2018-day-5.fsx

open System.IO
open System

let input = File.ReadLines("2018-day-5.txt")

let rec polymerExploder input: string =
  let polymerSome =
    input
    |> Seq.toArray
    |> Seq.pairwise
    |> Seq.tryFind (fun (a, b) -> a <> b && Char.ToLower(a) = Char.ToLower(b))

  if polymerSome.IsNone then input else

  let polymer = polymerSome.Value |> (fun (a, b) -> string a + string b)
  let updatedSequence = input.Replace(polymer, "")

  polymerExploder updatedSequence

// part 1
input
|> Seq.head
|> polymerExploder
|> Seq.length
|> printfn "%A"

// part 2
let replacedInput c input =
  input
  |> Seq.head
  |> fun s -> (string s).Replace((string c).ToLower(), "")
  |> fun s -> (string s).Replace((string c).ToUpper(), "")

['a' .. 'z']
  |> Seq.map (fun c -> c, replacedInput c input |> polymerExploder |> Seq.length)
  |> Seq.sortBy snd
  |> Seq.head
  |> printfn "%A"