
#else
         // Process line-by-line.
         while (first < last) {
            auto beg_ptr{first}; first = find_lf(first, last);
            auto end_ptr{first};

            // include trailing '\n'
            if (re.match(std::basic_string(beg_ptr, end_ptr - beg_ptr + 1)))
               ++matched_lines;

            ++lines;
            ++first;
         }
#endif

         // Output completed chunk, orderly by chunk_id.
         //
         if (nthds > 1 && chunk_id > 1)
            sem[chunk_id % nthds]->acquire();

         // std::cerr << "Chunk " << chunk_id << ": done\n";
         // std::cout << std::basic_string(first, last - first + 1);

         if (nthds > 1)
            sem[(chunk_id + 1) % nthds]->release();
      }
   }

   // Destroy the dynamically allocated semaphores.
   for (int i = 0; i < nthds; ++i)
      delete sem[i];

   std::cerr \
      << fname << ": " << mode << " (" << nthds << ") Total Lines: " << lines \
      << ", Matching Lines: " << matched_lines << '\n';
}

int main(int argc, char* argv[])
{
   if (argc < 2) {
      if (argc > 0)
         std::cerr << "Usage: grep-count-chunk regular-expression [ file... ]\n";
      return 1;
   }

#ifdef _OPENMP
   // Determine the number of threads.
   int nthds = std::thread::hardware_concurrency();
   const char* env_nthds1 = std::getenv("OMP_NUM_THREADS");
   const char* env_nthds2 = std::getenv("NUM_THREADS");

   if (env_nthds1 && strlen(env_nthds1))
      nthds = ::atoi(env_nthds1);
   else if (env_nthds2 && strlen(env_nthds2))
      nthds = ::atoi(env_nthds2);

   omp_set_dynamic(false);
   omp_set_num_threads(nthds);
#else
   int nthds = 1;
#endif

   jp::Regex re(std::string{"^.*"} + argv[1], "mS");
   // anchor pattern to allow searching once per chunk
   // 'm' modifier enables multi-line mode for the regex
   // 'S' modifier is an optimization mode
   if (!re) {
      std::cerr << "Invalid regular expression: " << re.getErrorMessage() << '\n';
      return 1;
   }

   if (argc == 2) {
      grep_file("STDIN", std::cin, re, nthds);
   }
   else {
      int    nfiles = argc - 2;
      char** fname  = &argv[2];
      for (int i = 0; i < nfiles; ++i) {
         std::ifstream fin(fname[i], std::ifstream::binary);
         if (!fin.is_open()) {
            std::cerr << "Error opening '" << fname[i] << "' : " << strerror(errno) << '\n';
            continue;
         }
         grep_file(fname[i], fin, re, nthds);
         fin.close();
      }
   }

   return 0;
}