defmodule Totpex do
  @moduledoc """
  Generate a Time-Based One-Time Password used from 2 factor authentication.
  Official specification: https://tools.ietf.org/html/rfc6238
  """

  defp generate_hmac(secret, period) do
    # Clean unwanted character from the secret and decode it using Base32 "encoding"
    key =
      secret
      |> String.replace(" ", "")
      |> String.upcase()
      |> Base.decode32!()

    # Generate the moving mactor
    moving_factor =
      DateTime.utc_now()
      |> DateTime.to_unix()
      |> Integer.floor_div(period)
      |> Integer.to_string(16)
      |> String.pad_leading(16, "0")
      |> String.upcase()
      |> Base.decode16!()

    # Generate SHA-1
    :crypto.hmac(:sha, key, moving_factor)
  end

  defp hmac_dynamic_truncation(hmac) do
    # Get the offset from last  4-bits
    <<_::19-binary, _::4, offset::4>> = hmac

    # Get the 4-bytes starting from the offset
    <<_::size(offset)-binary, p::4-binary, _::binary>> = hmac

    # Return the last 31-bits
    <<_::1, truncation::31>> = p

    truncation
  end

  defp generate_hotp(truncated_hmac) do
    truncated_hmac
    |> rem(1_000_000)
    |> Integer.to_string()
  end

  @doc """
  Generate Time-Based One-Time Password.
  The default period used to calculate the moving factor is 30s
  """
  def generate_totp(secret, period \\ 30) do
    secret
    |> generate_hmac(period)
    |> hmac_dynamic_truncation
    |> generate_hotp
  end
end