plumbing: commitgraph, Add generation v2 support

This PR adds in support for generation v2 support and a couple of new
walkers to match --date-order etc options on log. This PR also fixes a
bug in the chain code and adds more tests.

Signed-off-by: Andrew Thornton <art27@cantab.net>

1 files changed, 17 insertions, 0 deletions

diff --git a/plumbing/format/commitgraph/v2/commitgraph.go b/plumbing/format/commitgraph/v2/commitgraph.go
index 7c67b63..9c89cd9 100644
--- a/plumbing/format/commitgraph/v2/commitgraph.go
+++ b/plumbing/format/commitgraph/v2/commitgraph.go
@@ -2,6 +2,7 @@ package v2
 
 import (
 	"io"
+	"math"
 	"time"
 
 	"github.com/go-git/go-git/v5/plumbing"
@@ -19,10 +20,22 @@ type CommitData struct {
 	// Generation number is the pre-computed generation in the commit graph
 	// or zero if not available.
 	Generation uint64
+	// GenerationV2 stores the corrected commit date for the commits
+	// It combines the contents of the GDA2 and GDO2 sections of the commit-graph
+	// with the commit time portion of the CDAT section.
+	GenerationV2 uint64
 	// When is the timestamp of the commit.
 	When time.Time
 }
 
+// GenerationV2Data returns the corrected commit date for the commits
+func (c *CommitData) GenerationV2Data() uint64 {
+	if c.GenerationV2 == 0 || c.GenerationV2 == math.MaxUint64 {
+		return 0
+	}
+	return c.GenerationV2 - uint64(c.When.Unix())
+}
+
 // Index represents a representation of commit graph that allows indexed
 // access to the nodes using commit object hash
 type Index interface {
@@ -35,6 +48,10 @@ type Index interface {
 	GetCommitDataByIndex(i uint32) (*CommitData, error)
 	// Hashes returns all the hashes that are available in the index
 	Hashes() []plumbing.Hash
+	// HasGenerationV2 returns true if the commit graph has the corrected commit date data
+	HasGenerationV2() bool
+	// MaximumNumberOfHashes returns the maximum number of hashes within the index
+	MaximumNumberOfHashes() uint32
 
 	io.Closer
 }