openvcdiff | 311c714 | 2008-08-26 19:29:25 +0000 | [diff] [blame] | 1 | // Copyright 2006 Google Inc. |
| 2 | // Authors: Sanjay Ghemawat, Jeff Dean, Chandra Chereddi, Lincoln Smith |
| 3 | // |
| 4 | // Licensed under the Apache License, Version 2.0 (the "License"); |
| 5 | // you may not use this file except in compliance with the License. |
| 6 | // You may obtain a copy of the License at |
| 7 | // |
| 8 | // http://www.apache.org/licenses/LICENSE-2.0 |
| 9 | // |
| 10 | // Unless required by applicable law or agreed to in writing, software |
| 11 | // distributed under the License is distributed on an "AS IS" BASIS, |
| 12 | // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| 13 | // See the License for the specific language governing permissions and |
| 14 | // limitations under the License. |
| 15 | // |
| 16 | // Implementation of the Bentley/McIlroy algorithm for finding differences. |
| 17 | // Bentley, McIlroy. DCC 1999. Data Compression Using Long Common Strings. |
| 18 | // http://citeseer.ist.psu.edu/555557.html |
| 19 | |
| 20 | #ifndef OPEN_VCDIFF_BLOCKHASH_H_ |
| 21 | #define OPEN_VCDIFF_BLOCKHASH_H_ |
| 22 | |
| 23 | #include <config.h> |
openvcdiff | 28db807 | 2008-10-10 23:29:11 +0000 | [diff] [blame^] | 24 | #include <stddef.h> // size_t |
openvcdiff | 311c714 | 2008-08-26 19:29:25 +0000 | [diff] [blame] | 25 | #include <stdint.h> // uint32_t |
openvcdiff | 311c714 | 2008-08-26 19:29:25 +0000 | [diff] [blame] | 26 | #include <vector> |
| 27 | |
| 28 | namespace open_vcdiff { |
| 29 | |
| 30 | // A generic hash table which will be used to keep track of byte runs |
| 31 | // of size kBlockSize in both the incrementally processed target data |
| 32 | // and the preprocessed source dictionary. |
| 33 | // |
| 34 | // A custom hash table implementation is used instead of the standard |
| 35 | // hash_map template because we know that there will be exactly one |
| 36 | // entry in the BlockHash corresponding to each kBlockSize bytes |
| 37 | // in the source data, which makes certain optimizations possible: |
| 38 | // * The memory for the hash table and for all hash entries can be allocated |
| 39 | // in one step rather than incrementally for each insert operation. |
| 40 | // * A single integer can be used to represent both |
| 41 | // the index of the next hash entry in the chain |
| 42 | // and the position of the entry within the source data |
| 43 | // (== kBlockSize * block_number). This greatly reduces the size |
| 44 | // of a hash entry. |
| 45 | // |
| 46 | class BlockHash { |
| 47 | public: |
| 48 | // Block size as per Bentley/McIlroy; must be a power of two. |
| 49 | // |
| 50 | // Using (for example) kBlockSize = 4 guarantees that no match smaller |
| 51 | // than size 4 will be identified, that some matches having sizes |
| 52 | // 4, 5, or 6 may be identified, and that all matches |
| 53 | // having size 7 or greater will be identified (because any string of |
| 54 | // 7 bytes must contain a complete aligned block of 4 bytes.) |
| 55 | // |
| 56 | // Increasing kBlockSize by a factor of two will halve the amount of |
| 57 | // memory needed for the next block table, and will halve the setup time |
| 58 | // for a new BlockHash. However, it also doubles the minimum |
| 59 | // match length that is guaranteed to be found in FindBestMatch(), |
| 60 | // so that function will be less effective in finding matches. |
| 61 | // |
| 62 | // Computational effort in FindBestMatch (which is the inner loop of |
| 63 | // the encoding algorithm) will be proportional to the number of |
| 64 | // matches found, and a low value of kBlockSize will waste time |
| 65 | // tracking down small matches. On the other hand, if this value |
| 66 | // is set too high, no matches will be found at all. |
| 67 | // |
| 68 | // It is suggested that different values of kBlockSize be tried against |
| 69 | // a representative data set to find the best tradeoff between |
| 70 | // memory/CPU and the effectiveness of FindBestMatch(). |
