build numbers might exhibit some uniformity that makes it problematic with...

build numbers might exhibit some uniformity that makes it problematic with %4==0, so switching to random.

The average memory saving should be about the same.

core/src/main/java/hudson/tasks/Fingerprinter.java

@@ -70,6 +70,7 @@ import java.util.List;
 import java.util.ListIterator;
 import java.util.Map;
 import java.util.Map.Entry;
+import java.util.Random;
 import java.util.Set;
 import java.util.TreeMap;
 import java.util.logging.Level;
@@ -295,6 +296,8 @@ public class Fingerprinter extends Recorder implements Serializable, DependecyDe
         
         private final AbstractBuild build;
 
+        private static final Random rand = new Random();
+
         /**
          * From file name to the digest.
          */
@@ -341,9 +344,9 @@ public class Fingerprinter extends Recorder implements Serializable, DependecyDe
 
         public void onLoad() {
             // share data structure with nearby builds, but to keep lazy loading efficient,
-            // don't go back the history forever. By %4!=0, 4 neighboring builds will share
+            // don't go back the history forever. By RAND!=0, 4 neighboring builds will share
             // the data structure, so we'll get good enough saving.
-            if (build.getNumber()%4!=0) {
+            if (rand.nextInt(4)!=0) {
                 Run pb = build.getPreviousBuild();
                 if (pb!=null) {
                     FingerprintAction a = pb.getAction(FingerprintAction.class);