defmodule CRC do
  @moduledoc """
  This module is used to calculate CRC (Cyclic Redundancy Check) values
  for binary data. It uses NIF functions written in C to interate over
  the given binary calculating the CRC checksum value.

  CRC implementations have been tested against these online calculators to
  validate their correctness to the best of our ability.

  https://www.lammertbies.nl/comm/info/crc-calculation.html
  http://www.sunshine2k.de/coding/javascript/crc/crc_js.html
  """

  @spec crc(:crc_algorithm.params(), iodata()) :: :crc_algorithm.value()
  defdelegate crc(params, input), to: :crc

  @spec crc_init(:crc_algorithm.params()) :: :crc_algorithm.resource()
  defdelegate crc_init(params), to: :crc

  @spec crc_update(:crc_algorithm.resource(), iodata()) :: :crc_algorithm.resource()
  defdelegate crc_update(resource, input), to: :crc

  @spec crc_final(:crc_algorithm.resource()) :: :crc_algorithm.value()
  defdelegate crc_final(resource), to: :crc

  @model_list :crc_nif.crc_list() |> Map.to_list() |> Enum.map(fn {model, map} -> {model, Map.get(map, model)} end)
  @doc """
  Returns a list of all the pre-defined CRC models
  """
  @spec list() :: [{atom, String.t}]
  def list() do
    @model_list
  end

  @doc """
  Returns a list of all pre-defined CRC Models that match the filter given.

  Filter is compiled into a regular expression and matched against the model name
  and description.
  """
  @spec list(binary) :: [{atom, String.t}]
  def list(filter) do
    @model_list
    |> Enum.filter(&(list_filter(&1, filter)))
  end

  defp list_filter({model_atom, model_name}, filter) do
    atom_string = Atom.to_string(model_atom)

    {:ok, rfilter} = Regex.compile(filter)
    Regex.match?(rfilter, atom_string) or Regex.match?(rfilter, model_name)
  end

  @doc """
  Calculates a 8-bit CRC with polynomial x^8+x^6+x^3+x^2+1, 0x14D.
  Chosen based on Koopman, et al. (0xA6 in his notation = 0x14D >> 1):
  http://www.ece.cmu.edu/~koopman/roses/dsn04/koopman04_crc_poly_embedded.pdf

  seed defaults to 0xFF if one is not given
  """
  @spec crc_8(binary, number) :: number
  defdelegate crc_8(input, seed \\ 0xFF), to: :crc

  @doc """
  Calculates a 16-bit ANSI CRC checksum for the provided binary
  """
  @spec crc_16(binary) :: number
  def crc_16(input), do: :crc_fast.calc(:crc_16, input)

  @doc """
  Calculates a 16-bit CCITT CRC with the given seed,
  seed defaults to 0xFFFF if one is not given.

  This CCIT method uses a 0x1021 polynomial.
  """
  @spec ccitt_16(binary) :: number
  def ccitt_16(input), do: :crc_fast.calc(:crc_16_ccitt_false, input)

  @spec ccitt_16(binary, number) :: number
  def ccitt_16(input, seed) do
    extend_model_seed(:crc_16_ccitt_false, seed)
    |> :crc_fast.calc(input)
  end

  @doc """
  Calculates a 16-bit CCITT Kermit CRC

  This CCIT method uses a 0x8408 polynomial.
  """
  @spec ccitt_16_kermit(binary) :: number
  def ccitt_16_kermit(input), do: :crc_fast.calc(:crc_16_kermit, input)
  @spec ccitt_16_kermit(binary, number) :: number
  def ccitt_16_kermit(input, seed) do
    extend_model_seed(:crc_16_kermit, seed)
    |> :crc_fast.calc(input)
  end

  @doc """
  Calculates a 16-bit CCITT XMODEM CRC

  This CCIT method uses a 0x1021 polynomial.
  """
  @spec ccitt_16_xmodem(binary) :: number
  def ccitt_16_xmodem(input), do: :crc_fast.calc(:xmodem, input)

  @doc """
  Calculates a 16-bit CCITT 0x1D0F CRC

  This CCIT method uses a 0x1021 polynomial.
  """
  @spec ccitt_16_1D0F(binary) :: number
  def ccitt_16_1D0F(input) do
    extend_model_seed(:crc_16_ccitt_false, 0x1D0F)
    |> :crc_fast.calc(input)
  end

  @doc """
  Calculates a 16-bit modbus CRC
  """
  @spec crc_16_modbus(binary) :: number
  def crc_16_modbus(input), do: :crc_fast.calc(:crc_16_modbus, input)

  @doc """
  Calculates a 16-bit Sick CRC
  """
  @spec crc_16_sick(binary) :: number
  def crc_16_sick(input), do: :crc_fast.calc(:crc_16_sick, input)

  @doc """
  Calculates a 16-bit DNP CRC
  """
  @spec crc_16_dnp(binary) :: number
  def crc_16_dnp(input), do: :crc_fast.calc(:crc_16_dnp, input)

  @doc """
  Calculates a 32-bit CRC
  """
  @spec crc_32(binary) :: number
  def crc_32(input), do: :crc_fast.calc(:crc_32, input)

  @doc """
  Calculates an XOR checksum for the given binary
  """
  @spec checksum_xor(binary) :: number
  defdelegate checksum_xor(input), to: :crc

  defp extend_model_seed(model, seed), do: %{extend: model, init: seed}
end