Round Robin requesters (#1225)

* Round Robin requesters

When a delivery task has been assigned to a requester we move that
requester to the end of its list so that if another requester frame
requests the same item type, it will be selected for the task the next
time around.

See #1203

src/main/java/me/desht/pneumaticcraft/common/drone/LogisticsManager.java

@@ -56,7 +56,8 @@ public PriorityQueue<LogisticsTask> getTasks(Object holdingStack, boolean droneA
         FluidStack fluid = holdingStack instanceof FluidStack ? (FluidStack) holdingStack : FluidStack.EMPTY;
         PriorityQueue<LogisticsTask> tasks = new PriorityQueue<>();
         for (int priority = logistics.size() - 1; priority >= 0; priority--) {
-            for (AbstractLogisticsFrameEntity requester : logistics.get(priority)) {
+            for (int requesterId = 0; requesterId < logistics.get(priority).size(); requesterId++) {
+                AbstractLogisticsFrameEntity requester = logistics.get(priority).get(requesterId);
                 if (droneAccess && requester.isObstructed(PathComputationType.AIR)) continue;
                 for (int i = 0; i < priority; i++) {
                     for (AbstractLogisticsFrameEntity provider : logistics.get(i)) {
@@ -82,7 +83,15 @@ public PriorityQueue<LogisticsTask> getTasks(Object holdingStack, boolean droneA
                             // it could be that the drone is carrying some item or fluid it can't drop off right now
                             // however it might still be able to transfer the other resource type (i.e. transfer items if
                             // it's holding a fluid, and vice versa)
+                            int taskCount = tasks.size();
                             tryProvide(provider, requester, tasks, item.isEmpty(), fluid.isEmpty());
+
+                            // if we provided something to the requester, move the requester to last in its list
+                            // so another requester gets the next delivery, effectively round robin.
+                            List<AbstractLogisticsFrameEntity> requesters = logistics.get(priority);
+                            if (tasks.size() > taskCount && requesters.size() > 1) {
+                                requesters.add(requesters.remove(requesterId));
+                            }
                         }
                     }
                 }