credence

defmodule Credence.Pattern.UnnecessaryGraphemeChunking do
  @moduledoc """
  Detects the common n-gram generation pipeline that converts a string
  to graphemes, creates sliding window chunks of step 1, and joins them
  back into strings. This pattern is automatically fixed by replacing it
  with `String.slice/3` which avoids intermediate list allocations.

  ## Bad

      string
      |> String.graphemes()
      |> Enum.chunk_every(n, 1, :discard)
      |> Enum.map(&Enum.join/1)

  ## Good

      for i <- 0..(String.length(string) - n)//1 do
        String.slice(string, i, n)
      end

  `String.length/1` and `String.slice/3` both operate on grapheme clusters, so
  this is grapheme-equivalent. The `//1` step keeps it equivalent even when the
  string is shorter than the chunk size: the range is then empty, matching
  `Enum.chunk_every(_, n, 1, :discard)` (a stepless `0..-k` would descend and
  emit bogus slices).
  """

  use Credence.Pattern.Rule
  alias Credence.Issue

  @impl true
  def check(ast, _opts) do
    {_ast, issues} =
      Macro.prewalk(ast, [], fn node, acc ->
        case detect_fixable_pipeline(node) do
          {:ok, _subject, _n} ->
            {node, [trigger_issue(node) | acc]}

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

    Enum.reverse(issues)
  end

  @impl true
  def fix_patches(ast, _opts) do
    Credence.RuleHelpers.patches_from_postwalk(ast, fn node ->
      case detect_fixable_pipeline(node) do
        {:ok, subject, n} -> build_replacement(subject, n)
        :error -> node
      end
    end)
  end

  # Pipeline detection
  # Matches: subject |> String.graphemes() |> Enum.chunk_every(n, 1, ...) |> Enum.map(join_fn)
  defp detect_fixable_pipeline(
         {:|>, _,
          [
            {:|>, _,
             [
               {:|>, _, [subject, graphemes_call]},
               chunk_call
             ]},
            map_call
          ]}
       ) do
    if graphemes_call?(graphemes_call) &&
         chunk_every_sliding?(chunk_call) &&
         map_join?(map_call) do
      case extract_chunk_size(chunk_call) do
        nil -> :error
        n -> {:ok, subject, n}
      end
    else
      :error
    end
  end

  defp detect_fixable_pipeline(_), do: :error
  # AST predicates
  defp graphemes_call?({{:., _, [{:__aliases__, _, [:String]}, :graphemes]}, _, _}), do: true
  defp graphemes_call?(_), do: false

  defp chunk_every_sliding?({{:., _, [{:__aliases__, _, [:Enum]}, :chunk_every]}, _, args}) do
    case args do
      [_, n] -> int_value(n) == 1
      [_, n, discard] -> int_value(n) == 1 and atom_value(discard) == :discard
      _ -> false
    end
  end

  defp chunk_every_sliding?(_), do: false

  defp extract_chunk_size({{:., _, _}, _, [n | _]}), do: n
  defp extract_chunk_size(_), do: nil

  defp int_value({:__block__, _, [n]}) when is_integer(n), do: n
  defp int_value(_), do: nil

  # Atoms in function-name / module positions are bare; argument-position
  # atoms are wrapped in `:__block__`.
  defp atom_value({:__block__, _, [a]}) when is_atom(a), do: a
  defp atom_value(a) when is_atom(a), do: a
  defp atom_value(_), do: nil

  defp map_join?({{:., _, [{:__aliases__, _, [:Enum]}, :map]}, _, [join_fn]}),
    do: join_function?(join_fn)

  defp map_join?(_), do: false

  # Specific patterns — these match exact AST structures

  # &Enum.join/1
  defp join_function?(
         {:&, _, [{:/, _, [{{:., _, [{:__aliases__, _, [:Enum]}, :join]}, _, _}, 1]}]}
       ),
       do: true

  # &Enum.join(&1)
  defp join_function?({:&, _, [{{:., _, [{:__aliases__, _, [:Enum]}, :join]}, _, [{:&, _, 1}]}]}),
    do: true

  # &Enum.join(&1, "")
  defp join_function?(
         {:&, _, [{{:., _, [{:__aliases__, _, [:Enum]}, :join]}, _, [{:&, _, 1}, ""]}]}
       ),
       do: true

  # fn x -> Enum.join(x) end
  defp join_function?(
         {:fn, _, [{:->, _, [[_], {{:., _, [{:__aliases__, _, [:Enum]}, :join]}, _, [_]}]}]}
       ),
       do: true

  # fn x -> Enum.join(x, "") end
  defp join_function?(
         {:fn, _, [{:->, _, [[_], {{:., _, [{:__aliases__, _, [:Enum]}, :join]}, _, [_, ""]}]}]}
       ),
       do: true

  # Catch-all: handles any AST form whose string representation is a call to
  # Enum.join (e.g. parser-generated variants we didn't anticipate).
  # Safe because Enum.join is always a join — there's no false-positive path.
  defp join_function?(ast) do
    ast
    |> Macro.to_string()
    |> String.contains?("Enum.join")
  end

  # Replacement builder
  # Builds: for i <- 0..(String.length(subject) - n)//1, do: String.slice(subject, i, n)
  #
  # The `//1` step is load-bearing: when `String.length(subject) < n` the end is
  # negative, and a stepless `0..-k` range descends (`0..-1` = `[0, -1]`),
  # producing bogus slices. With `//1` the range is empty for any negative end —
  # matching `Enum.chunk_every(_, n, 1, :discard)`, which yields `[]` when the
  # string is shorter than the chunk size.
  defp build_replacement(subject, n) do
    length_call =
      {{:., [], [{:__aliases__, [], [:String]}, :length]}, [], [subject]}

    range =
      {:..//, [], [0, {:-, [], [length_call, n]}, 1]}

    body =
      {{:., [], [{:__aliases__, [], [:String]}, :slice]}, [], [subject, {:i, [], nil}, n]}

    {:for, [], [{:<-, [], [{:i, [], nil}, range]}, [do: body]]}
  end

  # Issue
  defp trigger_issue(node) do
    %Issue{
      rule: :unnecessary_grapheme_chunking,
      message: """
      This pipeline converts a string to graphemes, creates sliding window chunks,
      and joins them back into strings. This can be replaced with `String.slice/3`
      which avoids the intermediate list allocations:

          for i <- 0..(String.length(string) - n) do
            String.slice(string, i, n)
          end
      """,
      meta: %{line: get_line(node)}
    }
  end

  defp get_line({:|>, meta, _}), do: Keyword.get(meta, :line)
  defp get_line(_), do: nil
end