defmodule QuickChex do
  @moduledoc """
  main module with the main macros that you can use when writing property-based
  tests.

  add `use QuickChex` at the top of your ExUnit test module

  ## Example

      defmodule MyModuleTest do
        use ExUnit.Case, async: true
        use QuickChex # <- add this!
      end
  """
  import QuickChex.Generators, warn: false

  @default_iterations_number Application.get_env(:quick_chex, :iterations, 100)

  @doc false
  defmacro __using__(_opts) do
    quote do
      import unquote(__MODULE__)
      import QuickChex.Generators

      ExUnit.plural_rule("property", "properties")

      Module.register_attribute __MODULE__, :qc_properties, accumulate: true
    end
  end

  @doc """
  specify a property with a single generate parameter

  A property has a `name` to be identified by checks, a `param1` that will
  be generated, and a body to specify the requirements

  ## Example

  for a **negate** property of a `Test.negate/1` function you could write a
  property like this:

  ```
  property :negate, value do
    assert not value === Test.negate(value)
  end

  property :negate_two_time_means_doing_nothing, value do
    res = value
    |> Test.negate
    |> Test.negate
    assert value === res
  end
  ```
  """
  defmacro property(name, param1, do: contents) do
    contents = Macro.escape(contents)
    param1 = Macro.escape(param1)
    quote bind_quoted: [name: name, param1: param1, contents: contents] do
      @qc_properties name
      func_name = "quick_chex_property_#{name}" |> String.to_atom
      def unquote(func_name)(unquote(param1)) do
        unquote(contents)
      end
    end
  end

  @doc """
  same as property/2 with two generated parameters

  ## Example

  a property for the Test.add/2 function

  ```
  property :add_is_commutative, n1, n2 do
    assert Test.add(n1, n2) === Test.add(n2, n1)
  end
  ```
  """
  defmacro property(name, param1, param2, do: contents) do
    contents = Macro.escape(contents)
    param1 = Macro.escape(param1)
    param2 = Macro.escape(param2)
    quote bind_quoted: [name: name, param1: param1, param2: param2,
    contents: contents] do
      @qc_properties name
      func_name = "quick_chex_property_#{name}" |> String.to_atom
      def unquote(func_name)(unquote(param1), unquote(param2)) do
        unquote(contents)
      end
    end
  end

  @doc """
  same as property/2 with three generated parameters
  """
  defmacro property(name, param1, param2, param3, do: contents) do
    contents = Macro.escape(contents)
    param1 = Macro.escape(param1)
    param2 = Macro.escape(param2)
    param3 = Macro.escape(param3)
    quote bind_quoted: [name: name, param1: param1, param2: param2,
    param3: param3, contents: contents] do
      @qc_properties name
      func_name = "quick_chex_property_#{name}" |> String.to_atom
      def unquote(func_name)(unquote(param1), unquote(param2),
      unquote(param3)) do
        unquote(contents)
      end
    end
  end

  @doc """
  check a property by giving the property name and a list of settings

  settings:

    * **with** a *list* of generators (or values) to be passed to the property.
      **Be careful!** the number of generators **must** match the number of
      arguments of the property
    * **iterations** *integer* representing the number of iterations
    * **only_if** a *function* with the same arity as the number of generators
      in the with settings.
      If the function resturns true than the generated values are considered
      acceptable for the property, otherwise another attempt is made until a
      suitable array of values is found. Use this with caution, as it could slow
      down your test suite considerably.

  *a full example*

      check :my_property,
        with: [non_neg_integer, non_neg_integer],
        itarations: 100,
        only_if: fn num1, num2 -> num1 > num2 end

  this checks a property with two non negative integer args. The *only_if*
  function ensures that the first number is greater than the second one.

  usually there is a better approach

      check :my_property,
        with: [{:non_neg_integer, [0, 1000], {:non_neg_integer, [1001, 2000]}],
        itarations: 100

  by better specifying the generators we can omit the only_if block and speed up
  our test suite. Use the only_if block only if generator constraints are not
  enough.
  """
  defmacro check(name, check_name \\ nil, settings) do
    generators = settings
    |> Keyword.get(:with)
    |> Macro.escape
    iterations = settings[:iterations] || @default_iterations_number
    only_if = settings[:only_if]

    quote bind_quoted: [name: name, check_name: check_name, settings: settings,
    generators: generators, iterations: iterations, only_if: only_if] do
      func_name = "quick_chex_property_#{name}" |> String.to_atom
      1..iterations
      |> Enum.map(fn num ->
        args = calculate_args(generators, only_if)
        test_func_name = ExUnit.Case.register_test(__ENV__, :property,
          register_name(name, check_name, num), [[args: args]])
        args = args |> Macro.escape
        def unquote(test_func_name)(_) do
          apply(__MODULE__, unquote(func_name), unquote(args))
        end
      end)
    end
  end

  @doc false
  def register_name(name, nil, iteration_num) do
    "#{name} - iteration #{iteration_num}"
  end
  def register_name(name, check_name, iteration_num) do
    "#{name} - #{check_name} - iteration #{iteration_num}"
  end

  @doc false
  def calculate_args(generators = {:fn, _, _}, _) do
    {func, _} = Code.eval_quoted(generators, [], __ENV__)
    func.()
  end
  def calculate_args(generators, nil) do
    {values, _} = Code.eval_quoted(generators, [], __ENV__)
    values
  end
  def calculate_args(generators, check_function) do
    do_calculate_args(:next, generators, check_function)
  end
  defp do_calculate_args({:ok, values}, _, _), do: values
  defp do_calculate_args(:next, generators, check_function) do
    {values, _} = Code.eval_quoted(generators, [], __ENV__)
    res = if apply(check_function, values) do
      {:ok, values}
    else
      :next
    end
    do_calculate_args(res, generators, check_function)
  end
end