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(* FizzBuzz *)

let main input = 
  let f x = if x % 3 = 0 then "Fizz" else ""
  let b x = if x % 5 = 0 then "Buzz" else ""
  let ans = (f input) + (b input)
  if ans = "" then input.ToString() else ans

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let advance dist loc = 
  match loc.dir with
  | North -> 
    let npt = { x = loc.pt.x; y = loc.pt.y + dist }
    { loc with pt = npt }
  | South -> 
    let npt = { x = loc.pt.x; y = loc.pt.y - dist }
    { loc with pt = npt }
  | West -> 
    let npt = { x = loc.pt.x - dist; y = loc.pt.y }

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type Direction = 
  | North 
  | South 
  | East 
  | West

type TurnTo = 
  | R
  | L

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let startPt = {
  x = 0
  y = 0
}

let startLoc = {
  dir = North
  pt = startPt
}

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let toTurn = function 
  | 'R' -> R
  | 'L' -> L
  | _ -> failwith "Unknown turn type"

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let turn way loc = 
  match way with
  | L -> 
    match loc.dir with
    | North -> { loc with dir = West }
    | West -> { loc with dir = South }
    | South -> { loc with dir = East }
    | East -> { loc with dir = North }
  | R ->
    match loc.dir with

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let advance dist loc = 
  match loc.dir with
  | North -> 
    let npt = { x = loc.pt.x; y = loc.pt.y + dist }
    { loc with pt = npt }
  | South -> 
    let npt = { x = loc.pt.x; y = loc.pt.y - dist }
    { loc with pt = npt }
  | West -> 
    let npt = { x = loc.pt.x - dist; y = loc.pt.y }

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let move way dist = (turn way) >> (advance dist)

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let moveAll strList = 
  Regex.Matches (strList, @"[LR]\d+")
  |> Seq.cast
  |> Seq.fold 
    (fun oldloc (instr : System.Text.RegularExpressions.Match) -> 
      move 
        (instr.Value.[0] |> toTurn) 
        (instr.Value.Substring 1 |> int)
        oldloc
    )

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let distance stloc endloc = (abs (stloc.x - endloc.x)) + (abs (stloc.y - endloc.y))

let partOne strList = 
  strList |> moveAll |> fun x -> distance startPt x.pt