Day9.fs

module AoC2020.Day9

open AoC2020.Utils


let getWindow preample numbers =
    let stream =
        numbers |> Seq.windowed preample |> Seq.cache

    Seq.zip stream stream
    |> Seq.map (fun (a, b) ->
        Seq.allPairs a b
        |> Seq.filter (fun (a, b) -> a <> b)
        |> Seq.map (fun (a, b) -> a + b))

let findNumber input preampleSize =
    let toBeTested = input |> Seq.skip preampleSize
    let preamples = getWindow preampleSize input
    let stream = Seq.zip toBeTested preamples
    stream
    |> Seq.find (fun (n, preample) -> preample |> Seq.exists ((=) n) |> not)
    |> fst

let day9 fn preampleSize () =
    let input =
        readInput fn |> Seq.map int64 |> Seq.cache

    findNumber input preampleSize

let day9part2 fn preampleSize () =
    let input =
        readInput fn |> Seq.map int64 |> Seq.cache

    let n = findNumber input preampleSize

    let windows =
        Seq.initInfinite (fun i -> input |> Seq.map int64 |> Seq.windowed (i + 2))
        |> Seq.concat

    let found =
        windows
        |> Seq.find (Seq.sum >> (=) n)
        |> Seq.cache

    Seq.min found + Seq.max found