add Recovery.Attributes module

lib/feed_reader/recovery/attributes.ex

@@ -0,0 +1,98 @@
+defmodule FeedReader.Recovery.Attributes do
+  @moduledoc """
+  Repairs malformed quotes in XML attributes.
+
+  A small number of feeds place literal quotes inside a quoted attribute value.
+  This module repairs those quotes only within one complete markup element, so
+  valid quotes in neighboring elements cannot be rewritten accidentally.
+  Incomplete elements are retained as state for the next input chunk.
+  """
+
+  @doc "Repairs literal quotes in streamed XML attributes."
+  def normalize(input) do
+    Stream.transform(
+      input,
+      fn -> <<>> end,
+      fn chunk, pending ->
+        {output, pending} = scan(pending <> chunk, [])
+        {if(output == [], do: [], else: [IO.iodata_to_binary(Enum.reverse(output))]), pending}
+      end,
+      fn
+        <<>> -> []
+        pending -> [pending]
+      end
+    )
+  end
+
+  defp scan(<<>>, output), do: {output, <<>>}
+
+  defp scan(data, output) do
+    case :binary.match(data, "<") do
+      :nomatch ->
+        {[data | output], <<>>}
+
+      {position, 1} when position > 0 ->
+        {prefix, rest} = :erlang.split_binary(data, position)
+        scan(rest, [prefix | output])
+
+      {0, 1} ->
+        case markup_end(data) do
+          :incomplete -> {output, data}
+          {markup, rest} -> scan(rest, [repair(markup) | output])
+        end
+    end
+  end
+
+  defp markup_end(<<"<![CDATA[", _rest::binary>> = data), do: special_end(data, "]]>")
+  defp markup_end(<<"<!--", _rest::binary>> = data), do: special_end(data, "-->")
+
+  defp markup_end(data) do
+    case :binary.match(data, ">") do
+      :nomatch -> :incomplete
+      {position, 1} -> :erlang.split_binary(data, position + 1)
+    end
+  end
+
+  defp special_end(data, closing) do
+    case :binary.match(data, closing) do
+      :nomatch -> :incomplete
+      {position, length} -> :erlang.split_binary(data, position + length)
+    end
+  end
+
+  defp repair(<<"<![CDATA[", _rest::binary>> = markup), do: markup
+  defp repair(<<"<!--", _rest::binary>> = markup), do: markup
+  defp repair(<<"<", rest::binary>>), do: "<" <> repair_tag(rest, nil, false, [])
+  defp repair_tag(<<>>, _quote, _expect, output), do: IO.iodata_to_binary(Enum.reverse(output))
+
+  defp repair_tag(<<char, rest::binary>>, nil, expect, output) when char in [32, 9, 10, 13],
+    do: repair_tag(rest, nil, expect, [<<char>> | output])
+
+  defp repair_tag(<<char, rest::binary>>, nil, true, output) when char in [34, 39],
+    do: repair_tag(rest, {char, false}, false, [<<char>> | output])
+
+  defp repair_tag(<<char, rest::binary>>, nil, _expect, output),
+    do: repair_tag(rest, nil, char == ?=, [<<char>> | output])
+
+  defp repair_tag(<<quote, rest::binary>>, {quote, has_content}, _expect, output) do
+    cond do
+      delimiter?(rest) ->
+        repair_tag(rest, nil, false, [<<quote>> | output])
+
+      empty_quote?(rest, quote, has_content) ->
+        <<_quote, after_quote::binary>> = rest
+        repair_tag(after_quote, nil, false, [<<quote>> | output])
+
+      true ->
+        repair_tag(rest, {quote, true}, false, ["&quot;" | output])
+    end
+  end
+
+  defp repair_tag(<<char, rest::binary>>, {quote, _has_content}, _expect, output),
+    do: repair_tag(rest, {quote, true}, false, [<<char>> | output])
+
+  defp delimiter?(<<char, _rest::binary>>) when char in [32, 9, 10, 13, 47, 62, 63], do: true
+  defp delimiter?(_rest), do: false
+  defp empty_quote?(<<quote, rest::binary>>, quote, false), do: delimiter?(rest)
+  defp empty_quote?(_rest, _quote, _has_content), do: false
+end