Advent of Code (Day 17)

Day17_findPaths.fsx

let findPaths input =
  [| ((0, 0), input) |]
  |> Seq.unfold (fun paths -> 
    paths 
    |> Array.filter (fun (pos, _) -> pos <> (3, 3))
    |> Array.collect (fun (pos, path) ->
      let hashVals = hash path |> Seq.take 4
      hashVals 
      |> Seq.zip [| up; down; left; right |]
      |> Seq.choose (fun (f, hashVal) ->
        if hashVal > 'a' then f pos path else None)
      |> Seq.toArray)
    |> function 
      | [||] -> None 
      | next -> Some(next, next))
  |> Seq.collect id
  |> Seq.choose (function
    | (3, 3), path -> Some <| path.Substring(input.Length)
    | _            -> None)