ScoreboardManager

package somePackage;

import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.UUID;

import org.bukkit.Bukkit;
import org.bukkit.entity.Player;
import org.bukkit.scoreboard.DisplaySlot;
import org.bukkit.scoreboard.Objective;
import org.bukkit.scoreboard.Team;

public class Scoreboard {

        static {
                manager = new ScoreboardManager();
        }

        private static final ScoreboardManager manager;
        private static org.bukkit.scoreboard.Scoreboard board;
        private static Objective obj;

        /**
         * Returns the scoreboard manager
         * @return the scoreboard manager
         */
        public static ScoreboardManager getManager() {
                return manager;
        }

        private final String name;
        private List<String> lastUpdate = new ArrayList<>();
        private List<String> scores = new ArrayList<>();
        private String header;

        private Scoreboard(Player p) {
                name = p == null ? null : p.getName();
                if(p != null) {
                        p.setScoreboard(board);
                        header = p.getName();
                }
        }

        /**
         * Returns the scoreboard header of the player
         * @return the scoreboard header
         */
        public String getHeader() {
                return header;
        }

        /**
         * Sets the header
         * @param header the new header
         * @return the scoreboard instance, useful for chaining actions
         */
        public Scoreboard setHeader(String header) {
                this.header = header;
                return this;
        }

        /**
         * Returns the scoreboard's player
         * @return the scoreboard's player
         */
        @SuppressWarnings("deprecation")
        public Player getPlayer() {
                return Bukkit.getPlayer(name);
        }

        /**
         * Returns the scores
         * @return the scores, where the keys are the score names and the values their score value
         */
        public List<String> getScores() {
                return scores;
        }

        /**
         * Sets the scoreboard scores in descending order
         * @param scores the score names
         * @return the scoreboard instance
         */
        public Scoreboard setScores(List<String> scores) {
                this.scores = new ArrayList<String>(scores);
                return this;
        }

        /**
         * Sets the scoreboard scores in descending order, this method simply calls setScores(Arrays.asList(scores))
         * @param scores the score names
         * @return the scoreboard instance
         */
        public Scoreboard setScores(String... scores) {
                return setScores(Arrays.asList(scores));
        }

        /**
         * Updates the scoreboard with the current scores and header
         * @return the scoreboard instance
         */
        public Scoreboard update() {
                Player p = getPlayer();
                if(p == null) return this;
                if(scores.size() == lastUpdate.size()) {
                        int size = scores.size() - 1;
                        for(int i=0;i<=size;i++) {
                                String entry = scores.get(i);
                                String last = lastUpdate.get(i);
                                if(!entry.equals(last)) {
                                        Object remove = manager.getRemovePacket(obj, last);
                                        Object add = manager.getChangePacket(obj, entry, size - i);
                                        ReflectUtil.sendPacket(p, remove, add);
                                }
                        }
                } else {
                        for(int i=0;i<lastUpdate.size();i++) {
                                Object packet = manager.getRemovePacket(obj, lastUpdate.get(i));
                                ReflectUtil.sendPacket(p, packet);
                        }
                        int size = scores.size() - 1;
                        for(int i=0;i<=size;i++) {
                                Object packet = manager.getChangePacket(obj, scores.get(i), size-i);
                                ReflectUtil.sendPacket(p, packet);
                        }
                }
                try {
                        Object packet = ReflectUtil.getNMSClass(NMSClass.PACKET_OBJECTIVE).newInstance();
                        NMSClass clazz = NMSClass.ENUM_HEALTH_DISPLAY;

                        String[] keys = "a, b, c, d".split(", ");
                        Object[] values;
                        if(clazz.name != null)
                            values = new Object[] {
                                "Board", header, ReflectUtil.getField(ReflectUtil.getNMSClass(clazz),
                                        null, "INTEGER"), 2};
                        else values = new Object[] {"Board", header, 2};
                        for(int i=0;i<values.length;i++)
                                ReflectUtil.setField(packet.getClass(), packet, keys[i], values[i]);
                        ReflectUtil.sendPacket(p, packet);
                } catch (Exception ex) {
                        ex.printStackTrace();
                }
                this.lastUpdate = new ArrayList<String>(scores);
                return this;
        }

