Slimes.java

package com.demonwav.slimes;

import org.bukkit.Chunk;
import org.bukkit.entity.Player;
import org.bukkit.plugin.java.JavaPlugin;

import java.util.Random;

public final class Slimes extends JavaPlugin {

    @Override
    public void onEnable() {
        getCommand("checkslimechunk").setExecutor((sender, command, label, args) -> {
            if (!(sender instanceof Player)) {
                sender.sendMessage("You must be a player to use this command.");
                return true;
            }

            final Player player = (Player) sender;

            final Chunk chunk = player.getWorld().getChunkAt(player.getLocation());
            final int x = chunk.getX();
            final int z = chunk.getZ();

            final long seed = player.getWorld().getSeed();

            final Random rand = new Random(seed + (long) (x * x * 0x4c1906) + (long) (x * 0x5ac0db) + (long) (z * z) * 0x4307a7L + (long) (z * 0x5f24f) ^ 0x3ad8025f);

            if (rand.nextInt(10) == 0) {
                player.sendMessage("You are in a slime chunk.");
            } else {
                player.sendMessage("You are not in a slime chunk.");
            }
            return true;
        });
    }
}