defmodule Sonyflake do
  @moduledoc """
  Documentation for `Sonyflake`.
  """
  import Bitwise

  @bit_len_time 39
  @bit_len_sequence 8
  @bit_len_machine_id 63 - (@bit_len_sequence + @bit_len_time)
  @sonyflake_time_unit 1.0e7
  @sonyflake_epoch DateTime.from_iso8601("2014-09-01T00:00:00Z")

  @enforce_keys [:machine_id, :start_time, :sequence]
  defstruct [:machine_id, :start_time, elapsed_time: 0, msb: 0, sequence: 0]

  defp is_private_ipv4(ip) do
    case ip do
      {10, _x, _y, _z} ->
        true

      {192, 168, _x, _y} ->
        true

      {172, 16, _x, _y} ->
        true

      {172, 32, _x, _y} ->
        true

      _ ->
        false
    end
  end

  defp private_ipv4() do
    {:ok, addrs} = :inet.getif()

    filtered =
      Enum.filter(
        addrs,
        fn address ->
          ip = elem(address, 0)
          is_private_ipv4(ip)
        end
      )

    elem(hd(filtered), 0)
  end

  defp lower_16_bit_private_ip() do
    {_, _, b3, b4} = private_ipv4()
    (b3 <<< 8) + b4
  end

  @spec new :: %Sonyflake{
          elapsed_time: 0,
          machine_id: number,
          msb: 0,
          sequence: 255,
          start_time: integer
        }
  @doc """
  Create an instance of `Sonyflake` unique ID generator.
  """
  def new() do
    {:ok, time, _} = @sonyflake_epoch
    start_time = to_sonyflake_time(time)

    %Sonyflake{
      sequence: (1 <<< @bit_len_sequence) - 1,
      msb: 0,
      machine_id: lower_16_bit_private_ip(),
      start_time: start_time
    }
  end

  @spec current_elapsed_time(%Sonyflake{start_time: number}) :: integer
  @doc """
  Get time elapsed since the SonyFlake ID generator was initialised.
  """
  def current_elapsed_time(%Sonyflake{start_time: start_time}) do
    (to_sonyflake_time(DateTime.utc_now()) - start_time) |> trunc()
  end

  def to_sonyflake_time(datetime) do
    (DateTime.to_unix(datetime, :nanosecond) / @sonyflake_time_unit) |> trunc()
  end

  @spec next_id(%Sonyflake{
          elapsed_time: integer,
          machine_id: integer,
          msb: any,
          sequence: integer,
          start_time: number
        }) ::
          {:ok,
           %Sonyflake{
             elapsed_time: integer,
             machine_id: integer,
             msb: any,
             sequence: integer,
             start_time: number
           }, integer}
  @doc """
  Generates and returns the next unique ID.

  Raises a `TimeoutError` after the `SonyFlake` time overflows.
  """
  def next_id(sonyflake) do
    mask_sequence = (1 <<< @bit_len_sequence) - 1
    current_time = current_elapsed_time(sonyflake)

    sonyflake =
      if sonyflake.elapsed_time < current_time do
        %Sonyflake{sonyflake | elapsed_time: current_time, sequence: 0}
      else
        sequence = sonyflake.sequence + 1 &&& mask_sequence

        temp =
          if sequence == 0,
            do:
              (
                new_elapsed_time = sonyflake.elapsed_time + 1
                overtime = new_elapsed_time - current_time
                Process.sleep(overtime)
                %Sonyflake{sonyflake | sequence: sequence, elapsed_time: new_elapsed_time}
              )

        temp
      end

    {:ok, sonyflake, to_id(sonyflake)}
  end

  defp to_id(%Sonyflake{elapsed_time: elapsed_time, sequence: sequence, machine_id: machine_id}) do
    time = elapsed_time <<< (@bit_len_sequence + @bit_len_machine_id)
    sequence = sequence <<< @bit_len_machine_id
    time ||| sequence ||| machine_id
  end

  @spec decompose(non_neg_integer) :: [
          {:id, non_neg_integer}
          | {:machine_id, non_neg_integer}
          | {:msb, non_neg_integer}
          | {:sequence, non_neg_integer}
          | {:time, non_neg_integer},
          ...
        ]
  @doc """
  Decompose a generated sonyflake id back to its components.
  """
  def decompose(id) when is_number(id) do
    mask_sequence = ((1 <<< @bit_len_sequence) - 1) <<< @bit_len_machine_id
    mask_machine_id = (1 <<< @bit_len_machine_id) - 1
    msb = id >>> 63
    time = id >>> (@bit_len_sequence + @bit_len_machine_id)
    sequence = (id &&& mask_sequence) >>> @bit_len_machine_id
    machine_id = id &&& mask_machine_id
    [id: id, msb: msb, time: time, sequence: sequence, machine_id: machine_id]
  end
end