chromium/components/sync/model/string_ordinal.h

// Copyright 2012 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef COMPONENTS_SYNC_MODEL_STRING_ORDINAL_H_
#define COMPONENTS_SYNC_MODEL_STRING_ORDINAL_H_

#include <cstddef>
#include <cstdint>
#include <string>

namespace mojo {
template <typename DataViewType, typename T>
struct StructTraits;
}

namespace syncer {

namespace mojom {
class StringOrdinalDataView;
}

// WARNING, DO NOT USE! Use UniquePosition instead.
//
//
// A StringOrdinal is an object that can be used for ordering. The
// StringOrdinal class has an unbounded dense strict total order, which
// mean for any StringOrdinals a, b and c:
//
//  - a < b and b < c implies a < c (transitivity);
//  - exactly one of a < b, b < a and a = b holds (trichotomy);
//  - if a < b, there is a StringOrdinal x such that a < x < b (density);
//  - there are Ordinals<T> x and y such that x < a < y (unboundedness).
//
// This means that when StringOrdinal is used for sorting a list, if any
// item changes its position in the list, only its StringOrdinal value
// has to change to represent the new order, and all the other values
// can stay the same.
//
// A StringOrdinal is internally represented as an array of bytes, so it
// can be serialized to and deserialized from disk.
//
// A StringOrdinal is valid iff its corresponding string has at least
// kMinLength characters, does not contain any characters less than
// kZeroDigit or greater than kMaxDigit, is not all zero digits, and
// does not have any unnecessary trailing zero digits.
//
// Since StringOrdinals contain only printable characters, it is safe
// to store as a string in a protobuf.
class StringOrdinal { … };

bool operator==(const StringOrdinal& lhs, const StringOrdinal& rhs);

static_assert …;
static_assert …;
static_assert …;
static_assert …;
static_assert …;
static_assert …;
static_assert …;

}  // namespace syncer

#endif  // COMPONENTS_SYNC_MODEL_STRING_ORDINAL_H_