I need to calculate the exact value of $1,000,000,000!$ It can't be an approximation. As far as I am aware, the only way to do this would be to calculate it starting from the beginning $(1\cdot2\cdot3\cdot\cdots\cdot1,000,000,000)$.
So, I have a program setup to do this, and I improved performance a small amount by grabbing the next 100 numbers, multiplying them together, doing that 100 times, then multiplying them together again, and finally multiplying the result by the running factorial. For example, I am currently at $390,000$. so what the program is doing next is grabbing $390,001$ thru $390,100$ and multiplying those together, repeating this until it reaches $400,000$. At that point it will multiply the numbers from each 100 step together, and multiply that result by the factorial of $390,000$.
So, doing it this way is still way too slow. Is there a better way I could do this?