AdventOfCode_Day10.fsx

open System
open System.Text

module List =
   let rec skip n xs =
      match (n, xs) with
      | 0, _ -> xs
      | _, [] -> []
      | n, _::xs -> skip (n-1) xs

let rec countChar (c : char) (acc : int) (xs : char list) =
    match (c, xs) with
    | _, [] -> acc
    | a, b::c when a = b -> countChar a (acc + 1) c
    | _ -> acc

let rec seeAndSayLoop (acc : (int * char) list) (input : char list) : (int * char) list =
    if input.IsEmpty then
        acc
    else
        let f = input.Head
        let c = countChar f 0 input
        seeAndSayLoop ((c, f) :: acc) (input |> List.skip c)

let seeAndSay (input : char list) =
    seeAndSayLoop List.empty input
    |> List.rev
    |> List.fold (fun (b : StringBuilder) (i, c) -> b.Append(i).Append(c)) (new StringBuilder())
    |> (fun s -> s.ToString().ToCharArray() |> Array.toList)

let foldingSeeAndSay (input : string) (c : int) =
    let n = input.ToCharArray() |> Array.toList
    [1..c]
    |> Seq.fold (fun a i -> seeAndSay a) n
    |> (fun x -> x.Length)

let part1 = foldingSeeAndSay "3113322113" 40
let part2 = foldingSeeAndSay "3113322113" 50