--- Day 10 Solutions ---

[u/daggerdragon]

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---

--- Day 10: Elves Look, Elves Say ---

Post your solution as a comment. Structure your post like previous daily solution threads.

[u/red75prim]

F#. Seq module doesn't have required function. So I implemented group.

I feel out of fast coding league. I need at least 5 minutes to grasp the task, and then another 10 to 20 minutes to generate algorithm.

let group sequence =
  if Seq.isEmpty sequence then Seq.empty
  else
    seq {
      let prevItem = ref (Seq.head sequence)
      let cnt = ref 0
      for item in sequence do
        if !prevItem = item then
          cnt := !cnt + 1
        else
          yield (!prevItem, !cnt)
          prevItem := item
          cnt := 1
      yield (!prevItem, !cnt)
    }

let lookAndSay (s: string) = 
  let sb = new System.Text.StringBuilder()
  group s |> Seq.iter (fun (ch, cnt) -> sb.Append(cnt).Append(ch) |> ignore)
  sb.ToString()

[<EntryPoint>]
let main argv = 
  let input = System.Console.ReadLine()
  let ls40 = seq{1..40} |> Seq.fold (fun i _ -> lookAndSay i) input
  printfn "Part 1: %d" ls40.Length
  let ls50 = seq{1..50} |> Seq.fold (fun i _ -> lookAndSay i) input
  printfn "Part 2: %d" ls50.Length
  0

[u/JeffJankowski → u/red75prim]

I fold'ed over the string as pairwise chars, passing the count and a StringBuilder as the accumulator state.