The core is database of >9k songs tagged by mood (and a fun fact!). Coming up with the right 2 axes for the mood (solitary <-> social, chill <-> intense) took a few iterations. I think I need to get the number of songs to about 20–30k soon, to add more variety.
Then there's an algorithm to determine which song to sample given the mood/year combo and song attributes, which forms a radio schedule. You can see some of those song attributes by visiting a song's detailed page (click on the "now playing" marque text or visit /stats and search).
I've added keyboard shortcuts and the rest is just polish!