[Sync] Generalize StringOrdinal to handle ordinal_in_parent field

sync/api/string_ordinal.h

+// Copyright (c) 2012 The Chromium Authors. All rights reserved.

[rlarocque, 2012/09/05 21:43:46]

Why is this file under sync/api, while other seemingly related files are under
sync/internal_api/public/base?

[akalin, 2012/09/06 19:25:14]

Because this is the only file that users of the "new" sync API should use. 
NodeOrdinal is definitely used only internally, and Ordinal in some sense is
internal-only.

+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef SYNC_API_STRING_ORDINAL_H_
+#define SYNC_API_STRING_ORDINAL_H_
+
+#include "base/basictypes.h"
+#include "sync/internal_api/public/base/ordinal.h"
+
+namespace syncer {
+
+// A StringOrdinal is an Ordinal with range 'a'-'z' (for
+// printability).  It should be used for data types that want to
+// maintain one or more orderings for nodes.
+
+struct StringOrdinalTraits {
+  static const uint8 kZeroDigit = 'a';
+  static const uint8 kMaxDigit = 'z';
+  static const size_t kMinLength = 1;
+};
+
+typedef Ordinal<StringOrdinalTraits> StringOrdinal;
+
+COMPILE_ASSERT(StringOrdinal::kZeroDigit == 'a',
+               StringOrdinalHasCorrectZeroDigit);
+COMPILE_ASSERT(StringOrdinal::kOneDigit == 'b',
+               StringOrdinalHasCorrectOneDigit);
+COMPILE_ASSERT(StringOrdinal::kMidDigit == 'n',
+               StringOrdinalHasCorrectMidDigit);
+COMPILE_ASSERT(StringOrdinal::kMaxDigit == 'z',
+               StringOrdinalHasCorrectMaxDigit);
+COMPILE_ASSERT(StringOrdinal::kMidDigitValue == 13,
+               StringOrdinalHasCorrectMidDigitValue);
+COMPILE_ASSERT(StringOrdinal::kMaxDigitValue == 25,
+               StringOrdinalHasCorrectMaxDigitValue);
+COMPILE_ASSERT(StringOrdinal::kRadix == 26,
+               StringOrdinalHasCorrectRadix);
+
+}  // namespace syncer
+
+#endif  // SYNC_API_STRING_ORDINAL_H_