I wrote about doing the [advent of code 2022 in zig]({{< ref "advent-of-code-2022-in-zig.md" >}}), but I did not complete the year. I stopped on using zig on day 15 when I hit a bug when using hashmaps that I could not solve in time and continued in JavaScript until [day 22](https://adventofcode.com/2022/day/22). On day 22 part 2, you need to fold a cube and move on it keeping track of your orientation... It was hard!
Last week I wanted to warm up for the current advent of code and therefore took it up again... it was (almost) easy with Haskell!
## Day 22 - Monkey Map
You get an input that looks like this:
```
...#
.#..
#...
....
...#.......#
........#...
..#....#....
..........#.
...#....
.....#..
.#......
......#.
10R5L5R10L4R5L5
```
The `.` are floor tiles, the `#` are impassable walls. You have a cursor starting on the leftmost tile on the first line. The cursor moves and the empty spaces do not exist: if you step out you wrap around: easy enough... until part 2!
Here is how I parse the input:
```haskell
type Line = V.Vector Char
type Map = V.Vector Line
data Instruction = Move Int | L | R deriving Show
data Input = Input Map [Instruction] deriving Show
type Parser = Parsec Void String
parseMapLine :: Parser Line
parseMapLine = do
line <-some(char'.'<|> char ' ' <|> char '#') <* eol
return $ V.generate (length line) (line !!)
parseMap :: Parser Map
parseMap = do
lines <-someparseMapLine<*eol
return $ V.generate (length lines) (lines !!)
parseInstruction :: Parser Instruction
parseInstruction = (Move . read <$> some digitChar)
<|> (char 'L' $> L)
<|> (char 'R' $> R)
parseInput' :: Parser Input
parseInput' = Input <$> parseMap
<*> some parseInstruction <* eol <* eof
```
In part 2 you learn that your input pattern is in fact 6 squares that can be folded to form a cube. Now instead of simply wrapping the empty spaces, when stepping out you need to find out were you end up on the cube and with which orientation.
Here is a visualization I made in excalidraw to understand how folding the cube based on my input would work (this does not match the example above but matched the players' input):
The whole code is available [on my git server](https://git.adyxax.org/adyxax/advent-of-code/tree/2022/22-Monkey-Map/second.hs) but here is the core of my solver for this puzzle:
```haskell
stepOutside :: Map -> Int -> Int -> Int -> Heading -> Int -> Cursor
stepOutside m s x y h i | (t, h) == (a, N) = proceed fw (fn + rx) E
This `stepOutside` function takes in argument the map, its size, the cursor's `(x, y)` position and heading `h`, while i is the number of steps to perform. I first compute on which face the cursor is, and based on its heading where it should end up. I then use the faces coordinates to compute the final position, being careful to follow on the schematic how the transition is performed.
## Conclusion
The next days where quite a lot easier than this one. Haskell is really a great language for puzzle solving thanks to its excellent parsing capabilities and its incredible type system.
A great thing that should speak of Haskell's qualities is that it is the second year of advent of code that I completed all 25 days: both times it was thanks to Haskell! I think I should revisit the years 2021 that I did with Go next: I stopped on day 19 because it involved a three dimensional puzzle that was quite difficult.
