Add /glist command.

A much-needed base command, this is now available in Velocity directly.
It is partially inspired by the RedisBungee /glist command, which I also
created.

By default, /glist only provides the player count of the proxy. You can
view the player list for an individual server with /glist <server>, or
you can view the information for all players with /glist all.

proxy/src/main/java/com/velocitypowered/proxy/VelocityServer.java

@@ -17,6 +17,7 @@ import com.velocitypowered.api.proxy.server.ServerInfo;
 import com.velocitypowered.api.util.Favicon;
 import com.velocitypowered.api.util.GameProfile;
 import com.velocitypowered.api.util.ProxyVersion;
+import com.velocitypowered.proxy.command.GlistCommand;
 import com.velocitypowered.proxy.command.ServerCommand;
 import com.velocitypowered.proxy.command.ShutdownCommand;
 import com.velocitypowered.proxy.command.VelocityCommand;
@@ -151,6 +152,7 @@ public class VelocityServer implements ProxyServer {
     commandManager.register(new VelocityCommand(this), "velocity");
     commandManager.register(new ServerCommand(this), "server");
     commandManager.register(new ShutdownCommand(this), "shutdown", "end");
+    commandManager.register(new GlistCommand(this), "glist");
 
     try {
       Path configPath = Paths.get("velocity.toml");

proxy/src/main/java/com/velocitypowered/proxy/command/GlistCommand.java

@@ -0,0 +1,112 @@
+package com.velocitypowered.proxy.command;
+
+import com.google.common.collect.ImmutableList;
+import com.velocitypowered.api.command.Command;
+import com.velocitypowered.api.command.CommandSource;
+import com.velocitypowered.api.permission.Tristate;
+import com.velocitypowered.api.proxy.Player;
+import com.velocitypowered.api.proxy.ProxyServer;
+import com.velocitypowered.api.proxy.server.RegisteredServer;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Optional;
+import java.util.stream.Collectors;
+import net.kyori.text.TextComponent;
+import net.kyori.text.format.TextColor;
+import org.checkerframework.checker.nullness.qual.NonNull;
+
+public class GlistCommand implements Command {
+
+  private final ProxyServer server;
+
+  public GlistCommand(ProxyServer server) {
+    this.server = server;
+  }
+
+  @Override
+  public void execute(CommandSource source, String @NonNull [] args) {
+    if (args.length == 0) {
+      sendTotalProxyCount(source);
+      source.sendMessage(
+          TextComponent.builder("To view all players on servers, use ", TextColor.YELLOW)
+              .append("/glist all", TextColor.DARK_AQUA)
+              .append(".", TextColor.YELLOW)
+              .build());
+    } else if (args.length == 1) {
+      String arg = args[0];
+      if (arg.equalsIgnoreCase("all")) {
+        for (RegisteredServer server : server.getAllServers()) {
+          sendServerPlayers(source, server, true);
+        }
+        sendTotalProxyCount(source);
+      } else {
+        Optional<RegisteredServer> registeredServer = server.getServer(arg);
+        if (!registeredServer.isPresent()) {
+          source.sendMessage(
+              TextComponent.of("Server " + arg + " doesn't exist.", TextColor.RED));
+          return;
+        }
+        sendServerPlayers(source, registeredServer.get(), false);
+      }
+    } else {
+      source.sendMessage(TextComponent.of("Too many arguments.", TextColor.RED));
+    }
+  }
+
+  private void sendTotalProxyCount(CommandSource target) {
+    target.sendMessage(TextComponent.builder("There are ", TextColor.YELLOW)
+        .append(Integer.toString(server.getAllPlayers().size()), TextColor.GREEN)
+        .append(" player(s) online.", TextColor.YELLOW)
+        .build());
+  }
+
+  private void sendServerPlayers(CommandSource target, RegisteredServer server, boolean fromAll) {
+    List<Player> onServer = ImmutableList.copyOf(server.getPlayersConnected());
+    if (onServer.isEmpty() && fromAll) {
+      return;
+    }
+
+    TextComponent.Builder builder = TextComponent.builder()
+        .append(TextComponent.of("[" + server.getServerInfo().getName() + "] ",
+            TextColor.DARK_AQUA))
+        .append("(" + onServer.size() + ")", TextColor.GRAY)
+        .append(": ")
+        .resetStyle();
+
+    for (int i = 0; i < onServer.size(); i++) {
+      Player player = onServer.get(i);
+      builder.append(player.getUsername());
+
+      if (i + 1 < onServer.size()) {
+        builder.append(", ");
+      }
+    }
+
+    target.sendMessage(builder.build());
+  }
+
+  @Override
+  public List<String> suggest(CommandSource source, String @NonNull [] currentArgs) {
+    ImmutableList.Builder<String> options = ImmutableList.builder();
+    for (RegisteredServer server : server.getAllServers()) {
+      options.add(server.getServerInfo().getName());
+    }
+    options.add("all");
+
+    switch (currentArgs.length) {
+      case 0:
+        return options.build();
+      case 1:
+        return options.build().stream()
+            .filter(o -> o.regionMatches(true, 0, currentArgs[0], 0, currentArgs[0].length()))
+            .collect(ImmutableList.toImmutableList());
+      default:
+        return ImmutableList.of();
+    }
+  }
+
+  @Override
+  public boolean hasPermission(CommandSource source, String @NonNull [] args) {
+    return source.getPermissionValue("velocity.command.glist") == Tristate.TRUE;
+  }
+}