Refactor tpll command

Fix permission checks when sender is not the target.
Prepare to make it work with any entity.
Send messages asynchronously.
Code cleanup.

src/main/java/de/btegermany/terraplusminus/commands/CommandHelper.java

@@ -0,0 +1,80 @@
+package de.btegermany.terraplusminus.commands;
+
+import org.bukkit.Bukkit;
+import org.bukkit.Location;
+import org.bukkit.command.CommandSender;
+import org.bukkit.entity.Entity;
+import org.bukkit.entity.Player;
+import org.jetbrains.annotations.NotNull;
+
+import java.util.Collection;
+import java.util.Collections;
+import java.util.Comparator;
+import java.util.List;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import static java.util.Comparator.comparing;
+
+/**
+ * Utility class to help with commands.
+ *
+ * @author Smyler
+ */
+public final class CommandHelper {
+
+    /**
+     * Parses a <a href="https://minecraft.fandom.com/wiki/Target_selectors">command target selector</a> like '@a'.
+     * Currently supported selectors:
+     * <ul>
+     *     <li><code>@a</code></li>
+     *     <li><code>@p</code> (doesn't select the sender)</li>
+     * </ul>
+     * Target selector arguments are not supported.
+     * It's a shame that Bukkit does not expose the vanilla command system...
+     *
+     * @param sender    the {@link CommandSender command sender}
+     * @param selector  the selector string
+     * @return the collection of {@link Entity entities} that match the selector
+     *
+     * @throws InvalidTargetSelectorException if the selector is invalid, either syntactically or in the specific context
+     */
+    public static Collection<Entity> parseTargetSelector(@NotNull CommandSender sender, String selector) throws InvalidTargetSelectorException {
+        if (selector.startsWith("@") && selector.length() >= 2) {
+            char selectorChar = selector.charAt(1);
+            if (selectorChar == 'a') {
+                return Bukkit.getOnlinePlayers().stream()
+                        .map(p -> (Entity)p)
+                        .collect(Collectors.toList());
+            } else if (selectorChar == 'p' && sender instanceof Entity) {
+                Entity entitySender = (Entity) sender;
+                Location senderLocation = entitySender.getLocation();
+                return Collections.singleton(
+                        Bukkit.getOnlinePlayers().stream()
+                                .filter(p -> p != sender)
+                                .min(comparing(p -> p.getLocation().distanceSquared(senderLocation)))
+                                .orElseThrow(InvalidTargetSelectorException::new)
+                );
+            }
+        } else {
+            Player player = Bukkit.getPlayerExact(selector);
+            if (player == null || !player.isOnline()) {
+                throw new InvalidTargetSelectorException();
+            } else {
+                return List.of(player);
+            }
+        }
+
+        throw new InvalidTargetSelectorException();
+
+    }
+
+    public static class InvalidTargetSelectorException extends Exception {
+
+    }
+
+    private CommandHelper() {
+        throw new IllegalStateException();
+    }
+
+}