[Sync] Generalize StringOrdinal to handle ordinal_in_parent field

sync/internal_api/public/base/node_ordinal.cc

+// 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.
+
+#include "sync/internal_api/public/base/node_ordinal.h"
+
+#include <algorithm>
+
+namespace syncer {
+
+NodeOrdinal Int64ToNodeOrdinal(int64 x) {
+  uint64 y = static_cast<uint64>(x);
+  y ^= 0x8000000000000000ULL;
+  std::string bytes(NodeOrdinal::kMinLength, '\x00');
+  if (y == 0) {
+    // 0 is a special case since |bytes| must not be all zeros.
+    bytes.push_back('\x80');
+  } else {
+    for (int i = 7; i >= 0; --i) {
+      bytes[i] = static_cast<uint8>(y);
+      y >>= 8;
+    }
+  }
+  NodeOrdinal ordinal(bytes);
+  DCHECK(ordinal.IsValid());
+  return ordinal;
+}
+
+int64 NodeOrdinalToInt64(const NodeOrdinal& ordinal) {
+  uint64 y = 0;
+  const std::string& s = ordinal.ToInternalValue();
+  size_t l = NodeOrdinal::kMinLength;
+  if (s.length() < l) {
+    NOTREACHED();
+    l = s.length();
+  }
+  for (size_t i = 0; i < l; ++i) {
+    const uint8 byte = s[l - i - 1];
+    y |= static_cast<uint64>(byte) << (i * 8);
+  }
+  y ^= 0x8000000000000000ULL;
+  // This is technically implementation-defined if y > INT64_MAX, so
+  // we're assuming that we're on a twos-complement machine.
+  return static_cast<int64>(y);
+}
+
+}  // namespace syncer