Index: chrome/browser/safe_browsing/prefix_set.h |
diff --git a/chrome/browser/safe_browsing/prefix_set.h b/chrome/browser/safe_browsing/prefix_set.h |
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+// 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 |
+// be stored as: |
+// A pair {20, 0} in |index_|. |
+// 5, 16, 65391 in |deltas_|. |
+// A pair {150000, 3} in |index_|. |
+// 10000 in |deltas_|. |
+// |index_.size()| will be 2, |deltas_.size()| will be 4. |
+// |
+// This structure is intended for storage of sparse uniform sets of |
+// prefixes of a certain size. As of this writing, my safe-browsing |
+// database contains: |
+// 653132 add prefixes |
+// 6446 are duplicates (from different chunks) |
+// 24301 w/in 2^8 of the prior prefix |
+// 622337 w/in 2^16 of the prior prefix |
+// 47 further than 2^16 from the prior prefix |
+// For this input, the memory usage is approximately 2 bytes per |
+// prefix, a bit over 1.2M. The bloom filter used 25 bits per prefix, |
+// a bit over 1.9M on this data. |
+// |
+// 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: |
+// 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" |
+ |
+namespace safe_browsing { |
+ |
+class PrefixSet { |
+ public: |
+ explicit PrefixSet(const std::vector<SBPrefix>& prefixes); |
+ |
+ // |true| if |prefix| was in |prefixes| passed to the constructor. |
+ bool Exists(SBPrefix prefix) const; |
+ |
+ 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; |
+ |
+ // 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_ |