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* feat: add Sieve of Eratosthenes Originally created by @thechubbypanda. * updating DIRECTORY.md --------- Co-authored-by: github-actions[bot] <github-actions@users.noreply.github.com> Co-authored-by: Keval Kapdee <keval@thechubbypanda.net>
119 lines
3.4 KiB
C++
119 lines
3.4 KiB
C++
/**
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* @file
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* @brief [The Sieve of
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* Eratosthenes](https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes)
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* @details
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* Store an array of booleans where a true value indicates that it's index is
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* prime. For all the values in the array starting from 2 which we know is
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* prime, we walk the array in multiples of the current outer value setting them
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* to not prime. If we remove all multiples of a value as we see it, we'll be
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* left with just primes.
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*
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* Pass "print" as a command line arg to see the generated list of primes
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* @author [Keval Kapdee](https://github.com/thechubbypanda)
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*/
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#include <cassert> /// For assert
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#include <chrono> /// For timing the sieve
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#include <iostream> /// For IO operations
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#include <string> /// For string handling
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#include <vector> /// For std::vector
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/**
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* @namespace math
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* @brief Mathematical algorithms
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*/
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namespace math {
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/**
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* @brief Performs the sieve
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* @param vec Array of bools, all initialised to true, where the number of
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* elements is the highest number we wish to check for primeness
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* @returns void
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*/
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void sieve(std::vector<bool> *vec) {
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(*vec)[0] = false;
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(*vec)[1] = false;
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// The sieve sets values to false as they are found not prime
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for (uint64_t n = 2; n < vec->size(); n++) {
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for (uint64_t multiple = n << 1; multiple < vec->size();
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multiple += n) {
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(*vec)[multiple] = false;
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}
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}
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}
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/**
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* @brief Prints all the indexes of true values in the passed std::vector
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* @param primes The vector that has been passed through `sieve(...)`
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* @returns void
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*/
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void print_primes(std::vector<bool> const &primes) {
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for (uint64_t i = 0; i < primes.size(); i++) {
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if (primes[i]) {
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std::cout << i << std::endl;
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}
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}
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}
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} // namespace math
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/**
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* @brief Self-tests the sieve function for major inconsistencies
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* @returns void
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*/
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static void test() {
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auto primes = std::vector<bool>(10, true);
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math::sieve(&primes);
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assert(primes[0] == false);
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assert(primes[1] == false);
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assert(primes[2] == true);
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assert(primes[3] == true);
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assert(primes[4] == false);
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assert(primes[5] == true);
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assert(primes[6] == false);
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assert(primes[7] == true);
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assert(primes[8] == false);
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assert(primes[9] == false);
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std::cout << "All tests have successfully passed!\n";
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}
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/**
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* @brief Main function
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* @param argc commandline argument count
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* @param argv commandline array of arguments
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* @returns 0 on exit
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*/
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int main(int argc, char *argv[]) {
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test(); // run self-test implementations
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// The largest prime we will check for
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auto max = 10000;
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// Store a boolean for every number which states if that index is prime or
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// not
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auto primes = std::vector<bool>(max, true);
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// Store the algorithm start time
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auto start = std::chrono::high_resolution_clock::now();
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// Run the sieve
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math::sieve(&primes);
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// Time difference calculation
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auto time = std::chrono::duration_cast<
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std::chrono::duration<double, std::ratio<1>>>(
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std::chrono::high_resolution_clock::now() - start)
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.count();
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// Print the primes if we see that "print" was passed as an arg
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if (argc > 1 && argv[1] == std::string("print")) {
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math::print_primes(primes);
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}
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// Print the time taken we found earlier
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std::cout << "Time taken: " << time << " seconds" << std::endl;
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return 0;
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}
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