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// Copyright 2013 Google Inc. All Rights Reserved.
//
// 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.
//
// Function to find backward reference copies.
#include "./backward_references.h"
#include <algorithm>
#include <vector>
#include "./command.h"
namespace brotli {
template<typename Hasher, bool kUseCostModel, bool kUseDictionary>
void CreateBackwardReferences(size_t num_bytes,
size_t position,
const uint8_t* ringbuffer,
size_t ringbuffer_mask,
const float* literal_cost,
size_t literal_cost_mask,
const size_t max_backward_limit,
const double base_min_score,
const int quality,
Hasher* hasher,
int* dist_cache,
int* last_insert_len,
Command* commands,
int* num_commands) {
if (num_bytes >= 3 && position >= 3) {
// Prepare the hashes for three last bytes of the last write.
// These could not be calculated before, since they require knowledge
// of both the previous and the current block.
hasher->Store(&ringbuffer[(position - 3) & ringbuffer_mask],
position - 3);
hasher->Store(&ringbuffer[(position - 2) & ringbuffer_mask],
position - 2);
hasher->Store(&ringbuffer[(position - 1) & ringbuffer_mask],
position - 1);
}
const Command * const orig_commands = commands;
int insert_length = *last_insert_len;
size_t i = position & ringbuffer_mask;
const int i_diff = position - i;
const size_t i_end = i + num_bytes;
// For speed up heuristics for random data.
const int random_heuristics_window_size = quality < 9 ? 64 : 512;
int apply_random_heuristics = i + random_heuristics_window_size;
double average_cost = 5.4;
if (kUseCostModel) {
average_cost = 0.0;
for (int k = position; k < position + num_bytes; ++k) {
average_cost += literal_cost[k & literal_cost_mask];
}
if (num_bytes > 0) {
average_cost /= num_bytes;
}
}
// M1 match is for considering for two repeated copies, if moving
// one literal form the previous copy to the current one allows the
// current copy to be more efficient (because the way static dictionary
// codes words). M1 matching improves text compression density by ~0.15 %.
bool match_found_M1 = false;
int best_len_M1 = 0;
int best_len_code_M1 = 0;
int best_dist_M1 = 0;
double best_score_M1 = 0;
while (i + 3 < i_end) {
int max_length = i_end - i;
size_t max_distance = std::min(i + i_diff, max_backward_limit);
double min_score = base_min_score;
if (kUseCostModel && insert_length < 8) {
double cost_diff[8] =
{ 0.1, 0.038, 0.019, 0.013, 0.001, 0.001, 0.001, 0.001 };
min_score += cost_diff[insert_length];
}
int best_len = 0;
int best_len_code = 0;
int best_dist = 0;
double best_score = min_score;
bool match_found = hasher->FindLongestMatch(
ringbuffer, ringbuffer_mask,
literal_cost, literal_cost_mask, average_cost,
dist_cache, i + i_diff, max_length, max_distance,
&best_len, &best_len_code, &best_dist, &best_score);
if (match_found) {
if (kUseDictionary && match_found_M1 && best_score_M1 > best_score) {
// Two copies after each other. Take the last literal from the
// last copy, and use it as the first of this one.
Command prev_cmd = commands[-1];
commands[-1] = Command(prev_cmd.insert_len_,
prev_cmd.copy_len_ - 1,
prev_cmd.copy_len_ - 1,
prev_cmd.DistanceCode());
hasher->Store(ringbuffer + i, i + i_diff);
--i;
best_len = best_len_M1;
best_len_code = best_len_code_M1;
best_dist = best_dist_M1;
best_score = best_score_M1;
} else {
// Found a match. Let's look for something even better ahead.
int delayed_backward_references_in_row = 0;
for (;;) {
--max_length;
int best_len_2 = quality < 4 ? std::min(best_len - 1, max_length) : 0;
int best_len_code_2 = 0;
int best_dist_2 = 0;
double best_score_2 = min_score;
max_distance = std::min(i + i_diff + 1, max_backward_limit);
hasher->Store(ringbuffer + i, i + i_diff);
match_found = hasher->FindLongestMatch(
ringbuffer, ringbuffer_mask,
literal_cost, literal_cost_mask, average_cost,
dist_cache, i + i_diff + 1, max_length, max_distance,
&best_len_2, &best_len_code_2, &best_dist_2, &best_score_2);
double cost_diff_lazy = 7.0;
if (kUseCostModel) {
cost_diff_lazy = 0.0;
if (best_len >= 4) {
cost_diff_lazy +=
literal_cost[(i + 4) & literal_cost_mask] - average_cost;
}
{
const int tail_length = best_len_2 - best_len + 1;
for (int k = 0; k < tail_length; ++k) {
cost_diff_lazy -=
literal_cost[(i + best_len + k) & literal_cost_mask] -
average_cost;
}
}
// If we are not inserting any symbols, inserting one is more
// expensive than if we were inserting symbols anyways.
