Remember neighbourhood answers

- Each thread keeps its recent final hasColumnsAround answers next to its chunk slots, so the object and hydrology gates skip the nine-chunk scan on repeat

core/src/main/java/art/arcane/iris/generation/hydrology/HydrologyTileCache.java

@@ -38,6 +38,9 @@ public final class HydrologyTileCache implements AutoCloseable {
     private static final int CHUNK_SIZE = 16;
     private static final int CHUNK_COLUMN_COUNT = CHUNK_SIZE * CHUNK_SIZE;
     private static final int LOCAL_CHUNK_SLOTS = 16;
+    private static final byte AROUND_UNKNOWN = 0;
+    private static final byte AROUND_EMPTY = 1;
+    private static final byte AROUND_PUBLISHED = 2;
     private static final int MAXIMUM_PREGENERATION_TILES = 4096;
     private static final int PREGENERATION_HALO_BLOCKS = 512;
     private static final HydrologyColumnSample[] NO_COLUMNS = new HydrologyColumnSample[CHUNK_COLUMN_COUNT];
@@ -547,9 +550,25 @@ public final class HydrologyTileCache implements AutoCloseable {
     /**
      * Whether any planned footprint column lies in the chunk or one of its eight neighbours, which
      * decides whether the chunk can hold published hydrology at all. Tiles are planned like a sample
-     * would plan them.
+     * would plan them, and each thread remembers its recent final answers.
      */
     public boolean hasColumnsAround(int chunkX, int chunkZ) {
+        long epoch = cacheEpoch.get();
+        LocalChunks local = localChunks(epoch);
+        int slot = LocalChunks.slot(chunkX, chunkZ);
+        long key = RiverFootprint.pack(chunkX, chunkZ);
+        if (local.around[slot] != AROUND_UNKNOWN && local.aroundKeys[slot] == key) {
+            return local.around[slot] == AROUND_PUBLISHED;
+        }
+        boolean published = publishesAround(chunkX, chunkZ);
+        if (!waitsForbidden() && local.epoch == epoch) {
+            local.aroundKeys[slot] = key;
+            local.around[slot] = published ? AROUND_PUBLISHED : AROUND_EMPTY;
+        }
+        return published;
+    }
+
+    private boolean publishesAround(int chunkX, int chunkZ) {
         for (int offsetZ = -1; offsetZ <= 1; offsetZ++) {
             for (int offsetX = -1; offsetX <= 1; offsetX++) {
                 if (!chunkColumns((chunkX + offsetX) * CHUNK_SIZE, (chunkZ + offsetZ) * CHUNK_SIZE).empty()) {
@@ -839,6 +858,7 @@ public final class HydrologyTileCache implements AutoCloseable {
         LocalChunks local = localChunks.get();
         if (local.epoch != epoch) {
             Arrays.fill(local.slots, null);
+            Arrays.fill(local.around, AROUND_UNKNOWN);
             local.prefetched = null;
             local.epoch = epoch;
         }
@@ -1667,6 +1687,8 @@ public final class HydrologyTileCache implements AutoCloseable {
     /** Direct-mapped over a 4 by 4 chunk neighbourhood, so a placement crossing chunk edges keeps its chunks. */
     private static final class LocalChunks {
         private final ChunkColumns[] slots = new ChunkColumns[LOCAL_CHUNK_SLOTS];
+        private final long[] aroundKeys = new long[LOCAL_CHUNK_SLOTS];
+        private final byte[] around = new byte[LOCAL_CHUNK_SLOTS];
         private long epoch = Long.MIN_VALUE;
         private TileBounds prefetched;
 

core/src/test/java/art/arcane/iris/generation/hydrology/HydrologyTileCacheTest.java

@@ -2577,4 +2577,27 @@ public class HydrologyTileCacheTest {
         }
     }
 
+    @Test
+    public void neighbourhoodAnswersFollowPublishedColumnsAndResetWithTheEpoch() {
+        HydrologyPlanner planner = mock(HydrologyPlanner.class);
+        HydrologyTile publishing = mock(HydrologyTile.class);
+        HydrologyTile empty = mock(HydrologyTile.class);
+        HydrologyPlannerSettings settings = stalePrefetchSettings();
+        when(planner.settings()).thenReturn(settings);
+        AtomicReference<HydrologyTile> current = new AtomicReference<>(publishing);
+        when(planner.plan(any(HydrologyTileKey.class))).thenAnswer(invocation -> current.get());
+        publishesInto(publishing, RiverFootprint.pack(0, 0));
+        when(publishing.columnAt(0, 0)).thenReturn(Optional.of(
+                new HydrologyColumnSample(0, 0, 90, 63, false, "parent", List.of())));
+        HydrologyTileCache cache = new HydrologyTileCache(planner, 64);
+
+        assertTrue(cache.hasColumnsAround(1, 1));
+        assertTrue(cache.hasColumnsAround(-1, 0));
+        assertFalse(cache.hasColumnsAround(2, 0));
+        assertTrue(cache.hasColumnsAround(1, 1));
+        current.set(empty);
+        cache.clear();
+        assertFalse(cache.hasColumnsAround(1, 1));
+        assertFalse(cache.hasColumnsAround(0, 0));
+    }
 }