probably harder than 7kyu.

I actually don't agree. I think it's 8 kyu.... very very easy.

I agree to an extent.

I think this is def harder then a 7kyu for most, but I can only imagine this becomeing second nature once learned.
So idk how you would really rank this. It's probably too easy for anyone higher in rank but is harder for anyone lower.

I personally had some trouble with it and only slightly knew how to achieve it due to being exposed to it a bit in eloquent javascript.

I think this is a decent kata, but probably harder than 7kyu.

Good and thanks!

What an awesome kata! I had an ok Python solution that computed dbl_linear(60000) in about 500ms. However, the tests kept timing out, which forced me to come up with something better. When the aha moment finally came, it was over 10x faster than my original. I am glad the tests forced me to do better!