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);
}

// 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