if (insert_length < 1) {
cost_diff_lazy += 0.97;
}
// Add bias to slightly avoid lazy matching.
cost_diff_lazy += 2.0 + delayed_backward_references_in_row * 0.2;
cost_diff_lazy += 0.04 * literal_cost[i & literal_cost_mask];
}
if (match_found && best_score_2 >= best_score + cost_diff_lazy) {
// Ok, let's just write one byte for now and start a match from the
// next byte.
++i;
++insert_length;
best_len = best_len_2;
best_len_code = best_len_code_2;
best_dist = best_dist_2;
best_score = best_score_2;
if (++delayed_backward_references_in_row < 4) {
continue;
}
}
break;
}
}
apply_random_heuristics =
i + 2 * best_len + random_heuristics_window_size;
max_distance = std::min(i + i_diff, max_backward_limit);
int distance_code = best_dist + 16;
if (best_dist <= max_distance) {
if (best_dist == dist_cache[0]) {
distance_code = 1;
} else if (best_dist == dist_cache[1]) {
distance_code = 2;
} else if (best_dist == dist_cache[2]) {
distance_code = 3;
} else if (best_dist == dist_cache[3]) {
distance_code = 4;
} else if (quality > 1 && best_dist >= 6) {
for (int k = 4; k < kNumDistanceShortCodes; ++k) {
int idx = kDistanceCacheIndex[k];
int candidate = dist_cache[idx] + kDistanceCacheOffset[k];
static const int kLimits[16] = { 0, 0, 0, 0,
6, 6, 11, 11,
11, 11, 11, 11,
12, 12, 12, 12 };
if (best_dist == candidate && best_dist >= kLimits[k]) {
distance_code = k + 1;
break;
}
}
}
if (distance_code > 1) {
dist_cache[3] = dist_cache[2];
dist_cache[2] = dist_cache[1];
dist_cache[1] = dist_cache[0];
dist_cache[0] = best_dist;
}
}
Command cmd(insert_length, best_len, best_len_code, distance_code);
*commands++ = cmd;
insert_length = 0;
if (kUseDictionary) {
++i;
// Copy all copied literals to the hasher, except the last one.
// We cannot store the last one yet, otherwise we couldn't find
// the possible M1 match.
for (int j = 1; j < best_len - 1; ++j) {
if (i + 3 < i_end) {
hasher->Store(ringbuffer + i, i + i_diff);
}
++i;
}
// Prepare M1 match.
if (hasher->HasStaticDictionary() &&
best_len >= 4 && i + 20 < i_end && best_dist <= max_distance) {
max_distance = std::min(i + i_diff, max_backward_limit);
best_score_M1 = min_score;
match_found_M1 = hasher->FindLongestMatch(
ringbuffer, ringbuffer_mask,
literal_cost, literal_cost_mask, average_cost,
dist_cache, i + i_diff, i_end - i, max_distance,
&best_len_M1, &best_len_code_M1, &best_dist_M1, &best_score_M1);
} else {
match_found_M1 = false;
}
if (kUseCostModel) {
// This byte is just moved from the previous copy to the current,
// that is no gain.
best_score_M1 -= literal_cost[i & literal_cost_mask];
// Adjust for losing the opportunity for lazy matching.
best_score_M1 -= 3.75;
}
// Store the last one of the match.
if (i + 3 < i_end) {
hasher->Store(ringbuffer + i, i + i_diff);
}
++i;
} else {
// Put the hash keys into the table, if there are enough
// bytes left.
for (int j = 1; j < best_len; ++j) {
hasher->Store(&ringbuffer[i + j], i + i_diff + j);
}
i += best_len;
}
} else {
match_found_M1 = false;
++insert_length;
hasher->Store(ringbuffer + i, i + i_diff);
++i;
// If we have not seen matches for a long time, we can skip some
// match lookups. Unsuccessful match lookups are very very expensive
// and this kind of a heuristic speeds up compression quite
// a lot.
