| // Copyright 2006, 2008 Google Inc. |
| // Authors: Chandra Chereddi, Lincoln Smith |
| // |
| // Licensed under the Apache License, Version 2.0 (the "License"); |
| // you may not use this file except in compliance with the License. |
| // You may obtain a copy of the License at |
| // |
| // http://www.apache.org/licenses/LICENSE-2.0 |
| // |
| // Unless required by applicable law or agreed to in writing, software |
| // distributed under the License is distributed on an "AS IS" BASIS, |
| // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| // See the License for the specific language governing permissions and |
| // limitations under the License. |
| |
| #include <config.h> |
| #include "vcdiffengine.h" |
| #include <stdint.h> // uint32_t |
| #include <string.h> // memcpy |
| #include "blockhash.h" |
| #include "codetablewriter_interface.h" |
| #include "logging.h" |
| #include "rolling_hash.h" |
| |
| namespace open_vcdiff { |
| |
| VCDiffEngine::VCDiffEngine(const char* dictionary, size_t dictionary_size) |
| // If dictionary_size == 0, then dictionary could be NULL. Guard against |
| // using a NULL value. |
| : dictionary_((dictionary_size > 0) ? new char[dictionary_size] : ""), |
| dictionary_size_(dictionary_size), |
| hashed_dictionary_(NULL) { |
| if (dictionary_size > 0) { |
| memcpy(const_cast<char*>(dictionary_), dictionary, dictionary_size); |
| } |
| } |
| |
| VCDiffEngine::~VCDiffEngine() { |
| delete hashed_dictionary_; |
| if (dictionary_size_ > 0) { |
| delete[] dictionary_; |
| } |
| } |
| |
| bool VCDiffEngine::Init() { |
| if (hashed_dictionary_) { |
| VCD_DFATAL << "Init() called twice for same VCDiffEngine object" |
| << VCD_ENDL; |
| return false; |
| } |
| hashed_dictionary_ = BlockHash::CreateDictionaryHash(dictionary_, |
| dictionary_size()); |
| if (!hashed_dictionary_) { |
| VCD_DFATAL << "Creation of dictionary hash failed" << VCD_ENDL; |
| return false; |
| } |
| RollingHash<BlockHash::kBlockSize>::Init(); |
| return true; |
| } |
| |
| // This helper function tries to find an appropriate match within |
| // hashed_dictionary_ for the block starting at the current target position. |
| // If target_hash is not NULL, this function will also look for a match |
| // within the previously encoded target data. |
| // |
| // If a match is found, this function will generate an ADD instruction |
| // for all unencoded data that precedes the match, |
| // and a COPY instruction for the match itself; then it returns |
| // the number of bytes processed by both instructions, |
| // which is guaranteed to be > 0. |
| // If no appropriate match is found, the function returns 0. |
| // |
| // The first four parameters are input parameters which are passed |
| // directly to BlockHash::FindBestMatch; please see that function |
| // for a description of their allowable values. |
| template<bool look_for_target_matches> |
| inline size_t VCDiffEngine::EncodeCopyForBestMatch( |
| uint32_t hash_value, |
| const char* target_candidate_start, |
| const char* unencoded_target_start, |
| size_t unencoded_target_size, |
| const BlockHash* target_hash, |
| CodeTableWriterInterface* coder) const { |
| // When FindBestMatch() comes up with a match for a candidate block, |
| // it will populate best_match with the size, source offset, |
| // and target offset of the match. |
| BlockHash::Match best_match; |
| |
| // First look for a match in the dictionary. |
| hashed_dictionary_->FindBestMatch(hash_value, |
| target_candidate_start, |
| unencoded_target_start, |
| unencoded_target_size, |
| &best_match); |
| // If target matching is enabled, then see if there is a better match |
| // within the target data that has been encoded so far. |
| if (look_for_target_matches) { |
| target_hash->FindBestMatch(hash_value, |
| target_candidate_start, |
| unencoded_target_start, |
| unencoded_target_size, |
| &best_match); |
| } |
| if (!ShouldGenerateCopyInstructionForMatchOfSize(best_match.size())) { |
| return 0; |
| } |
| if (best_match.target_offset() > 0) { |
| // Create an ADD instruction to encode all target bytes |
| // from the end of the last COPY match, if any, up to |
| // the beginning of this COPY match. |
| coder->Add(unencoded_target_start, best_match.target_offset()); |
| } |
| coder->Copy(best_match.source_offset(), best_match.size()); |
| return best_match.target_offset() // ADD size |
| + best_match.size(); // + COPY size |
| } |
| |
| // Once the encoder loop has finished checking for matches in the target data, |
| // this function creates an ADD instruction to encode all target bytes |
| // from the end of the last COPY match, if any, through the end of |
