Okay, maybe I'm missing something.  Since Lua is new to me and I started a day late, I used C for the day one problem, to get the mechanics of the website worked out.  Like Philippe, I used Excel for the first star, but then I saw the issue with that approach for the second star and switched to C.  I used a trivial list to store all "frequencies" as they occurred, and the addFrequency() function returned an indication that the new value was already there.  My data stream was 959 items and the program required 38 seconds on a Macbook Pro.  Since I don't even know what cyclic arithmetic is, maybe I'm missing something really important, and bonus points to anyone who'll enlighten me.  For instance, are performance issues like runtime and memory consumption counted? Sadly, now I'm a) hooked and b) committed to the "different language every day" idea.  I'll try to do days 2 and 3 tomorrow. On 12/2/18 1:11 PM, Philippe Verdy wrote: interesting. The first star on day one is elementary to solve just with a basic very Excel sheet, but the second star is much more difficult as it involves cyclic aruthmetic (you can solve it by brute force but it rapidly quite computing intensive as each test requires decomposing 1016 integers into primes in order to test which position to look at (otherwise the search loops are unbounded): you need some knowledge of cyclic arithmetic to solve it with an efficient algorithm which is ensured to find a solution in a bounded and in fact quite small time. Your current test code in Haskell uses the brute force approach to repeat searches in more and more repeated cycles, but even if you find a repeated frequency with N cycles of the list, you could still find a lower frequency located below in the input list. There's a more efficient solution by sorting the frequencies produced by the first cycle. But then you need to apply the algorithm to compute a lowest common multiplier and see if it's divisible by a position in the sorted list. Finally you have to aplky a second sort... Technically you can still do that within Excel by several operations involding copying computed values from one column to another (without their formula) and then apply a custom sort. over a group of columns. All this could as well be done in Lua of course (instead of Excel, you need Lua tables) Le dim. 2 déc. 2018 à 15:10, Personal a écrit : This is interesting.  An interesting twist might be to use a different language each day. -- Daryl Lee Sent from my iPhone On Dec 2, 2018, at 4:52 AM, Pierre Chapuis wrote: Advent of Code [1] is a coding problem advent calendar: every day from December 1 to December 25, they publish two code problems that can be solved in any language. Like last year, I am doing it in Lua (I may solve the problem in another language as well some days, but I intend to do all of them in Lua at least). I publish my solutions [2] on GitHub. I am not interested in leaderboards (based on resolution time since publication), first because for Europeans the only way to be competitive would be to wake up very early or stay up very late, but also because I only do this because it is somehow fun to me. Sometimes I may not have the time to play at all on a given day and catch up later in the week. Anyway, I thought it might interest some people on this list. -- Pierre Chapuis ```-- Daryl Lee All our discontents about what we want appeared to me to spring from the want of thankfulness for what we have. -- Daniel Defoe ```