Just played your game too and it was really fun :D. Left a comment on your page.
The beatmap system was honestly pretty simple. Just an array of timestamps plus what color note to play. When the song timer reached a timestamp the note would appear and travel down the screen. I feel like the majority of the game design came down to how the disc handled.
This was my first forray into any kind of audio programming, so getting the gameplay to sync up correctly was hell. But I enjoyed making this game alot!