chromium/net/disk_cache/blockfile/addr.h

// Copyright 2012 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// This is an internal class that handles the address of a cache record.
// See net/disk_cache/disk_cache.h for the public interface of the cache.

#ifndef NET_DISK_CACHE_BLOCKFILE_ADDR_H_
#define NET_DISK_CACHE_BLOCKFILE_ADDR_H_

#include <stddef.h>
#include <stdint.h>

#include "base/notreached.h"
#include "net/base/net_export.h"
#include "net/disk_cache/blockfile/disk_format_base.h"

namespace disk_cache {

enum FileType { … };

const int kMaxBlockSize = …;
const int16_t kMaxBlockFile = …;
const int kMaxNumBlocks = …;
const int16_t kFirstAdditionalBlockFile = …;

// Defines a storage address for a cache record
//
// Header:
//   1000 0000 0000 0000 0000 0000 0000 0000 : initialized bit
//   0111 0000 0000 0000 0000 0000 0000 0000 : file type
//
// File type values:
//   0 = separate file on disk
//   1 = rankings block file
//   2 = 256 byte block file
//   3 = 1k byte block file
//   4 = 4k byte block file
//   5 = external files block file
//   6 = active entries block file
//   7 = evicted entries block file
//
// If separate file:
//   0000 1111 1111 1111 1111 1111 1111 1111 : file#  0 - 268,435,456 (2^28)
//
// If block file:
//   0000 1100 0000 0000 0000 0000 0000 0000 : reserved bits
//   0000 0011 0000 0000 0000 0000 0000 0000 : number of contiguous blocks 1-4
//   0000 0000 1111 1111 0000 0000 0000 0000 : file selector 0 - 255
//   0000 0000 0000 0000 1111 1111 1111 1111 : block#  0 - 65,535 (2^16)
//
// Note that an Addr can be used to "point" to a variety of different objects,
// from a given type of entry to random blobs of data. Conceptually, an Addr is
// just a number that someone can inspect to find out how to locate the desired
// record. Most users will not care about the specific bits inside Addr, for
// example, what parts of it point to a file number; only the code that has to
// select a specific file would care about those specific bits.
//
// From a general point of view, an Addr has a total capacity of 2^24 entities,
// in that it has 24 bits that can identify individual records. Note that the
// address space is bigger for independent files (2^28), but that would not be
// the general case.
class NET_EXPORT_PRIVATE Addr { … };

}  // namespace disk_cache

#endif  // NET_DISK_CACHE_BLOCKFILE_ADDR_H_