c769403631
This CL allows aapt2 to obfuscate resource paths within the output apk and move resources to shorter obfuscated paths. This reduces apk size when there is a large number of resources since the path metadata exists in 4 places in the apk. This CL adds two arguments to aapt2, one to enable resource path obfuscation and one to point to a path to output the path map to (for later debugging). Test: make aapt2_tests Bug: b/75965637 Change-Id: I9cacafe1d17800d673566b2d61b0b88f3fb8d60c
113 lines
3.8 KiB
C++
113 lines
3.8 KiB
C++
/*
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* Copyright (C) 2018 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#include "optimize/ResourcePathShortener.h"
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#include <math.h>
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#include <unordered_set>
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#include "androidfw/StringPiece.h"
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#include "ResourceTable.h"
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#include "ValueVisitor.h"
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static const std::string base64_chars =
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"ABCDEFGHIJKLMNOPQRSTUVWXYZ"
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"abcdefghijklmnopqrstuvwxyz"
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"0123456789-_";
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namespace aapt {
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ResourcePathShortener::ResourcePathShortener(
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std::map<std::string, std::string>& path_map_out)
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: path_map_(path_map_out) {
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}
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std::string ShortenFileName(const android::StringPiece& file_path, int output_length) {
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std::size_t hash_num = std::hash<android::StringPiece>{}(file_path);
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std::string result = "";
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// Convert to (modified) base64 so that it is a proper file path.
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for (int i = 0; i < output_length; i++) {
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uint8_t sextet = hash_num & 0x3f;
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hash_num >>= 6;
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result += base64_chars[sextet];
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}
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return result;
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}
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// Calculate the optimal hash length such that an average of 10% of resources
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// collide in their shortened path.
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// Reference: http://matt.might.net/articles/counting-hash-collisions/
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int OptimalShortenedLength(int num_resources) {
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int num_chars = 2;
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double N = 64*64; // hash space when hash is 2 chars long
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double max_collisions = num_resources * 0.1;
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while (num_resources - N + N * pow((N - 1) / N, num_resources) > max_collisions) {
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N *= 64;
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num_chars++;
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}
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return num_chars;
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}
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std::string GetShortenedPath(const android::StringPiece& shortened_filename,
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const android::StringPiece& extension, int collision_count) {
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std::string shortened_path = "res/" + shortened_filename.to_string();
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if (collision_count > 0) {
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shortened_path += std::to_string(collision_count);
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}
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shortened_path += extension;
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return shortened_path;
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}
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bool ResourcePathShortener::Consume(IAaptContext* context, ResourceTable* table) {
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// used to detect collisions
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std::unordered_set<std::string> shortened_paths;
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std::unordered_set<FileReference*> file_refs;
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for (auto& package : table->packages) {
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for (auto& type : package->types) {
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for (auto& entry : type->entries) {
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for (auto& config_value : entry->values) {
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FileReference* file_ref = ValueCast<FileReference>(config_value->value.get());
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if (file_ref) {
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file_refs.insert(file_ref);
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}
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}
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}
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}
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}
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int num_chars = OptimalShortenedLength(file_refs.size());
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for (auto& file_ref : file_refs) {
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android::StringPiece res_subdir, actual_filename, extension;
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util::ExtractResFilePathParts(*file_ref->path, &res_subdir, &actual_filename, &extension);
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std::string shortened_filename = ShortenFileName(*file_ref->path, num_chars);
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int collision_count = 0;
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std::string shortened_path = GetShortenedPath(shortened_filename, extension, collision_count);
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while (shortened_paths.find(shortened_path) != shortened_paths.end()) {
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collision_count++;
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shortened_path = GetShortenedPath(shortened_filename, extension, collision_count);
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}
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shortened_paths.insert(shortened_path);
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path_map_.insert({*file_ref->path, shortened_path});
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file_ref->path = table->string_pool.MakeRef(shortened_path, file_ref->path.GetContext());
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}
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return true;
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}
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} // namespace aapt
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