credence

defmodule Credence.Pattern.NoKeywordGetIntegerKey do
  @moduledoc """
  Detects `Keyword.get(list, integer)` where the key is an integer literal.

  `Keyword.get/2` requires atom keys — its guard is `when is_atom(key)`.
  Passing an integer key always crashes at runtime with `FunctionClauseError`.

  LLMs produce this when translating Python's `list[-1]` (last element)
  or `dict[index]` into Elixir, reaching for `Keyword.get` as the
  closest-looking dictionary lookup.

  ## Bad

      Keyword.get(acc, -1)
      Keyword.get(list, 0)
      acc |> Keyword.get(-1)

  ## Good

      List.last(acc)
      List.first(list)
      acc |> List.last()

  ## What is flagged

  Any call to `Keyword.get` with exactly two arguments where the second
  is an integer literal (direct call), or one argument that is an integer
  literal (piped call). Three-argument calls are not flagged.

  ## Auto-fix

  Rewrites based on the index value:

      Keyword.get(var, -1)  →  List.last(var)
      Keyword.get(var, 0)   →  List.first(var)
      Keyword.get(var, n)   →  Enum.at(var, n)

  Only fixes when the list argument is a simple variable name.
  """

  use Credence.Pattern.Rule
  alias Credence.Issue

  @impl true
  def fixable?, do: true

  # ── Check ──────────────────────────────────────────────────────

  @impl true
  def check(ast, _opts) do
    {_ast, issues} =
      Macro.prewalk(ast, [], fn node, acc ->
        case detect(node) do
          {:ok, meta} -> {node, [build_issue(meta) | acc]}
          :skip -> {node, acc}
        end
      end)

    Enum.reverse(issues)
  end

  # Direct call: Keyword.get(list, integer_key)
  defp detect({{:., _, [{:__aliases__, _, [:Keyword]}, :get]}, meta, [_list, key]}) do
    if integer_literal?(key), do: {:ok, meta}, else: :skip
  end

  # Piped call: expr |> Keyword.get(integer_key)
  defp detect({{:., _, [{:__aliases__, _, [:Keyword]}, :get]}, meta, [key]}) do
    if integer_literal?(key), do: {:ok, meta}, else: :skip
  end

  defp detect(_), do: :skip

  # In the AST, positive integers are bare values (is_integer/1).
  # Negative integers are {:-, _, [positive_integer]} (unary minus).
  defp integer_literal?(n) when is_integer(n), do: true
  defp integer_literal?({:-, _, [n]}) when is_integer(n), do: true
  defp integer_literal?(_), do: false

  # ── Fix ────────────────────────────────────────────────────────

  # Direct: Keyword.get(var, integer)
  @direct_re ~r/Keyword\.get\((\w+),\s*(-?\d+)\)/
  # Piped: |> Keyword.get(integer)
  @piped_re ~r/Keyword\.get\((-?\d+)\)/

  @impl true
  def fix(source, _opts) do
    source
    |> String.split("\n")
    |> Enum.map(&fix_line/1)
    |> Enum.join("\n")
  end

  defp fix_line(line) do
    line
    |> fix_direct()
    |> fix_piped()
  end

  defp fix_direct(line) do
    Regex.replace(@direct_re, line, fn _full, var, index_str ->
      direct_replacement(var, String.to_integer(index_str))
    end)
  end

  defp fix_piped(line) do
    Regex.replace(@piped_re, line, fn _full, index_str ->
      piped_replacement(String.to_integer(index_str))
    end)
  end

  # ── Replacements ───────────────────────────────────────────────

  defp direct_replacement(var, -1), do: "List.last(#{var})"
  defp direct_replacement(var, 0), do: "List.first(#{var})"
  defp direct_replacement(var, n), do: "Enum.at(#{var}, #{n})"

  defp piped_replacement(-1), do: "List.last()"
  defp piped_replacement(0), do: "List.first()"
  defp piped_replacement(n), do: "Enum.at(#{n})"

  # ── Issue ──────────────────────────────────────────────────────

  defp build_issue(meta) do
    %Issue{
      rule: :no_keyword_get_integer_key,
      message: """
      `Keyword.get/2` requires atom keys — integer keys always crash \
      at runtime with `FunctionClauseError`.

      This is usually a Python `list[-1]` translation. Use list \
      access functions instead:

          Keyword.get(list, -1)    →    List.last(list)
          Keyword.get(list, 0)     →    List.first(list)
          Keyword.get(list, n)     →    Enum.at(list, n)
      """,
      meta: %{line: Keyword.get(meta, :line)}
    }
  end
end