2 files changed, 65 insertions, 5 deletions
diff --git a/plumbing/format/idxfile/writer.go b/plumbing/format/idxfile/writer.go
index 3c5a00e..ea54081 100644
--- a/plumbing/format/idxfile/writer.go
+++ b/plumbing/format/idxfile/writer.go
@@ -18,12 +18,16 @@ type Writer struct {
 	count    uint32
 	checksum plumbing.Hash
 	objects  objects
+	offset64 uint32
+	idx      *MemoryIndex
 }
 
 // Create index returns a filled MemoryIndex with the information filled by
 // the observer callbacks.
 func (w *Writer) CreateIndex() (*MemoryIndex, error) {
 	idx := new(MemoryIndex)
+	w.idx = idx
+
 	sort.Sort(w.objects)
 
 	// unmap all fans by default
@@ -60,13 +64,13 @@ func (w *Writer) CreateIndex() (*MemoryIndex, error) {
 
 		idx.Names[bucket] = append(idx.Names[bucket], o.Hash[:]...)
 
-		// TODO: implement 64 bit offsets
-		if o.Offset > math.MaxInt32 {
-			panic("64 bit offsets not implemented")
+		offset := o.Offset
+		if offset > math.MaxInt32 {
+			offset = w.addOffset64(offset)
 		}
 
 		buf.Truncate(0)
-		binary.WriteUint32(buf, uint32(o.Offset))
+		binary.WriteUint32(buf, uint32(offset))
 		idx.Offset32[bucket] = append(idx.Offset32[bucket], buf.Bytes()...)
 
 		buf.Truncate(0)
@@ -78,12 +82,23 @@ func (w *Writer) CreateIndex() (*MemoryIndex, error) {
 		idx.Fanout[j] = uint32(len(w.objects))
 	}
 
+	idx.Version = VersionSupported
 	idx.PackfileChecksum = w.checksum
-	// TODO: fill IdxChecksum
 
 	return idx, nil
 }
 
+func (w *Writer) addOffset64(pos uint64) uint64 {
+	buf := new(bytes.Buffer)
+	binary.WriteUint64(buf, pos)
+	w.idx.Offset64 = append(w.idx.Offset64, buf.Bytes()...)
+
+	index := uint64(w.offset64 | (1 << 31))
+	w.offset64++
+
+	return index
+}
+
 // Add appends new object data.
 func (w *Writer) Add(h plumbing.Hash, pos uint64, crc uint32) {
 	w.objects = append(w.objects, Entry{h, crc, pos})
diff --git a/plumbing/format/idxfile/writer_test.go b/plumbing/format/idxfile/writer_test.go
new file mode 100644
index 0000000..92d2046
--- /dev/null
+++ b/plumbing/format/idxfile/writer_test.go
@@ -0,0 +1,45 @@
+package idxfile_test
+
+import (
+	"bytes"
+	"io/ioutil"
+
+	"gopkg.in/src-d/go-git.v4/plumbing/format/idxfile"
+	"gopkg.in/src-d/go-git.v4/plumbing/format/packfile"
+
+	. "gopkg.in/check.v1"
+	"gopkg.in/src-d/go-git-fixtures.v3"
+)
+
+type IndexSuite struct {
+	fixtures.Suite
+}
+
+var _ = Suite(&IndexSuite{})
+
+func (s *IndexSuite) TestIndexWriter(c *C) {
+	f := fixtures.Basic().One()
+	scanner := packfile.NewScanner(f.Packfile())
+
+	obs := new(idxfile.Writer)
+	parser := packfile.NewParser(scanner, obs)
+
+	_, err := parser.Parse()
+	c.Assert(err, IsNil)
+
+	idx, err := obs.CreateIndex()
+	c.Assert(err, IsNil)
+
+	idxFile := f.Idx()
+	expected, err := ioutil.ReadAll(idxFile)
+	c.Assert(err, IsNil)
+	idxFile.Close()
+
+	buf := new(bytes.Buffer)
+	encoder := idxfile.NewEncoder(buf)
+	n, err := encoder.Encode(idx)
+	c.Assert(err, IsNil)
+	c.Assert(n, Equals, len(expected))
+
+	c.Assert(buf.Bytes(), DeepEquals, expected)
+}