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January 10, 2022 09:56 am GMT

namespace ra :: Random Prime No. Generator

Aim:

To create a random number engine that generates prime number on the go! Here the link GitHub repo

Plan of Attack:

Since finding prime number in constant time is impossible by the math at the moment, So We are going to use a database-based approach. Lets store all the prime number in primeDB file
Since C++ already have random engines, Let us create a wrapper around that using templates

Objective:

Container that takes in engine class and override all the engine functions, also takes in filepath for database
Functions to override
- () -> returns random prime number
- min() -> returns min prime number
- max() -> returns max prime number
- << -> save the state to std::stringstream
- >> -> restores the state from std::stringstream

Code:

Private Members:
- template <typename Engine> Engine eng;
- int min_prime;
- int max_prime;
- char const *db_filepath;
Public Members:
- Constructor
- int operator()();
- friend stringstream &operator<<
- friend stringstream &operator>>

Constructor: takes in `seed` and `filepath`

#define DB_FILEPATH "primeDB"template <typename Engine>random_prime_engine<Engine>::random_prime_engine(unsigned long rand_seed, char const *db_filepath=DB_FILEPATH){    // Get DB path    this->db_filepath = db_filepath;    eng.seed(rand_seed);}

// Usagera::random_prime_engine<std::default_random_engine> rpe_2(rand_seed);

`()` operator,

#define DB_SIZE 10000template <typename Engine>int random_prime_engine<Engine>::operator()(){    int idx = eng() % DB_SIZE; // to get a index within file    int rand_prime;    // Open the file    std::ifstream file;    file.open(db_filepath);    // update rand_prime till it gets the value at index    for (int i = 0; i <= idx; i++)        file >> rand_prime;    file.close();    return (rand_prime);}

// Usagestd::cout << rpe()

min and max

# pseudo code# initialize min/max with 0, for lazy initialization{    # return min or max if once found    if bool(min/max):        return min/max    min = file.first_line    max = file.last_line    return min/max}

`>>` and `<<`

// Friend Functions#define stringstream SSfunction SS &operator<>(SS&in/out, rpe){    in/out >> rpe.eng;    return in/out}

// Usagestd::stringstream state;state << rpe; //save current statestate >> rpe; //restore old state

All the code can be found at the below link:
https://github.com/ra101/Random-Prime-Number-Generator-Engine-cpp

Original Link: https://dev.to/ra101/namespace-ra-random-prime-no-generator-2d8d

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