Update voxel update order to random selection

src/arcade/potts/sim/Potts.java

@@ -159,13 +159,19 @@ public void step(SimState simstate) {
             HashSet<Integer> uniqueIDTargets = getUniqueIDs(x, y, z);
             HashSet<Integer> uniqueRegionTargets = getUniqueRegions(x, y, z);
 
-            // Select unique ID (if there is one), otherwise select unique
-            // region (if there is one). If there are neither, then skip.
-            if (uniqueIDTargets.size() > 0) {
+            // Check if there are valid unique targets.
+            boolean hasIDTargets = uniqueIDTargets.size() > 0;
+            boolean hasRegionTargets = uniqueRegionTargets.size() > 0;
+            boolean check = simstate.random.nextDouble() < 0.5;
+
+            // Select unique ID or unique region (if they exist). If there is
+            // a unique ID and unique region target, then randomly select. If
+            // there are neither, then skip.
+            if (hasIDTargets && (!hasRegionsCell || !hasRegionTargets || check)) {
                 int i = simstate.random.nextInt(uniqueIDTargets.size());
                 int targetID = (int) uniqueIDTargets.toArray()[i];
                 flip(ids[z][x][y], targetID, x, y, z, r);
-            } else if (hasRegionsCell && uniqueRegionTargets.size() > 0) {
+            } else if (hasRegionsCell && hasRegionTargets) {
                 int i = simstate.random.nextInt(uniqueRegionTargets.size());
                 int targetRegion = (int) uniqueRegionTargets.toArray()[i];
                 flip(ids[z][x][y], regions[z][x][y], targetRegion, x, y, z, r);