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 #include "optimize/ResourcePathShortener.h"
18
19 #include <set>
20 #include <unordered_set>
21
22 #include "androidfw/StringPiece.h"
23
24 #include "ResourceTable.h"
25 #include "ValueVisitor.h"
26 #include "util/Util.h"
27
28
29 static const std::string base64_chars =
30 "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
31 "abcdefghijklmnopqrstuvwxyz"
32 "0123456789-_";
33
34 namespace aapt {
35
ResourcePathShortener(std::map<std::string,std::string> & path_map_out)36 ResourcePathShortener::ResourcePathShortener(
37 std::map<std::string, std::string>& path_map_out)
38 : path_map_(path_map_out) {
39 }
40
ShortenFileName(const android::StringPiece & file_path,int output_length)41 std::string ShortenFileName(const android::StringPiece& file_path, int output_length) {
42 std::size_t hash_num = std::hash<android::StringPiece>{}(file_path);
43 std::string result = "";
44 // Convert to (modified) base64 so that it is a proper file path.
45 for (int i = 0; i < output_length; i++) {
46 uint8_t sextet = hash_num & 0x3f;
47 hash_num >>= 6;
48 result += base64_chars[sextet];
49 }
50 return result;
51 }
52
53
54 // Return the optimal hash length such that at most 10% of resources collide in
55 // their shortened path.
56 // Reference: http://matt.might.net/articles/counting-hash-collisions/
OptimalShortenedLength(int num_resources)57 int OptimalShortenedLength(int num_resources) {
58 if (num_resources > 4000) {
59 return 3;
60 } else {
61 return 2;
62 }
63 }
64
GetShortenedPath(const android::StringPiece & shortened_filename,const android::StringPiece & extension,int collision_count)65 std::string GetShortenedPath(const android::StringPiece& shortened_filename,
66 const android::StringPiece& extension, int collision_count) {
67 std::string shortened_path = "res/" + shortened_filename.to_string();
68 if (collision_count > 0) {
69 shortened_path += std::to_string(collision_count);
70 }
71 shortened_path += extension;
72 return shortened_path;
73 }
74
75 // implement custom comparator of FileReference pointers so as to use the
76 // underlying filepath as key rather than the integer address. This is to ensure
77 // determinism of output for colliding files.
78 struct PathComparator {
operator ()aapt::PathComparator79 bool operator() (const FileReference* lhs, const FileReference* rhs) const {
80 return lhs->path->compare(*rhs->path);
81 }
82 };
83
Consume(IAaptContext * context,ResourceTable * table)84 bool ResourcePathShortener::Consume(IAaptContext* context, ResourceTable* table) {
85 // used to detect collisions
86 std::unordered_set<std::string> shortened_paths;
87 std::set<FileReference*, PathComparator> file_refs;
88 for (auto& package : table->packages) {
89 for (auto& type : package->types) {
90 for (auto& entry : type->entries) {
91 for (auto& config_value : entry->values) {
92 FileReference* file_ref = ValueCast<FileReference>(config_value->value.get());
93 if (file_ref) {
94 file_refs.insert(file_ref);
95 }
96 }
97 }
98 }
99 }
100 int num_chars = OptimalShortenedLength(file_refs.size());
101 for (auto& file_ref : file_refs) {
102 android::StringPiece res_subdir, actual_filename, extension;
103 util::ExtractResFilePathParts(*file_ref->path, &res_subdir, &actual_filename, &extension);
104
105 // Android detects ColorStateLists via pathname, skip res/color*
106 if (util::StartsWith(res_subdir, "res/color"))
107 continue;
108
109 std::string shortened_filename = ShortenFileName(*file_ref->path, num_chars);
110 int collision_count = 0;
111 std::string shortened_path = GetShortenedPath(shortened_filename, extension, collision_count);
112 while (shortened_paths.find(shortened_path) != shortened_paths.end()) {
113 collision_count++;
114 shortened_path = GetShortenedPath(shortened_filename, extension, collision_count);
115 }
116 shortened_paths.insert(shortened_path);
117 path_map_.insert({*file_ref->path, shortened_path});
118 file_ref->path = table->string_pool.MakeRef(shortened_path, file_ref->path.GetContext());
119 }
120 return true;
121 }
122
123 } // namespace aapt
124