More random recent image selection (#1542)

time-based randomness is biased - if someone uploaded 100 images in
hour, one week ago, and I select a random point in time, their last
image is way more likely to come up than anything else.

With this, there is still bias towards choosing one of the last N
in any burst of uploads (where N is the number of recent changes
fetched) but it's a bit better than before.

Author: ejegg

app/src/main/java/fr/free/nrw/commons/media/RecentChangesImageUtils.java

@@ -3,6 +3,8 @@
 import org.w3c.dom.Element;
 import org.w3c.dom.NodeList;
 
+import java.util.Random;
+
 import javax.annotation.Nullable;
 
 public class RecentChangesImageUtils {
@@ -13,8 +15,23 @@ public class RecentChangesImageUtils {
     @Nullable
     public static String findImageInRecentChanges(NodeList childNodes) {
         String imageTitle;
-        for (int i = 0; i < childNodes.getLength(); i++) {
-            Element e = (Element)childNodes.item(i);
+        Random r = new Random();
+        int count = childNodes.getLength();
+        // Build a range array
+        int[] randomIndexes = new int[count];
+        for (int i = 0; i < count; i++) {
+            randomIndexes[i] = i;
+        }
+        // Then shuffle it
+        for (int i = 0; i < count; i++) {
+            int swapIndex = r.nextInt(count);
+            int temp = randomIndexes[i];
+            randomIndexes[i] = randomIndexes[swapIndex];
+            randomIndexes[swapIndex] = temp;
+        }
+        for (int i = 0; i < count; i++) {
+            int randomIndex = randomIndexes[i];
+            Element e = (Element) childNodes.item(randomIndex);
             if (e.getAttribute("type").equals("log") && !e.getAttribute("old_revid").equals("0")) {
                 // For log entries, we only want ones where old_revid is zero, indicating a new file
                 continue;