zip_cd_entry_map.h

/*
 * Copyright (C) 2020 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#pragma once

#include <stdint.h>

#include <map>
#include <memory>
#include <string_view>
#include <utility>

#include "zip_error.h"

// This class is the interface of the central directory entries map. The map
// helps to locate a particular cd entry based on the filename.
class CdEntryMapInterface {
 public:
  virtual ~CdEntryMapInterface() = default;
  // Adds an entry to the map. The |name| should internally points to the
  // filename field of a cd entry. And |start| points to the beginning of the
  // central directory. Returns 0 on success.
  virtual ZipError AddToMap(std::string_view name, const uint8_t* start) = 0;
  // For the zip entry |entryName|, finds the offset of its filename field in
  // the central directory. Returns a pair of [status, offset]. The value of
  // the status is 0 on success.
  virtual std::pair<ZipError, uint64_t> GetCdEntryOffset(std::string_view name,
                                                         const uint8_t* cd_start) const = 0;
  // Resets the iterator to the beginning of the map.
  virtual void ResetIteration() = 0;
  // Returns the [name, cd offset] of the current element. Also increments the
  // iterator to points to the next element. Returns an empty pair we have read
  // past boundary.
  virtual std::pair<std::string_view, uint64_t> Next(const uint8_t* cd_start) = 0;
};

/**
 * More space efficient string representation of strings in an mmaped zipped
 * file than std::string_view. Using std::string_view as an entry in the
 * ZipArchive hash table wastes space. std::string_view stores a pointer to a
 * string (on 64 bit, 8 bytes) and the length to read from that pointer,
 * 2 bytes. Because of alignment, the structure consumes 16 bytes, wasting
 * 6 bytes.
 *
 * ZipStringOffset stores a 4 byte offset from a fixed location in the memory
 * mapped file instead of the entire address, consuming 8 bytes with alignment.
 */
struct ZipStringOffset {
  uint32_t name_offset;
  uint16_t name_length;

  const std::string_view ToStringView(const uint8_t* start) const {
    return std::string_view{reinterpret_cast<const char*>(start + name_offset), name_length};
  }
};

// This implementation of CdEntryMap uses an array hash table. It uses less
// memory than std::map; and it's used as the default implementation for zip
// archives without zip64 extension.
class CdEntryMapZip32 : public CdEntryMapInterface {
 public:
  static std::unique_ptr<CdEntryMapInterface> Create(uint16_t num_entries);

  ZipError AddToMap(std::string_view name, const uint8_t* start) override;
  std::pair<ZipError, uint64_t> GetCdEntryOffset(std::string_view name,
                                                 const uint8_t* cd_start) const override;
  void ResetIteration() override;
  std::pair<std::string_view, uint64_t> Next(const uint8_t* cd_start) override;

 private:
  explicit CdEntryMapZip32(uint16_t num_entries);

  // We know how many entries are in the Zip archive, so we can have a
  // fixed-size hash table. We define a load factor of 0.75 and over
  // allocate so the maximum number entries can never be higher than
  // ((4 * UINT16_MAX) / 3 + 1) which can safely fit into a uint32_t.
  uint32_t hash_table_size_{0};
  std::unique_ptr<ZipStringOffset[], decltype(&free)> hash_table_{nullptr, free};

  // The position of element for the current iteration.
  uint32_t current_position_{0};
};

// This implementation of CdEntryMap uses a std::map
class CdEntryMapZip64 : public CdEntryMapInterface {
 public:
  static std::unique_ptr<CdEntryMapInterface> Create();

  ZipError AddToMap(std::string_view name, const uint8_t* start) override;
  std::pair<ZipError, uint64_t> GetCdEntryOffset(std::string_view name,
                                                 const uint8_t* cd_start) const override;
  void ResetIteration() override;
  std::pair<std::string_view, uint64_t> Next(const uint8_t* cd_start) override;

 private:
  CdEntryMapZip64() = default;

  std::map<std::string_view, uint64_t> entry_table_;

  std::map<std::string_view, uint64_t>::iterator iterator_;
};