Chunk a stream based on orderedness, sizer and joiner.

chunker.ex

@spec chunker(Enumerable.t(), non_neg_integer(), boolean(), (any() -> non_neg_integer()), (Enumerable.t() -> any())) :: Enumerable.t()
def chunker(
        chunks,
        chunk_size,
        ordered \\ true,
        sizer \\ &String.length/1,
        joiner \\ &Enum.join/1
      ) do
  zero = {0, []}
  convert =
    if ordered do
      fn chunks -> chunks |> Enum.reverse() |> then(joiner) end
    else
      joiner
    end

  final = fn {_, chunks} -> {:cont, convert.(chunks), zero} end
  add_chunk = fn {size, chunks}, chunk -> {size + sizer.(chunk), [chunk | chunks]} end

  step = fn chunk, accum = {size, chunks} ->
    if size >= chunk_size do
      {:cont, convert.(chunks), add_chunk.(zero, chunk)}
    else
      {:cont, add_chunk.(accum, chunk)}
    end
  end

  Stream.chunk_while(chunks, zero, step, final)
end