Add default DifficultyCommand. Partially fixes BUKKIT-2671

By: EvilSeph <evilseph@gmail.com>

paper-api/src/main/java/org/bukkit/command/SimpleCommandMap.java

@@ -53,6 +53,7 @@ public class SimpleCommandMap implements CommandMap {
         fallbackCommands.add(new BanListCommand());
         fallbackCommands.add(new DefaultGameModeCommand());
         fallbackCommands.add(new SeedCommand());
+        fallbackCommands.add(new DifficultyCommand());
     }
 
     public SimpleCommandMap(final Server server) {

paper-api/src/main/java/org/bukkit/command/defaults/DifficultyCommand.java

@@ -0,0 +1,86 @@
+package org.bukkit.command.defaults;
+
+import com.google.common.collect.ImmutableList;
+import org.apache.commons.lang.Validate;
+import org.bukkit.Bukkit;
+import org.bukkit.ChatColor;
+import org.bukkit.command.Command;
+import org.bukkit.command.CommandSender;
+import org.bukkit.util.StringUtil;
+import org.bukkit.Difficulty;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class DifficultyCommand extends VanillaCommand {
+    private static final List<String> DIFFICULTY_NAMES = ImmutableList.of("peaceful", "easy", "normal", "hard");
+
+    public DifficultyCommand() {
+        super("difficulty");
+        this.description = "Sets the game difficulty";
+        this.usageMessage = "/difficulty <new difficulty> ";
+        this.setPermission("bukkit.command.difficulty");
+    }
+
+    @Override
+    public boolean execute(CommandSender sender, String currentAlias, String[] args) {
+        if (!testPermission(sender)) return true;
+        if (args.length != 1 || args[0].length() == 0) {
+            sender.sendMessage(ChatColor.RED + "Usage: " + usageMessage);
+            return false;
+        }
+
+        Difficulty difficulty = Difficulty.getByValue(getDifficultyForString(sender, args[0]));
+
+        if (Bukkit.isHardcore()) {
+            difficulty = Difficulty.HARD;
+        }
+
+        Bukkit.getWorlds().get(0).setDifficulty(difficulty);
+
+        int levelCount = 1;
+        if (Bukkit.getAllowNether()) {
+            Bukkit.getWorlds().get(levelCount).setDifficulty(difficulty);
+            levelCount++;
+        }
+
+        if (Bukkit.getAllowEnd()) {
+            Bukkit.getWorlds().get(levelCount).setDifficulty(difficulty);
+        }
+
+        Command.broadcastCommandMessage(sender, "Set difficulty to " + difficulty.toString());
+        return true;
+    }
+
+    protected int getDifficultyForString(CommandSender sender, String name) {
+        if (name.equalsIgnoreCase("peaceful") || name.equalsIgnoreCase("p")) {
+            return 0;
+        } else if (name.equalsIgnoreCase("easy") || name.equalsIgnoreCase("e")) {
+            return 1;
+        } else if (name.equalsIgnoreCase("normal") || name.equalsIgnoreCase("n")) {
+            return 2;
+        } else if (name.equalsIgnoreCase("hard") || name.equalsIgnoreCase("h")) {
+            return 3;
+        } else {
+            return getInteger(sender, name, 0, 3);
+        }
+    }
+
+    @Override
+    public boolean matches(String input) {
+        return input.equalsIgnoreCase("difficulty");
+    }
+
+    @Override
+    public List<String> tabComplete(CommandSender sender, String alias, String[] args) {
+        Validate.notNull(sender, "Sender cannot be null");
+        Validate.notNull(args, "Arguments cannot be null");
+        Validate.notNull(alias, "Alias cannot be null");
+
+        if (args.length == 1) {
+            return StringUtil.copyPartialMatches(args[0], DIFFICULTY_NAMES, new ArrayList<String>(DIFFICULTY_NAMES.size()));
+        }
+
+        return ImmutableList.of();
+    }
+}