Changed DB to store node positions as Ordinals.

sync/internal_api/public/base/ordinal.h

 // Copyright (c) 2012 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 SYNC_INTERNAL_API_PUBLIC_BASE_ORDINAL_H_
 #define SYNC_INTERNAL_API_PUBLIC_BASE_ORDINAL_H_
 
 #include <algorithm>
 #include <cstddef>
 #include <string>
@@ skipping 60 matching lines @@
     EqualsFn();
 
     bool operator()(const Ordinal<Traits>& lhs,
                     const Ordinal<Traits>& rhs) const;
   };
 
   // Creates an Ordinal from the given string of bytes. The Ordinal
   // may be valid or invalid.
   explicit Ordinal(const std::string& bytes);
 
-  // Creates an invalid Ordinal.
+  // Creates a valid initial Ordinal.
   Ordinal();
 
   // Creates a valid initial Ordinal. This is called to create the first
   // element of Ordinal list (i.e. before we have any other values we can
   // generate from).
   static Ordinal CreateInitialOrdinal();
 
   // Returns true iff this Ordinal is valid.  This takes constant
   // time.
   bool IsValid() const;
@@ skipping 133 matching lines @@
                                            const Ordinal<Traits>& rhs) const {
   return lhs.Equals(rhs);
 }
 
 template <typename Traits>
 Ordinal<Traits>::Ordinal(const std::string& bytes)
     : bytes_(bytes),
       is_valid_(IsValidOrdinalBytes(bytes_)) {}
 
 template <typename Traits>
-Ordinal<Traits>::Ordinal() : is_valid_(false) {}
+Ordinal<Traits>::Ordinal()
+    : bytes_(Traits::kMinLength, kZeroDigit),
+      is_valid_(true){
+  bytes_[0] = kMidDigit;
+}
 
 template <typename Traits>
 Ordinal<Traits> Ordinal<Traits>::CreateInitialOrdinal() {
   std::string bytes(Traits::kMinLength, kZeroDigit);
   bytes[0] = kMidDigit;
   return Ordinal(bytes);
 }
 
 template <typename Traits>
 bool Ordinal<Traits>::IsValid() const {
@@ skipping 231 matching lines @@
   DCHECK_LT(midpoint, end_bytes);
 
   Ordinal<Traits> midpoint_ordinal(midpoint);
   DCHECK(midpoint_ordinal.IsValid());
   return midpoint_ordinal;
 }
 
 }  // namespace syncer
 
 #endif  // SYNC_INTERNAL_API_PUBLIC_BASE_ORDINAL_H_