defmodule Credence.Rule.NoSortThenReverse do
  @moduledoc """
  Performance & readability rule: Detects the pattern of calling `Enum.sort/1`
  followed by `Enum.reverse/1` on the result.

  Sorting ascending then reversing is equivalent to `Enum.sort(list, :desc)`
  (or `Enum.sort(list, &>=/2)`) but wastes a full O(n) pass for the reversal.

  ## Bad

      sorted = Enum.sort(nums)
      top = Enum.reverse(sorted)

      # or in a pipeline
      nums |> Enum.sort() |> Enum.reverse()

  ## Good

      sorted_desc = Enum.sort(nums, :desc)
  """
  @behaviour Credence.Rule
  alias Credence.Issue

  @impl true
  def check(ast, _opts) do
    {_ast, issues} =
      Macro.prewalk(ast, [], fn
        # Pipeline form: ... |> Enum.sort(...) |> Enum.reverse()
        # `a |> b |> c` = {:|>, _, [{:|>, _, [a, b]}, c]}
        # We check: right == Enum.reverse, and the rightmost call on the left is Enum.sort.
        {:|>, meta, [left, right]} = node, issues ->
          if remote_call?(right, :Enum, :reverse) and remote_call?(rightmost(left), :Enum, :sort) do
            {node, [build_issue(meta) | issues]}
          else
            {node, issues}
          end

        # Nested call form: Enum.reverse(Enum.sort(...))
        {{:., _, [{:__aliases__, _, [:Enum]}, :reverse]}, meta,
         [
           {{:., _, [{:__aliases__, _, [:Enum]}, :sort]}, _, _}
         ]} = node,
        issues ->
          {node, [build_issue(meta) | issues]}

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

    # Second pass: detect variable-mediated sort-then-reverse
    bound_issues = find_variable_sort_reverse(ast)

    Enum.reverse(issues) ++ bound_issues
  end

  defp find_variable_sort_reverse(ast) do
    # Pass 1: collect all variable names bound to Enum.sort(...)
    {_ast, sort_vars} =
      Macro.prewalk(ast, MapSet.new(), fn
        # var = Enum.sort(...)
        {:=, _, [{var_name, _, nil}, {{:., _, [{:__aliases__, _, [:Enum]}, :sort]}, _, _}]} =
            node,
        acc
        when is_atom(var_name) ->
          {node, MapSet.put(acc, var_name)}

        # var = x |> Enum.sort()
        {:=, _,
         [
           {var_name, _, nil},
           {:|>, _, [_, {{:., _, [{:__aliases__, _, [:Enum]}, :sort]}, _, _}]}
         ]} = node,
        acc
        when is_atom(var_name) ->
          {node, MapSet.put(acc, var_name)}

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

    if MapSet.size(sort_vars) == 0 do
      []
    else
      # Pass 2: find Enum.reverse(var) or var |> Enum.reverse()
      {_ast, issues} =
        Macro.prewalk(ast, [], fn
          # Enum.reverse(sorted_var)
          {{:., _, [{:__aliases__, _, [:Enum]}, :reverse]}, meta, [{var_name, _, nil}]} = node,
          acc
          when is_atom(var_name) ->
            if MapSet.member?(sort_vars, var_name) do
              {node, [build_issue(meta) | acc]}
            else
              {node, acc}
            end

          # sorted_var |> Enum.reverse()
          {:|>, meta,
           [
             {var_name, _, nil},
             {{:., _, [{:__aliases__, _, [:Enum]}, :reverse]}, _, _}
           ]} = node,
          acc
          when is_atom(var_name) ->
            if MapSet.member?(sort_vars, var_name) do
              {node, [build_issue(meta) | acc]}
            else
              {node, acc}
            end

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

      Enum.reverse(issues)
    end
  end

  defp rightmost({:|>, _, [_, right]}), do: right
  defp rightmost(other), do: other

  defp remote_call?(node, mod, func) do
    match?({{:., _, [{:__aliases__, _, [^mod]}, ^func]}, _, _}, node)
  end

  defp build_issue(meta) do
    %Issue{
      rule: :no_sort_then_reverse,
      severity: :warning,
      message:
        "Avoid `Enum.sort/1` followed by `Enum.reverse/1`. Use `Enum.sort(list, :desc)` instead to sort in descending order in a single pass.",
      meta: %{line: Keyword.get(meta, :line)}
    }
  end
end