visually connect descendant replies in threads

closes sk22#256
closes sk22#510

mastodon/src/androidTest/java/org/joinmastodon/android/fragments/ThreadFragmentTest.java

@@ -0,0 +1,58 @@
+package org.joinmastodon.android.fragments;
+
+import static org.junit.Assert.*;
+
+import android.util.Pair;
+
+import org.joinmastodon.android.model.Status;
+import org.joinmastodon.android.model.StatusContext;
+import org.junit.Test;
+
+import java.util.List;
+import java.util.stream.Collectors;
+
+public class ThreadFragmentTest {
+
+	private Status fakeStatus(String id, String inReplyTo) {
+		Status status = Status.ofFake(id, null, null);
+		status.inReplyToId = inReplyTo;
+		return status;
+	}
+
+	@Test
+	public void countAncestryLevels() {
+		StatusContext context = new StatusContext();
+		context.ancestors = List.of(
+				fakeStatus("oldest ancestor", null),
+				fakeStatus("younger ancestor", "oldest ancestor")
+		);
+		context.descendants = List.of(
+				fakeStatus("first reply", "main"),
+				fakeStatus("reply to first reply", "first reply"),
+				fakeStatus("third level reply", "reply to first reply"),
+				fakeStatus("another reply", "main")
+		);
+		List<Pair<String, Integer>> actual =
+				ThreadFragment.countAncestryLevels("main status", context);
+
+		List<Pair<String, Integer>> expected = List.of(
+				Pair.create("oldest ancestor", -2),
+				Pair.create("younger ancestor", -1),
+				Pair.create("main status", 0),
+				Pair.create("first reply", 1),
+				Pair.create("reply to first reply", 2),
+				Pair.create("third level reply", 3),
+				Pair.create("another reply", 1)
+		);
+		assertEquals(
+				"status ids are in the right order",
+				expected.stream().map(p -> p.first).collect(Collectors.toList()),
+				actual.stream().map(p -> p.first).collect(Collectors.toList())
+		);
+		assertEquals(
+				"counted levels match",
+				expected.stream().map(p -> p.second).collect(Collectors.toList()),
+				actual.stream().map(p -> p.second).collect(Collectors.toList())
+		);
+	}
+}