| 71 | // |
| 72 | // If you change kBlockSize to a smaller value, please increase |
| 73 | // kMaxMatchesToCheck accordingly. |
| 74 | static const int kBlockSize = 32; |
| 75 | |
| 76 | // This class is used to store the best match found by FindBestMatch() |
| 77 | // and return it to the caller. |
| 78 | class Match { |
| 79 | public: |
| 80 | Match() : size_(0), source_offset_(-1), target_offset_(-1) { } |
| 81 | |
| 82 | void ReplaceIfBetterMatch(size_t candidate_size, |
| 83 | int candidate_source_offset, |
| 84 | int candidate_target_offset) { |
| 85 | if (candidate_size > size_) { |
| 86 | size_ = candidate_size; |
| 87 | source_offset_ = candidate_source_offset; |
| 88 | target_offset_ = candidate_target_offset; |
| 89 | } |
| 90 | } |
| 91 | |
| 92 | size_t size() const { return size_; } |
| 93 | int source_offset() const { return source_offset_; } |
| 94 | int target_offset() const { return target_offset_; } |
| 95 | |
| 96 | private: |
| 97 | // The size of the best (longest) match passed to ReplaceIfBetterMatch(). |
| 98 | size_t size_; |
| 99 | |
| 100 | // The source offset of the match, including the starting_offset_ |
| 101 | // of the BlockHash for which the match was found. |
| 102 | int source_offset_; |
| 103 | |
| 104 | // The target offset of the match. An offset of 0 corresponds to the |
| 105 | // data at target_start, which is an argument of FindBestMatch(). |
| 106 | int target_offset_; |
| 107 | |
| 108 | // Making these private avoids implicit copy constructor |
| 109 | // & assignment operator |
| 110 | Match(const Match&); // NOLINT |
| 111 | void operator=(const Match&); |
| 112 | }; |
| 113 | |
| 114 | // A BlockHash is created using a buffer of source data. The hash table |
| 115 | // will contain one entry for each kBlockSize-byte block in the |
| 116 | // source data. |
| 117 | // |
| 118 | // See the comments for starting_offset_, below, for a description of |
| 119 | // the starting_offset argument. For a hash of source (dictionary) data, |
| 120 | // starting_offset_ will be zero; for a hash of previously encoded |
| 121 | // target data, starting_offset_ will be equal to the dictionary size. |
| 122 | // |
| 123 | BlockHash(const char* source_data, size_t source_size, int starting_offset); |
| 124 | |
| 125 | ~BlockHash(); |
| 126 | |
| 127 | // Initializes the object before use. |
| 128 | // This method must be called after constructing a BlockHash object, |
| 129 | // and before any other method may be called. This is because |
| 130 | // Init() dynamically allocates hash_table_ and next_block_table_. |
| 131 | // Returns true if initialization succeeded, or false if an error occurred, |
| 132 | // in which case no other method except the destructor may then be used |
| 133 | // on the object. |
| 134 | // |
| 135 | // If populate_hash_table is true, then AddAllBlocks() will be called |
| 136 | // to populate the hash table. If populate_hash_table is false, then |
| 137 | // classes that inherit from BlockHash are expected to call AddBlock() |
| 138 | // to incrementally populate individual blocks of data. |
| 139 | // |
| 140 | bool Init(bool populate_hash_table); |
| 141 | |
| 142 | // In the context of the open-vcdiff encoder, BlockHash is used for two |
| 143 | // purposes: to hash the source (dictionary) data, and to hash |
| 144 | // the previously encoded target data. The main differences between |
| 145 | // a dictionary BlockHash and a target BlockHash are as follows: |
| 146 | // |
| 147 | // 1. The best_match->source_offset() returned from FindBestMatch() |
| 148 | // for a target BlockHash is computed in the following manner: |
| 149 | // the starting offset of the first byte in the target data |
| 150 | // is equal to the dictionary size. FindBestMatch() will add |
| 151 | // starting_offset_ to any best_match->source_offset() value it returns, |
| 152 | // in order to produce the correct offset value for a target BlockHash. |
