Prime sieve in C++ and Python

primes.cpp

/*
 Copyright (c) 2017 Chris Calderon
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal
 in the Software without restriction, including without limitation the rights
 to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 copies of the Software, and to permit persons to whom the Software is
 furnished to do so, subject to the following conditions:
 
 The above copyright notice and this permission notice shall be included in all
 copies or substantial portions of the Software.
 
 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 SOFTWARE.
 */

#include <iostream>  // for cout, cin and endl
#include <cmath>  // for sqrt
#include <cstdlib>  //for atoi
#include <string>  // for string
#include <fstream>  // for writing output
#include <ctime>  // for timing
#include <iomanip>  // for printing time spent correctly

#define FROM_INDEX(x) 2*(x) + 3
#define TO_INDEX(x) ((x) - 3)/2

int main(int argc, const char * argv[]) {
    
    if(argc < 2){
        std::string full_name = std::string(argv[0]);
        auto start = full_name.find_last_of('/');
        std::string short_name = full_name.substr(start + 1);
        std::cout << "Usage: " << short_name << " limit" << "[-p]" << std::endl;
        return 1;
    }
    
    bool print = false;
    std::string print_opt = std::string("-p");
    if(argc > 2){
        if(!print_opt.compare(argv[2]))
            print=true;
    }
    
    int limit = std::atoi(argv[1]);
    int primes_len = limit/2 - 1;
    bool *primes = new bool[primes_len]();
    
    int largest_num_to_check = TO_INDEX((int) sqrt(limit));
    
    std::cout << "Finding all primes less than " << limit << std::endl;
    auto start_time = time(NULL);
    for(int i=0; i <= largest_num_to_check; i++){
        if(!primes[i]){
            int step = FROM_INDEX(i);
            int start = TO_INDEX(step*step);
            for(int j=start; j < primes_len; j+=step)
                primes[j] = true;
        }
    }
    std::cout.precision(10);
    std::cout << "done in "<< std::scientific << difftime(time(NULL), start_time) << " seconds." << std::endl;
    
    if(print){
        bool first=true;
        std::cout << '[';
        for(int i=0; i < primes_len; i++){
            if(!primes[i]){
                if(first){
                    std::cout << FROM_INDEX(i);
                    first=false;
                } else {
                    std::cout << ", " << FROM_INDEX(i);
                }
            }
        }
        std::cout << ']' << std::endl;
    } else {
        std::cout << "Writing results to primes.bin" << std::endl;
        auto output = std::fstream("primes.bin", std::fstream::out|std::fstream::binary);
        for(int i=0; i<primes_len; i++){
            if(!primes[i]){
                int temp = FROM_INDEX(i);
                output.write(reinterpret_cast<const char*>(&temp), sizeof(int));
            }
        }
    }
    
    delete [] primes;
    return 0;
}

primes.py

#!/usr/bin/env python3

# Copyright (c) 2017 Chris Calderon
# 
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
# 
# The above copyright notice and this permission notice shall be included in all
# copies or substantial portions of the Software.
# 
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
# SOFTWARE.

import time
import sys
import os


def main():
    if len(sys.argv) < 2:
        shortname = os.path.basename(sys.argv[0])
        print('Usage:', shortname, 'limit [-p]')
        return 1

    _print = False
    if len(sys.argv) > 2:
        if sys.argv[2] == '-p':
            _print = True

    limit = int(sys.argv[1])
    primes_len = limit//2 - 1
    primes = bytearray(primes_len)
    largest_num_to_check = (int(limit**0.5) - 3)//2
    
    print('Finding all primes less than', limit)
    start_time = time.time()

    for i in range(largest_num_to_check + 1):
        if not primes[i]:
            step = 2*i + 3
            start = (step*step - 3)//2
            for j in range(start, primes_len, step):
                primes[j] = 1

    print('done in %.10e' %  (time.time() - start_time), 'seconds')

    if _print:
        first = True
        print('[', sep='', end='', flush=True)
        for i, e in enumerate(primes):
            if not e:
                if first:
                    print(2*i + 3, sep='', end='', flush=True)
                    first = False
                else:
                    print(', ', 2*i + 3, sep='', end='', flush=True)
        print(']')
    else:
        print('Writing results to primes2.bin')
        output = open('primes2.bin', 'wb')
        for i, e in enumerate(primes):
            if not e:
                output.write((2*i + 3).to_bytes(4, 'little'))
        output.close()
    return 0

if __name__ == '__main__':
    sys.exit(main())