3 files changed, 18 insertions, 0 deletions
diff --git a/plumbing/object/commitnode.go b/plumbing/object/commitnode.go
index 22927f4..ce25487 100644
--- a/plumbing/object/commitnode.go
+++ b/plumbing/object/commitnode.go
@@ -25,6 +25,9 @@ type CommitNode interface {
 	ParentNode(i int) (CommitNode, error)
 	// ParentHashes returns hashes of the parent commits for a specified node
 	ParentHashes() []plumbing.Hash
+	// Generation returns the generation of the commit for reachability analysis.
+	// Objects with newer generation are not reachable from objects of older generation.
+	Generation() uint64
 	// Commit returns the full commit object from the node
 	Commit() (*Commit, error)
 }
diff --git a/plumbing/object/commitnode_graph.go b/plumbing/object/commitnode_graph.go
index b2a6f57..ac790ab 100644
--- a/plumbing/object/commitnode_graph.go
+++ b/plumbing/object/commitnode_graph.go
@@ -110,6 +110,13 @@ func (c *graphCommitNode) ParentHashes() []plumbing.Hash {
 	return c.commitData.ParentHashes
 }
 
+func (c *graphCommitNode) Generation() uint64 {
+	// If the commit-graph file was generated with older Git version that
+	// set the generation to zero for every commit the generation assumption
+	// is still valid. It is just less useful.
+	return uint64(c.commitData.Generation)
+}
+
 func (c *graphCommitNode) Commit() (*Commit, error) {
 	return GetCommit(c.gci.s, c.hash)
 }
diff --git a/plumbing/object/commitnode_object.go b/plumbing/object/commitnode_object.go
index 52316f8..9ac42d2 100644
--- a/plumbing/object/commitnode_object.go
+++ b/plumbing/object/commitnode_object.go
@@ -1,6 +1,7 @@
 package object
 
 import (
+	"math"
 	"time"
 
 	"gopkg.in/src-d/go-git.v4/plumbing"
@@ -76,6 +77,13 @@ func (c *objectCommitNode) ParentHashes() []plumbing.Hash {
 	return c.commit.ParentHashes
 }
 
+func (c *objectCommitNode) Generation() uint64 {
+	// Commit nodes representing objects outside of the commit graph can never
+	// be reached by objects from the commit-graph thus we return the highest
+	// possible value.
+	return math.MaxUint64
+}
+
 func (c *objectCommitNode) Commit() (*Commit, error) {
 	return c.commit, nil
 }