| 153 | // 2. For a dictionary BlockHash, the entire data set is hashed at once |
| 154 | // when Init() is called with the parameter populate_hash_table = true. |
| 155 | // For a target BlockHash, because the previously encoded target data |
| 156 | // includes only the data seen up to the current encoding position, |
| 157 | // the data blocks are hashed incrementally as the encoding position |
| 158 | // advances, using AddOneIndexHash() and AddAllBlocksThroughIndex(). |
| 159 | // |
| 160 | // The following two factory functions can be used to create BlockHash |
| 161 | // objects for each of these two purposes. Each factory function calls |
| 162 | // the object constructor and also calls Init(). If an error occurs, |
| 163 | // NULL is returned; otherwise a valid BlockHash object is returned. |
| 164 | // Since a dictionary BlockHash is not expected to be modified after |
| 165 | // initialization, a const object is returned. |
| 166 | // The caller is responsible for deleting the returned object |
| 167 | // (using the C++ delete operator) once it is no longer needed. |
| 168 | static const BlockHash* CreateDictionaryHash(const char* dictionary_data, |
| 169 | size_t dictionary_size); |
| 170 | static BlockHash* CreateTargetHash(const char* target_data, |
| 171 | size_t target_size, |
| 172 | size_t dictionary_size); |
| 173 | |
| 174 | // This function will be called to add blocks incrementally to the target hash |
| 175 | // as the encoding position advances through the target data. It will be |
| 176 | // called for every kBlockSize-byte block in the target data, regardless |
| 177 | // of whether the block is aligned evenly on a block boundary. The |
| 178 | // BlockHash will only store hash entries for the evenly-aligned blocks. |
| 179 | // |
| 180 | void AddOneIndexHash(int index, uint32_t hash_value) { |
| 181 | if (index == NextIndexToAdd()) { |
| 182 | AddBlock(hash_value); |
| 183 | } |
| 184 | } |
| 185 | |
| 186 | // Calls AddBlock() for each kBlockSize-byte block in the range |
| 187 | // (last_block_added_ * kBlockSize, end_index), exclusive of the endpoints. |
| 188 | // If end_index <= the last index added (last_block_added_ * kBlockSize), |
| 189 | // this function does nothing. |
| 190 | // |
| 191 | // A partial block beginning anywhere up to (end_index - 1) is also added, |
| 192 | // unless it extends outside the end of the source data. Like AddAllBlocks(), |
| 193 | // this function computes the hash value for each of the blocks in question |
| 194 | // from scratch, so it is not a good option if the hash values have already |
| 195 | // been computed for some other purpose. |
| 196 | // |
| 197 | // Example: assume kBlockSize = 4, last_block_added_ = 1, and there are |
| 198 | // 14 bytes of source data. |
| 199 | // If AddAllBlocksThroughIndex(9) is invoked, then it will call AddBlock() |
| 200 | // only for block number 2 (at index 8). |
| 201 | // If, after that, AddAllBlocksThroughIndex(14) is invoked, it will not call |
| 202 | // AddBlock() at all, because block 3 (beginning at index 12) would |
| 203 | // fall outside the range of source data. |
| 204 | // |
| 205 | // VCDiffEngine::Encode (in vcdiffengine.cc) uses this function to |
| 206 | // add a whole range of data to a target hash when a COPY instruction |
| 207 | // is generated. |
| 208 | void AddAllBlocksThroughIndex(int end_index); |
| 209 | |
| 210 | // FindBestMatch takes a position within the unencoded target data |
| 211 | // (target_candidate_start) and the hash value of the kBlockSize bytes |
| 212 | // beginning at that position (hash_value). It attempts to find a matching |
| 213 | // set of bytes within the source (== dictionary) data, expanding |
| 214 | // the match both below and above the target block. It cannot expand |
| 215 | // the match outside the bounds of the source data, or below |
| 216 | // target_start within the target data, or past |
| 217 | // the end limit of (target_start + target_length). |
| 218 | // |
