In the last AoC challenge you have to put marbles in a circle (linked list), sometimes moving back 7 places from the current position. Stripped version of the my code for that is below. I thought that the straight forward

current.prev.prev.prev.prev.prev.prev.prev

should be the fastest way possible, but out of curiosity I also did it as

generateSequence(current) { it.prev }.take(8).last()

and to my surprise, when I measured times of execution I got this : -

.prev

chain -> 2389ms -

generateSequence

-> 460ms So why is

generateSequence

5 times faster? On my machine I get results like that when looping up to 10mln, after that

generateSequence

suddenly takes 3s to finish.

If you are doing AoC, there are quite a few of us in #advent-of-code!

Thanks! Will link my question there 🙂

It appears to hit some worst-case scenario in the interpreter, but after a couple of iterations I get

60ms

for the sequence and

40ms

for the

.prev.prev

thing as expected.

I know that after JIT kicks in times get better. But how can I verify it is the case and what exactly makes it hit the worse case scenario? Also, is there any way to improve it for a single run (like running it once as .kts)?

Maybe running it with native makes it better?