refactor: marshal/unmashal on index

internal/store/segment/block/file.go

@@ -431,8 +431,7 @@ func (b *FileBlock) persistIndex(ctx context.Context) error {
 	buf := make([]byte, length)
 	for i, index := range b.indexes {
 		off := length - (i+1)*v1IndexLength
-		binary.BigEndian.PutUint64(buf[off:off+8], uint64(index.offset))
-		binary.BigEndian.PutUint32(buf[off+8:off+12], uint32(index.length))
+		_, _ = index.MarshalTo(buf[off : off+v1IndexLength])
 	}
 
 	if _, err := b.f.WriteAt(buf, b.cap-int64(length)); err != nil {
@@ -468,10 +467,7 @@ func (b *FileBlock) loadIndexFromFile() error {
 	b.indexes = make([]index, num)
 	for i := range b.indexes {
 		off := length - (i+1)*v1IndexLength
-		b.indexes[i] = index{
-			offset: int64(binary.BigEndian.Uint64(data[off : off+8])),
-			length: int32(binary.BigEndian.Uint32(data[off+8 : off+12])),
-		}
+		b.indexes[i], _ = unmashalIndex(data[off : off+v1IndexLength])
 	}
 
 	return nil

internal/store/segment/block/index.go

@@ -14,6 +14,11 @@
 
 package block
 
+import (
+	"bytes"
+	"encoding/binary"
+)
+
 const (
 	v1IndexLength = 8 + 4
 )
@@ -22,3 +27,25 @@ type index struct {
 	offset int64
 	length int32
 }
+
+func (i index) MarshalTo(data []byte) (int, error) {
+	if len(data) < v1IndexLength {
+		// TODO(james.yin): correct error.
+		return 0, bytes.ErrTooLarge
+	}
+	binary.BigEndian.PutUint64(data[0:8], uint64(i.offset))
+	binary.BigEndian.PutUint32(data[8:12], uint32(i.length))
+	return v1IndexLength, nil
+}
+
+func unmashalIndex(data []byte) (index, error) {
+	if len(data) < v1IndexLength {
+		// TODO(james.yin): correct error.
+		return index{}, bytes.ErrTooLarge
+	}
+	i := index{
+		offset: int64(binary.BigEndian.Uint64(data[0:8])),
+		length: int32(binary.BigEndian.Uint32(data[8:12])),
+	}
+	return i, nil
+}