day09_part01.fs

module day09_part01

open System

open AdventOfCode_Utilities
open AdventOfCode_2020.Modules

let path = "day09/day09_input.txt"

let inputLines = LocalHelper.GetLinesFromFile(path) |> Array.map (fun x -> Convert.ToUInt64(x)) |> List.ofArray

let preambleSize = 25

let numberIsValid (value: uint64) (listChecker: uint64 list) =
   let permu = combination 2 listChecker
   (permu |> List.exists(fun x -> x.Item(0) + x.Item(1) = value), value)

let rec findInvalidValue (elements: uint64 list) (preamble: int) : (bool * uint64)=
   match elements.Length = 0 with
   | true -> (false, uint64(0))
   | false ->
       let checkList = elements |> List.take(preamble + 1)
       let valid = numberIsValid (checkList |> List.rev |> List.head) (checkList |> List.take(preamble))
       match fst valid with 
       | false -> (true, snd valid)
       |  true -> findInvalidValue (elements |> List.skip(1)) preamble

let execute =
    snd (findInvalidValue inputLines preambleSize)