safe_browsing::PrefixSet persistence code.

chrome/browser/safe_browsing/prefix_set.h

 // Copyright (c) 2011 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.
 //
 // A read-only set implementation for |SBPrefix| items.  Prefixes are
 // sorted and stored as 16-bit deltas from the previous prefix.  An
 // index structure provides quick random access, and also handles
 // cases where 16 bits cannot encode a delta.
 //
 // For example, the sequence {20, 25, 41, 65432, 150000, 160000} would
@@ skipping 19 matching lines @@
 // Experimenting with random selections of the above data, storage
 // size drops almost linearly as prefix count drops, until the index
 // overhead starts to become a problem a bit under 200k prefixes.  The
 // memory footprint gets worse than storing the raw prefix data around
 // 75k prefixes.  Fortunately, the actual memory footprint also falls.
 // If the prefix count increases the memory footprint should increase
 // approximately linearly.  The worst-case would be 2^16 items all
 // 2^16 apart, which would need 512k (versus 256k to store the raw
 // data).
 //
-// TODO(shess): Write serialization code.  Something like this should
-// work:
+// The on-disk format looks like:
 //         4 byte magic number
 //         4 byte version number
 //         4 byte |index_.size()|
 //         4 byte |deltas_.size()|
 //     n * 8 byte |&index_[0]..&index_[n]|
 //     m * 2 byte |&deltas_[0]..&deltas_[m]|
 //        16 byte digest
 
 #ifndef CHROME_BROWSER_SAFE_BROWSING_PREFIX_SET_H_
 #define CHROME_BROWSER_SAFE_BROWSING_PREFIX_SET_H_
 #pragma once
 
 #include <vector>
 
 #include "chrome/browser/safe_browsing/safe_browsing_util.h"
 
+class FilePath;
+
 namespace safe_browsing {
 
 class PrefixSet {
  public:
   explicit PrefixSet(const std::vector<SBPrefix>& sorted_prefixes);
   ~PrefixSet();
 
   // |true| if |prefix| was in |prefixes| passed to the constructor.
   bool Exists(SBPrefix prefix) const;
 
+  // Persist the set on disk.
+  static PrefixSet* LoadFile(const FilePath& filter_name);
+  bool WriteFile(const FilePath& filter_name) const;
+
   // Regenerate the vector of prefixes passed to the constructor into
   // |prefixes|.  Prefixes will be added in sorted order.
   void GetPrefixes(std::vector<SBPrefix>* prefixes);
 
  private:
   // Maximum delta that can be encoded in a 16-bit unsigned.
   static const unsigned kMaxDelta = 256 * 256;
 
   // Maximum number of consecutive deltas to encode before generating
   // a new index entry.  This helps keep the worst-case performance
   // for |Exists()| under control.
   static const size_t kMaxRun = 100;
 
+  // Helper for |LoadFile()|.  Steals the contents of |index| and

[lzheng, 2011/03/08 01:15:51]

Are there two spaces between "." and "Steals?"

[Scott Hess - ex-Googler, 2011/03/08 01:26:41]

Yeah?  All my prose has two spaces after sentence-ending punctuation.

+  // |deltas| using |swap()|.
+  PrefixSet(std::vector<std::pair<SBPrefix,size_t> > *index,
+            std::vector<uint16> *deltas);
+
   // Top-level index of prefix to offset in |deltas_|.  Each pair
   // indicates a base prefix and where the deltas from that prefix
   // begin in |deltas_|.  The deltas for a pair end at the next pair's
   // index into |deltas_|.
   std::vector<std::pair<SBPrefix,size_t> > index_;
 
   // Deltas which are added to the prefix in |index_| to generate
   // prefixes.  Deltas are only valid between consecutive items from
   // |index_|, or the end of |deltas_| for the last |index_| pair.
   std::vector<uint16> deltas_;
 
   DISALLOW_COPY_AND_ASSIGN(PrefixSet);
 };
 
 }  // namespace safe_browsing
 
 #endif  // CHROME_BROWSER_SAFE_BROWSING_PREFIX_SET_H_