Minor optimizations for MinecraftCompressorAndLengthEncoder and friends

No need to bounce around changing the writer index, we can just set the value directly.

Also pull out the handshake checks into a separate function, to improve inlining.

proxy/src/main/java/com/velocitypowered/proxy/protocol/ProtocolUtils.java

@@ -150,7 +150,7 @@ public enum ProtocolUtils {
       BinaryTagTypes.COMPOUND, BinaryTagTypes.INT_ARRAY, BinaryTagTypes.LONG_ARRAY};
   private static final QuietDecoderException BAD_VARINT_CACHED =
       new QuietDecoderException("Bad VarInt decoded");
-  private static final int[] VAR_INT_LENGTHS = new int[65];
+  private static final int[] VAR_INT_LENGTHS = new int[33];
 
   static {
     for (int i = 0; i <= 32; ++i) {
@@ -250,16 +250,15 @@ public enum ProtocolUtils {
   }
 
   /**
-   * Writes the specified {@code value} as a 21-bit Minecraft VarInt to the specified {@code buf}.
+   * Directly encodes a 21-bit Minecraft VarInt, ready to be written with {@link ByteBuf#writeMedium(int)}.
    * The upper 11 bits will be discarded.
    *
-   * @param buf   the buffer to read from
-   * @param value the integer to write
+   * @param value the value to encode
+   * @return the encoded value
    */
-  public static void write21BitVarInt(ByteBuf buf, int value) {
+  public static int encode21BitVarInt(int value) {
     // See https://steinborn.me/posts/performance/how-fast-can-you-write-a-varint/
-    int w = (value & 0x7F | 0x80) << 16 | ((value >>> 7) & 0x7F | 0x80) << 8 | (value >>> 14);
-    buf.writeMedium(w);
+    return (value & 0x7F | 0x80) << 16 | ((value >>> 7) & 0x7F | 0x80) << 8 | (value >>> 14);
   }
 
   public static String readString(ByteBuf buf) {

proxy/src/main/java/com/velocitypowered/proxy/protocol/netty/MinecraftCompressorAndLengthEncoder.java

@@ -46,7 +46,7 @@ public class MinecraftCompressorAndLengthEncoder extends MessageToByteEncoder<By
     if (uncompressed < threshold) {
       // Under the threshold, there is nothing to do.
       ProtocolUtils.writeVarInt(out, uncompressed + 1);
-      ProtocolUtils.writeVarInt(out, 0);
+      out.writeByte(0);
       out.writeBytes(msg);
     } else {
       handleCompressed(ctx, msg, out);
@@ -57,7 +57,7 @@ public class MinecraftCompressorAndLengthEncoder extends MessageToByteEncoder<By
       throws DataFormatException {
     int uncompressed = msg.readableBytes();
 
-    ProtocolUtils.write21BitVarInt(out, 0); // Dummy packet length
+    out.writeMedium(0); // Reserve the packet length
     ProtocolUtils.writeVarInt(out, uncompressed);
     ByteBuf compatibleIn = MoreByteBufUtils.ensureCompatible(ctx.alloc(), compressor, msg);
 
@@ -72,11 +72,8 @@ public class MinecraftCompressorAndLengthEncoder extends MessageToByteEncoder<By
       throw new DataFormatException("The server sent a very large (over 2MiB compressed) packet.");
     }
 
-    int writerIndex = out.writerIndex();
     int packetLength = out.readableBytes() - 3;
-    out.writerIndex(0);
-    ProtocolUtils.write21BitVarInt(out, packetLength); // Rewrite packet length
-    out.writerIndex(writerIndex);
+    out.setMedium(0, ProtocolUtils.encode21BitVarInt(packetLength)); // Rewrite packet length
   }
 
   @Override

proxy/src/main/java/com/velocitypowered/proxy/protocol/netty/MinecraftVarintFrameDecoder.java

@@ -83,9 +83,8 @@ public class MinecraftVarintFrameDecoder extends ByteToMessageDecoder {
 
     // try to read the length of the packet
     in.markReaderIndex();
-    int preIndex = in.readerIndex();
     int length = readRawVarInt21(in);
-    if (preIndex == in.readerIndex()) {
+    if (packetStart == in.readerIndex()) {
       return;
     }
     if (length < 0) {
@@ -94,6 +93,23 @@ public class MinecraftVarintFrameDecoder extends ByteToMessageDecoder {
 
     if (length > 0) {
       if (state == StateRegistry.HANDSHAKE && direction == ProtocolUtils.Direction.SERVERBOUND) {
+        if (validateServerboundHandshakePacket(in, length)) {
+          return;
+        }
+      }
+    }
+
+    // note that zero-length packets are ignored
+    if (length > 0) {
+      if (in.readableBytes() < length) {
+        in.resetReaderIndex();
+      } else {
+        out.add(in.readRetainedSlice(length));
+      }
+    }
+  }
+
+  private boolean validateServerboundHandshakePacket(ByteBuf in, int length) throws Exception {
     StateRegistry.PacketRegistry.ProtocolRegistry registry =
         state.getProtocolRegistry(direction, ProtocolVersion.MINIMUM_VERSION);
 
@@ -102,7 +118,7 @@ public class MinecraftVarintFrameDecoder extends ByteToMessageDecoder {
     // Index hasn't changed, we've read nothing
     if (index == in.readerIndex()) {
       in.resetReaderIndex();
-          return;
+      return true;
     }
     final int payloadLength = length - ProtocolUtils.varIntBytes(packetId);
 
@@ -124,19 +140,8 @@ public class MinecraftVarintFrameDecoder extends ByteToMessageDecoder {
       throw handleUnderflow(packet, expectedMaxLen, in.readableBytes());
     }
 
-
     in.readerIndex(index);
-      }
-    }
-
-    // note that zero-length packets are ignored
-    if (length > 0) {
-      if (in.readableBytes() < length) {
-        in.resetReaderIndex();
-      } else {
-        out.add(in.readRetainedSlice(length));
-      }
-    }
+    return false;
   }
 
   @Override

proxy/src/test/java/com/velocitypowered/proxy/protocol/ProtocolUtilsTest.java

@@ -17,6 +17,7 @@
 
 package com.velocitypowered.proxy.protocol;
 
+import static com.velocitypowered.proxy.protocol.ProtocolUtils.encode21BitVarInt;
 import static org.junit.jupiter.api.Assertions.assertArrayEquals;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 
@@ -83,7 +84,7 @@ public class ProtocolUtilsTest {
 
   private void writeReadTest3Bytes(ByteBuf buf, int test) {
     buf.clear();
-    ProtocolUtils.write21BitVarInt(buf, test);
+    buf.writeMedium(encode21BitVarInt(test));
     assertEquals(test, ProtocolUtils.readVarInt(buf));
   }