coercion

defmodule Coercion do
  @moduledoc """
  Rigorous coercion of untrusted values to native primitive types
  """

  @type supported_type :: :integer | :boolean | :string | :atom | :datetime | :naive_datetime
  @type output_type ::
          String.t() | integer | boolean | Date.t() | DateTime.t() | NaiveDateTime.t()

  @spec coerce(any, supported_type()) :: {:ok | :invalid | :blank, output_type()}

  @doc """
  Coerce and validate a value to the given type.

  ## Examples

      iex> import Coercion # For demo purposes
      Coercion

      iex> coerce(" 20 ", :integer)
      {:ok, 20}
      iex> coerce("  x", :integer)
      {:invalid, 0}

      iex> coerce(" TRue ", :boolean)
      {:ok, true}
      iex> coerce(" T ", :boolean)
      {:ok, true}
      iex> coerce(" y ", :boolean)
      {:ok, true}
      iex> coerce(" Yes ", :boolean)
      {:ok, true}
      iex> coerce(" 1 ", :boolean)
      {:ok, true}
      iex> coerce(" TRu ", :boolean)
      {:invalid, false}
      iex> coerce(" F ", :boolean)
      {:ok, false}
      iex> coerce(" N ", :boolean)
      {:ok, false}
      iex> coerce(" 0 ", :boolean)
      {:ok, false}

      iex> coerce(" hello ", :string)
      {:ok, "hello"}
      iex> coerce("  ", :string)
      {:blank, ""}
      iex> coerce(true, :string)
      {:ok, "true"}
      iex> coerce(10.5, :string)
      {:ok, "10.5"}
      iex> :bellow
      :bellow
      iex> coerce("bellow", :atom)
      {:ok, :bellow}
      iex> coerce("   bellow  ", :atom)
      {:ok, :bellow}

      iex> coerce("2020-04-02", :date)
      {:ok, ~D[2020-04-02]}
      iex> coerce(:hello, :date)
      {:invalid, nil}
      iex> coerce("2020-04-02T12:00:01Z", :datetime)
      {:ok, ~U[2020-04-02 12:00:01Z]}
      iex> coerce("2020-04-02T12:00:01Z", :naive_datetime)
      {:ok, ~N[2020-04-02 12:00:01]}
  """

  # Integer
  def coerce(value, :integer) when is_integer(value), do: {:ok, value}
  def coerce(value, :integer) when is_float(value), do: {:ok, Kernel.round(value)}

  def coerce(value, :integer) when is_bitstring(value) do
    value
    |> String.trim()
    |> coerce(:integer, :trimmed)
  end

  def coerce(_value, :integer), do: {:invalid, 0}

  # Boolean
  def coerce(value, :boolean) when is_boolean(value), do: {:ok, value}

  def coerce(value, :boolean) when is_bitstring(value) do
    value
    |> String.trim()
    |> coerce(:boolean, :trimmed)
  end

  def coerce(_value, :boolean), do: {:invalid, false}

  # Date
  def coerce(%Date{} = value, :date), do: {:ok, value}

  def coerce(value, :date) when is_binary(value) do
    case Date.from_iso8601(value) do
      {:ok, date} -> {:ok, date}
      {:error, _error} -> {:invalid, nil}
    end
  end

  def coerce(_value, :date) do
    {:invalid, nil}
  end

  # DateTime
  def coerce(%DateTime{} = value, :datetime), do: {:ok, value}

  def coerce(value, :datetime) when is_binary(value) do
    case DateTime.from_iso8601(value) do
      # Note: The DateTime.from_iso8601/1 documentation claims that ISO 8601 does not include a proper timezone
      # I don't normally use DateTime, so maybe someone else can resolve offset issues.
      {:ok, dt, _} -> {:ok, dt}
      {:error, _error} -> {:invalid, nil}
    end
  end

  def coerce(_value, :datetime) do
    {:invalid, nil}
  end

  # NaiveDateTime
  def coerce(%NaiveDateTime{} = value, :naive_datetime), do: {:ok, value}

  def coerce(value, :naive_datetime) when is_binary(value) do
    case NaiveDateTime.from_iso8601(value) do
      {:ok, dt} -> {:ok, dt}
      {:error, _error} -> {:invalid, nil}
    end
  end

  def coerce(_value, :naive_datetime) do
    {:invalid, nil}
  end

  # String
  def coerce(value, :string) when is_integer(value), do: coerce(Integer.to_string(value), :string)
  def coerce(value, :string) when is_float(value), do: coerce(Float.to_string(value), :string)
  def coerce(_value = true, :string), do: coerce("true", :string)
  def coerce(_value = false, :string), do: coerce("false", :string)
  def coerce(value, :string) when is_nil(value), do: coerce("", :string)

  def coerce(value, :string) when is_bitstring(value) do
    value
    |> String.trim()
    |> coerce(:string, :trimmed)
  end

  def coerce(_value, :string), do: {:invalid, ""}

  # Atom
  def coerce(value, :atom) when is_bitstring(value) do
    atom_value =
      value
      |> String.trim()
      |> String.to_existing_atom()

    {:ok, atom_value}
  end

  def coerce(value, :atom) when is_integer(value), do: coerce(Integer.to_string(value), :atom)
  def coerce(_value, :atom), do: {:invalid, ""}

  # Value
  def value(value, atom) do
    {_, value} = coerce(value, atom)
    value
  end

  #
  # Private functions
  #

  # Integer from a trimmed string
  defp coerce(_value = "", :integer, :trimmed), do: {:blank, 0}

  defp coerce(value, :integer, :trimmed) do
    Integer.parse(value, 10)
    |> coerce(:integer, :parsed)
  end

  # From the result of `Integer.parse`
  defp coerce(:error, :integer, :parsed), do: {:invalid, 0}
  defp coerce({int, ""}, :integer, :parsed), do: {:ok, int}
  defp coerce({int, _}, :integer, :parsed), do: {:invalid, int}

  # From a trimmed string
  defp coerce(_value = "", :string, :trimmed), do: {:blank, ""}
  defp coerce(value, :string, :trimmed), do: {:ok, value}

  # Parse boolean from trimmed string
  defp coerce(value, :boolean, :trimmed), do: coerce(String.downcase(value), :boolean, :downcased)
  # Parse boolean from trimmed, downcased string
  defp coerce(_value = "true", :boolean, :downcased), do: {:ok, true}
  defp coerce(_value = "false", :boolean, :downcased), do: {:ok, false}
  defp coerce(_value = "t", :boolean, :downcased), do: {:ok, true}
  defp coerce(_value = "f", :boolean, :downcased), do: {:ok, false}
  defp coerce(_value = "yes", :boolean, :downcased), do: {:ok, true}
  defp coerce(_value = "no", :boolean, :downcased), do: {:ok, false}
  defp coerce(_value = "y", :boolean, :downcased), do: {:ok, true}
  defp coerce(_value = "n", :boolean, :downcased), do: {:ok, false}
  defp coerce(_value = "on", :boolean, :downcased), do: {:ok, true}
  defp coerce(_value = "off", :boolean, :downcased), do: {:ok, false}
  defp coerce(_value = "1", :boolean, :downcased), do: {:ok, true}
  defp coerce(_value = "0", :boolean, :downcased), do: {:ok, false}
  defp coerce(_value = "", :boolean, :downcased), do: {:blank, false}
  defp coerce(_value, :boolean, :downcased), do: {:invalid, false}
end