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// 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_) {
LOG(DFATAL) << "Init() called twice for same VCDiffEngine object"
<< LOG_ENDL;
return false;
}
hashed_dictionary_ = BlockHash::CreateDictionaryHash(dictionary_,
dictionary_size());
if (!hashed_dictionary_) {
LOG(DFATAL) << "Creation of dictionary hash failed" << LOG_ENDL;
return false;
}
if (!RollingHash<BlockHash::kBlockSize>::Init()) {
LOG(DFATAL) << "RollingHash initialization failed" << LOG_ENDL;
return false;
}
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())) {
LOG(DFATAL) << "Internal error in VCDiffEngine::Encode: "
"original target size (" << target_size
<< ") does not match number of bytes processed ("
<< coder->target_length() << ")" << LOG_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_) {
LOG(DFATAL) << "Internal error: VCDiffEngine::Encode() "
"called before VCDiffEngine::Init()" << LOG_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) {
LOG(DFATAL) << "Instantiation of target hash failed" << LOG_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