defmodule Excrc32c do
  @moduledoc """
  A pure Elixir implementation of the CRC32C algorithm.

  ### Usage

  ```elixir
  iex> Excrc32c.crc32c("123456789")
  3808858755
  iex> Excrc32c.crc32c("DYB|O")
  0
  ```

  """
  import Bitwise

  @crc32c_poly 0x82F63B78
  @uint32_mask 0xFFFFFFFF

  @crc32c_table (for byte <- 0..255 do
                   Enum.reduce(1..8, byte, fn _bit, crc ->
                     if (crc &&& 1) == 1 do
                       bxor(crc >>> 1, @crc32c_poly)
                     else
                       crc >>> 1
                     end
                   end)
                 end)
                |> List.to_tuple()

  @compile {:inline, crc32c: 1, crc32c_loop: 2}

  @doc """
  Calculates the CRC32C of the binary.
  """
  @spec crc32c(binary()) :: non_neg_integer()
  def crc32c(data) when is_binary(data) do
    crc32c_loop(data, @uint32_mask)
  end

  defp crc32c_loop(<<>>, crc), do: bxor(crc, @uint32_mask)

  defp crc32c_loop(<<byte::8, rest::binary>>, crc) do
    table_index = bxor(crc, byte) &&& 0xFF
    crc32c_loop(rest, bxor(elem(@crc32c_table, table_index), crc >>> 8))
  end
end