From 27c7fdd5a67eb082d3f44747e589be48c1b1011f Mon Sep 17 00:00:00 2001
From: Runxi Yu
Date: Thu, 26 Mar 2026 04:39:38 +0000
Subject: object/id: Empty tree

---
 object/id/algorithms.go    | 12 +++++++++++-
 object/id/objectid_test.go | 45 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 56 insertions(+), 1 deletion(-)

diff --git a/object/id/algorithms.go b/object/id/algorithms.go
index 404d3bbf..f3540c42 100644
--- a/object/id/algorithms.go
+++ b/object/id/algorithms.go
@@ -24,6 +24,7 @@ type algorithmDetails struct {
 	packHashID uint32
 	sum        func([]byte) ObjectID
 	new        func() hash.Hash
+	emptyTree  ObjectID
 }
 
 //nolint:gochecknoglobals
@@ -69,12 +70,15 @@ var (
 )
 
 func init() { //nolint:gochecknoinits
+	emptyTreeInput := []byte("tree 0\x00")
+
 	for algo := Algorithm(0); int(algo) < len(algorithmTable); algo++ {
-		info := algorithmTable[algo]
+		info := &algorithmTable[algo]
 		if info.name == "" {
 			continue
 		}
 
+		info.emptyTree = info.sum(emptyTreeInput)
 		algorithmByName[info.name] = algo
 		supportedAlgorithms = append(supportedAlgorithms, algo)
 	}
@@ -135,6 +139,12 @@ func (algo Algorithm) New() (hash.Hash, error) {
 	return newFn(), nil
 }
 
+// EmptyTree returns the object ID of an empty tree ("tree 0\x00") for this
+// algorithm.
+func (algo Algorithm) EmptyTree() ObjectID {
+	return algo.info().emptyTree
+}
+
 func (algo Algorithm) info() algorithmDetails {
 	return algorithmTable[algo]
 }
diff --git a/object/id/objectid_test.go b/object/id/objectid_test.go
index 2559be0a..dc25832b 100644
--- a/object/id/objectid_test.go
+++ b/object/id/objectid_test.go
@@ -182,3 +182,48 @@ func TestAlgorithmSum(t *testing.T) {
 		t.Fatalf("sha1 and sha256 should differ")
 	}
 }
+
+func TestAlgorithmEmptyTree(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		algo objectid.Algorithm
+		want string
+	}{
+		{
+			name: "sha1",
+			algo: objectid.AlgorithmSHA1,
+			want: "4b825dc642cb6eb9a060e54bf8d69288fbee4904",
+		},
+		{
+			name: "sha256",
+			algo: objectid.AlgorithmSHA256,
+			want: "6ef19b41225c5369f1c104d45d8d85efa9b057b53b14b4b9b939dd74decc5321",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			got := tt.algo.EmptyTree()
+			if got.Algorithm() != tt.algo {
+				t.Fatalf("EmptyTree() algorithm = %v, want %v", got.Algorithm(), tt.algo)
+			}
+
+			if got.String() != tt.want {
+				t.Fatalf("EmptyTree() = %q, want %q", got.String(), tt.want)
+			}
+		})
+	}
+}
+
+func TestUnknownAlgorithmEmptyTree(t *testing.T) {
+	t.Parallel()
+
+	got := objectid.AlgorithmUnknown.EmptyTree()
+	if got != (objectid.ObjectID{}) {
+		t.Fatalf("EmptyTree() for unknown algorithm = %#v, want zero value", got)
+	}
+}
-- 
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