From 44567627d04c184ba0d20c9e879adfd2b658542a Mon Sep 17 00:00:00 2001
From: Elliott Eggleston <ejegg@ejegg.com>
Date: Sun, 20 May 2018 05:45:44 -0500
Subject: [PATCH] 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.
---
 .../media/RecentChangesImageUtils.java        | 21 +++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/app/src/main/java/fr/free/nrw/commons/media/RecentChangesImageUtils.java b/app/src/main/java/fr/free/nrw/commons/media/RecentChangesImageUtils.java
index e7962efd8..713ae1d06 100644
--- a/app/src/main/java/fr/free/nrw/commons/media/RecentChangesImageUtils.java
+++ b/app/src/main/java/fr/free/nrw/commons/media/RecentChangesImageUtils.java
@@ -3,6 +3,8 @@ package fr.free.nrw.commons.media;
 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;