DensePacketRegistryMap's internal hash map should size based on a power of 2

proxy/src/main/java/com/velocitypowered/proxy/network/registry/packet/DensePacketRegistryMap.java

@@ -17,6 +17,8 @@
 
 package com.velocitypowered.proxy.network.registry.packet;
 
+import static com.velocitypowered.proxy.util.MathUtil.nextHighestPowerOfTwo;
+
 import com.velocitypowered.api.network.ProtocolVersion;
 import com.velocitypowered.proxy.network.ProtocolUtils;
 import com.velocitypowered.proxy.network.packet.Packet;
@@ -41,11 +43,12 @@ public class DensePacketRegistryMap implements PacketRegistryMap {
 
   public DensePacketRegistryMap(Int2ObjectMap<PacketMapping<?>> mappings) {
     int size = mappings.keySet().stream().mapToInt(x -> x).max().orElse(0) + 1;
+    int hashTableSize = nextHighestPowerOfTwo(size);
 
     this.readersById = new PacketReader[size];
-    this.writersByClass = new PacketWriter[size * 2];
-    this.classesByKey = new Class[size * 2];
-    this.idsByKey = new int[size * 2];
+    this.writersByClass = new PacketWriter[hashTableSize];
+    this.classesByKey = new Class[hashTableSize];
+    this.idsByKey = new int[hashTableSize];
 
     for (PacketMapping<?> value : mappings.values()) {
       this.readersById[value.id] = value.reader;

proxy/src/main/java/com/velocitypowered/proxy/util/MathUtil.java

@@ -0,0 +1,39 @@
+/*
+ * Copyright (C) 2018 Velocity Contributors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package com.velocitypowered.proxy.util;
+
+public class MathUtil {
+
+  /**
+   * Rounds {@code v} to the next highest power of 2.
+   *
+   * @param v the number to round up
+   * @return the next-highest power of 2 from {@code v}
+   */
+  public static int nextHighestPowerOfTwo(int v) {
+    // https://graphics.stanford.edu/~seander/bithacks.html#RoundUpPowerOf2
+    v--;
+    v |= v >> 1;
+    v |= v >> 2;
+    v |= v >> 4;
+    v |= v >> 8;
+    v |= v >> 16;
+    v++;
+    return v;
+  }
+}