Advent of Code F# – Day 8

ps. look out for all my other solutions for Advent of Code challenges here.


Day 8

See details of the challenge here.

The input for today’s challenge looks like this:

rect 1×1
rotate row y=0 by 10
rect 1×1
rotate row y=0 by 10
rect 1×1
rotate row y=0 by 5
rotate column x=43 by 1
rotate column x=40 by 2
rotate column x=38 by 1

So, our first task is to parse this input into something more workable (something like the Cmd union type we’ve declared below).

Notice that I used active patterns instead of plain functions:

  • StartsWith returns the substring after the specified prefix
  • SepBy returns the 2 numbers separated by either “x” or “by” (eg. “1×1”, “0 by 5”)

I find this style of parsing code easier to understand as it structurally matches the input line and mentally I don’t have to follow any branching logic.


Next, let’s model the screen and the different operations against it – rect, rotate row and rotate column.

Here I’ve gone with an imperative approach for efficiency sake, duplicating the 50×6 2D array repeatedly (at least once per command) just doesn’t sit well with me.

Anyhow, once we apply the commands parsed from the input file we need to count the no. of true elements in our 2D array to answer part 1.


Part 2

You notice that the screen is only capable of displaying capital letters; in the
font it uses, each letter is 5 pixels wide and 6 tall.

After you swipe your card, what code is the screen trying to display?

To solve part 2, the easiest way I could think of was to print it out and see what letters are spelled out.

which in my case spelled out:





  1. Mikhail   •  

    I’m doing these exercises for my own fun, so I don’t want to make compromises on immutability, just to force myself into solving problems in more functional way.

    In this case Array2D is the first option, but I ended up replacing it with a Set and it works great. A sketch of operations:

    type Screen = Set

    let rect w h screen =
    seq { for x in 0..w-1 do for y in 0..h-1 -> x, y }
    |> Set.ofSeq
    |> Set.union screen

    let rotateRow index by = (fun (x, y) -> if y = index then (x + by) % Width, y else x, y)

    let rotateColumn index by = (fun (x, y) -> if x = index then x, (y + by) % Height else x, y)

    Full code in my github:

    And thanks for your blog!

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