Probe history digest

- Adds an opt-in digest of semantic and saved biome records per chunk

probe/build.gradle

@@ -180,6 +180,7 @@ tasks.register('genProbe', JavaExec) {
     systemProperty('iris.probe.pregenProfile', providers.gradleProperty('probePregenProfile').orElse('false').get())
     systemProperty('iris.probe.hydrologyPreplan', providers.gradleProperty('probeHydrologyPreplan').orElse('false').get())
     systemProperty('iris.probe.historyRouter', providers.gradleProperty('probeHistoryRouter').orElse('false').get())
+    systemProperty('iris.probe.historyDigest', providers.gradleProperty('probeHistoryDigest').orElse('false').get())
     systemProperty('iris.probe.productionBuffers', providers.gradleProperty('probeProductionBuffers').orElse('false').get())
     String probeCounters = providers.gradleProperty('probeCounters').orElse('false').get()
     systemProperty('iris.generation.counters', probeCounters)

probe/src/main/java/art/arcane/iris/probe/GenerationProbe.java

@@ -177,7 +177,7 @@ public final class GenerationProbe {
     }
 
     record ProbeOptions(int threads, Path dumpFile, boolean pregenProfile, boolean hydrologyPreplan,
-                        boolean historyRouter, boolean productionBuffers, boolean counters) {
+                        boolean historyRouter, boolean productionBuffers, boolean counters, boolean historyDigest) {
         ProbeOptions {
             if (threads < 1) {
                 throw new IllegalArgumentException("Thread count must be at least 1.");
@@ -196,7 +196,8 @@ public final class GenerationProbe {
                     booleanProperty("iris.probe.hydrologyPreplan"),
                     booleanProperty("iris.probe.historyRouter"),
                     booleanProperty("iris.probe.productionBuffers"),
-                    booleanProperty("iris.generation.counters"));
+                    booleanProperty("iris.generation.counters"),
+                    booleanProperty("iris.probe.historyDigest"));
         }
 
         private static boolean booleanProperty(String key) {
@@ -437,7 +438,8 @@ public final class GenerationProbe {
                     + " timeMs=" + String.format(Locale.ROOT, "%.3f", nanosToMillis(engineReadyNanos)));
             System.out.println("IRIS_GENPROBE_MANTLE version=1 radius=" + engine.getMantle().getRadius()
                     + " real_radius=" + engine.getMantle().getRealRadius());
-            ProbeChunkGenerator generator = new ProbeChunkGenerator(engine, router, options.productionBuffers());
+            ProbeChunkGenerator generator = new ProbeChunkGenerator(engine, router, options.productionBuffers(),
+                    options.historyDigest());
             long nativeSamplesBefore = FractalBillowSimplexNoise.nativeSampleCount();
             GenerationResult generation = generateWithProfile(generator, configuration, options);
             verifyBackend(nativeSamplesBefore);
@@ -445,6 +447,9 @@ public final class GenerationProbe {
                 System.out.println("IRIS_GENPROBE_BUFFERS version=1 mode=production biome_expanded_chunks="
                         + generator.expandedBiomeHunks());
             }
+            if (generator.historySummary() != null) {
+                System.out.println(generator.historySummary());
+            }
             String status = generation.failedChunks() == 0 ? "PASS" : "FAIL";
             printGenerationFailures(generation);
             return new ProbeResult(

