episode cleanup 12 hour option: add unit test to prepare for using hours

rather than days as data structure

core/src/main/java/de/danoeh/antennapod/core/storage/APCleanupAlgorithm.java

@@ -80,12 +80,17 @@ public class APCleanupAlgorithm extends EpisodeCleanupAlgorithm {
         return counter;
     }
 
+    @VisibleForTesting
+    Date calcMostRecentDateForDeletion(@NonNull Date currentDate) {
+        return minusDays(currentDate, numberOfDaysAfterPlayback);
+    }
+
     @NonNull
     private List<FeedItem> getCandidates() {
         List<FeedItem> candidates = new ArrayList<>();
         List<FeedItem> downloadedItems = DBReader.getDownloadedItems();
 
-        Date mostRecentDateForDeletion = minusDays(new Date(), numberOfDaysAfterPlayback);
+        Date mostRecentDateForDeletion = calcMostRecentDateForDeletion(new Date());
         for (FeedItem item : downloadedItems) {
             if (item.hasMedia()
                     && item.getMedia().isDownloaded()

core/src/test/java/de/danoeh/antennapod/core/storage/APCleanupAlgorithmTest.java

@@ -0,0 +1,20 @@
+package de.danoeh.antennapod.core.storage;
+
+import org.junit.Test;
+
+import java.text.SimpleDateFormat;
+import java.util.Date;
+
+import static org.junit.Assert.assertEquals;
+
+public class APCleanupAlgorithmTest {
+
+    @Test
+    public void testCalcMostRecentDateForDeletion() throws Exception {
+        APCleanupAlgorithm algo = new APCleanupAlgorithm(1.0f);
+        Date curDateForTest = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssZ").parse("2018-11-13T14:08:56-0800");
+        Date resExpected = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssZ").parse("2018-11-12T14:08:56-0800");
+        Date resActual = algo.calcMostRecentDateForDeletion(curDateForTest);
+        assertEquals("cutoff for retaining most recent 1 day", resExpected, resActual);
+    }
+}