This ecto validator checks the provided value is valid using the Luhn algorithm. This is useful for credit cards and other common administrative values.

The validator accepts an optional :transformer function that can modify the value before applying the Luhn check. This is useful in cases where the input value contains other characters or letters instead of only numbers. The transformed value will be Luhn-checked without replacing the value provided in the params.

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_code: :string}
iex> params = %{admin_code: "7A0123450"}
iex> Ecto.Changeset.cast({%{}, types}, params, Map.keys(types))
...> |> validate_luhn(:admin_code, transformer: &(String.replace(&1, "7A", "74")))
#Ecto.Changeset<action: nil, changes: %{admin_code: "7A0123450"}, errors: [], data: %{}, valid?: true>

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>