Initial commit of sync engine code to browser/sync....

chrome/browser/sync/syncable/syncable_changes_version.h

+// Copyright (c) 2009 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.
+
+#ifndef CHROME_BROWSER_SYNC_SYNCABLE_SYNCABLE_CHANGES_VERSION_H_
+#define CHROME_BROWSER_SYNC_SYNCABLE_SYNCABLE_CHANGES_VERSION_H_
+
+namespace syncable {
+
+// For the most part, the sync engine treats version numbers as opaque values.
+// However, there are parts of our code base that break this abstraction, and
+// depend on the following two invariants:
+//
+//  1.  CHANGES_VERSION is less than 0.
+//  2.  The server only issues positive version numbers.
+//
+// Breaking these abstractions makes some operations 10 times
+// faster.  If either of these invariants change, then those queries
+// must be revisited.
+
+enum {
+  CHANGES_VERSION = -1
+};
+
+#define CHANGES_VERSION_STRING "-1"
+
+}  // namespace syncable
+
+#endif  // CHROME_BROWSER_SYNC_SYNCABLE_SYNCABLE_CHANGES_VERSION_H_