Generates a random integer and rounds it to the nearest prime number

round_to_prime.py

#!/usr/bin/env python

import sys, os
from random import randint
from cPickle import dump,load

REUSE_FILE="round_to_prime.data"
MAX_VALUE = 1000


def is_prime(n):
    result = True
    for i in range(2, (n//2)+1):
        if (n % i == 0):
            result = False
            break
    return result


def round_to_prime(n):

    result = None

    for i in range(0, (n//2)+1):
        if is_prime(n+i):
            result = n+i
            break
        elif (n-i) > 0 and is_prime(n-i):
            result = n-i
            break

    return result


def load_reuse_data(filename):
    data = []
    if os.path.exists(filename):
        with open(filename, 'r') as fh:
            data = load(fh)
    return data


def save_reuse_data(filename, data):
    with open(filename, 'w') as fh:
        dump(data, fh)
    return None


def find_unused(n, reuse_data):

    result = None

    for i in range(0, max(MAX_VALUE-n, n)+1 ):
        if n+i <= MAX_VALUE and not n+i in reuse_data:
            result = n+i
            break
        elif n-i > 0 and not n-i in reuse_data:
            result = n-i
            break

    return result


def _main():

    reuse_data = load_reuse_data(REUSE_FILE)

    if len(reuse_data) >= MAX_VALUE:
        raise Exception("Pool depleted. Increase MAX_VALUE")
        
    n = find_unused(randint(0, MAX_VALUE), reuse_data)
    print "The nearest prime to %d is %d" % (n, round_to_prime(n))

    reuse_data.append(n)
    save_reuse_data(REUSE_FILE, reuse_data)

    return 0


if __name__ == "__main__":
    sys.exit(_main())