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

  alias Carny.Statement

  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, 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?("start", %Carny.Identifier{}, %Carny.Policy{statements: [%Carny.Statement{privileges: ["start"], resources: [%Carny.Identifier{}]}]})
      true
  """
  def authorized?(privilege, id, 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?(privilege, id, statement)}

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