| // the target data. In the worst case, if no matches were found at all, |
| // this function will create one big ADD instruction |
| // for the entire buffer of target data. |
| inline void VCDiffEngine::AddUnmatchedRemainder( |
| const char* unencoded_target_start, |
| size_t unencoded_target_size, |
| CodeTableWriterInterface* coder) const { |
| if (unencoded_target_size > 0) { |
| coder->Add(unencoded_target_start, unencoded_target_size); |
| } |
| } |
| |
| // This helper function tells the coder to finish the encoding and write |
| // the results into the output string "diff". |
| inline void VCDiffEngine::FinishEncoding( |
| size_t target_size, |
| OutputStringInterface* diff, |
| CodeTableWriterInterface* coder) const { |
| if (target_size != static_cast<size_t>(coder->target_length())) { |
| VCD_DFATAL << "Internal error in VCDiffEngine::Encode: " |
| "original target size (" << target_size |
| << ") does not match number of bytes processed (" |
| << coder->target_length() << ")" << VCD_ENDL; |
| } |
| coder->Output(diff); |
| } |
| |
| template<bool look_for_target_matches> |
| void VCDiffEngine::EncodeInternal(const char* target_data, |
| size_t target_size, |
| OutputStringInterface* diff, |
| CodeTableWriterInterface* coder) const { |
| if (!hashed_dictionary_) { |
| VCD_DFATAL << "Internal error: VCDiffEngine::Encode() " |
| "called before VCDiffEngine::Init()" << VCD_ENDL; |
| return; |
| } |
| if (target_size == 0) { |
| return; // Do nothing for empty target |
| } |
| // Special case for really small input |
| if (target_size < static_cast<size_t>(BlockHash::kBlockSize)) { |
| AddUnmatchedRemainder(target_data, target_size, coder); |
| FinishEncoding(target_size, diff, coder); |
| return; |
| } |
| RollingHash<BlockHash::kBlockSize> hasher; |
| BlockHash* target_hash = NULL; |
| if (look_for_target_matches) { |
| // Check matches against previously encoded target data |
| // in this same target window, as well as against the dictionary |
| target_hash = BlockHash::CreateTargetHash(target_data, |
| target_size, |
| dictionary_size()); |
| if (!target_hash) { |
| VCD_DFATAL << "Instantiation of target hash failed" << VCD_ENDL; |
| return; |
| } |
| } |
| const char* const target_end = target_data + target_size; |
| const char* const start_of_last_block = target_end - BlockHash::kBlockSize; |
| // Offset of next bytes in string to ADD if NOT copied (i.e., not found in |
| // dictionary) |
| const char* next_encode = target_data; |
| // candidate_pos points to the start of the kBlockSize-byte block that may |
| // begin a match with the dictionary or previously encoded target data. |
| const char* candidate_pos = target_data; |
| uint32_t hash_value = hasher.Hash(candidate_pos); |
| while (1) { |
| const size_t bytes_encoded = |
| EncodeCopyForBestMatch<look_for_target_matches>( |
| hash_value, |
| candidate_pos, |
| next_encode, |
| (target_end - next_encode), |
| target_hash, |
| coder); |
| if (bytes_encoded > 0) { |
| next_encode += bytes_encoded; // Advance past COPYed data |
| candidate_pos = next_encode; |
| if (candidate_pos > start_of_last_block) { |
| break; // Reached end of target data |
| } |
| // candidate_pos has jumped ahead by bytes_encoded bytes, so UpdateHash |
| // can't be used to calculate the hash value at its new position. |
| hash_value = hasher.Hash(candidate_pos); |
| if (look_for_target_matches) { |
| // Update the target hash for the ADDed and COPYed data |
| target_hash->AddAllBlocksThroughIndex( |
| static_cast<int>(next_encode - target_data)); |
| } |
| } else { |
| // No match, or match is too small to be worth a COPY instruction. |
| // Move to the next position in the target data. |
| if ((candidate_pos + 1) > start_of_last_block) { |
| break; // Reached end of target data |
| } |
| if (look_for_target_matches) { |
| target_hash->AddOneIndexHash( |
| static_cast<int>(candidate_pos - target_data), |
| hash_value); |
| } |
| hash_value = hasher.UpdateHash(hash_value, |
| candidate_pos[0], |
| candidate_pos[BlockHash::kBlockSize]); |
| ++candidate_pos; |
| } |
| } |
| AddUnmatchedRemainder(next_encode, target_end - next_encode, coder); |
| FinishEncoding(target_size, diff, coder); |
| delete target_hash; |
| } |
| |
| void VCDiffEngine::Encode(const char* target_data, |
| size_t target_size, |
| bool look_for_target_matches, |
| OutputStringInterface* diff, |
| CodeTableWriterInterface* coder) const { |
| if (look_for_target_matches) { |
| EncodeInternal<true>(target_data, target_size, diff, coder); |
| } else { |
| EncodeInternal<false>(target_data, target_size, diff, coder); |
| } |
| } |
| |
| } // namespace open_vcdiff |