| 219 | // target_candidate_start is the start of the candidate block within the |
| 220 | // target data for which a match will be sought, while |
| 221 | // target_start (which is <= target_candidate_start) |
| 222 | // is the start of the target data that has yet to be encoded. |
| 223 | // |
| 224 | // If a match is found whose size is greater than the size |
| 225 | // of best_match, this function populates *best_match with the |
| 226 | // size, source_offset, and target_offset of the match found. |
| 227 | // best_match->source_offset() will contain the index of the start of the |
| 228 | // matching source data, plus starting_offset_ |
| 229 | // (see description of starting_offset_ for details); |
| 230 | // best_match->target_offset() will contain the offset of the match |
| 231 | // beginning with target_start = offset 0, such that |
| 232 | // 0 <= best_match->target_offset() |
| 233 | // <= (target_candidate_start - target_start); |
| 234 | // and best_match->size() will contain the size of the match. |
| 235 | // If no such match is found, this function leaves *best_match unmodified. |
| 236 | // |
| 237 | // On calling FindBestMatch(), best_match must |
| 238 | // point to a valid Match object, and cannot be NULL. |
| 239 | // The same Match object can be passed |
| 240 | // when calling FindBestMatch() on a different BlockHash object |
| 241 | // for the same candidate data block, in order to find |
| 242 | // the best match possible across both objects. For example: |
| 243 | // |
| 244 | // open_vcdiff::BlockHash::Match best_match; |
| 245 | // uint32_t hash_value = |
| 246 | // RollingHash<BlockHash::kBlockSize>::Hash(target_candidate_start); |
| 247 | // bh1.FindBestMatch(hash_value, |
| 248 | // target_candidate_start, |
| 249 | // target_start, |
| 250 | // target_length, |
| 251 | // &best_match); |
| 252 | // bh2.FindBestMatch(hash_value, |
| 253 | // target_candidate_start, |
| 254 | // target_start, |
| 255 | // target_length, |
| 256 | // &best_match); |
| 257 | // if (best_size >= 0) { |
| 258 | // // a match was found; its size, source offset, and target offset |
| 259 | // // can be found in best_match |
| 260 | // } |
| 261 | // |
| 262 | // hash_value is passed as a separate parameter from target_candidate_start, |
| 263 | // (rather than calculated within FindBestMatch) in order to take |
| 264 | // advantage of the rolling hash, which quickly calculates the hash value |
| 265 | // of the block starting at target_candidate_start based on |
| 266 | // the known hash value of the block starting at (target_candidate_start - 1). |
| 267 | // See vcdiffengine.cc for more details. |
| 268 | // |
| 269 | // Example: |
| 270 | // kBlockSize: 4 |
| 271 | // target text: "ANDREW LLOYD WEBBER" |
| 272 | // 1^ 5^2^ 3^ |
| 273 | // dictionary: "INSURANCE : LLOYDS OF LONDON" |
| 274 | // 4^ |
| 275 | // hashed dictionary blocks: |
| 276 | // "INSU", "RANC", "E : ", "LLOY", "DS O", "F LON" |
| 277 | // |
| 278 | // 1: target_start (beginning of unencoded data) |
| 279 | // 2: target_candidate_start (for the block "LLOY") |
| 280 | // 3: target_length (points one byte beyond the last byte of data.) |
| 281 | // 4: best_match->source_offset() (after calling FindBestMatch) |
| 282 | // 5: best_match->target_offset() (after calling FindBestMatch) |
| 283 | // |
| 284 | // Under these conditions, FindBestMatch will find a matching |
| 285 | // hashed dictionary block for "LLOY", and will extend the beginning of |
| 286 | // this match backwards by one byte, and the end of the match forwards |
| 287 | // by one byte, finding that the best match is " LLOYD" |
| 288 | // with best_match->source_offset() = 10 |
| 289 | // (offset of " LLOYD" in the source string), |
| 290 | // best_match->target_offset() = 6 |
| 291 | // (offset of " LLOYD" in the target string), |
| 292 | // and best_match->size() = 6. |
| 293 | // |
| 294 | void FindBestMatch(uint32_t hash_value, |
| 295 | const char* target_candidate_start, |
| 296 | const char* target_start, |
