components/sync/base/node_ordinal.h

// Copyright 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 COMPONENTS_SYNC_BASE_NODE_ORDINAL_H_
#define COMPONENTS_SYNC_BASE_NODE_ORDINAL_H_

#include <stddef.h>
#include <stdint.h>

#include "components/sync/base/ordinal.h"

namespace syncer {

// A NodeOrdinal is an Ordinal whose internal value comes from the
// ordinal_in_parent field of SyncEntity (see sync.proto).  It uses
// the entire uint8_t range for backwards compatibility with the old
// int64_t-based positioning.

struct NodeOrdinalTraits {
  static const uint8_t kZeroDigit = 0;
  static const uint8_t kMaxDigit = UINT8_MAX;
  static const size_t kMinLength = 8;
};

using NodeOrdinal = Ordinal<NodeOrdinalTraits>;

static_assert(static_cast<char>(NodeOrdinal::kZeroDigit) == '\x00',
              "NodeOrdinal has incorrect zero digit");
static_assert(static_cast<char>(NodeOrdinal::kOneDigit) == '\x01',
              "NodeOrdinal has incorrect one digit");
static_assert(static_cast<char>(NodeOrdinal::kMidDigit) == '\x80',
              "NodeOrdinal has incorrect mid digit");
static_assert(static_cast<char>(NodeOrdinal::kMaxDigit) == '\xff',
              "NodeOrdinal has incorrect max digit");
static_assert(NodeOrdinal::kMidDigitValue == 128,
              "NodeOrdinal has incorrect mid digit value");
static_assert(NodeOrdinal::kMaxDigitValue == 255,
              "NodeOrdinal has incorrect max digit value");
static_assert(NodeOrdinal::kRadix == 256, "NodeOrdinal has incorrect radix");

// Converts an int64_t position (usually from the position_in_parent
// field of SyncEntity) to a NodeOrdinal.  This transformation
// preserves the ordering relation: a < b under integer ordering if
// and only if Int64ToNodeOrdinal(a) < Int64ToNodeOrdinal(b).
NodeOrdinal Int64ToNodeOrdinal(int64_t x);

// The inverse of Int64ToNodeOrdinal.  This conversion is, in general,
// lossy: NodeOrdinals can have arbitrary fidelity, while numeric
// positions contain only 64 bits of information (in fact, this is the
// reason we've moved away from them).
int64_t NodeOrdinalToInt64(const NodeOrdinal& ordinal);

}  // namespace syncer

#endif  // COMPONENTS_SYNC_BASE_NODE_ORDINAL_H_