Componentize visitedlinks to src/components/visitedlink

chrome/common/visitedlink_common.h

-// Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#ifndef CHROME_COMMON_VISITEDLINK_COMMON_H__
-#define CHROME_COMMON_VISITEDLINK_COMMON_H__
-
-#include <vector>
-
-#include "base/basictypes.h"
-
-class GURL;
-
-// number of bytes in the salt
-#define LINK_SALT_LENGTH 8
-
-// A multiprocess-safe database of the visited links for the browser. There
-// should be exactly one process that has write access (implemented by
-// VisitedLinkMaster), while all other processes should be read-only
-// (implemented by VisitedLinkSlave). These other processes add links by calling
-// the writer process to add them for it. The writer may also notify the readers
-// to replace their table when the table is resized.
-//
-// IPC is not implemented in these classes. This is done through callback
-// functions supplied by the creator of these objects to allow more flexibility,
-// especially for testing.
-//
-// This class defines the common base for these others. We implement accessors
-// for looking things up in the hash table, and for computing hash values and
-// fingerprints. Both the master and the slave inherit from this, and add their
-// own code to set up and change these values as their design requires. The
-// slave pretty much just sets up the shared memory and saves the pointer. The
-// master does a lot of work to manage the table, reading and writing it to and
-// from disk, and resizing it when it gets too full.
-//
-// To ask whether a page is in history, we compute a 64-bit fingerprint of the
-// URL. This URL is hashed and we see if it is in the URL hashtable. If it is,
-// we consider it visited. Otherwise, it is unvisited. Note that it is possible
-// to get collisions, which is the penalty for not storing all URL strings in
-// memory (which could get to be more than we want to have in memory). We use
-// a salt value for the links on one computer so that an attacker can not
-// manually create a link that causes a collision.
-class VisitedLinkCommon {
- public:
-  // A number that identifies the URL.
-  typedef uint64 Fingerprint;
-  typedef std::vector<Fingerprint> Fingerprints;
-
-  // A hash value of a fingerprint
-  typedef int32 Hash;
-
-  // A fingerprint or hash value that does not exist
-  static const Fingerprint null_fingerprint_;
-  static const Hash null_hash_;
-
-  VisitedLinkCommon();
-  virtual ~VisitedLinkCommon();
-
-  // Returns the fingerprint for the given URL.
-  Fingerprint ComputeURLFingerprint(const char* canonical_url,
-                                    size_t url_len) const {
-    return ComputeURLFingerprint(canonical_url, url_len, salt_);
-  }
-
-  // Looks up the given key in the table. The fingerprint for the URL is
-  // computed if you call one with the string argument. Returns true if found.
-  // Does not modify the hastable.
-  bool IsVisited(const char* canonical_url, size_t url_len) const;
-  bool IsVisited(const GURL& url) const;
-  bool IsVisited(Fingerprint fingerprint) const;
-
-#ifdef UNIT_TEST
-  // Returns statistics about DB usage
-  void GetUsageStatistics(int32* table_size,
-                          VisitedLinkCommon::Fingerprint** fingerprints) {
-    *table_size = table_length_;
-    *fingerprints = hash_table_;
-  }
-#endif
-
- protected:
-  // This structure is at the beginning of the shared memory so that the slaves
-  // can get stats on the table
-  struct SharedHeader {
-    // see goes into table_length_
-    uint32 length;
-
-    // goes into salt_
-    uint8 salt[LINK_SALT_LENGTH];
-  };
-
-  // Returns the fingerprint at the given index into the URL table. This
-  // function should be called instead of accessing the table directly to
-  // contain endian issues.
-  Fingerprint FingerprintAt(int32 table_offset) const {
-    if (!hash_table_)
-      return null_fingerprint_;
-    return hash_table_[table_offset];
-  }
-
-  // Computes the fingerprint of the given canonical URL. It is static so the
-  // same algorithm can be re-used by the table rebuilder, so you will have to
-  // pass the salt as a parameter. See the non-static version above if you
-  // want to use the current class' salt.
-  static Fingerprint ComputeURLFingerprint(const char* canonical_url,
-                                           size_t url_len,
-                                           const uint8 salt[LINK_SALT_LENGTH]);
-
-  // Computes the hash value of the given fingerprint, this is used as a lookup
-  // into the hashtable.
-  static Hash HashFingerprint(Fingerprint fingerprint, int32 table_length) {
-    if (table_length == 0)
-      return null_hash_;
-    return static_cast<Hash>(fingerprint % table_length);
-  }
-  // Uses the current hashtable.
-  Hash HashFingerprint(Fingerprint fingerprint) const {
-    return HashFingerprint(fingerprint, table_length_);
-  }
-
-  // pointer to the first item
-  VisitedLinkCommon::Fingerprint* hash_table_;
-
-  // the number of items in the hash table
-  int32 table_length_;
-
-  // salt used for each URL when computing the fingerprint
-  uint8 salt_[LINK_SALT_LENGTH];
-
- private:
-  DISALLOW_COPY_AND_ASSIGN(VisitedLinkCommon);
-};
-
-#endif  // CHROME_COMMON_VISITEDLINK_COMMON_H_