From 618bb285911a9fb9b1386ed38a717db30253d797 Mon Sep 17 00:00:00 2001
From: Brian Neumann-Fopiano <mypsn1550@gmail.com>
Date: Fri, 30 Oct 2026 22:47:20 -0400
Subject: [PATCH] Fixed lattice eviction race

- Lattice eviction sorts a snapshot of tile access stamps
- Concurrent tile use during eviction no longer breaks the sort
- Test covers eviction under concurrent fills
---
 .../hydrology/grid/HeightLattice.java         | 23 ++++++++------
 .../hydrology/grid/HeightLatticeTest.java     | 31 +++++++++++++++++++
 2 files changed, 44 insertions(+), 10 deletions(-)

diff --git a/core/src/main/java/art/arcane/iris/generation/hydrology/grid/HeightLattice.java b/core/src/main/java/art/arcane/iris/generation/hydrology/grid/HeightLattice.java
index cc1804606..68cf87401 100644
--- a/core/src/main/java/art/arcane/iris/generation/hydrology/grid/HeightLattice.java
+++ b/core/src/main/java/art/arcane/iris/generation/hydrology/grid/HeightLattice.java
@@ -138,19 +138,19 @@ public final class HeightLattice {
             return;
         }
         try {
-            List<Map.Entry<Long, Tile>> idle = new ArrayList<>(tiles.size());
+            List<IdleTile> idle = new ArrayList<>(tiles.size());
             for (Map.Entry<Long, Tile> entry : tiles.entrySet()) {
-                if (entry.getValue().pending.get() == 0) {
-                    idle.add(entry);
-                }
-            }
-            idle.sort(Comparator.comparingLong(entry -> entry.getValue().used));
-            int target = capacityTiles - (capacityTiles >> 3);
-            for (int index = 0; index < idle.size() && tiles.size() > target; index++) {
-                Map.Entry<Long, Tile> entry = idle.get(index);
                 Tile tile = entry.getValue();
                 if (tile.pending.get() == 0) {
-                    tiles.remove(entry.getKey(), tile);
+                    idle.add(new IdleTile(entry.getKey(), tile, tile.used));
+                }
+            }
+            idle.sort(Comparator.comparingLong(IdleTile::used));
+            int target = capacityTiles - (capacityTiles >> 3);
+            for (int index = 0; index < idle.size() && tiles.size() > target; index++) {
+                IdleTile candidate = idle.get(index);
+                if (candidate.tile().pending.get() == 0) {
+                    tiles.remove(candidate.key(), candidate.tile());
                 }
             }
         } finally {
@@ -386,6 +386,9 @@ public final class HeightLattice {
         int cellZ(int index);
     }
 
+    private record IdleTile(Long key, Tile tile, long used) {
+    }
+
     private record Span(int cellX0, int cellZ0, int width, int stride) implements Nodes {
         @Override
         public int cellX(int index) {
diff --git a/core/src/test/java/art/arcane/iris/generation/hydrology/grid/HeightLatticeTest.java b/core/src/test/java/art/arcane/iris/generation/hydrology/grid/HeightLatticeTest.java
index 5e4da3c2d..bce372d77 100644
--- a/core/src/test/java/art/arcane/iris/generation/hydrology/grid/HeightLatticeTest.java
+++ b/core/src/test/java/art/arcane/iris/generation/hydrology/grid/HeightLatticeTest.java
@@ -100,6 +100,37 @@ public class HeightLatticeTest {
         assertEquals(DISTINCT_ACROSS_NINE, source.samples());
     }
 
+    @Test
+    public void evictionToleratesConcurrentTileUse() throws Exception {
+        HeightLattice lattice = new HeightLattice(new CountingSource(), 1, false, 96 * 1024);
+        int threads = 12;
+        CyclicBarrier start = new CyclicBarrier(threads);
+        ExecutorService pool = Executors.newFixedThreadPool(threads);
+        try {
+            List<Future<Void>> results = new ArrayList<>();
+            for (int thread = 0; thread < threads; thread++) {
+                long seed = 31L * thread + 7L;
+                Callable<Void> task = () -> {
+                    Random random = new Random(seed);
+                    HydrologySampleBuffer out = new HydrologySampleBuffer(4);
+                    start.await();
+                    for (int fill = 0; fill < 40_000; fill++) {
+                        int tileX = random.nextInt(24);
+                        int tileZ = random.nextInt(24);
+                        lattice.fill(tileX * 32 + random.nextInt(31), tileZ * 32 + random.nextInt(31), 2, 2, out);
+                    }
+                    return null;
+                };
+                results.add(pool.submit(task));
+            }
+            for (Future<Void> result : results) {
+                result.get(120, TimeUnit.SECONDS);
+            }
+        } finally {
+            pool.shutdownNow();
+        }
+    }
+
     @Test
     public void returnsBitIdenticalValues() {
         HeightLattice full = new HeightLattice(new CountingSource(), 8, false, 1 << 16);