### Advent of Code 2020 review

A look back on the event

Starting with Attoparsec

Day 5 was one of a complex description hiding a simple problem. All the guff about boarding numbers and the like obscured the fact that we were dealing with numbers written in base 2: binary. `B`

and `R`

represented the digit `1`

; `F`

and `L`

represented `0`

.

Haskell already has a function for converting strings of arbitrary "digits" in arbitrary bases into numbers: `readInt`

. That needs a couple of supplementary functions for converting characters into digits, and it produces more output than we need, but the conversion is fairly painless.

```
directionToInt :: Char -> Int
directionToInt dir = if dir `elem` "BR" then 1 else 0
convert :: String -> Int
convert = fst . head . readInt 2 (`elem` "FBLR") directionToInt
```

Now I have essentially a list of numbers as input, finding the largest is trivial.

```
part1 = maximum . map convert
```

For part 2, finding the gap, I created the set of numbers I expected (in the range lowest to highest), and the set of numbers I had, subtracted one from the other, and that was the answer.

```
part2 passes = head $ expecteds \\ ns
where ns = map convert passes
highest = maximum ns
lowest = minimum ns
expecteds = [lowest..highest]
```

I could have used a `Set`

, but the set-like operations in `Data.List`

were sufficient for this case.

You can find the code here or on Gitlab.