add Recovery.Prefix module

test/feed_reader/recovery/prefix_test.exs

@@ -0,0 +1,28 @@
+defmodule FeedReader.Recovery.PrefixTest do
+  use ExUnit.Case, async: true
+
+  alias FeedReader.Recovery.Prefix
+
+  test "removes a BOM and leading XML whitespace" do
+    assert normalize([<<0xEF, 0xBB, 0xBF, 13, 10, 9, 32>>, "<rss/>"]) == "<rss/>"
+  end
+
+  test "removes a BOM split across chunks" do
+    assert normalize([<<0xEF>>, <<0xBB>>, <<0xBF>>, " \n", "<rss/>"]) == "<rss/>"
+  end
+
+  test "removes leading whitespace split across chunks" do
+    assert normalize(["\r", "\n", "\t", " ", "<rss/>"]) == "<rss/>"
+  end
+
+  test "preserves content that does not have a removable prefix" do
+    assert normalize(["<rss/>", "<tail/>"]) == "<rss/><tail/>"
+  end
+
+  test "returns an empty stream for empty or whitespace-only input" do
+    assert normalize([]) == ""
+    assert normalize([" ", "\n", "\r\t"]) == ""
+  end
+
+  defp normalize(chunks), do: chunks |> Prefix.normalize() |> Enum.join()
+end