defmodule Mex do
  alias Mex.CLI
  defdelegate set_width(num_chars), to: Mex.CLI

  @name [
    none: &Mex.no_expansion/2,
    once: &Macro.expand_once/2,
    expand: &Macro.expand/2,
    all: &Mex.expand_all/2
  ]

  @pos [[:none, :once, :expand, :all], [:all], [:expand, :all], [:once, :expand, :all]]

  def by_name(atom), do: @name[atom]

  def by_column_count(i) do
    Enum.map(Enum.at(@pos, i) || List.first(@pos), &by_name/1)
  end

  @doc """
  Call with a `do..end` block, preceded by an optional parameter:

  - a number 1-3, to limit number of expansions to show side by side
  - a single named expander, one of `:none, :once, :expand, :all`

  If you'd like to compare the output of your own expanders side by side,
  see `Mex.pp_compare`.
  """
  defmacro mex(num \\ 0, rest)

  defmacro mex(num, do: node) when is_integer(num) do
    pp_compare(node, __CALLER__, by_column_count(num))
    quote do: :ok
  end

  defmacro mex(atom, do: node) when is_atom(atom) do
    pp_compare(node, __CALLER__, [by_name(atom)])
    quote do: :ok
  end

  @doc """
  Display the expansion of a `node` side-by-side in `env`,
  expanded by `expanders`, a list of anonymous expansion functions.
  """
  def pp_compare(node, env, expanders) do
    {texts, errors} = try_expansions(node, env, expanders)

    IO.puts(expanders |> Enum.map(&inspect/1) |> CLI.headers())
    IO.puts(texts |> CLI.side_by_side())
    IO.puts(errors |> Enum.reject(&is_nil/1) |> Enum.map_join("\n", &(" " <> &1)))
  end

  def expand_all(n, env) do
    Macro.prewalk(n, &Macro.expand(&1, env))
  end

  def no_expansion(node, _env), do: node

  # error handling
  def try_expansions(node, env, fns) do
    fns
    |> Enum.map(&try_expand(&1, node, env))
    |> Enum.reduce({[], []}, &accum_expansion/2)
  end

  defp accum_expansion({_status, txt, err}, {pages, errs}) do
    {pages ++ [txt], errs ++ [err]}
  end

  defp try_expand(f, node, env) do
    import IO.ANSI

    try do
      {:ok, f.(node, env) |> Macro.to_string(), nil}
    rescue
      e ->
        {:err, "ExpansionError", "#{bright()}#{inspect(f)}#{normal()} - #{e.message}"}
    end
  end
end