1 files changed, 62 insertions, 0 deletions
diff --git a/plumbing/format/idxfile/idxfile.go b/plumbing/format/idxfile/idxfile.go
new file mode 100644
index 0000000..8329c23
--- /dev/null
+++ b/plumbing/format/idxfile/idxfile.go
@@ -0,0 +1,62 @@
+package idxfile
+
+import "gopkg.in/src-d/go-git.v4/plumbing"
+
+const (
+	// VersionSupported is the only idx version supported.
+	VersionSupported = 2
+)
+
+var (
+	idxHeader = []byte{255, 't', 'O', 'c'}
+)
+
+// An Idxfile represents an idx file in memory.
+type Idxfile struct {
+	Version          uint32
+	Fanout           [255]uint32
+	ObjectCount      uint32
+	Entries          EntryList
+	PackfileChecksum [20]byte
+	IdxChecksum      [20]byte
+}
+
+// An Entry represents data about an object in the packfile: its hash,
+// offset and CRC32 checksum.
+type Entry struct {
+	Hash   plumbing.Hash
+	CRC32  uint32
+	Offset uint64
+}
+
+func (idx *Idxfile) Add(h plumbing.Hash, offset uint64, crc32 uint32) {
+	idx.Entries = append(idx.Entries, Entry{
+		Hash:   h,
+		Offset: offset,
+		CRC32:  crc32,
+	})
+}
+
+func (idx *Idxfile) isValid() bool {
+	fanout := idx.calculateFanout()
+	for k, c := range idx.Fanout {
+		if fanout[k] != c {
+			return false
+		}
+	}
+
+	return true
+}
+
+func (idx *Idxfile) calculateFanout() [256]uint32 {
+	fanout := [256]uint32{}
+	for _, e := range idx.Entries {
+		fanout[e.Hash[0]]++
+	}
+
+	for i := 1; i < 256; i++ {
+		fanout[i] += fanout[i-1]
+	}
+
+	return fanout
+}