add Recovery.Entities module

lib/feed_reader/recovery/entities.ex

@@ -0,0 +1,131 @@
+defmodule FeedReader.Recovery.Entities do
+  @moduledoc """
+  Recovers entity and markup irregularities in streamed XML.
+
+  Some feeds contain raw ampersands or malformed entity references that Saxy
+  rejects. This module preserves valid named and numeric entities, escapes
+  invalid references, and leaves CDATA and comments untouched. Its scanner
+  carries incomplete entities and markup delimiters between chunks so the
+  result does not depend on the input chunk size.
+  """
+
+  @doc "Escapes invalid entity references while preserving CDATA and comments."
+  def normalize(input) do
+    Stream.transform(input, fn -> {:normal, <<>>} end, &normalize_chunk/2, &flush/1)
+  end
+
+  defp normalize_chunk(chunk, {mode, pending}) do
+    {output, mode, pending} = scan(pending <> chunk, mode, [])
+    emit(output, {mode, pending})
+  end
+
+  defp scan(data, :normal, output) do
+    case next_markup(data) do
+      :none ->
+        case partial_prefix(data) do
+          :none ->
+            {append(output, data), :normal, <<>>}
+
+          length ->
+            split = byte_size(data) - length
+
+            {append(output, binary_part(data, 0, split)), :normal,
+             binary_part(data, split, length)}
+        end
+
+      {position, :cdata} ->
+        {prefix, rest} = :erlang.split_binary(data, position)
+        scan(binary_part(rest, 9, byte_size(rest) - 9), :cdata, ["<![CDATA[", prefix | output])
+
+      {position, :comment} ->
+        {prefix, rest} = :erlang.split_binary(data, position)
+        scan(binary_part(rest, 4, byte_size(rest) - 4), :comment, ["<!--", prefix | output])
+
+      {position, :entity} ->
+        {prefix, rest} = :erlang.split_binary(data, position)
+        normalize_reference(binary_part(rest, 1, byte_size(rest) - 1), [prefix | output])
+    end
+  end
+
+  defp scan(data, :cdata, output), do: scan_delimited(data, output, "]]>", :cdata)
+  defp scan(data, :comment, output), do: scan_delimited(data, output, "-->", :comment)
+
+  defp scan_delimited(data, output, closing, mode) do
+    case :binary.match(data, closing) do
+      :nomatch ->
+        keep = min(byte_size(data), 2)
+        split = byte_size(data) - keep
+        {append(output, binary_part(data, 0, split)), mode, binary_part(data, split, keep)}
+
+      {position, length} ->
+        {prefix, rest} = :erlang.split_binary(data, position + length)
+        scan(rest, :normal, [prefix | output])
+    end
+  end
+
+  defp next_markup(data) do
+    [
+      {:cdata, :binary.match(data, "<![CDATA[")},
+      {:comment, :binary.match(data, "<!--")},
+      {:entity, :binary.match(data, "&")}
+    ]
+    |> Enum.reduce(:none, &earliest/2)
+  end
+
+  defp partial_prefix(<<>>), do: :none
+
+  defp partial_prefix(data) do
+    max_length = min(byte_size(data), 8)
+
+    Enum.find_value(max_length..1//-1, :none, fn length ->
+      suffix = binary_part(data, byte_size(data) - length, length)
+
+      if suffix in [binary_part("<![CDATA[", 0, length), binary_part("<!--", 0, min(length, 4))],
+        do: length,
+        else: false
+    end)
+  end
+
+  defp earliest({_kind, :nomatch}, result), do: result
+  defp earliest({kind, {position, _length}}, :none), do: {position, kind}
+
+  defp earliest({kind, {position, _length}}, {current, _}) when position < current,
+    do: {position, kind}
+
+  defp earliest(_candidate, result), do: result
+
+  defp normalize_reference(data, output) do
+    case consume(data, "&") do
+      {:complete, entity, rest} ->
+        scan(rest, :normal, [if(valid?(entity), do: entity, else: escape(entity)) | output])
+
+      {:invalid, entity, rest} ->
+        scan(rest, :normal, [escape(entity) | output])
+
+      :incomplete ->
+        {output, :normal, "&" <> data}
+    end
+  end
+
+  defp consume(<<>>, _entity), do: :incomplete
+  defp consume(<<";", rest::binary>>, entity), do: {:complete, entity <> ";", rest}
+
+  defp consume(<<char, rest::binary>>, entity) when char in [9, 10, 13, 32, 34, 38, 39, 60],
+    do: {:invalid, entity, <<char, rest::binary>>}
+
+  defp consume(<<char, rest::binary>>, entity), do: consume(rest, entity <> <<char>>)
+  defp valid?(entity), do: Regex.match?(~r/^&(amp|lt|gt|quot|apos|#\d+|#x[0-9a-f]+);$/i, entity)
+  defp escape(entity), do: "&amp;" <> binary_part(entity, 1, byte_size(entity) - 1)
+  defp append(output, <<>>), do: output
+  defp append(output, chunk), do: [chunk | output]
+
+  defp emit(output, state),
+    do:
+      if(output == [],
+        do: {[], state},
+        else: {[output |> Enum.reverse() |> IO.iodata_to_binary()], state}
+      )
+
+  defp flush({_mode, <<>>}), do: []
+  defp flush({_mode, pending}), do: [pending]
+end