| 297 | size_t target_size, |
| 298 | Match* best_match) const; |
| 299 | |
| 300 | protected: |
| 301 | // FindBestMatch() will not process more than this number |
| 302 | // of matching hash entries. |
| 303 | // |
| 304 | // It is necessary to have a limit on the maximum number of matches |
| 305 | // that will be checked in order to avoid the worst-case performance |
| 306 | // possible if, for example, all the blocks in the dictionary have |
| 307 | // the same hash value. See the unit test SearchStringFindsTooManyMatches |
| 308 | // for an example of such a case. The encoder uses a loop in |
| 309 | // VCDiffEngine::Encode over each target byte, containing a loop in |
| 310 | // BlockHash::FindBestMatch over the number of matches (up to a maximum |
| 311 | // of the number of source blocks), containing two loops that extend |
| 312 | // the match forwards and backwards up to the number of source bytes. |
| 313 | // Total complexity in the worst case is |
| 314 | // O([target size] * source_size_ * source_size_) |
| 315 | // Placing a limit on the possible number of matches checked changes this to |
| 316 | // O([target size] * source_size_ * kMaxMatchesToCheck) |
| 317 | // |
| 318 | // In empirical testing on real HTML text, using a block size of 4, |
| 319 | // the number of true matches per call to FindBestMatch() did not exceed 78; |
| 320 | // with a block size of 32, the number of matches did not exceed 3. |
| 321 | // |
| 322 | // The expected number of true matches scales super-linearly |
| 323 | // with the inverse of kBlockSize, but here a linear scale is used |
| 324 | // for block sizes smaller than 32. |
| 325 | static const int kMaxMatchesToCheck = (kBlockSize >= 32) ? 8 : |
| 326 | (8 * (32 / kBlockSize)); |
| 327 | |
| 328 | // Do not skip more than this number of non-matching hash collisions |
| 329 | // to find the next matching entry in the hash chain. |
| 330 | static const int kMaxProbes = 16; |
| 331 | |
| 332 | // Internal routine which calculates a hash table size based on kBlockSize and |
| 333 | // the dictionary_size. Will return a power of two if successful, or 0 if an |
| 334 | // internal error occurs. Some calculations (such as GetHashTableIndex()) |
| 335 | // depend on the table size being a power of two. |
| 336 | static const size_t CalcTableSize(const size_t dictionary_size); |
| 337 | |
| 338 | const size_t GetNumberOfBlocks() const { |
| 339 | return source_size_ / kBlockSize; |
| 340 | } |
| 341 | |
| 342 | // Use the lowest-order bits of the hash value |
| 343 | // as the index into the hash table. |
| 344 | uint32_t GetHashTableIndex(uint32_t hash_value) const { |
| 345 | return hash_value & hash_table_mask_; |
| 346 | } |
| 347 | |
| 348 | // The index within source_data_ of the next block |
| 349 | // for which AddBlock() should be called. |
| 350 | int NextIndexToAdd() const { |
| 351 | return (last_block_added_ + 1) * kBlockSize; |
| 352 | } |
| 353 | |
| 354 | static inline bool TooManyMatches(int* match_counter); |
| 355 | |
| 356 | const char* const source_data() { return source_data_; } |
| 357 | const size_t source_size() { return source_size_; } |
| 358 | |
| 359 | // Adds an entry to the hash table for one block of source data of length |
| 360 | // kBlockSize, starting at source_data_[block_number * kBlockSize], |
| 361 | // where block_number is always (last_block_added_ + 1). That is, |
| 362 | // AddBlock() must be called once for each block in source_data_ |
| 363 | // in increasing order. |
| 364 | void AddBlock(uint32_t hash_value); |
| 365 | |
| 366 | // Calls AddBlock() for each complete kBlockSize-byte block between |
| 367 | // source_data_ and (source_data_ + source_size_). It is equivalent |
| 368 | // to calling AddAllBlocksThroughIndex(source_data + source_size). |
| 369 | // This function is called when Init(true) is invoked. |
| 370 | void AddAllBlocks(); |
| 371 | |
| 372 | // Returns true if the contents of the kBlockSize-byte block |
