# Advent of Code F# – Day 2

Yan Cui

I help clients go faster for less using serverless technologies.

Table of Content

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

## Day 2

See details of the challenge here.

Let’s start by capturing the model and input data in a module. Since I’m armed with the hindsight of having seen both parts of today’s challenge I’ve refactored the common logic into a solve function:

• after each move (U, D, L or R) we need to check if the new position is a button, and ignore it if not
• capture the button we finished at at the end of each line

The interesting thing here is the no doubt the solve function.

After we parse the input string we end up with a Direction[][], which we’ll scan over to get the positions on the keypad we end up at after each line of the input.

However, Array.scan includes the initial value we passed in, which we can solve with Seq.skip 1.

The rest, is just a case of stiching together the actual values from the keypad.

Now that we have the core flow of the solution in place, let’s fill in the bits we need to solve part 1:

### Part 2

You finally arrive at the bathroom (it’s a several minute walk from the lobby so visitors can behold the many fancy conference rooms and water coolers on this floor) and go to punch in the code. Much to your bladder’s dismay, the keypad is not at all like you imagined it. Instead, you are confronted with the result of hundreds of man-hours of bathroom-keypad-design meetings:

``````    1
2 3 4
5 6 7 8 9
A B C
D
``````

You still start at “5” and stop when you’re at an edge, but given the same instructions as above, the outcome is very different:

• You start at “5” and don’t move at all (up and left are both edges), ending at `5`.
• Continuing from “5”, you move right twice and down three times (through “6”, “7”, “B”, “D”, “D”), ending at `D`.
• Then, from “D”, you move five more times (through “D”, “B”, “C”, “C”, “B”), ending at `B`.
• Finally, after five more moves, you end at `3`.

So, given the actual keypad layout, the code would be `5DB3`.

Using the same instructions in your puzzle input, what is the correct bathroom code?

We can present the keypad in part 2 as a Option<char>[][].

The only other difference to part 1 is that we need to deal with the empty spaces on the keypad, hereby represented as None.

With these two pieces, part 2 is pretty straight forward too.