packfile: A copy operation cannot be bigger than 64kb

More info here: https://github.com/git/git/blob/f7466e94375b3be27f229c78873f0acf8301c0a5/diff-delta.c#L428

2 files changed, 35 insertions, 1 deletions

diff --git a/plumbing/format/packfile/delta_test.go b/plumbing/format/packfile/delta_test.go
index 43253b0..9ee3499 100644
--- a/plumbing/format/packfile/delta_test.go
+++ b/plumbing/format/packfile/delta_test.go
@@ -2,6 +2,7 @@ package packfile
 
 import (
 	"fmt"
+	"math/rand"
 
 	. "gopkg.in/check.v1"
 )
@@ -61,9 +62,25 @@ func (s *DeltaSuite) SetUpSuite(c *C) {
 			{"4", 400}, {"5", 23}},
 		target: []piece{{"1", 30}, {"2", 20}, {"7", 40}, {"4", 400},
 			{"5", 10}},
+	}, {
+		description: "A copy operation bigger tan 64kb",
+		base:        []piece{{bigRandStr, 1}, {"1", 200}},
+		target:      []piece{{bigRandStr, 1}},
 	}}
 }
 
+var bigRandStr = randStringBytes(100 * 1024)
+
+const letterBytes = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
+
+func randStringBytes(n int) string {
+	b := make([]byte, n)
+	for i := range b {
+		b[i] = letterBytes[rand.Intn(len(letterBytes))]
+	}
+	return string(b)
+}
+
 func (s *DeltaSuite) TestAddDelta(c *C) {
 	for _, t := range s.testCases {
 		baseBuf := genBytes(t.base)
diff --git a/plumbing/format/packfile/diff_delta.go b/plumbing/format/packfile/diff_delta.go
index e3438aa..60a04d9 100644
--- a/plumbing/format/packfile/diff_delta.go
+++ b/plumbing/format/packfile/diff_delta.go
@@ -15,6 +15,10 @@ import (
 const (
 	// Standard chunk size used to generate fingerprints
 	s = 16
+
+	// https://github.com/git/git/blob/f7466e94375b3be27f229c78873f0acf8301c0a5/diff-delta.c#L428
+	// Max size of a copy operation (64KB)
+	maxCopySize = 64 * 1024
 )
 
 // GetDelta returns an EncodedObject of type OFSDeltaObject. Base and Target object,
@@ -70,7 +74,20 @@ func DiffDelta(src []byte, tgt []byte) []byte {
 			ibuf.WriteByte(tgt[i])
 		} else {
 			encodeInsertOperation(ibuf, buf)
-			buf.Write(encodeCopyOperation(offset, l))
+
+			rl := l
+			aOffset := offset
+			for {
+				if rl < maxCopySize {
+					buf.Write(encodeCopyOperation(aOffset, rl))
+					break
+				}
+
+				buf.Write(encodeCopyOperation(aOffset, maxCopySize))
+				rl -= maxCopySize
+				aOffset += maxCopySize
+			}
+
 			i += l - 1
 		}
 	}