Test and fix scheduler cycle budgets

src/main/java/com/livingworld/core/simulation/SimulationScheduler.java

@@ -37,7 +37,8 @@ public final class SimulationScheduler {
     }
 
     public boolean shouldRunSimulationCycle() {
-        return minecraftTickCounter % config.getSimulationIntervalTicks() == 0;
+        return minecraftTickCounter > 0
+                && minecraftTickCounter % config.getSimulationIntervalTicks() == 0;
     }
 
     public void queueRegion(RegionUpdateJob job) {

src/test/java/com/livingworld/core/simulation/SchedulerBudgetTest.java

@@ -0,0 +1,86 @@
+package com.livingworld.core.simulation;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import com.livingworld.config.SimulationConfig;
+import com.livingworld.regions.RegionCoordinate;
+import java.util.List;
+import java.util.Set;
+import org.junit.jupiter.api.Test;
+
+class SchedulerBudgetTest {
+
+    @Test
+    void oneCycleReturnsAtMostConfiguredRegionBudget() {
+        SimulationScheduler scheduler = scheduler(50, 100);
+        for (int index = 0; index < 500; index++) {
+            scheduler.queueRegion(job(index, index % 10));
+        }
+
+        List<RegionUpdateJob> jobs = scheduler.pollJobsForCycle();
+
+        assertEquals(50, jobs.size());
+        assertEquals(450, scheduler.getQueuedJobCount());
+    }
+
+    @Test
+    void higherPriorityJobsAreReturnedFirst() {
+        SimulationScheduler scheduler = scheduler(3, 100);
+        scheduler.queueRegion(job(0, 1));
+        scheduler.queueRegion(job(1, 20));
+        scheduler.queueRegion(job(2, 5));
+        scheduler.queueRegion(job(3, 10));
+
+        assertEquals(
+                List.of(20, 10, 5),
+                scheduler.pollJobsForCycle().stream()
+                        .map(RegionUpdateJob::priority)
+                        .toList());
+    }
+
+    @Test
+    void jobsOutsideCurrentBudgetRemainQueued() {
+        SimulationScheduler scheduler = scheduler(2, 100);
+        scheduler.queueRegion(job(0, 3));
+        scheduler.queueRegion(job(1, 2));
+        scheduler.queueRegion(job(2, 1));
+
+        scheduler.pollJobsForCycle();
+
+        assertEquals(1, scheduler.getQueuedJobCount());
+        assertEquals(1, scheduler.pollJobsForCycle().getFirst().priority());
+    }
+
+    @Test
+    void simulationIntervalStartsAfterTheConfiguredNumberOfTicks() {
+        SimulationScheduler scheduler = scheduler(50, 3);
+
+        assertFalse(scheduler.shouldRunSimulationCycle());
+        scheduler.onMinecraftTick();
+        assertFalse(scheduler.shouldRunSimulationCycle());
+        scheduler.onMinecraftTick();
+        assertFalse(scheduler.shouldRunSimulationCycle());
+        scheduler.onMinecraftTick();
+        assertTrue(scheduler.shouldRunSimulationCycle());
+        scheduler.onMinecraftTick();
+        assertFalse(scheduler.shouldRunSimulationCycle());
+    }
+
+    private static SimulationScheduler scheduler(int maxRegionsPerCycle, int intervalTicks) {
+        SimulationConfig config = new SimulationConfig();
+        config.setMaxRegionsPerCycle(maxRegionsPerCycle);
+        config.setSimulationIntervalTicks(intervalTicks);
+        return new SimulationScheduler(config);
+    }
+
+    private static RegionUpdateJob job(int coordinateX, int priority) {
+        return new RegionUpdateJob(
+                new RegionCoordinate("minecraft:overworld", coordinateX, 0),
+                priority,
+                0,
+                Set.of(),
+                UpdateReason.NORMAL_ROLLING_UPDATE);
+    }
+}