AdventOfCode 2020, December 23

AdventOfCode2020.12.23.fsx

module TwentyThree =

    open System

    let testInput = "389125467"
    let input = "942387615"
    let cups = (Seq.map ((fun x -> x.ToString()) >> Int32.Parse) input) |> Seq.toList

    let rec findDestination (cups : int list) pickup destination =
        if not (pickup |> List.contains destination) then 
            destination
        else
            findDestination cups pickup (if destination = 1 then cups.Length else destination-1)

    let makeMove cups =
        let current = cups |> List.head
        let pickup = cups |> List.skip 1 |> List.take 3
        let destination = findDestination cups pickup (if current = 1 then cups.Length else current-1)
        let destIndex = cups |> List.findIndex (fun x -> x = destination)
        let res =
            List.concat [
                cups |> List.skip 4 |> List.truncate (destIndex-4) 
                destination :: (List.append pickup (cups |> List.skip (destIndex+1))) @ [current]
            ]
        res

    let rec play cups n =
        if n = 0 then cups
        else play (makeMove cups) (n - 1)

    play cups 100
    |> fun cups ->
        let ndx = cups |> List.findIndex (fun x -> x = 1)
        [cups |> List.skip (ndx+1); cups |> List.take ndx] |> List.concat