Day2.fsx

open System

let (|Prefix|_|) (p: string) (s: string) =
    if s.StartsWith(p) then
        Some(s.Substring(p.Length))
    else
        None

type OpCode =
    | Forward of int64
    | Up of int64
    | Down of int64

let parse s =
    match s with
    | Prefix "forward " num -> Forward(Int64.Parse num)
    | Prefix "up " num -> Up(Int64.Parse num)
    | Prefix "down " num -> Down(Int64.Parse num)
    | _ -> failwith "failed to parse"

let input =
    IO.File.ReadAllLines "Day2.txt" |> Seq.map parse

let depth =
    input
    |> Seq.fold
        (fun acc c ->
            acc
            + match c with
              | Up num -> (num * -1L)
              | Down num -> num
              | _ -> 0L)
        0L

let horizontal =
    input
    |> Seq.fold
        (fun acc c ->
            acc
            + match c with
              | Forward num -> num
              | _ -> 0L)
        0L

let problem1 = depth * horizontal

let depth2 =
    input
    |> Seq.fold
        (fun (aim, currDepth) c ->
            match c with
            | Up num -> (aim - num, currDepth)
            | Down num -> (aim + num, currDepth)
            | Forward num -> (aim, currDepth + aim * num))
        (0L, 0L)
    |> snd

let problem2 = horizontal * depth2