1 files changed, 92 insertions, 0 deletions
diff --git a/storage/seekable/internal/index/index.go b/storage/seekable/internal/index/index.go
new file mode 100644
index 0000000..8e041bd
--- /dev/null
+++ b/storage/seekable/internal/index/index.go
@@ -0,0 +1,92 @@
+package index
+
+import (
+	"fmt"
+	"io"
+
+	"gopkg.in/src-d/go-git.v3/core"
+	"gopkg.in/src-d/go-git.v3/formats/idxfile"
+	"gopkg.in/src-d/go-git.v3/formats/packfile"
+)
+
+// Index is a database of objects and their offset in a packfile.
+// Objects are identified by their hash.
+type Index map[core.Hash]int64
+
+// NewFromIdx returns a new index from an idx file reader.
+func NewFromIdx(r io.Reader) (Index, error) {
+	d := idxfile.NewDecoder(r)
+	idx := &idxfile.Idxfile{}
+	err := d.Decode(idx)
+	if err != nil {
+		return nil, err
+	}
+
+	ind := make(Index)
+	for _, e := range idx.Entries {
+		if _, ok := ind[e.Hash]; ok {
+			return nil, fmt.Errorf("duplicated hash: %s", e.Hash)
+		}
+		ind[e.Hash] = int64(e.Offset)
+	}
+
+	return ind, nil
+}
+
+// NewFrompackfile returns a new index from a packfile reader.
+func NewFromPackfile(rs io.ReadSeeker) (Index, error) {
+	index := make(Index)
+
+	r := packfile.NewSeekable(rs)
+	p := packfile.NewParser(r)
+
+	count, err := p.ReadHeader()
+	if err != nil {
+		return nil, err
+	}
+
+	for i := 0; i < int(count); i++ {
+		offset, err := r.Offset()
+		if err != nil {
+			return nil, err
+		}
+
+		obj, err := p.ReadObject()
+		if err != nil {
+			return nil, err
+		}
+
+		err = r.Remember(offset, obj)
+		if err != nil {
+			return nil, err
+		}
+
+		err = index.Set(obj.Hash(), offset)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return index, nil
+}
+
+// Get returns the offset that an object has the packfile.
+func (i Index) Get(h core.Hash) (int64, error) {
+	o, ok := i[h]
+	if !ok {
+		return 0, core.ErrObjectNotFound
+	}
+
+	return o, nil
+}
+
+// Set adds a new hash-offset pair to the index, or substitutes an existing one.
+func (i Index) Set(h core.Hash, o int64) error {
+	if _, ok := i[h]; ok {
+		return fmt.Errorf("index.Set failed: duplicated key: %s", h)
+	}
+
+	i[h] = o
+
+	return nil
+}