add tests

test/feed_reader/format_contract_test.exs

@@ -0,0 +1,38 @@
+defmodule FeedReader.FormatContractTest do
+  use ExUnit.Case, async: true
+
+  alias FeedReader.{Feed, FormatRegistry, Item}
+  alias FeedReader.TestSupport.ExternalFormat
+
+  test "built-in registry has a fixed format priority" do
+    assert FormatRegistry.builtins() == [
+             FeedReader.Formats.RSS,
+             FeedReader.Formats.Atom,
+             FeedReader.Formats.RDF
+           ]
+  end
+
+  test "per-call formats append after built-ins without duplicate modules" do
+    formats = FormatRegistry.formats([ExternalFormat, FeedReader.Formats.RSS])
+
+    assert formats == FormatRegistry.builtins() ++ [ExternalFormat]
+  end
+
+  test "external formats implement detection, parser state, and normalization callbacks" do
+    sink = fn event -> send(self(), {:format_event, event}) end
+    state = ExternalFormat.new_parser_state(sink, mode: :test)
+
+    assert ExternalFormat.id() == :external_test
+    assert ExternalFormat.matches_prefix?("<external-feed>")
+    refute ExternalFormat.matches_prefix?("<other-feed>")
+    assert ExternalFormat.handler() == FeedReader.TestSupport.ExternalFormatHandler
+    assert state.event_sink == sink
+    assert state.opts == [mode: :test]
+
+    feed = ExternalFormat.normalize_feed(state, [])
+    assert %Feed{title: "External", type: :rss} = feed
+
+    assert ExternalFormat.normalize_item(%{title: "Entry"}, feed, []) ==
+             {:ok, %Item{title: "Entry"}}
+  end
+end