        /**
         * Unregisters the scoreboard from the system
         */
        public void unregister() {
                Player p = getPlayer();
                if(p != null) {
                        manager.boards.remove(p.getUniqueId());
                        p.setScoreboard(Bukkit.getScoreboardManager().getMainScoreboard());
                }
        }

        public static class ScoreboardManager {

                private final Map<UUID, Scoreboard> boards;

                private ScoreboardManager() {
                        boards = new HashMap<>();
                }

                /**
                 * Sets a scoreboard up for the player, this should be called in a join listener, use getScoreboard(Player) to get a player's board
                 * @param p the player to set the board for
                 * @return the newly created scoreboard instance
                 */
                public Scoreboard setupBoard(Player p) {
                        Scoreboard board = new Scoreboard(p);
                        boards.put(p.getUniqueId(), board);
                        return board;
                }

                /**
                 * Returns the player's scoreboard
                 * @param p the player's scoreboard
                 * @return the scoreboard of the player
                 */
                public Scoreboard getScoreboard(Player p) {
                        return boards.get(p.getUniqueId());
                }

                /**
                 * Gets the main scoreboard, use this to register/unregister/manipulate teams
                 * @return
                 */
                public org.bukkit.scoreboard.Scoreboard getScoreboard() {
                        return board;
                }

                /**
                 * Resets all teams
                 */
                public void resetTeams() {
                        //Converting it to an array because when unregistering a team, it gets removed from the board's teams list,
                        //and even by calling Iterator#remove() it will throw a ConcurrentModificationException
                        Team[] teams = board.getTeams().toArray(new Team[0]);
                        for(Team team : teams) {
                                team.unregister();
                        }
                }

                private Object getChangePacket(Objective obj, String entry, int score) {
                        try {
                                Object packet = ReflectUtil.getNMSClass(NMSClass.PACKET_SCORE).newInstance();
                                NMSClass clazz = NMSClass.ENUM_ACTION;
                                String[] keys = "a, b, c, d".split(", ");
                                Object[] values;
                                if(clazz.name != null)
                                    values = new Object[] {entry, obj.getName(), score,
                                            ReflectUtil.getField(ReflectUtil.getNMSClass(clazz),
                                                            null, "CHANGE")};
                                else values = new Object[] {entry, obj.getName(), score, 0};
                                for(int i=0;i<keys.length;i++)
                                        ReflectUtil.setField(packet.getClass(), packet, keys[i], values[i]);
                                return packet;
                        } catch (Exception ex) {
                                ex.printStackTrace();
                                return null;
                        }
                }

                private Object getRemovePacket(Objective obj, String entry) {
                        Object handle = ReflectUtil.getHandle(obj);
                        try {
                            Class<?> objective = ReflectUtil.getNMSClassByName("ScoreboardObjective");
                            if(ReflectUtil.isAbove("v1_8_R1"))
                                return ReflectUtil.getNMSClass(NMSClass.PACKET_SCORE)
                                                .getConstructor(String.class, objective)
                                                .newInstance(entry, handle);
                            else {
                                return ReflectUtil.getNMSClass(NMSClass.PACKET_SCORE)
                                        .getConstructor(String.class).newInstance(entry);
                            }
                        } catch (Exception ex) {
                                ex.printStackTrace();
                                return null;
                        }
                }

                public void start() {
                        board = Bukkit.getScoreboardManager().getNewScoreboard();
                        obj = board.registerNewObjective("Board", "dummy");
                        obj.setDisplaySlot(DisplaySlot.SIDEBAR);
                }
        }

        private static enum NMSClass {

            PACKET_OBJECTIVE(new String[0], "PacketPlayOutScoreboardObjective"),
            ENUM_HEALTH_DISPLAY(new String[] {"v1_8_R1"}, null, "EnumScoreboardHealthDisplay"),
            PACKET_SCORE(new String[0], "PacketPlayOutScoreboardScore"),
            ENUM_ACTION(new String[] {"v1_8_R1"}, null, "EnumScoreboardAction");

