Reapply "Optimize varint writing"

Inspired by the approach described at the bottom of https://richardstartin.github.io/posts/dont-use-protobuf-for-telemetry

Given that we do a lot of varint writing as well, this should provide a small performance boost for larger/complex packets whilst not regressing hard on smaller packets.

This includes a test to ensure that the behavior is as expected and fixes the initialization loop so that the correct results will be given. Much thanks to @octylFractal for acting as my duck while trying to figure this out.

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

@@ -62,6 +62,17 @@ public enum ProtocolUtils {
   private static final int DEFAULT_MAX_STRING_SIZE = 65536; // 64KiB
   private static final QuietDecoderException BAD_VARINT_CACHED =
       new QuietDecoderException("Bad varint decoded");
+  private static final int[] VAR_INT_BYTE_LENGTHS = new int[33];
+
+  static {
+    // Inspired by https://richardstartin.github.io/posts/dont-use-protobuf-for-telemetry
+    // This has been slightly modified in that we reduce the length to 32-bit only, since Velocity
+    // doesn't look at any part of the Minecraft protocol that requires us to look at VarLongs.
+    for (int i = 0; i <= 32; ++i) {
+      VAR_INT_BYTE_LENGTHS[i] = (int) Math.ceil((31d - (i - 1)) / 7d);
+    }
+    VAR_INT_BYTE_LENGTHS[32] = 1; // Special case for the number 0.
+  }
 
   /**
    * Reads a Minecraft-style VarInt from the specified {@code buf}.
@@ -97,21 +108,22 @@ public enum ProtocolUtils {
     return Integer.MIN_VALUE;
   }
 
+  public static int varintBytes(int value) {
+    return VAR_INT_BYTE_LENGTHS[Integer.numberOfLeadingZeros(value)];
+  }
+
   /**
    * Writes a Minecraft-style VarInt to the specified {@code buf}.
    * @param buf the buffer to read from
    * @param value the integer to write
    */
   public static void writeVarInt(ByteBuf buf, int value) {
-    while (true) {
-      if ((value & 0xFFFFFF80) == 0) {
-        buf.writeByte(value);
-        return;
-      }
-
-      buf.writeByte(value & 0x7F | 0x80);
+    int length = varintBytes(value);
+    for (int i = 0; i < length; ++i) {
+      buf.writeByte(((byte) ((value & 0x7F) | 0x80)));
       value >>>= 7;
     }
+    buf.writeByte((byte) value);
   }
 
   public static String readString(ByteBuf buf) {

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

@@ -0,0 +1,42 @@
+package com.velocitypowered.proxy.protocol;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+public class ProtocolUtilsTest {
+
+  @Test
+  void negativeVarIntBytes() {
+    assertEquals(5, ProtocolUtils.varintBytes(-1));
+    assertEquals(5, ProtocolUtils.varintBytes(Integer.MIN_VALUE));
+  }
+
+  @Test
+  void zeroVarIntBytes() {
+    assertEquals(1, ProtocolUtils.varintBytes(0));
+    assertEquals(1, ProtocolUtils.varintBytes(1));
+  }
+
+  @Test
+  void ensureConsistencyAcrossNumberBits() {
+    for (int i = 0; i <= 31; i++) {
+      int number = (1 << i) - 1;
+      assertEquals(conventionalWrittenBytes(number), ProtocolUtils.varintBytes(number),
+          "mismatch with " + i + "-bit number");
+    }
+  }
+
+  private int conventionalWrittenBytes(int value) {
+    int wouldBeWritten = 0;
+    while (true) {
+      if ((value & ~0x7FL) == 0) {
+        wouldBeWritten++;
+        return wouldBeWritten;
+      } else {
+        wouldBeWritten++;
+        value >>>= 7;
+      }
+    }
+  }
+}