output =
  println(format(fmt,
    [seqLength, initSeq, tailSeq] ++ toList(longestPair)
  ))

fmt = """getSeq(27) (length): {}
getSeq(27) (first 4): {}
getSeq(27) (last 4): {}
max length {} for n = {}"""

-----------------------------------------------------------

seq       = getSeq(27)
seqLength = length(seq)
initSeq   = take(4, seq)
tailSeq   = drop(seqLength - 4, seq)

-----------------------------------------------------------

longestPair =
  range(1, 99999)
  |> map(n => (length(getSeq(n)), n))
  |> argmax(at(0))

-----------------------------------------------------------
-- generate full sequence

getSeq(n) =
  iterate(step, n)
  |> takeUntil(eq(1))

-----------------------------------------------------------
-- get the next number in a sequence

step(n) =
  if n % 2 == 0 then round(n / 2) else n * 3 + 1