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- // 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.
- //
- // Protobuf representation of the UniquePosition class.
- // If you change or add any fields in this file, update proto_visitors.h and
- // potentially proto_enum_conversions.{h, cc}.
- syntax = "proto2";
- option java_multiple_files = true;
- option java_package = "org.chromium.components.sync.protocol";
- option optimize_for = LITE_RUNTIME;
- package sync_pb;
- // A UniquePosition is a string of bytes.
- //
- // Unique positions are unique per-item, since they are guaranteed to end with a
- // fixed-length suffix that is unique per-item. The position string may not end
- // with a '\0' byte.
- //
- // Prior to the suffix is a series of arbitrary bytes of arbitrary length.
- // Items under the same parent are positioned relative to each other by a
- // lexicographic comparison of their UniquePosition values.
- message UniquePosition {
- // History:
- //
- // Unique positions were first introduced in M28. This change was rolled out
- // in such a way that it would try to maintain backwards compatibilty with
- // clients that understood only the old int64-based positions.
- //
- // At first, clients supported only the 'value' field. This version never
- // made it to stable. We later added support for the 'compressed_value'
- // field, and clients would populate either one or the other.
- //
- // In M30, we added the custom_compressed_v1 representation. This
- // representation was better than the previous implementations in almost every
- // way. However, we could not use it right away, since older clients would
- // not understand it. We decided to write both the old-style ('value' or
- // 'custom_compressed') representation and the 'custom_compressed_v1'
- // repersentations to every protobuf during the transition period. Protobufs
- // written during this transition period would be readable by clients who
- // understand at least one of the two formats.
- //
- // In M33, we dropped support for writing the backwards-compatibility fields.
- // Protobufs written by this version or later are not be intelligible by
- // clients with version M29 or older. Those clients will end up making use of
- // the old int64 position fallback mechanism.
- // The uncompressed string of bytes representing the position.
- //
- // Deprecated. See history note above.
- optional bytes value = 1 [deprecated = true];
- // The client may choose to write a compressed position to this field instead
- // of populating the 'value' above. If it chooses to use compression, the
- // 'value' field above must be empty. The position value will be compressed
- // with gzip and stored in the compressed_value field. The position's
- // uncompressed length must be specified and written to the
- // uncompressed_length field.
- //
- // Deprecated. See history note above.
- optional bytes compressed_value = 2 [deprecated = true];
- optional uint64 uncompressed_length = 3 [deprecated = true];
- // This encoding uses compression scheme designed especially for unique
- // positions. It has the property that X < Y precisely when Compressed(X) <
- // Compressed(Y), which is very useful when the most common operation is to
- // compare these positions against each other. Their values may remain
- // compressed in memory.
- //
- // The compression scheme is implemented and documented in
- // sync/core_impl/base/unique_position.cc.
- //
- // As of M30, this is the preferred encoding. Newer clients may continue to
- // populate the 'value' and 'compressed_value' fields to ensure backwards
- // compatibility, but they will always try to read from this field first.
- optional bytes custom_compressed_v1 = 4;
- }
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