Advent of Code 2015 in F# - Day 1

Day 1: Not Quite Lisp.

We get a long list of parentheses as input. A ( means “go up one floor”, a ) means “go down one floor”.

Part One 

The goal is to find the final floor after following all instructions. To achieve this, we parse the parentheses as either +1 or -1, and then simply sum them up.

let parse c =
    match c with
    | '(' -> 1
    | ')' -> -1
    | _ -> failwithf "Unexpected character %c" c

let input = System.IO.File.ReadAllText(filename).Trim() |> Seq.map parse

input |> Seq.sum |> printfn "Part 1: %i"

Part Two 

We need to find the index of the first instruction that causes Santa to enter the basement floor. Let’s start with the full solution, and then break it down:

input
|> Seq.scan (+) 0
|> Seq.findIndex ((=) -1)
|> printfn "Part 2: %i"

The trick here is Seq.scan:

|> Seq.scan (+) 0

Seq.scan works like Seq.fold, but instead of returning the final result as a single value, each step of the fold is saved and a list of these steps is returned instead. An example:

Seq.scan (+) 0 [1; 0; 2] // Returns [1; 1; 3]

Once we have this, Seq.findIndex gives us our solution.

Full solution.