if (i > apply_random_heuristics) {
// Going through uncompressible data, jump.
if (i > apply_random_heuristics + 4 * random_heuristics_window_size) {
// It is quite a long time since we saw a copy, so we assume
// that this data is not compressible, and store hashes less
// often. Hashes of non compressible data are less likely to
// turn out to be useful in the future, too, so we store less of
// them to not to flood out the hash table of good compressible
// data.
int i_jump = std::min(i + 16, i_end - 4);
for (; i < i_jump; i += 4) {
hasher->Store(ringbuffer + i, i + i_diff);
insert_length += 4;
}
} else {
int i_jump = std::min(i + 8, i_end - 3);
for (; i < i_jump; i += 2) {
hasher->Store(ringbuffer + i, i + i_diff);
insert_length += 2;
}
}
}
}
}
insert_length += (i_end - i);
*last_insert_len = insert_length;
*num_commands += (commands - orig_commands);
}
void CreateBackwardReferences(size_t num_bytes,
size_t position,
const uint8_t* ringbuffer,
size_t ringbuffer_mask,
const float* literal_cost,
size_t literal_cost_mask,
const size_t max_backward_limit,
const double base_min_score,
const int quality,
Hashers* hashers,
int hash_type,
int* dist_cache,
int* last_insert_len,
Command* commands,
int* num_commands) {
switch (hash_type) {
case 1:
CreateBackwardReferences<Hashers::H1, false, false>(
num_bytes, position, ringbuffer, ringbuffer_mask,
literal_cost, literal_cost_mask, max_backward_limit, base_min_score,
quality, hashers->hash_h1.get(), dist_cache, last_insert_len,
commands, num_commands);
break;
case 2:
CreateBackwardReferences<Hashers::H2, false, false>(
num_bytes, position, ringbuffer, ringbuffer_mask,
literal_cost, literal_cost_mask, max_backward_limit, base_min_score,
quality, hashers->hash_h2.get(), dist_cache, last_insert_len,
commands, num_commands);
break;
case 3:
CreateBackwardReferences<Hashers::H3, false, false>(
num_bytes, position, ringbuffer, ringbuffer_mask,
literal_cost, literal_cost_mask, max_backward_limit, base_min_score,
quality, hashers->hash_h3.get(), dist_cache, last_insert_len,
commands, num_commands);
break;
case 4:
CreateBackwardReferences<Hashers::H4, false, false>(
num_bytes, position, ringbuffer, ringbuffer_mask,
literal_cost, literal_cost_mask, max_backward_limit, base_min_score,
quality, hashers->hash_h4.get(), dist_cache, last_insert_len,
commands, num_commands);
break;
case 5:
CreateBackwardReferences<Hashers::H5, false, false>(
num_bytes, position, ringbuffer, ringbuffer_mask,
literal_cost, literal_cost_mask, max_backward_limit, base_min_score,
quality, hashers->hash_h5.get(), dist_cache, last_insert_len,
commands, num_commands);
break;
case 6:
CreateBackwardReferences<Hashers::H6, false, false>(
num_bytes, position, ringbuffer, ringbuffer_mask,
literal_cost, literal_cost_mask, max_backward_limit, base_min_score,
quality, hashers->hash_h6.get(), dist_cache, last_insert_len,
commands, num_commands);
break;
case 7:
CreateBackwardReferences<Hashers::H7, false, false>(
num_bytes, position, ringbuffer, ringbuffer_mask,
literal_cost, literal_cost_mask, max_backward_limit, base_min_score,
quality, hashers->hash_h7.get(), dist_cache, last_insert_len,
commands, num_commands);
break;
case 8:
CreateBackwardReferences<Hashers::H8, true, true>(
num_bytes, position, ringbuffer, ringbuffer_mask,
literal_cost, literal_cost_mask, max_backward_limit, base_min_score,
quality, hashers->hash_h8.get(), dist_cache, last_insert_len,
commands, num_commands);
break;
case 9:
CreateBackwardReferences<Hashers::H9, true, false>(
num_bytes, position, ringbuffer, ringbuffer_mask,
literal_cost, literal_cost_mask, max_backward_limit, base_min_score,
quality, hashers->hash_h9.get(), dist_cache, last_insert_len,
commands, num_commands);
break;
default:
break;
}
}
} // namespace brotli