From c928a8d246b99596f55f9b3480dddf2dd182aa89 Mon Sep 17 00:00:00 2001
From: Klaus Post <klauspost@gmail.com>
Date: Mon, 11 Dec 2023 14:21:02 +0100
Subject: [PATCH] Use a bit for extra copy length in TagCopy4.

---
 s2/encode_best.go | 6 +++++-
 s2/encode_go.go   | 7 ++++++-
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/s2/encode_best.go b/s2/encode_best.go
index 1e66d332c8..22e741e88f 100644
--- a/s2/encode_best.go
+++ b/s2/encode_best.go
@@ -717,7 +717,11 @@ emitRemainder:
 func emitCopySize(offset, length int) int {
 	if offset >= 65536 {
 		// 3 Byte offset + Variable length (base length 4).
-		return 3 + emitRepeatSize(offset, length-3)
+		length -= 3
+		if length > 28 {
+			length -= 28
+		}
+		return 3 + emitRepeatSize(offset, length)
 	}
 
 	// Offset no more than 2 bytes.
diff --git a/s2/encode_go.go b/s2/encode_go.go
index 8fe687b702..967611d371 100644
--- a/s2/encode_go.go
+++ b/s2/encode_go.go
@@ -199,8 +199,13 @@ func encodeLength60(dst []byte, tag uint8, length int) int {
 func emitCopy(dst []byte, offset, length int) int {
 	offset--
 	if offset >= 65536 {
+		length = length - 3
 		// Encode tag+length as up to 4 bytes.
-		n := encodeLength(dst, tagCopy4, length-3)
+		if length > 28 {
+			offset |= 1 << 23
+			length -= 28
+		}
+		n := encodeLength(dst, tagCopy4, length)
 		// Encode offset as 3 bytes.
 		dst[n+2] = uint8(offset >> 16)
 		dst[n+1] = uint8(offset >> 8)