defmodule Carny.Policy do
  @moduledoc """
  Policies are lists of statements that you can validate an identifier against.
  """

  alias __MODULE__
  alias Carny.{Statement, Identifier}

  defstruct statements: []

  @doc ~S"""
  Adds a statement to a given policy

  ## Examples

      iex> Carny.Policy.add_statement(%Carny.Policy{statements: [%Carny.Statement{effect: :deny}]}, %Carny.Statement{})
      %Carny.Policy{statements: [%Carny.Statement{}, %Carny.Statement{effect: :deny}]}
  """
  def add_statement(%Policy{} = policy, %Statement{} = statement) do
    Map.update!(policy, :statements, fn statements ->
      [statement | statements]
    end)
  end

  @doc """
  Does the id pass a specific policy? The use case here is we have
  already looked up a policy for our given resource and want to make
  sure that it has access to some other resource (id).

  ## Examples

      iex> Carny.Policy.authorized?(%Carny.Identifier{}, %Carny.Policy{statements: [%Carny.Statement{resources: [%Carny.Identifier{}]}]})
      true
  """
  def authorized?(%Identifier{} = id, %Policy{} = policy) do
    # As soon as we reach a "true" value then halt
    # because we know it should have access
    Enum.reduce_while(policy.statements, false, fn
      statement, false ->
        {:cont, Statement.passes?(id, statement)}

      _statement, true ->
        {:halt, true}
    end)
  end
end