Not repeating an operation to cutdown runtime

[DreamShore]

Problem here is you need either remember the result, or calculate them again. And recalculating is seemingly more expensive.

A most simple way is a constant sized buffer, that you know the work won't exceed that buffer.

If you are not sure about the working size, dynamic-sized buffer.

A chain of buffers is a little complex, but could be more efficent.

If you are handling something too large, write it to file. Files are good dynamic-sized buffers if you don't access them often.

p.s. maybe just loop until NUM / the_first_prime_you_found, or simpler NUM / 2, which is less efficient.