            String name;

            NMSClass(String[] versions, String... s) {
                if(versions.length == 0)
                    name = s[0];
                for(int i=versions.length - 1;i>=0;i--) {
                    if(ReflectUtil.isAbove(versions[i]))
                        name = s[i + 1];
                }
            }
            /*PacketPlayOutScoreboardObjective
            EnumScoreboardHealthDisplay
            PacketPlayOutScoreboardScore
            EnumScoreboardAction*/
        }

        private static class ReflectUtil {

                static final String CBK;
                static final boolean subclasses;

                static {
                        String pkg = Bukkit.getServer().getClass().getPackage().getName();
                        CBK = pkg.substring(pkg.lastIndexOf('.') + 1);
                        subclasses = isAbove("v1_8_R2");
                }

                static boolean isAbove(String version) {
                    String[] s0 = CBK.split("_");
                    int i0 = Integer.parseInt(s0[0].substring(1));
                    int j0 = Integer.parseInt(s0[1]);
                    int k0 = Integer.parseInt(s0[2].substring(1));

                    String[] s1 = version.split("_");

                    int i1 = Integer.parseInt(s1[0].substring(1));
                    int j1 = Integer.parseInt(s1[1]);
                    int k1 = Integer.parseInt(s1[2].substring(1));
                    return i0 > i1 ? true : i0 == i1 ? (j0 > j1 ? true : j0 == j1 ? k0 >= k1  : false) : false;
                }

                static Class<?> getNMSClassByName(String name) {
                        if(subclasses){
                                //From v1_8_R2, some classes have become subclasses
                                if(name.equals("EnumScoreboardAction"))
                                        name = "PacketPlayOutScoreboardScore$" + name;
                                else if(name.equals("EnumScoreboardHealthDisplay"))
                                        name = "IScoreboardCriteria$" + name;
                        }
                        try {
                                return Class.forName("net.minecraft.server." + CBK + "." + name);
                        } catch (ClassNotFoundException ex) {
                                ex.printStackTrace();
                                return null;
                        }
                }

                static Class<?> getNMSClass(NMSClass clazz) {
                    return getNMSClassByName(clazz.name);
                }

                static Object getField(Class<?> clazz, Object instance, String field) {
                        try {
                                Field f = clazz.getDeclaredField(field);
                                f.setAccessible(true);
                                return f.get(instance);
                        } catch (Exception ex) {
                                ex.printStackTrace();
                                return null;
                        }
                }

                static void setField(Class<?> clazz, Object instance, String field, Object value) {
                        try {
                                Field f = clazz.getDeclaredField(field);
                                f.setAccessible(true);
                                f.set(instance, value);
                        } catch (Exception ex) {
                                ex.printStackTrace();
                        }
                }

                static Object getHandle(Object obj) {
                        try {
                                return obj.getClass().getMethod("getHandle").invoke(obj);
                        } catch (Exception ex) {
                                ex.printStackTrace();
                                return null;
                        }
                }

                static void sendPacket(Player p, Object... packets) {
                        try {
                                Object handle = getHandle(p);
                                Object connection = handle.getClass().getDeclaredField("playerConnection").get(handle);
                                Method send = connection.getClass().getDeclaredMethod("sendPacket", getNMSClassByName("Packet"));
                                for(Object packet : packets)
                                        send.invoke(connection, packet);
                        } catch (Exception ex) {
                                ex.printStackTrace();
                        }
                }
        }
}