credence

defmodule Credence.Semantic.NoUnderscoreInExpression do
  @moduledoc """
  Fixes compiler errors caused by using `_` in expression position,
  such as a tuple key in a for-comprehension body.

  The Elixir compiler rejects `_` in expression position:

      for _ <- 0..(n - 1), into: %{} do
        {_, :infinity}    # ← error: invalid use of _
      end

  The fix renames the `_` generator to a fresh variable (`idx`)
  and updates all references in the comprehension body:

      for idx <- 0..(n - 1), into: %{} do
        {idx, :infinity}
      end
  """
  use Credence.Semantic.Rule

  alias Credence.Issue

  @impl true
  def match?(%{message: msg}) when is_binary(msg) do
    String.contains?(msg, "invalid use of _")
  end

  def match?(_), do: false

  @impl true
  def to_issue(diagnostic) do
    %Issue{
      rule: :no_underscore_in_expression,
      message: diagnostic.message,
      meta: %{line: line(diagnostic)}
    }
  end

  @impl true
  def fix(source, _diagnostic) do
    case parse(source) do
      {:ok, ast} ->
        transformed = transform_underscores(ast)

        if transformed == ast do
          source
        else
          patches = Credence.RuleHelpers.patches_from_diff(ast, transformed)

          case patches do
            [] -> source
            _ -> Sourceror.patch_string(source, patches)
          end
        end

      :error ->
        source
    end
  end

  defp line(%{position: {line, _col}}), do: line
  defp line(%{position: line}) when is_integer(line), do: line

  defp parse(source) do
    {:ok, Sourceror.parse_string!(source)}
  rescue
    _ -> :error
  end

  # Forms that introduce a *pattern* or a new binding/scope. If any of these
  # appear in the comprehension body we refuse to fix: a blind rename would
  # rewrite `_` that sit in pattern position (e.g. `{_, _} = a`, a `case`
  # wildcard, an `fn` arg), silently changing match semantics or shadowing a
  # binding. Inside a body free of all of these, every `_` is unambiguously in
  # expression position — exactly the invalid use the compiler rejects — so
  # renaming all of them to one fresh variable is the same-answer fix.
  @pattern_forms [
    :=,
    :case,
    :cond,
    :with,
    :for,
    :fn,
    :receive,
    :try,
    :&,
    :quote,
    :unquote,
    :<-,
    :->,
    :^
  ]

  defp transform_underscores(ast) do
    Macro.postwalk(ast, fn
      {:for, meta, args} = node when is_list(args) ->
        if fixable?(args) do
          fresh = fresh_var_name(args)
          {:for, meta, rename_underscore_in_for_args(args, fresh)}
        else
          node
        end

      node ->
        node
    end)
  end

  # Safe to fix only when there is exactly one `_` generator (so a body `_`
  # maps to it unambiguously) and the body uses `_` solely in expression
  # position.
  defp fixable?(args) do
    underscore_generator_count(args) == 1 and fixable_body?(args)
  end

  defp underscore_generator_count(args) when is_list(args) do
    Enum.count(args, fn
      {:<-, _, [{:_, _, nil}, _]} -> true
      _ -> false
    end)
  end

  defp fixable_body?(args) do
    case do_body(args) do
      {:ok, body} -> contains_underscore?(body) and not contains_pattern_form?(body)
      :error -> false
    end
  end

  defp do_body(args) do
    Enum.find_value(args, :error, fn
      [{{:__block__, _, [:do]}, body}] -> {:ok, body}
      _ -> false
    end)
  end

  defp contains_underscore?(ast) do
    {_, found} =
      Macro.prewalk(ast, false, fn
        {:_, _, ctx} = n, _acc when is_atom(ctx) -> {n, true}
        n, acc -> {n, acc}
      end)

    found
  end

  defp contains_pattern_form?(ast) do
    {_, found} =
      Macro.prewalk(ast, false, fn
        {head, _, _} = n, _acc when head in @pattern_forms -> {n, true}
        n, acc -> {n, acc}
      end)

    found
  end

  # A variable name not used anywhere in the comprehension, so the renamed
  # generator cannot collide with (or shadow) anything the body references.
  defp fresh_var_name(args) do
    used = collect_var_names(args)
    candidates = [:idx, :i, :index] ++ Enum.map(0..999, &:"idx_#{&1}")
    Enum.find(candidates, :idx, fn name -> not MapSet.member?(used, name) end)
  end

  defp collect_var_names(ast) do
    {_, names} =
      Macro.prewalk(ast, MapSet.new(), fn
        {name, _, ctx} = n, acc when is_atom(name) and is_atom(ctx) ->
          {n, MapSet.put(acc, name)}

        n, acc ->
          {n, acc}
      end)

    names
  end

  defp rename_underscore_in_for_args(args, fresh) do
    Enum.map(args, fn
      {:<-, arrow_meta, [{:_, var_meta, nil}, range]} ->
        {:<-, arrow_meta, [{fresh, var_meta, nil}, range]}

      [{{:__block__, do_meta, [:do]}, body}] ->
        new_body =
          Macro.prewalk(body, fn
            {:_, u_meta, nil} -> {fresh, u_meta, nil}
            other -> other
          end)

        [{{:__block__, do_meta, [:do]}, new_body}]

      other ->
        other
    end)
  end
end