Add BiomeProvider structure search fast path to CustomWorldChunkManager

paper-api/src/main/java/org/bukkit/generator/BiomeProvider.java

@@ -1,6 +1,7 @@
 package org.bukkit.generator;
 
 import java.util.List;
+import java.util.Optional;
 import org.bukkit.block.Biome;
 import org.jetbrains.annotations.NotNull;
 
@@ -61,6 +62,29 @@ public Biome getBiome(@NotNull WorldInfo worldInfo, int x, int y, int z, @NotNul
         return getBiome(worldInfo, x, y, z);
     }
 
+    /**
+     * Return a biome for structure placement searching (e.g. stronghold ring
+     * position computation), bypassing expensive biome pipeline evaluation.
+     * <p>
+     * Implementations may return a coarse approximation sufficient to determine
+     * whether a position is eligible for a structure (e.g. land vs ocean).
+     * Return {@link Optional#empty()} to fall back to the full
+     * {@link #getBiome(WorldInfo, int, int, int)} path.
+     * <p>
+     * This is called from {@code findBiomeHorizontal} which runs on background
+     * executor threads. Implementations must be thread-safe.
+     *
+     * @param worldInfo The world info of the world being searched
+     * @param x The X block coordinate
+     * @param z The Z block coordinate
+     * @return An optional biome for fast structure placement eligibility
+     *         checking, or empty to use the full pipeline
+     */
+    @NotNull
+    public Optional<Biome> getStructurePlacementBiome(@NotNull WorldInfo worldInfo, int x, int z) {
+        return Optional.empty();
+    }
+
     /**
      * Returns a list with every biome the {@link BiomeProvider} will use for
      * the given world.

paper-server/src/main/java/org/bukkit/craftbukkit/generator/CustomWorldChunkManager.java

@@ -1,18 +1,42 @@
 package org.bukkit.craftbukkit.generator;
 
 import com.google.common.base.Preconditions;
+import com.mojang.datafixers.util.Pair;
 import com.mojang.serialization.MapCodec;
+import java.util.Optional;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.function.Predicate;
 import java.util.stream.Stream;
+import net.minecraft.core.BlockPos;
 import net.minecraft.core.Holder;
 import net.minecraft.core.QuartPos;
+import net.minecraft.util.RandomSource;
+import net.minecraft.world.level.biome.Biome;
 import net.minecraft.world.level.biome.BiomeSource;
 import net.minecraft.world.level.biome.Climate;
 import org.bukkit.craftbukkit.block.CraftBiome;
 import org.bukkit.generator.BiomeProvider;
 import org.bukkit.generator.WorldInfo;
+import org.jspecify.annotations.Nullable;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 public class CustomWorldChunkManager extends BiomeSource {
 
+    private static final Logger LOGGER = LoggerFactory.getLogger(CustomWorldChunkManager.class);
+    // One-shot guards: prevent flooding debug/warn logs across 400k+ biome queries per search.
+    private static final AtomicBoolean LOGGED_SEARCH_INTERCEPT = new AtomicBoolean(false);
+    private static final AtomicBoolean LOGGED_FAST_PATH = new AtomicBoolean(false);
+    private static final AtomicBoolean LOGGED_FAST_PATH_EMPTY = new AtomicBoolean(false);
+
+    // Set true on the calling thread while inside findBiomeHorizontal so that
+    // getNoiseBiome can use the fast path for structure placement searches.
+    // findBiomeHorizontal is the only caller of getNoiseBiome for stronghold
+    // ring position computation; normal chunk generation calls getNoiseBiome
+    // directly without going through findBiomeHorizontal.
+    private static final ThreadLocal<Boolean> IN_STRUCTURE_SEARCH =
+        ThreadLocal.withInitial(() -> false);
+
     private final WorldInfo worldInfo;
     private final BiomeProvider biomeProvider;
     public final BiomeSource vanillaBiomeSource;
@@ -28,8 +52,54 @@ protected MapCodec<? extends BiomeSource> codec() {
         throw new UnsupportedOperationException("Cannot serialize CustomWorldChunkManager");
     }
 
+    // Paper - structure search fast path: intercept the horizontal biome search used
+    // by ConcentricRingsStructurePlacement (strongholds). Sets a thread-local so
+    // getNoiseBiome can call BiomeProvider.getStructurePlacementBiome() instead of
+    // the full pipeline, avoiding 100-2000x amplification from pipeline chunk cache misses.
+    @Override
+    public @Nullable Pair<BlockPos, Holder<Biome>> findBiomeHorizontal(
+            int x, int y, int z, int searchRadius, int skipSteps,
+            Predicate<Holder<Biome>> allowed, RandomSource random,
+            boolean findClosest, Climate.Sampler sampler) {
+        if (LOGGED_SEARCH_INTERCEPT.compareAndSet(false, true)) {
+            LOGGER.debug("Structure search fast path active for world '{}'", this.worldInfo.getName());
+        }
+        IN_STRUCTURE_SEARCH.set(true);
+        try {
+            return super.findBiomeHorizontal(x, y, z, searchRadius, skipSteps,
+                                              allowed, random, findClosest, sampler);
+        } finally {
+            IN_STRUCTURE_SEARCH.set(false);
+        }
+    }
+
     @Override
     public Holder<net.minecraft.world.level.biome.Biome> getNoiseBiome(int x, int y, int z, Climate.Sampler noise) {
+        if (IN_STRUCTURE_SEARCH.get()) {
+            Optional<org.bukkit.block.Biome> fast =
+                this.biomeProvider.getStructurePlacementBiome(this.worldInfo, QuartPos.toBlock(x), QuartPos.toBlock(z));
+            if (fast.isPresent()) {
+                Holder<net.minecraft.world.level.biome.Biome> biome = CraftBiome.bukkitToMinecraftHolder(fast.get());
+                if (biome != null) {
+                    if (LOGGED_FAST_PATH.compareAndSet(false, true)) {
+                        LOGGER.debug("Structure search fast path hit for world '{}' — provider: {}, biome: {}",
+                            this.worldInfo.getName(), this.biomeProvider.getClass().getSimpleName(), fast.get());
+                    }
+                    return biome;
+                } else {
+                    if (LOGGED_FAST_PATH_EMPTY.compareAndSet(false, true)) {
+                        LOGGER.warn("Structure search fast path for world '{}': biome '{}' returned by {} is not in the biome registry; falling back to full pipeline",
+                            this.worldInfo.getName(), fast.get(), this.biomeProvider.getClass().getSimpleName());
+                    }
+                }
+            } else {
+                if (LOGGED_FAST_PATH_EMPTY.compareAndSet(false, true)) {
+                    LOGGER.debug("Structure search fast path not available for world '{}' — {} returned empty; using full pipeline",
+                        this.worldInfo.getName(), this.biomeProvider.getClass().getSimpleName());
+                }
+            }
+        }
+
         Holder<net.minecraft.world.level.biome.Biome> biome = CraftBiome.bukkitToMinecraftHolder(
             this.biomeProvider.getBiome(this.worldInfo, QuartPos.toBlock(x), QuartPos.toBlock(y), QuartPos.toBlock(z), CraftBiomeParameterPoint.createBiomeParameterPoint(noise, noise.sample(x, y, z)))
         );