credence

defmodule Credence.Rule.NoRedundantEnumJoinSeparator do
  @moduledoc """
  Readability rule: Detects `Enum.join("")` where the empty-string separator
  is passed explicitly.

  `Enum.join/1` already defaults to `""`, so the argument adds visual noise
  without changing behaviour.

  ## Bad

      graphemes |> Enum.join("")
      Enum.join(list, "")

  ## Good

      graphemes |> Enum.join()
      Enum.join(list)
  """
  @behaviour Credence.Rule
  alias Credence.Issue

  @impl true
  def check(ast, _opts) do
    {_ast, issues} =
      Macro.prewalk(ast, [], fn
        # Direct call: Enum.join(list, "")
        {{:., _, [{:__aliases__, _, [:Enum]}, :join]}, meta, [_list, ""]} = node, issues ->
          {node, [build_issue(meta) | issues]}

        # In a pipe the separator is the only explicit arg: ... |> Enum.join("")
        # The piped value becomes the first arg, so the AST call has [""]
        {{:., _, [{:__aliases__, _, [:Enum]}, :join]}, meta, [""]} = node, issues ->
          {node, [build_issue(meta) | issues]}

        node, issues ->
          {node, issues}
      end)

    Enum.reverse(issues)
  end

  defp build_issue(meta) do
    %Issue{
      rule: :no_redundant_enum_join_separator,
      severity: :info,
      message:
        "`Enum.join/1` already defaults to an empty string separator. " <>
          "Remove the redundant `\"\"` argument.",
      meta: %{line: Keyword.get(meta, :line)}
    }
  end
end