| 373 | // beginning at block1 are identical to the contents of |
| 374 | // the block beginning at block2; false otherwise. |
| 375 | static bool BlockContentsMatch(const char* block1, const char* block2); |
| 376 | |
| 377 | // Compares each machine word of the two (possibly unaligned) blocks, rather |
| 378 | // than each byte, thus reducing the number of test-and-branch instructions |
| 379 | // executed. Returns a boolean (do the blocks match?) rather than |
| 380 | // the signed byte difference returned by memcmp. |
| 381 | // |
| 382 | // BlockContentsMatch will use either this function or memcmp to do its work, |
| 383 | // depending on which is faster for a particular architecture. |
| 384 | // |
| 385 | // For gcc on x86-based architectures, this function has been shown to run |
| 386 | // about twice as fast as the library function memcmp(), and between five and |
| 387 | // nine times faster than the assembly instructions (repz and cmpsb) that gcc |
| 388 | // uses by default for builtin memcmp. On other architectures, or using |
| 389 | // other compilers, this function has not shown to be faster than memcmp. |
| 390 | static bool BlockCompareWords(const char* block1, const char* block2); |
| 391 | |
| 392 | // Finds the first block number within the hashed data |
| 393 | // that represents a match for the given hash value. |
| 394 | // Returns -1 if no match was found. |
| 395 | // |
| 396 | // Init() must have been called and returned true before using |
| 397 | // FirstMatchingBlock or NextMatchingBlock. No check is performed |
| 398 | // for this condition; the code will crash if this condition is violated. |
| 399 | // |
| 400 | // The hash table is initially populated with -1 (not found) values, |
| 401 | // so if this function is called before the hash table has been populated |
| 402 | // using AddAllBlocks() or AddBlock(), it will simply return -1 |
| 403 | // for any value of hash_value. |
| 404 | int FirstMatchingBlock(uint32_t hash_value, const char* block_ptr) const; |
| 405 | |
| 406 | // Given a block number returned by FirstMatchingBlock() |
| 407 | // or by a previous call to NextMatchingBlock(), returns |
| 408 | // the next block number that matches the same hash value. |
| 409 | // Returns -1 if no match was found. |
| 410 | int NextMatchingBlock(int block_number, const char* block_ptr) const; |
| 411 | |
| 412 | // Inline version of FirstMatchingBlock. This saves the cost of a function |
| 413 | // call when this routine is called from within the module. The external |
| 414 | // (non-inlined) version is called only by unit tests. |
| 415 | inline int FirstMatchingBlockInline(uint32_t hash_value, |
| 416 | const char* block_ptr) const; |
| 417 | |
| 418 | // Walk through the hash entry chain, skipping over any false matches |
| 419 | // (for which the lowest bits of the fingerprints match, |
| 420 | // but the actual block data does not.) Returns the block number of |
| 421 | // the first true match found, or -1 if no true match was found. |
| 422 | // If block_number is a matching block, the function will return block_number |
| 423 | // without skipping to the next block. |
| 424 | int SkipNonMatchingBlocks(int block_number, const char* block_ptr) const; |
| 425 | |
| 426 | // Returns the number of bytes to the left of source_match_start |
| 427 | // that match the corresponding bytes to the left of target_match_start. |
| 428 | // Will not examine more than max_bytes bytes, which is to say that |
| 429 | // the return value will be in the range [0, max_bytes] inclusive. |
| 430 | static int MatchingBytesToLeft(const char* source_match_start, |
| 431 | const char* target_match_start, |
| 432 | int max_bytes); |
| 433 | |
| 434 | // Returns the number of bytes starting at source_match_end |
| 435 | // that match the corresponding bytes starting at target_match_end. |
| 436 | // Will not examine more than max_bytes bytes, which is to say that |
| 437 | // the return value will be in the range [0, max_bytes] inclusive. |
