defmodule EctoCommons.LuhnValidator do
  @moduledoc """
  This ecto validator checks the provided value is valid
  using the Luhn algorithm. This is useful for credit cards and other
  common administrative values.

  ## Example:

      iex> types = %{admin_code: :string}
      iex> params = %{admin_code: "740123450"}
      iex> Ecto.Changeset.cast({%{}, types}, params, Map.keys(types))
      ...> |> validate_luhn(:admin_code)
      #Ecto.Changeset<action: nil, changes: %{admin_code: "740123450"}, errors: [], data: %{}, valid?: true>

      iex> types = %{admin_id: :string}
      iex> params = %{admin_id: "74012345123456"}
      iex> Ecto.Changeset.cast({%{}, types}, params, Map.keys(types))
      ...> |> validate_luhn(:admin_id)
      #Ecto.Changeset<action: nil, changes: %{admin_id: "74012345123456"}, errors: [admin_id: {"is not a valid code", [validation: :luhn]}], data: %{}, valid?: false>

      iex> types = %{admin_id: :string}
      iex> params = %{admin_id: "this is an incorrect value"}
      iex> Ecto.Changeset.cast({%{}, types}, params, Map.keys(types))
      ...> |> validate_luhn(:admin_id)
      #Ecto.Changeset<action: nil, changes: %{admin_id: "this is an incorrect value"}, errors: [admin_id: {"is not a valid code", [validation: :luhn]}], data: %{}, valid?: false>
  """
  import Ecto.Changeset

  def validate_luhn(changeset, field, opts \\ []) do
    transformer = Keyword.get(opts, :transformer, & &1)

    if transformer && !is_function(transformer),
      do: raise("Given `:transformer` is not a function")

    validate_change(changeset, field, {:luhn, opts}, fn _, value ->
      value = transformer.(value)

      try do
        case Luhn.valid?(value) do
          false ->
            [{field, {message(opts, "is not a valid code"), [validation: :luhn]}}]

          true ->
            []
        end
      rescue
        _e ->
          [{field, {message(opts, "is not a valid code"), [validation: :luhn]}}]
      end
    end)
  end

  defp message(opts, key \\ :message, default) do
    Keyword.get(opts, key, default)
  end
end