components/visitedlink/common/visitedlink_common.cc

   1 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "components/visitedlink/common/visitedlink_common.h"
   6
   7 #include <string.h>  // for memset()
   8
   9 #include "base/logging.h"
  10 #include "base/md5.h"
  11 #include "url/gurl.h"
  12
  13 namespace visitedlink {
  14
  15 const VisitedLinkCommon::Fingerprint VisitedLinkCommon::null_fingerprint_ = 0;
  16 const VisitedLinkCommon::Hash VisitedLinkCommon::null_hash_ = -1;
  17
  18 VisitedLinkCommon::VisitedLinkCommon()
  19     : hash_table_(NULL),
  20       table_length_(0) {
  21   memset(salt_, 0, sizeof(salt_));
  22 }
  23
  24 VisitedLinkCommon::~VisitedLinkCommon() {
  25 }
  26
  27 // FIXME: this uses linear probing, it should be replaced with quadratic
  28 // probing or something better. See VisitedLinkMaster::AddFingerprint
  29 bool VisitedLinkCommon::IsVisited(const char* canonical_url,
  30                                   size_t url_len) const {
  31   if (url_len == 0)
  32     return false;
  33   if (!hash_table_ || table_length_ == 0)
  34     return false;
  35   return IsVisited(ComputeURLFingerprint(canonical_url, url_len));
  36 }
  37
  38 bool VisitedLinkCommon::IsVisited(const GURL& url) const {
  39   return IsVisited(url.spec().data(), url.spec().size());
  40 }
  41
  42 bool VisitedLinkCommon::IsVisited(Fingerprint fingerprint) const {
  43   // Go through the table until we find the item or an empty spot (meaning it
  44   // wasn't found). This loop will terminate as long as the table isn't full,
  45   // which should be enforced by AddFingerprint.
  46   Hash first_hash = HashFingerprint(fingerprint);
  47   Hash cur_hash = first_hash;
  48   while (true) {
  49     Fingerprint cur_fingerprint = FingerprintAt(cur_hash);
  50     if (cur_fingerprint == null_fingerprint_)
  51       return false;  // End of probe sequence found.
  52     if (cur_fingerprint == fingerprint)
  53       return true;  // Found a match.
  54
  55     // This spot was taken, but not by the item we're looking for, search in
  56     // the next position.
  57     cur_hash++;
  58     if (cur_hash == table_length_)
  59       cur_hash = 0;
  60     if (cur_hash == first_hash) {
  61       // Wrapped around and didn't find an empty space, this means we're in an
  62       // infinite loop because AddFingerprint didn't do its job resizing.
  63       NOTREACHED();
  64       return false;
  65     }
  66   }
  67 }
  68
  69 // Uses the top 64 bits of the MD5 sum of the canonical URL as the fingerprint,
  70 // this is as random as any other subset of the MD5SUM.
  71 //
  72 // FIXME: this uses the MD5SUM of the 16-bit character version. For systems
  73 // where wchar_t is not 16 bits (Linux uses 32 bits, I think), this will not be
  74 // compatable. We should define explicitly what should happen here across
  75 // platforms, and convert if necessary (probably to UTF-16).
  76
  77 // static
  78 VisitedLinkCommon::Fingerprint VisitedLinkCommon::ComputeURLFingerprint(
  79     const char* canonical_url,
  80     size_t url_len,
  81     const uint8 salt[LINK_SALT_LENGTH]) {
  82   DCHECK(url_len > 0) << "Canonical URLs should not be empty";
  83
  84   base::MD5Context ctx;
  85   base::MD5Init(&ctx);
  86   base::MD5Update(&ctx, base::StringPiece(reinterpret_cast<const char*>(salt),
  87                                           LINK_SALT_LENGTH));
  88   base::MD5Update(&ctx, base::StringPiece(canonical_url, url_len));
  89
  90   base::MD5Digest digest;
  91   base::MD5Final(&digest, &ctx);
  92
  93   // This is the same as "return *(Fingerprint*)&digest.a;" but if we do that
  94   // direct cast the alignment could be wrong, and we can't access a 64-bit int
  95   // on arbitrary alignment on some processors. This reinterpret_casts it
  96   // down to a char array of the same size as fingerprint, and then does the
  97   // bit cast, which amounts to a memcpy. This does not handle endian issues.
  98   return bit_cast<Fingerprint, uint8[8]>(
  99       *reinterpret_cast<uint8(*)[8]>(&digest.a));
 100 }
 101
 102 }  // namespace visitedlink