So I was listening to this song and wanted to play a bit of it on my keyboard (the computer's one, not the piano). Many Java music libraries are available (jMusic, jFugue). First I tried with jMusic but it seems to be a bit unstable. I tried to figure out an easy way to write the parts of the song and decided to write a POC in Kotlin.
The main.kt contains the 5 first bars of the song. It's far from perfect (and relies on the MIDI sequencer) but you can easily understand the source and how it generates the music.
The smallest unit is a Chunk class that contains many sequences of keys. The magic of Kotlin allows some basic operations:
chunk + 12will transpose the chunk to the upper octavechunk * 4will repeat the chunk 4 timeschunk1 then chunk2will play the chunks in a rowchunk1 or chunk2will mix the chunks