1 /*
2  * Copyright (C) 2018 The Android Open Source Project
3  *
4  * Licensed under the Apache License, Version 2.0 (the "License");
5  * you may not use this file except in compliance with the License.
6  * You may obtain a copy of the License at
7  *
8  *      http://www.apache.org/licenses/LICENSE-2.0
9  *
10  * Unless required by applicable law or agreed to in writing, software
11  * distributed under the License is distributed on an "AS IS" BASIS,
12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13  * See the License for the specific language governing permissions and
14  * limitations under the License.
15  */
16 
17 #ifndef ART_LIBDEXFILE_DEX_COMPACT_OFFSET_TABLE_H_
18 #define ART_LIBDEXFILE_DEX_COMPACT_OFFSET_TABLE_H_
19 
20 #include <cstdint>
21 #include <vector>
22 
23 namespace art {
24 
25 // Compact offset table that aims to minimize size while still providing reasonable speed (10-20ns
26 // average time per lookup on host).
27 class CompactOffsetTable {
28  public:
29   // This value is coupled with the leb chunk bitmask. That logic must also be adjusted when the
30   // integer is modified.
31   static constexpr size_t kElementsPerIndex = 16;
32 
33   // Leb block format:
34   // [uint16_t] 16 bit mask for what indexes actually have a non zero offset for the chunk.
35   // [lebs] Up to 16 lebs encoded using leb128, one leb bit. The leb specifies how the offset
36   // changes compared to the previous index.
37 
38   class Accessor {
39    public:
40     // Read the minimum and table offsets from the data pointer.
41     explicit Accessor(const uint8_t* data_begin);
42 
43     Accessor(const uint8_t* data_begin, uint32_t minimum_offset, uint32_t table_offset);
44 
45     // Return the offset for the index.
46     uint32_t GetOffset(uint32_t index) const;
47 
48    private:
49     const uint32_t* const table_;
50     const uint32_t minimum_offset_;
51     const uint8_t* const data_begin_;
52   };
53 
54   // Version that also serializes the min offset and table offset.
55   static void Build(const std::vector<uint32_t>& offsets, std::vector<uint8_t>* out_data);
56 
57   // Returned offsets are all relative to out_min_offset.
58   static void Build(const std::vector<uint32_t>& offsets,
59                     std::vector<uint8_t>* out_data,
60                     uint32_t* out_min_offset,
61                     uint32_t* out_table_offset);
62 
63   // 32 bit aligned for the offset table.
64   static constexpr size_t kAlignment = sizeof(uint32_t);
65 };
66 
67 }  // namespace art
68 
69 #endif  // ART_LIBDEXFILE_DEX_COMPACT_OFFSET_TABLE_H_
70