| 438 | static int MatchingBytesToRight(const char* source_match_end, |
| 439 | const char* target_match_end, |
| 440 | int max_bytes); |
| 441 | |
| 442 | // The protected functions BlockContentsMatch, FirstMatchingBlock, |
| 443 | // NextMatchingBlock, MatchingBytesToLeft, and MatchingBytesToRight |
| 444 | // should be made accessible to unit tests. |
| 445 | friend class BlockHashTest; |
| 446 | |
| 447 | private: |
| 448 | const char* const source_data_; |
| 449 | const size_t source_size_; |
| 450 | |
| 451 | // The size of this array is determined using CalcTableSize(). It has at |
| 452 | // least one element for each kBlockSize-byte block in the source data. |
| 453 | // GetHashTableIndex() returns an index into this table for a given hash |
| 454 | // value. The value of each element of hash_table_ is the lowest block |
| 455 | // number in the source data whose hash value would return the same value from |
| 456 | // GetHashTableIndex(), or -1 if there is no matching block. This value can |
| 457 | // then be used as an index into next_block_table_ to retrieve the entire set |
| 458 | // of matching block numbers. |
| 459 | std::vector<int> hash_table_; |
| 460 | |
| 461 | // An array containing one element for each source block. Each element is |
| 462 | // either -1 (== not found) or the index of the next block whose hash value |
| 463 | // would produce a matching result from GetHashTableIndex(). |
| 464 | std::vector<int> next_block_table_; |
| 465 | |
| 466 | // This vector has the same size as next_block_table_. For every block number |
| 467 | // B that is referenced in hash_table_, last_block_table_[B] will contain |
| 468 | // the maximum block number that has the same GetHashTableIndex() value |
| 469 | // as block B. This number may be B itself. For a block number B' that |
| 470 | // is not referenced in hash_table_, the value of last_block_table_[B'] is -1. |
| 471 | // This table is used only while populating the hash table, not while looking |
| 472 | // up hash values in the table. Keeping track of the last block number in the |
| 473 | // chain allows us to construct the block chains as FIFO rather than LIFO |
| 474 | // lists, so that the match with the lowest index is returned first. This |
| 475 | // should result in a more compact encoding because the VCDIFF format favors |
| 476 | // smaller index values and repeated index values. |
| 477 | std::vector<int> last_block_table_; |
| 478 | |
| 479 | // Performing a bitwise AND with hash_table_mask_ will produce a value ranging |
| 480 | // from 0 to the number of elements in hash_table_. |
| 481 | uint32_t hash_table_mask_; |
| 482 | |
| 483 | // The offset of the first byte of source data (the data at source_data_[0]). |
| 484 | // For the purpose of computing offsets, the source data and target data |
| 485 | // are considered to be concatenated -- not literally in a single memory |
| 486 | // buffer, but conceptually as described in the RFC. |
| 487 | // The first byte of the previously encoded target data |
| 488 | // has an offset that is equal to dictionary_size, i.e., just after |
| 489 | // the last byte of source data. |
| 490 | // For a hash of source (dictionary) data, starting_offset_ will be zero; |
| 491 | // for a hash of previously encoded target data, starting_offset_ will be |
| 492 | // equal to the dictionary size. |
| 493 | const int starting_offset_; |
| 494 | |
| 495 | // The last index added by AddBlock(). This determines the block number |
| 496 | // for successive calls to AddBlock(), and is also |
| 497 | // used to determine the starting block for AddAllBlocksThroughIndex(). |
| 498 | int last_block_added_; |
| 499 | |
| 500 | // Making these private avoids implicit copy constructor & assignment operator |
| 501 | BlockHash(const BlockHash&); // NOLINT |
| 502 | void operator=(const BlockHash&); |
| 503 | }; |
| 504 | |
| 505 | } // namespace open_vcdiff |
| 506 | |
| 507 | #endif // OPEN_VCDIFF_BLOCKHASH_H_ |