# J, 17 bytes, score = 0.8235 (14/17) ([cracked][2])

    (+/@(!+:)&i.-)@>:

Computes [A002478][1].

The funny thing is that this version was originally the short one to be kept secret.

[1]: http://oeis.org/A002478
[2]: https://codegolf.stackexchange.com/a/89029/6710