credence

defmodule Credence.Semantic.UndefinedFunction do
  @moduledoc """
  Fixes compiler warnings about undefined or deprecated functions with known replacements.

  LLMs sometimes use functions that don't exist in the expected module or
  have been deprecated in favour of a replacement. This rule maintains a
  mapping of known corrections.

  ## Rename examples (module.function → module.function)

      # Warning: Enum.last/1 is undefined or private
      list |> Enum.last()
      # Fixed:
      list |> List.last()

      # Warning: Enum.partition/2 is deprecated. Use Enum.split_with/2 instead
      Enum.partition(list, &pred/1)
      # Fixed:
      Enum.split_with(list, &pred/1)

  ## Literal examples (hallucinated call → Elixir literal)

      # Warning: Float.NegInfinity/0 is undefined or private
      validate(root, Float.NegInfinity(), Float.PositiveInfinity())
      # Fixed:
      validate(root, :neg_infinity, :infinity)

      # Warning: Float.inf/0 is undefined or private (often used as -Float.inf)
      max_num = -Float.inf
      # Fixed:
      max_num = :neg_infinity
  """
  use Credence.Semantic.Rule
  alias Credence.Issue

  # ── Replacement table ──────────────────────────────────────────
  #
  #   {:rename, new_mod, new_fun}         — swap Module.function, keep args/parens
  #   {:literal, text}                    — replace Module.function() or Module.function with a literal
  #   {:literal_with_neg, pos, neg}       — like :literal, but if preceded by `-`, use the neg form

  @replacements %{
    # Wrong module for real function
    {"Enum", "last", 1} => {:rename, "List", "last"},
    {"Enum", "last", 0} => {:rename, "List", "last"},
    {"List", "reverse", 1} => {:rename, "Enum", "reverse"},

    # Deprecated
    {"Enum", "partition", 2} => {:rename, "Enum", "split_with"},

    # Hallucinated Float infinity (from Python float('inf') / Java Double.POSITIVE_INFINITY)
    {"Float", "NegInfinity", 0} => {:literal, ":neg_infinity"},
    {"Float", "PositiveInfinity", 0} => {:literal, ":infinity"},
    {"Float", "NegInf", 0} => {:literal, ":neg_infinity"},
    {"Float", "Infinity", 0} => {:literal, ":infinity"},

    # Float.inf — lowercase, often used as -Float.inf (Python float('-inf'))
    {"Float", "inf", 0} => {:literal_with_neg, ":infinity", ":neg_infinity"},

    # Hallucinated Integer bounds (from Java Integer.MIN_VALUE / MAX_VALUE)
    {"Integer", "min_value", 0} => {:literal, ":neg_infinity"},
    {"Integer", "max_value", 0} => {:literal, ":infinity"},

    # Hallucinated List.pop (from Python list.pop() — get last element)
    {"List", "pop", 1} => {:rename, "List", "last"},

    # Wrong module (List.drop doesn't exist — Enum.drop does)
    {"List", "drop", 2} => {:rename, "Enum", "drop"},

    # Wrong module (Enum.cycle doesn't exist — Stream.cycle does)
    {"Enum", "cycle", 1} => {:rename, "Stream", "cycle"}
  }

  @impl true
  def match?(%{severity: :warning, message: msg}) do
    (String.contains?(msg, "is undefined or private") or String.contains?(msg, "is deprecated")) and
      parse_function_ref(msg) != nil and
      Map.has_key?(@replacements, parse_function_ref(msg))
  end

  def match?(_), do: false

  @impl true
  def to_issue(%{message: msg, position: position}) do
    %Issue{
      rule: :undefined_function,
      message: msg,
      meta: %{line: extract_line(position)}
    }
  end

  @impl true
  def fix(source, %{message: msg, position: position}) do
    line_no = extract_line(position)

    case parse_function_ref(msg) do
      {mod, fun, _arity} = key ->
        case Map.get(@replacements, key) do
          {:rename, new_mod, new_fun} ->
            replace_on_line(source, line_no, "#{mod}.#{fun}", "#{new_mod}.#{new_fun}")

          {:literal, text} ->
            replace_literal(source, line_no, mod, fun, text)

          {:literal_with_neg, pos_text, neg_text} ->
            replace_literal_with_neg(source, line_no, mod, fun, pos_text, neg_text)

          nil ->
            source
        end

      nil ->
        source
    end
  end

  # ── Literal replacement (with parens fallback to without) ──────

  defp replace_literal(source, line_no, mod, fun, text) do
    call_with_parens = "#{mod}.#{fun}()"
    call_without_parens = "#{mod}.#{fun}"

    result = replace_on_line(source, line_no, call_with_parens, text)

    if result == source do
      replace_on_line(source, line_no, call_without_parens, text)
    else
      result
    end
  end

  # ── Literal replacement with negation awareness ────────────────
  # Tries negated forms first (-Float.inf → :neg_infinity),
  # then positive forms (Float.inf → :infinity).

  defp replace_literal_with_neg(source, line_no, mod, fun, pos_text, neg_text) do
    neg_with_parens = "-#{mod}.#{fun}()"
    neg_without_parens = "-#{mod}.#{fun}"
    pos_with_parens = "#{mod}.#{fun}()"
    pos_without_parens = "#{mod}.#{fun}"

    result = replace_on_line(source, line_no, neg_with_parens, neg_text)
    result = if result == source, do: replace_on_line(source, line_no, neg_without_parens, neg_text), else: result
    result = if result == source, do: replace_on_line(source, line_no, pos_with_parens, pos_text), else: result
    if result == source, do: replace_on_line(source, line_no, pos_without_parens, pos_text), else: result
  end

  # ── Helpers ────────────────────────────────────────────────────

  defp extract_line({line, _col}) when is_integer(line), do: line
  defp extract_line(line) when is_integer(line), do: line
  defp extract_line(_), do: nil

  defp parse_function_ref(msg) do
    case Regex.run(~r/(\w+)\.(\w+)\/(\d+) is (undefined or private|deprecated)/, msg) do
      [_, mod, fun, arity, _] -> {mod, fun, String.to_integer(arity)}
      _ -> nil
    end
  end

  defp replace_on_line(source, line_no, old, new) do
    source
    |> String.split("\n")
    |> Enum.with_index(1)
    |> Enum.map(fn
      {line, ^line_no} -> String.replace(line, old, new, global: false)
      {line, _} -> line
    end)
    |> Enum.join("\n")
  end
end