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…


This content originally appeared on DEV Community and was authored by Parth Agarwal

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);
}

// Usage

ra::random_prime_engine<std::default_random_engine> rpe_2(rand_seed);

() operator, 

#define DB_SIZE 10000

template <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);
}

// Usage

std::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 SS
function SS &operator<>(SS&in/out, rpe){
    in/out >> rpe.eng;
    return in/out
}

// Usage

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

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


This content originally appeared on DEV Community and was authored by Parth Agarwal


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