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 <math.h>
20 #include <unordered_set>
21
22 #include "androidfw/StringPiece.h"
23
24 #include "ResourceTable.h"
25 #include "ValueVisitor.h"
26
27
28 static const std::string base64_chars =
29 "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
30 "abcdefghijklmnopqrstuvwxyz"
31 "0123456789-_";
32
33 namespace aapt {
34
ResourcePathShortener(std::map<std::string,std::string> & path_map_out)35 ResourcePathShortener::ResourcePathShortener(
36 std::map<std::string, std::string>& path_map_out)
37 : path_map_(path_map_out) {
38 }
39
ShortenFileName(const android::StringPiece & file_path,int output_length)40 std::string ShortenFileName(const android::StringPiece& file_path, int output_length) {
41 std::size_t hash_num = std::hash<android::StringPiece>{}(file_path);
42 std::string result = "";
43 // Convert to (modified) base64 so that it is a proper file path.
44 for (int i = 0; i < output_length; i++) {
45 uint8_t sextet = hash_num & 0x3f;
46 hash_num >>= 6;
47 result += base64_chars[sextet];
48 }
49 return result;
50 }
51
52
53 // Calculate the optimal hash length such that an average of 10% of resources
54 // collide in their shortened path.
55 // Reference: http://matt.might.net/articles/counting-hash-collisions/
OptimalShortenedLength(int num_resources)56 int OptimalShortenedLength(int num_resources) {
57 int num_chars = 2;
58 double N = 64*64; // hash space when hash is 2 chars long
59 double max_collisions = num_resources * 0.1;
60 while (num_resources - N + N * pow((N - 1) / N, num_resources) > max_collisions) {
61 N *= 64;
62 num_chars++;
63 }
64 return num_chars;
65 }
66
GetShortenedPath(const android::StringPiece & shortened_filename,const android::StringPiece & extension,int collision_count)67 std::string GetShortenedPath(const android::StringPiece& shortened_filename,
68 const android::StringPiece& extension, int collision_count) {
69 std::string shortened_path = "res/" + shortened_filename.to_string();
70 if (collision_count > 0) {
71 shortened_path += std::to_string(collision_count);
72 }
73 shortened_path += extension;
74 return shortened_path;
75 }
76
Consume(IAaptContext * context,ResourceTable * table)77 bool ResourcePathShortener::Consume(IAaptContext* context, ResourceTable* table) {
78 // used to detect collisions
79 std::unordered_set<std::string> shortened_paths;
80 std::unordered_set<FileReference*> file_refs;
81 for (auto& package : table->packages) {
82 for (auto& type : package->types) {
83 for (auto& entry : type->entries) {
84 for (auto& config_value : entry->values) {
85 FileReference* file_ref = ValueCast<FileReference>(config_value->value.get());
86 if (file_ref) {
87 file_refs.insert(file_ref);
88 }
89 }
90 }
91 }
92 }
93 int num_chars = OptimalShortenedLength(file_refs.size());
94 for (auto& file_ref : file_refs) {
95 android::StringPiece res_subdir, actual_filename, extension;
96 util::ExtractResFilePathParts(*file_ref->path, &res_subdir, &actual_filename, &extension);
97
98 std::string shortened_filename = ShortenFileName(*file_ref->path, num_chars);
99 int collision_count = 0;
100 std::string shortened_path = GetShortenedPath(shortened_filename, extension, collision_count);
101 while (shortened_paths.find(shortened_path) != shortened_paths.end()) {
102 collision_count++;
103 shortened_path = GetShortenedPath(shortened_filename, extension, collision_count);
104 }
105 shortened_paths.insert(shortened_path);
106 path_map_.insert({*file_ref->path, shortened_path});
107 file_ref->path = table->string_pool.MakeRef(shortened_path, file_ref->path.GetContext());
108 }
109 return true;
110 }
111
112 } // namespace aapt
113