Edit : I just realized, I wrote a really crappy post.. I mean, a lot of the information is redundant, and really poorly worded. So, ignore the typos and missing words.
It's a simfile (as all BMS are) try asking in the simfiles forum, maybe they know what it is. It's not in IIDX, in case you're wondering... if it was, you would have heard about it.....
The beginning pretty much followed the same chord structure, and melody from the beginning of "Pink Rose" and that one was a KBM/DDR/IIDX song.
I don't know, something about that video just didn't seem right. I don't know if it was because the timing was loose (seriously, no IIDX timing has ever been as loose as that IIDX sim) the Just Great window seemed to be like 100 milliseconds.
I don't know, something about all of that just didn't sit right with me. I mean, the videos the guy has for other things don't seem right... I don't know, it just doesn't seem RIGHT... but I can't pinpoint what irritates me about them.
Anyways, in regards to your question about what the song is : It's a BMS file for a song some guy calling himself "Estish Carmine" (lawl, silly Japanese people and their made-up other-language names). The name of the song in the video is Mirai.
Here's a list of some other BMS that EST did :
BMS̾´Õ
And a link for the personal page of EST him (or her)self :
Estish Carmine
Download links for BMS and his "Musics?" He has a music section with downloadable music. As for his old songs (like Mirai and Angelic Snow), I have absolutely no clue. However, that should be enough information for you to be able to find it yourself.
I suggest trying the BMS of Fighters entries, as I think he enters his BMS into those. But who knows, I found out all the information I just told you in 5 minutes of googling.
Help identify~ 
