global_id.ex

defmodule GlobalId do
  @moduledoc """
  GlobalId module contains an implementation of a guaranteed globally unique id system.

  It can be used for generating 64-bit unique IDs at high scale.
  The IDs generated by this service are roughly time sortable.
  """

  # NOTE: On production don't change it once set
  epoch = {{1970, 1, 1}, {0, 0, 0}}
  @epoch :calendar.datetime_to_gregorian_seconds(epoch)

  @doc """
  Please implement the following function.

  Since the IDs use timestamp as the first component, they are time sortable.

  64 bit non negative integer output
  """
  # NOTE: On production track increasing sequence
  @spec get_id(sequence :: non_neg_integer) :: non_neg_integer
  def get_id(sequence) do
    <<new_id::unsigned-integer-64>> =
      <<timestamp()::unsigned-integer-42, node_id()::unsigned-integer-10,
        sequence::unsigned-integer-12>>

    new_id
  end

  @doc """
  Returns your node id as an integer.
  It will be greater than or equal to 0 and less than 1024.
  It is guaranteed to be globally unique.

  SIZE: 1024 = 2^10 - 10 bits
  """
  @spec node_id() :: non_neg_integer
  def node_id,
    # NOTE: On production rely on node discovery
    do: Enum.random(0..1024)

  @doc """
  Returns timestamp since the epoch in milliseconds.

  The maximum timestamp that can be represented using 42 bits is 2^42 - 1,
  or 4398046511103, which comes out to be Wednesday, May 15, 2109 7:35:11.103 AM.
  That gives us 139 years with respect to a custom epoch.

  defp epoch do
    2
    |> :math.pow(42)
    |> Kernel.-(1)
    |> round()
    |> DateTime.from_unix!(:millisecond)
  end
  """
  @spec timestamp() :: non_neg_integer
  def timestamp,
    do: System.os_time(:millisecond) - @epoch
end