Question

(C#, prime generator)
Heres some code a friend and I were poking around on:

public List<int> GetListToTop(int top)
{            
    top++;
    List<int> result = new List<int>();
    BitArray primes = new BitArray(top / 2);
    int root = (int)Math.Sqrt(top);
    for (int i = 3, count = 3; i <= root; i += 2, count++)
    {
        int n = i - count;
        if (!primes[n])
            for (int j = n + i; j < top / 2; j += i)
            {
                primes[j] = true;
            }
    }
    if (top >= 2)
        result.Add(2);            
    for (int i = 0, count = 3; i < primes.Length; i++, count++)
    {
        if (!primes[i])
        {
            int n = i + count;
            result.Add(n);
        }
    }

    return result;
}

On my dorky AMD x64 1800+ (dual core), for all primes below 1 billion in 34546.875ms. Problem seems to be storing more in the bit array. Trying to crank more than ~2billion is more than the bitarray wants to store. Any ideas on how to get around that?

Answer 1

I would “swap” parts of the array out to disk. By that, I mean, divide your bit array into half-billion bit chunks and store them on disk.

The have only a few chunks in memory at any one time. With C# (or any other OO language), it should be easy to encapsulate the huge array inside this chunking class.

You’ll pay for it with a slower generation time but I don’t see any way around that until we get larger address spaces and 128-bit compilers.