probe/src/main/java/art/arcane/iris/probe/HistoryDigest.java

@@ -0,0 +1,86 @@
+/*
+ * Iris is a World Generator for Minecraft Servers
+ * Copyright (c) 2026 Arcane Arts (Volmit Software)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package art.arcane.iris.probe;
+
+import art.arcane.iris.world.history.ChunkGenerationSemantics;
+import art.arcane.iris.world.history.GenerationHistoryRuntimeRouter;
+import art.arcane.iris.world.history.SavedBiomeChunk;
+
+import java.io.IOException;
+import java.nio.ByteBuffer;
+import java.nio.charset.StandardCharsets;
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
+import java.util.HexFormat;
+import java.util.Map;
+import java.util.Optional;
+import java.util.concurrent.ConcurrentSkipListMap;
+
+final class HistoryDigest {
+    private final GenerationHistoryRuntimeRouter router;
+    private final Map<Long, byte[]> chunks = new ConcurrentSkipListMap<>();
+
+    HistoryDigest(GenerationHistoryRuntimeRouter router) {
+        this.router = router;
+    }
+
+    void record(int chunkX, int chunkZ) throws IOException {
+        StringBuilder canonical = new StringBuilder(4096);
+        Optional<ChunkGenerationSemantics> semantics = router.history().semantics(chunkX, chunkZ);
+        canonical.append(semantics.map(value -> value + "|" + value.pointsOfInterest()).orElse("none")).append('\n');
+        Optional<SavedBiomeChunk> biomes = router.biomes().snapshot(chunkX, chunkZ);
+        if (biomes.isEmpty()) {
+            canonical.append("none");
+        } else {
+            SavedBiomeChunk chunk = biomes.get();
+            canonical.append(chunk.header());
+            for (int localZ = 0; localZ < 16; localZ++) {
+                for (int localX = 0; localX < 16; localX++) {
+                    canonical.append('\n').append(chunk.column(localX, localZ));
+                }
+            }
+        }
+        chunks.put(((long) chunkX << 32) | (chunkZ & 0xFFFFFFFFL), sha256(canonical.toString().getBytes(StandardCharsets.UTF_8)));
+    }
+
+    String summary() {
+        MessageDigest digest = newDigest();
+        ByteBuffer key = ByteBuffer.allocate(Long.BYTES);
+        for (Map.Entry<Long, byte[]> entry : chunks.entrySet()) {
+            key.clear();
+            key.putLong(entry.getKey());
+            digest.update(key.array());
+            digest.update(entry.getValue());
+        }
+        return "IRIS_GENPROBE_HISTORY version=1 chunks=" + chunks.size()
+                + " digest=" + HexFormat.of().formatHex(digest.digest(), 0, 8);
+    }
+
+    private static byte[] sha256(byte[] bytes) {
+        return newDigest().digest(bytes);
+    }
+
+    private static MessageDigest newDigest() {
+        try {
+            return MessageDigest.getInstance("SHA-256");
+        } catch (NoSuchAlgorithmException missing) {
+            throw new IllegalStateException("SHA-256 is unavailable", missing);
+        }
+    }
+}

probe/src/main/java/art/arcane/iris/probe/ProbeChunkGenerator.java

@@ -36,12 +36,19 @@ final class ProbeChunkGenerator {
     private final Engine engine;
     private final GenerationHistoryRuntimeRouter router;
     private final boolean productionBuffers;
+    private final HistoryDigest historyDigest;
     private final AtomicInteger expandedBiomeHunks = new AtomicInteger();
 
-    ProbeChunkGenerator(Engine engine, GenerationHistoryRuntimeRouter router, boolean productionBuffers) {
+    ProbeChunkGenerator(Engine engine, GenerationHistoryRuntimeRouter router, boolean productionBuffers,
+                        boolean historyDigest) {
         this.engine = engine;
         this.router = router;
         this.productionBuffers = productionBuffers;
+        this.historyDigest = historyDigest && router != null ? new HistoryDigest(router) : null;
+    }
+
+    String historySummary() {
+        return historyDigest == null ? null : historyDigest.summary();
     }
 
     Engine engine() {
@@ -64,6 +71,13 @@ final class ProbeChunkGenerator {
             failures.add(e);
         }
         long elapsed = System.nanoTime() - started;
+        if (historyDigest != null && failures.isEmpty()) {
+            try {
+                historyDigest.record(coordinate.x(), coordinate.z());
+            } catch (Throwable e) {
+                failures.add(e);
+            }
+        }
         if (biomes instanceof ProductionBiomeHunk production && production.expanded()) {
             expandedBiomeHunks.incrementAndGet();
         }

probe/src/test/java/art/arcane/iris/probe/GenerationProbeTest.java

@@ -134,7 +134,7 @@ public final class GenerationProbeTest {
         assertEquals(1, defaults.threads());
         assertNull(defaults.dumpFile());
         assertThrows(IllegalArgumentException.class,
-                () -> new GenerationProbe.ProbeOptions(0, null, false, false, false, false, false));
+                () -> new GenerationProbe.ProbeOptions(0, null, false, false, false, false, false, false));
         assertFalse(defaults.hydrologyPreplan());
         assertFalse(defaults.historyRouter());
         assertFalse(defaults.productionBuffers());