1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
5 // Protobuf representation of the UniquePosition class.
7 // Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
8 // any fields in this file.
12 option optimize_for = LITE_RUNTIME;
13 option retain_unknown_fields = true;
17 // A UniquePosition is a string of bytes.
19 // Unique positions are unique per-item, since they are guaranteed to end with a
20 // fixed-length suffix that is unique per-item. The position string may not end
23 // Prior to the suffix is a series of arbitrary bytes of arbitrary length.
24 // Items under the same parent are positioned relative to each other by a
25 // lexicographic comparison of their UniquePosition values.
26 message UniquePosition {
27 // The uncompressed string of bytes representing the position.
28 optional bytes value = 1;
30 // The client may choose to write a compressed position to this field instead
31 // of populating the 'value' above. If it chooses to use compression, the
32 // 'value' field above must be empty. The position value will be compressed
33 // with gzip and stored in the compressed_value field. The position's
34 // uncompressed length must be specified and written to the
35 // uncompressed_length field.
36 optional bytes compressed_value = 2;
37 optional uint64 uncompressed_length = 3;
39 // This encoding uses compression scheme designed especially for unique
40 // positions. It has the property that X < Y precisely when Compressed(X) <
41 // Compressed(Y), which is very useful when the most common operation is to
42 // compare these positions against each other. Their values may remain
43 // compressed in memory.
45 // The compression scheme is implemented and documented in
46 // sync/internal_api/base/unique_position.cc.
48 // As of M30, this is the preferred encoding. Newer clients may continue to
49 // populate the 'value' and 'compressed_value' fields to ensure backwards
50 // compatibility, but they will always try to read from this field first.
51 optional bytes custom_compressed_v1 = 4;