overriding the seed for any dimension

paper-server/src/main/java/io/papermc/paper/configuration/GlobalConfiguration.java

@@ -149,6 +149,23 @@ public class Console extends ConfigurationPart {
         public boolean hasAllPermissions = false;
     }
 
+    public WorldGeneration worldGeneration;
+
+    public class WorldGeneration extends ConfigurationPart {
+        @Comment(
+            "Whether seed overrides should also be applied to dimensions that already have saved world generation settings. " +
+            "Existing chunks are not regenerated, so changing a dimension's seed may create abrupt chunk borders."
+        )
+        public boolean applyToExistingDimensions = false;
+
+        @Comment(
+            "Overrides the seed used to generate a dimension. " +
+            "Keys are dimension identifiers, for example minecraft:overworld or example:custom_dimension. " +
+            "By default, overrides are only applied when a dimension is created for the first time."
+        )
+        public Map<Identifier, Long> dimensionSeedOverrides = Map.of();
+    }
+
     public Watchdog watchdog;
 
     public class Watchdog extends ConfigurationPart {

paper-server/src/main/java/io/papermc/paper/world/PaperWorldLoader.java

@@ -4,6 +4,7 @@
 import io.papermc.paper.world.saveddata.PaperLevelOverrides;
 import io.papermc.paper.world.saveddata.PaperWorldMetadata;
 import io.papermc.paper.world.saveddata.PaperWorldPDC;
+import io.papermc.paper.world.settings.PaperWorldGenSettings;
 import java.io.IOException;
 import java.nio.file.Files;
 import java.util.Locale;
@@ -144,13 +145,18 @@ private void loadInitialWorld(final LevelStem stem, final boolean hasWorldData)
             return;
         }
 
-        final WorldGenSettings worldGenSettings = !hasWorldData
+        final WorldGenSettings storedWorldGenSettings = !hasWorldData
             ? this.server.getWorldGenSettings()
             : loadWorldGenSettings(
             this.server.storageSource,
             this.server.worldLoaderContext.datapackWorldgen(),
             loading.info().dimensionKey()
         );
+        final WorldGenSettings worldGenSettings = PaperWorldGenSettings.applySeedOverride(
+            storedWorldGenSettings,
+            loading.info().dimensionKey(),
+            hasWorldData
+        );
         final var worldDataAndGenSettings = new LevelDataAndDimensions.WorldDataAndGenSettings(this.server.getWorldData(), worldGenSettings);
 
         if (loading.info().dimensionKey() == Level.OVERWORLD) {

paper-server/src/main/java/io/papermc/paper/world/settings/PaperWorldGenSettings.java

@@ -0,0 +1,55 @@
+package io.papermc.paper.world.settings;
+
+import io.papermc.paper.configuration.GlobalConfiguration;
+import java.util.Map;
+import net.minecraft.resources.Identifier;
+import net.minecraft.resources.ResourceKey;
+import net.minecraft.world.level.Level;
+import net.minecraft.world.level.levelgen.WorldGenSettings;
+import net.minecraft.world.level.levelgen.WorldOptions;
+import org.jspecify.annotations.NullMarked;
+
+@NullMarked
+public final class PaperWorldGenSettings {
+
+    private PaperWorldGenSettings() {
+    }
+
+    public static WorldGenSettings applySeedOverride(
+        final WorldGenSettings settings,
+        final ResourceKey<Level> dimensionKey,
+        final boolean existingDimension
+    ) {
+        final GlobalConfiguration.WorldGeneration configuration = GlobalConfiguration.get().worldGeneration;
+        return applySeedOverride(
+            settings,
+            dimensionKey,
+            existingDimension,
+            configuration.applyToExistingDimensions,
+            configuration.dimensionSeedOverrides
+        );
+    }
+
+    static WorldGenSettings applySeedOverride(
+        final WorldGenSettings settings,
+        final ResourceKey<Level> dimensionKey,
+        final boolean existingDimension,
+        final boolean applyToExistingDimensions,
+        final Map<Identifier, Long> seedOverrides
+    ) {
+        if (existingDimension && !applyToExistingDimensions) {
+            return settings;
+        }
+
+        final Long seedOverride = seedOverrides.get(dimensionKey.identifier());
+        if (seedOverride == null) {
+            return settings;
+        }
+
+        final WorldOptions options = settings.options();
+        return new WorldGenSettings(
+            new WorldOptions(seedOverride, options.generateStructures(), options.generateBonusChest()),
+            settings.dimensions()
+        );
+    }
+}

paper-server/src/test/java/io/papermc/paper/world/settings/PaperWorldGenSettingsTest.java

@@ -0,0 +1,71 @@
+package io.papermc.paper.world.settings;
+
+import java.util.Map;
+import net.minecraft.core.registries.Registries;
+import net.minecraft.resources.Identifier;
+import net.minecraft.resources.ResourceKey;
+import net.minecraft.world.level.Level;
+import net.minecraft.world.level.levelgen.WorldDimensions;
+import net.minecraft.world.level.levelgen.WorldGenSettings;
+import net.minecraft.world.level.levelgen.WorldOptions;
+import org.bukkit.support.environment.Normal;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertSame;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.mockito.Mockito.mock;
+
+@Normal
+class PaperWorldGenSettingsTest {
+
+    private static final Identifier DIMENSION_ID = Identifier.parse("test:dimension");
+    private static final ResourceKey<Level> DIMENSION_KEY = ResourceKey.create(Registries.DIMENSION, DIMENSION_ID);
+
+    @Test
+    void returnsOriginalSettingsWithoutOverride() {
+        final WorldGenSettings settings = settings(123L, true, false);
+
+        final WorldGenSettings result = PaperWorldGenSettings.applySeedOverride(settings, DIMENSION_KEY, false, false, Map.of());
+
+        assertSame(settings, result);
+    }
+
+    @Test
+    void overridesOnlySeedForNewDimension() {
+        final WorldGenSettings settings = settings(123L, false, true);
+
+        final WorldGenSettings result = PaperWorldGenSettings.applySeedOverride(settings, DIMENSION_KEY, false, false, Map.of(DIMENSION_ID, 456L));
+
+        assertEquals(456L, result.options().seed());
+        assertFalse(result.options().generateStructures());
+        assertTrue(result.options().generateBonusChest());
+        assertSame(settings.dimensions(), result.dimensions());
+    }
+
+    @Test
+    void doesNotOverrideExistingDimensionByDefault() {
+        final WorldGenSettings settings = settings(123L, true, false);
+
+        final WorldGenSettings result = PaperWorldGenSettings.applySeedOverride(settings, DIMENSION_KEY, true, false, Map.of(DIMENSION_ID, 456L));
+
+        assertSame(settings, result);
+    }
+
+    @Test
+    void overridesExistingDimensionWhenEnabled() {
+        final WorldGenSettings settings = settings(123L, true, false);
+
+        final WorldGenSettings result = PaperWorldGenSettings.applySeedOverride(settings, DIMENSION_KEY, true, true, Map.of(DIMENSION_ID, 456L));
+
+        assertEquals(456L, result.options().seed());
+    }
+
+    private static WorldGenSettings settings(final long seed, final boolean generateStructures, final boolean generateBonusChest) {
+        return new WorldGenSettings(
+            new WorldOptions(seed, generateStructures, generateBonusChest),
+            mock(WorldDimensions.class)
+        );
+    }
+}