Separated deposit chance samples

- Clump seeds mix the generator seed and attempt index before sampling
- Deposit plans share immutable seeds across preparation workers
- Probability checks cover generator independence and adjacent clumps across batch boundaries

core/src/main/java/art/arcane/iris/engine/modifier/IrisDepositModifier.java

@@ -153,22 +153,21 @@ public class IrisDepositModifier extends EngineAssignedModifier<PlatformBlockSta
 
     private DepositPlan plan(IrisDepositGenerator generator, RNG rng) {
         if (generator.getSpawnChance() < rng.d()) {
-            return new DepositPlan(generator, rng, false, 0);
+            return new DepositPlan(generator, rng.getSeed(), false, 0);
         }
         boolean ore = generator.isOre(getData());
         int attempts = rng.i(generator.getMinPerChunk(), generator.getMaxPerChunk() + 1);
-        return new DepositPlan(generator, rng, ore, attempts);
+        return new DepositPlan(generator, rng.getSeed(), ore, attempts);
     }
 
     private PreparedDeposit prepare(DepositPlan plan, int first, int limit, int cx, int cz, HeightMap he, ChunkContext context) {
         IrisDepositGenerator k = plan.generator();
-        RNG rng = plan.rng();
         boolean oreDeposit = plan.ore();
         boolean needsCaveBiome = oreDeposit || k.usesCaveBiomeFilter();
         IrisDimensionCarvingResolver.State carvingState = needsCaveBiome ? new IrisDimensionCarvingResolver.State() : null;
         List<PreparedClump> clumps = new ArrayList<>(limit - first);
         for (int l = first; l < limit; l++) {
-            RNG clumpRng = rng.nextParallelRNG(l + 1L);
+            RNG clumpRng = new RNG(clumpSeed(plan.seed(), l));
             if (k.getPerClumpSpawnChance() < clumpRng.d()) {
                 continue;
             }
@@ -492,7 +491,14 @@ public class IrisDepositModifier extends EngineAssignedModifier<PlatformBlockSta
         return null;
     }
 
-    private record DepositPlan(IrisDepositGenerator generator, RNG rng, boolean ore, int attempts) {
+    static long clumpSeed(long seed, int attempt) {
+        long mixed = seed + 0x9e3779b97f4a7c15L * (attempt + 1L);
+        mixed = (mixed ^ (mixed >>> 30)) * 0xbf58476d1ce4e5b9L;
+        mixed = (mixed ^ (mixed >>> 27)) * 0x94d049bb133111ebL;
+        return mixed ^ (mixed >>> 31);
+    }
+
+    private record DepositPlan(IrisDepositGenerator generator, long seed, boolean ore, int attempts) {
     }
 
     private record PreparedDeposit(IrisDepositGenerator generator, boolean ore, List<PreparedClump> clumps) {

core/src/test/java/art/arcane/iris/engine/modifier/IrisDepositModifierContextTest.java

@@ -37,7 +37,6 @@ import static org.junit.Assert.assertSame;
 import static org.junit.Assert.assertTrue;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.anyString;
-import static org.mockito.ArgumentMatchers.anyLong;
 import static org.mockito.Mockito.RETURNS_DEEP_STUBS;
 import static org.mockito.Mockito.doAnswer;
 import static org.mockito.Mockito.doReturn;
@@ -103,7 +102,6 @@ public class IrisDepositModifierContextTest {
         RNG rng = mock(RNG.class);
         when(rng.d()).thenReturn(0.5D);
         when(rng.i(0, 5)).thenReturn(4, 0);
-        when(rng.nextParallelRNG(anyLong())).thenReturn(rng);
 
         new IrisDepositModifier(engine).generate(generator, null, null, rng, 0, 0, false, null);
 

core/src/test/java/art/arcane/iris/engine/modifier/IrisDepositModifierOrderingTest.java

@@ -88,7 +88,7 @@ public class IrisDepositModifierOrderingTest {
         RNG generatorRng = new RNG(0L).nextParallelRNG(0L);
         for (String name : List.of("dimension", "region", "biome")) {
             for (int i = 0; i < 25; i++) {
-                expectedSamples.add(name + ":" + generatorRng.nextParallelRNG(i + 1L).getSeed());
+                expectedSamples.add(name + ":" + IrisDepositModifier.clumpSeed(generatorRng.getSeed(), i));
             }
         }
         Collections.sort(expectedSamples);

core/src/test/java/art/arcane/iris/engine/modifier/IrisDepositModifierProbabilityTest.java

@@ -0,0 +1,36 @@
+package art.arcane.iris.engine.modifier;
+
+import art.arcane.volmlib.util.math.RNG;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+public class IrisDepositModifierProbabilityTest {
+    @Test
+    public void generatorAndClumpChancesRemainIndependent() {
+        int samples = 100_000;
+        int accepted = 0;
+        for (int seed = 0; seed < samples; seed++) {
+            if (new RNG(seed).d() <= 0.2D
+                    && new RNG(IrisDepositModifier.clumpSeed(seed, 0)).d() <= 0.2D) {
+                accepted++;
+            }
+        }
+        assertEquals(0.04D, accepted / (double) samples, 0.003D);
+    }
+
+    @Test
+    public void neighboringClumpChancesRemainIndependentAcrossBatchBoundaries() {
+        int samples = 100_000;
+        for (int first : new int[]{0, 7, 31}) {
+            int accepted = 0;
+            for (int seed = 0; seed < samples; seed++) {
+                if (new RNG(IrisDepositModifier.clumpSeed(seed, first)).d() <= 0.2D
+                        && new RNG(IrisDepositModifier.clumpSeed(seed, first + 1)).d() <= 0.2D) {
+                    accepted++;
+                }
+            }
+            assertEquals("Clump pair at " + first, 0.04D, accepted / (double) samples, 0.003D);
+        }
+    }
+}