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go/src/infra/libs/git/emptyObject.go

Index: go/src/infra/libs/git/emptyObject.go
diff --git a/go/src/infra/libs/git/emptyObject.go b/go/src/infra/libs/git/emptyObject.go
new file mode 100644
index 0000000000000000000000000000000000000000..035b6d3b84e30ff216b0c84f09e4ba4a0c4cb082
--- /dev/null
+++ b/go/src/infra/libs/git/emptyObject.go
@@ -0,0 +1,54 @@
+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package git
+
+import (
+	"fmt"
+)
+
+// Types ///////////////////////////////////////////////////////////////////////
+
+// EmptyObject is a placeholder Object which is not Internable, but can be
+// used to create a Complete() Tree. It can be of type "commit" or "blob".
+// It is never Complete().
+//
+// Implements:
+//	 fmt.Stringer
+//	 Object
+type EmptyObject struct {
+	id  *ObjectID
+	typ ObjectType
+}
+
+// Constructors  ///////////////////////////////////////////////////////////////
+
+// NewEmptyObject returns an Object of type |typ| with no data. For "tree"
+// types, this returns an empty Tree (which is mutable). All other types are an
+// EmptyObject.
+func NewEmptyObjectErr(typ ObjectType, id Identifiable) (Object, error) {
+	switch typ {
+	case BlobType, CommitType, TreeType:
+		return &EmptyObject{id.ID(), typ}, nil
+	default:
+		return nil, fmt.Errorf("unknown type %s", typ)
+	}
+}
+
+func NewEmptyObject(typ ObjectType, id Identifiable) Object {
+	r, err := NewEmptyObjectErr(typ, id)
+	if err != nil {
+		panic(err)
+	}
+	return r
+}
+
+// Member functions ////////////////////////////////////////////////////////////
+
+func (o *EmptyObject) ID() *ObjectID    { return o.id }
+func (o *EmptyObject) Type() ObjectType { return o.typ }
+func (o *EmptyObject) Complete() bool   { return false }
+func (o *EmptyObject) String() string {
+	return fmt.Sprintf("EmptyObject(%s, %s)", o.ID(), o.Type())
+}