Fix empty tree hash for SHA-256 repos

pkg/commands/models/commit.go

@@ -8,8 +8,18 @@ import (
 	"github.com/stefanhaller/git-todo-parser/todo"
 )
 
-// Special commit hash for empty tree object
+// Empty tree object hashes for SHA-1 and SHA-256 repos.
 const EmptyTreeCommitHash = "4b825dc642cb6eb9a060e54bf8d69288fbee4904"
+const EmptyTreeCommitHashSHA256 = "6ef19b41225c5369f1c104d45d8d85efa9b057b53b14b4b9b939dd74decc5321"
+
+// EmptyTreeHashForFormat returns the empty tree hash for the given object
+// format ("sha1" or "sha256"). Falls back to SHA-1 for unknown formats.
+func EmptyTreeHashForFormat(objectFormat string) string {
+	if objectFormat == "sha256" {
+		return EmptyTreeCommitHashSHA256
+	}
+	return EmptyTreeCommitHash
+}
 
 type CommitStatus uint8
 
@@ -122,11 +132,22 @@ func (c *Commit) ShortRefName() string {
 
 func (c *Commit) ParentRefName() string {
 	if c.IsFirstCommit() {
-		return EmptyTreeCommitHash
+		return c.emptyTreeHash()
 	}
 	return c.RefName() + "^"
 }
 
+// emptyTreeHash returns the empty tree object hash matching this repo's
+// object format. This is only called when IsFirstCommit() is true, so the
+// commit comes from git log with a full-length hash (40 for SHA-1, 64 for
+// SHA-256).
+func (c *Commit) emptyTreeHash() string {
+	if len(c.Hash()) == len(EmptyTreeCommitHashSHA256) {
+		return EmptyTreeCommitHashSHA256
+	}
+	return EmptyTreeCommitHash
+}
+
 func (c *Commit) Parents() []string {
 	return lo.Map(c.parents, func(s *string, _ int) string { return *s })
 }

pkg/gui/presentation/graph/graph.go

@@ -32,9 +32,10 @@ type Pipe struct {
 }
 
 var (
-	highlightStyle      = style.FgLightWhite.SetBold()
-	EmptyTreeCommitHash = models.EmptyTreeCommitHash
-	StartCommitHash     = "START"
+	highlightStyle            = style.FgLightWhite.SetBold()
+	EmptyTreeCommitHash       = models.EmptyTreeCommitHash
+	EmptyTreeCommitHashSHA256 = models.EmptyTreeCommitHashSHA256
+	StartCommitHash           = "START"
 )
 
 func (self Pipe) left() int16 {
@@ -142,7 +143,11 @@ func getNextPipes(prevPipes []Pipe, commit *models.Commit, getStyle func(c *mode
 
 	var toHash *string
 	if commit.IsFirstCommit() {
-		toHash = &EmptyTreeCommitHash
+		if len(commit.Hash()) == len(EmptyTreeCommitHashSHA256) {
+			toHash = &EmptyTreeCommitHashSHA256
+		} else {
+			toHash = &EmptyTreeCommitHash
+		}
 	} else {
 		toHash = commit.ParentPtrs()[0]
 	}

pkg/gui/presentation/graph/graph_test.go

@@ -531,6 +531,19 @@ func TestGetNextPipes(t *testing.T) {
 				{fromPos: 1, toPos: 1, fromHash: pool("root"), toHash: pool(models.EmptyTreeCommitHash), kind: STARTS, style: &style.FgDefault},
 			},
 		},
+		// SHA-256 repo: first commit with a 64-char hash should use the SHA-256 empty tree hash
+		{
+			prevPipes: []Pipe{
+				{fromPos: 0, toPos: 0, fromHash: pool("a"), toHash: pool("ba741fcd02b3cd42187ee46d4bdbfed10bf8fdbb15ccf739cb9ce583aefe5cae"), kind: TERMINATES, style: &style.FgDefault},
+			},
+			commit: models.NewCommit(hashPool, models.NewCommitOpts{
+				Hash:    "ba741fcd02b3cd42187ee46d4bdbfed10bf8fdbb15ccf739cb9ce583aefe5cae",
+				Parents: []string{},
+			}),
+			expected: []Pipe{
+				{fromPos: 1, toPos: 1, fromHash: pool("ba741fcd02b3cd42187ee46d4bdbfed10bf8fdbb15ccf739cb9ce583aefe5cae"), toHash: pool(models.EmptyTreeCommitHashSHA256), kind: STARTS, style: &style.FgDefault},
+			},
+		},
 	}
 
 	oldColorLevel := color.ForceSetColorLevel(terminfo.ColorLevelMillions)