defmodule Nicene.PublicFunctionsFirst do
  @moduledoc """
  Public functions in a module should be defined before private functions.
  """
  @explanation [check: @moduledoc]

  use Credo.Check, base_priority: :high, category: :readability

  alias Nicene.Traverse

  @doc false
  def run(source_file, params \\ []) do
    case Credo.Code.prewalk(source_file, &Traverse.get_funs/2) do
      [] ->
        []

      functions ->
        issue_meta = IssueMeta.for(source_file, params)

        functions
        |> Enum.max_by(fn
          {:def, _, _, line_no} -> line_no
          _ -> 0
        end)
        |> case do
          {:defp, _, _, _} ->
            []

          {_, _, _, last_public_function} ->
            Enum.reduce(
              functions,
              [],
              &check_function(&1, &2, issue_meta, last_public_function)
            )
        end
    end
  end

  defp check_function({:defp, _, _, line}, issues, issue_meta, public_line)
       when line < public_line do
    [issue_for(issue_meta, line) | issues]
  end

  defp check_function(_, issues, _, _), do: issues

  defp issue_for(issue_meta, line_no) do
    format_issue(issue_meta,
      message: "Private function is defined before a public function",
      line_no: line_no
    )
  end
end