credence

defmodule Credence.Pattern.NoCaseTrueFalse do
  @moduledoc """
  Detects `case expr do true -> …; false -> … end` that should be `if/else`.

  LLMs frequently transliterate Python's `if/else` through a `case` on a
  boolean expression with explicit `true`/`false` (or `_`) clauses. Idiomatic
  Elixir uses `if/else` when the condition is already a boolean.

  Only flags cases where the subject is a boolean expression (comparison,
  function call, operator) — not a plain variable, which may be a legitimate
  pattern match on a tristate value.

  ## Detected patterns

      case expr do true -> A; false -> B end
      case expr do false -> B; true -> A end
      case expr do true -> A; _ -> B end
      case expr do false -> B; _ -> A end

  ## Bad

      case rem(n, 2) == 0 do
        true -> :even
        false -> :odd
      end

  ## Good

      if rem(n, 2) == 0 do
        :even
      else
        :odd
      end

  ## Auto-fix

  Rewrites the `case` to `if/else`, placing the `true` body (or the wildcard
  counterpart) in the `do` block and the `false` body in `else`.
  """

  use Credence.Pattern.Rule
  alias Credence.Issue

  @impl true
  def fixable?, do: true

  # ── Check ─────────────────────────────────────────────────────────
  # Uses AST from Code.string_to_quoted (bare boolean literals).

  @impl true
  def check(ast, _opts) do
    {_ast, issues} =
      Macro.prewalk(ast, [], fn
        {:case, meta, [subject, [do: [clause_a, clause_b]]]} = node, acc ->
          if not plain_variable?(subject) and
               boolean_clause_pair?(clause_pattern(clause_a), clause_pattern(clause_b)) do
            {node, [build_issue(meta) | acc]}
          else
            {node, acc}
          end

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

    Enum.reverse(issues)
  end

  # ── Fix ───────────────────────────────────────────────────────────
  # Uses Sourceror for parsing; rewrites matching case nodes to if/else
  # via Macro.postwalk, then emits source with Sourceror.to_string().

  @impl true
  def fix(source, _opts) do
    case Sourceror.parse_string(source) do
      {:ok, ast} ->
        if has_fixable_case?(ast) do
          ast
          |> Macro.postwalk(&maybe_rewrite_case/1)
          |> Sourceror.to_string()
        else
          source
        end

      {:error, _} ->
        source
    end
  end

  # ── Check helpers ─────────────────────────────────────────────────

  # Extract the pattern from a case clause: {:->, _, [[pattern], body]}
  defp clause_pattern({:->, _, [[pattern], _body]}), do: pattern
  defp clause_pattern(_), do: :no_match

  # A plain variable like `some_flag` — legitimate pattern match, not flagged.
  defp plain_variable?({name, _meta, context})
       when is_atom(name) and is_atom(context),
       do: true

  defp plain_variable?(_), do: false

  # Recognises the boolean pairs we flag: true/false, true/_, false/_
  # and their flipped orderings.
  defp boolean_clause_pair?(a, b) do
    case {normalize_pattern(a), normalize_pattern(b)} do
      {true, false} -> true
      {false, true} -> true
      {true, :wildcard} -> true
      {:wildcard, true} -> true
      {false, :wildcard} -> true
      {:wildcard, false} -> true
      _ -> false
    end
  end

  defp normalize_pattern(true), do: true
  defp normalize_pattern(false), do: false
  defp normalize_pattern({:_, _, _}), do: :wildcard
  defp normalize_pattern(_), do: :other

  # ── Fix helpers ───────────────────────────────────────────────────

  # Extracts the clause list from a case node's keyword block.
  # Handles both Code.string_to_quoted format ([do: clauses]) and
  # Sourceror format ([{{:__block__, _, [:do]}, clauses}]).
  defp extract_do_clauses([{:do, clauses}]) when is_list(clauses), do: clauses

  defp extract_do_clauses([{{:__block__, _, [:do]}, clauses}]) when is_list(clauses),
    do: clauses

  defp extract_do_clauses(_), do: nil

  # Quick pre-scan: is there at least one case node we can rewrite?
  # Prevents unnecessary Sourceror.to_string() (which reformats the file).
  defp has_fixable_case?(ast) do
    {_, found} =
      Macro.prewalk(ast, false, fn
        _node, true ->
          {nil, true}

        {:case, _, [subject, kw]} = node, false when is_list(kw) ->
          fixable =
            case extract_do_clauses(kw) do
              [clause_a, clause_b] ->
                not plain_variable?(subject) and
                  rewrite_clauses(clause_a, clause_b) != :skip

              _ ->
                false
            end

          {node, fixable}

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

    found
  end

  # Postwalk callback: rewrite a matching case node to if/else.
  defp maybe_rewrite_case({:case, meta, [subject, kw]} = node) when is_list(kw) do
    case extract_do_clauses(kw) do
      [clause_a, clause_b] ->
        if not plain_variable?(subject) do
          case rewrite_clauses(clause_a, clause_b) do
            {:ok, do_body, else_body} ->
              {:if, meta, [subject, [do: do_body, else: else_body]]}

            :skip ->
              node
          end
        else
          node
        end

      _ ->
        node
    end
  end

  defp maybe_rewrite_case(node), do: node

  # Determines if two clauses form a fixable boolean pair and extracts
  # the correct body placement for if (do = true branch, else = false branch).
  #
  # Returns {:ok, do_body, else_body} or :skip.
  defp rewrite_clauses(clause_a, clause_b) do
    with {pat_a, body_a} <- extract_clause(clause_a),
         {pat_b, body_b} <- extract_clause(clause_b) do
      ua = unwrap_pattern(pat_a)
      ub = unwrap_pattern(pat_b)

      cond do
        # true -> A; false -> B
        ua == true and ub == false -> {:ok, body_a, body_b}
        # false -> B; true -> A
        ua == false and ub == true -> {:ok, body_b, body_a}
        # true -> A; _ -> B
        ua == true and ub == :wildcard -> {:ok, body_a, body_b}
        # false -> B; _ -> A  (wildcard covers the true case)
        ua == false and ub == :wildcard -> {:ok, body_b, body_a}
        # Wildcard-first variants (unreachable second clause) — don't fix
        true -> :skip
      end
    else
      _ -> :skip
    end
  end

  defp extract_clause({:->, _, [[pattern], body]}), do: {pattern, body}
  defp extract_clause(_), do: :error

  # Normalise a clause pattern, handling Sourceror's __block__ wrapping.
  defp unwrap_pattern({:__block__, _, [true]}), do: true
  defp unwrap_pattern({:__block__, _, [false]}), do: false
  defp unwrap_pattern(true), do: true
  defp unwrap_pattern(false), do: false
  defp unwrap_pattern({:_, _, _}), do: :wildcard
  defp unwrap_pattern(_), do: :other

  # ── Issue construction ────────────────────────────────────────────

  defp build_issue(meta) do
    %Issue{
      rule: :no_case_true_false,
      message:
        "`case` on a boolean expression with `true`/`false` clauses " <>
          "should be written as `if`/`else`.",
      meta: %{line: Keyword.get(meta, :line)}
    }
  end
end