A simple code-base for figuring out how objects move under the effects of gravity.
The official term for a code like this is an N-body integrator. There are many better codes for doing this (e.g. Rebound), but this code describes the general structure.
This code still needs a lot of work to become functional. There's an opportunity to do that, if you're interested! Alternatively, you could use a complete Rebound code for something, if you wanted to.