Elixir module to create unique identifiers of specified length, with serialization to and from strings. Put here to solicit feedback on coding style/correctness, as I am new to Elixir.

uuid.ex

defmodule Hello.Util.UUID do

  def new(byte_count // 16) do 
    dividend = div(byte_count, 16)
    remainder = rem(byte_count, 16)
    cnt = case remainder do
      r when r === 0 -> dividend 
      _ -> dividend + remainder
    end
    make cnt, byte_count, <<>>
  end

  defp make(0, byte_count, acc), do:
    binary_part(acc, 0, byte_count)
  defp make(cnt, byte_count, acc) do
    u = :ossp_uuid.make :v4, :binary
    make(cnt - 1, byte_count, acc <> u)
  end

  def valid?(uuid, bytes // 16)
  def valid?(uuid, bytes) when is_binary(uuid) do 
    case uuid do
      <<_::[size(bytes), binary]>> -> true
      _ -> false
    end
  end
  def valid?(uuid, bytes) when is_list(uuid) do 
    chars = bytes * 2 
    {:ok, r} = Regex.compile("[A-F0-9]{#{chars}}")
    Regex.match?(r, uuid) and length_valid?(uuid, chars) 
  end

  defp length_valid?(uuid, chars) when is_binary(uuid), do:
    length_valid?(bitstring_to_list(uuid), chars)
  defp length_valid?(uuid, chars) when is_list(uuid), do:
    length(uuid) === chars

  def to_string(uuid) when is_binary(uuid) do
    l = lc <<nibble::4>> inbits uuid, do: hd(integer_to_list(nibble, 16))
    :erlang.list_to_binary l
  end

  def from_string(uuid) when is_binary(uuid), do:
    from_string(bitstring_to_list(uuid))
  def from_string(uuid) when is_list(uuid), do:
    append_byte(uuid, <<>>)

  defp append_byte([], acc), do: acc
  defp append_byte([x,y|rest], acc), do:
    append_byte(rest,  acc <> <<list_to_integer([x, y], 16)>>)

end

Hi Peter, thanks for checking this out!

So, I've been using the if/unless/cond for nil tests, mostly, but I agree the case here obscures the intent. In fact, this code was wrong. Here's the corrected version.

  def new(byte_count // 16) do 
    dividend = div(byte_count, 16)
    add = if rem(byte_count, 16) > 0, do: 1, else: 0
    make (dividend + add), byte_count, <<>>
  end

Thanks for your feedback 😊