Finally came up with a solution, however not really idiomatic (you can tell I come from Python 😉 ) https://github.com/DnzzL/kotlin-advent-2018/blob/master/src/main/kotlin/tech/thomaslegrand/advent2018/Day05.kt

An easy improvement would be to have

doReactions

return whether anything changed, use that as your while condition and do away with all of the other checking.

Thank you for your feedback. I was not very proud of the code and I’ll try to apply this change