brotli/enc/hash.h
2014-03-25 16:48:25 +01:00

448 lines
16 KiB
C++

// Copyright 2010 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.
//
// A (forgetful) hash table to the data seen by the compressor, to
// help create backward references to previous data.
#ifndef BROTLI_ENC_HASH_H_
#define BROTLI_ENC_HASH_H_
#include <stddef.h>
#include <stdint.h>
#include <string.h>
#include <sys/types.h>
#include <algorithm>
#include <cstdlib>
#include <memory>
#include <string>
#include "./transform.h"
#include "./fast_log.h"
#include "./find_match_length.h"
#include "./port.h"
#include "./static_dict.h"
namespace brotli {
// kHashMul32 multiplier has these properties:
// * The multiplier must be odd. Otherwise we may lose the highest bit.
// * No long streaks of 1s or 0s.
// * There is no effort to ensure that it is a prime, the oddity is enough
// for this use.
// * The number has been tuned heuristically against compression benchmarks.
static const uint32_t kHashMul32 = 0x1e35a7bd;
template<int kShiftBits, int kMinLength>
inline uint32_t Hash(const uint8_t *data) {
if (kMinLength <= 3) {
// If kMinLength is 2 or 3, we hash the first 3 bytes of data.
uint32_t h = (BROTLI_UNALIGNED_LOAD32(data) & 0xffffff) * kHashMul32;
// The higher bits contain more mixture from the multiplication,
// so we take our results from there.
return h >> (32 - kShiftBits);
} else {
// If kMinLength is at least 4, we hash the first 4 bytes of data.
uint32_t h = BROTLI_UNALIGNED_LOAD32(data) * kHashMul32;
// The higher bits contain more mixture from the multiplication,
// so we take our results from there.
return h >> (32 - kShiftBits);
}
}
// Usually, we always choose the longest backward reference. This function
// allows for the exception of that rule.
//
// If we choose a backward reference that is further away, it will
// usually be coded with more bits. We approximate this by assuming
// log2(distance). If the distance can be expressed in terms of the
// last four distances, we use some heuristic constants to estimate
// the bits cost. For the first up to four literals we use the bit
// cost of the literals from the literal cost model, after that we
// use the average bit cost of the cost model.
//
// This function is used to sometimes discard a longer backward reference
// when it is not much longer and the bit cost for encoding it is more
// than the saved literals.
inline double BackwardReferenceScore(double average_cost,
double start_cost4,
double start_cost3,
double start_cost2,
int copy_length,
int backward_reference_offset) {
double retval = 0;
switch (copy_length) {
case 2: retval = start_cost2; break;
case 3: retval = start_cost3; break;
default: retval = start_cost4 + (copy_length - 4) * average_cost; break;
}
retval -= 1.20 * Log2Floor(backward_reference_offset);
return retval;
}
inline double BackwardReferenceScoreUsingLastDistance(double average_cost,
double start_cost4,
double start_cost3,
double start_cost2,
int copy_length,
int distance_short_code) {
double retval = 0;
switch (copy_length) {
case 2: retval = start_cost2; break;
case 3: retval = start_cost3; break;
default: retval = start_cost4 + (copy_length - 4) * average_cost; break;
}
static const double kDistanceShortCodeBitCost[16] = {
-0.6, 0.95, 1.17, 1.27,
0.93, 0.93, 0.96, 0.96, 0.99, 0.99,
1.05, 1.05, 1.15, 1.15, 1.25, 1.25
};
retval -= kDistanceShortCodeBitCost[distance_short_code];
return retval;
}
// A (forgetful) hash table to the data seen by the compressor, to
// help create backward references to previous data.
//
// This is a hash map of fixed size (kBucketSize) to a ring buffer of
// fixed size (kBlockSize). The ring buffer contains the last kBlockSize
// index positions of the given hash key in the compressed data.
template <int kBucketBits, int kBlockBits, int kMinLength>
class HashLongestMatch {
public:
HashLongestMatch()
: last_distance1_(4),
last_distance2_(11),
last_distance3_(15),
last_distance4_(16),
insert_length_(0),
average_cost_(5.4),
static_dict_(NULL) {
Reset();
}
void Reset() {
std::fill(&num_[0], &num_[sizeof(num_) / sizeof(num_[0])], 0);
}
void SetStaticDictionary(const StaticDictionary *dict) {
static_dict_ = dict;
}
bool HasStaticDictionary() const {
return static_dict_ != NULL;
}
// Look at 3 bytes at data.
// Compute a hash from these, and store the value of ix at that position.
inline void Store(const uint8_t *data, const int ix) {
const uint32_t key = Hash<kBucketBits, kMinLength>(data);
const int minor_ix = num_[key] & kBlockMask;
buckets_[key][minor_ix] = ix;
++num_[key];
}
// Store hashes for a range of data.
void StoreHashes(const uint8_t *data, size_t len, int startix, int mask) {
for (int p = 0; p < len; ++p) {
Store(&data[p & mask], startix + p);
}
}
// Find a longest backward match of &data[cur_ix] up to the length of
// max_length.
//
// Does not look for matches longer than max_length.
// Does not look for matches further away than max_backward.
// Writes the best found match length into best_len_out.
// Writes the index (&data[index]) offset from the start of the best match
// into best_distance_out.
// Write the score of the best match into best_score_out.
bool FindLongestMatch(const uint8_t * __restrict data,
const float * __restrict literal_cost,
const size_t ring_buffer_mask,
const uint32_t cur_ix,
uint32_t max_length,
const uint32_t max_backward,
size_t * __restrict best_len_out,
size_t * __restrict best_len_code_out,
size_t * __restrict best_distance_out,
double * __restrict best_score_out,
bool * __restrict in_dictionary) {
*in_dictionary = true;
*best_len_code_out = 0;
const size_t cur_ix_masked = cur_ix & ring_buffer_mask;
const double start_cost4 = literal_cost == NULL ? 20 :
literal_cost[cur_ix_masked] +
literal_cost[(cur_ix + 1) & ring_buffer_mask] +
literal_cost[(cur_ix + 2) & ring_buffer_mask] +
literal_cost[(cur_ix + 3) & ring_buffer_mask];
const double start_cost3 = literal_cost == NULL ? 15 :
literal_cost[cur_ix_masked] +
literal_cost[(cur_ix + 1) & ring_buffer_mask] +
literal_cost[(cur_ix + 2) & ring_buffer_mask] + 0.3;
double start_cost2 = literal_cost == NULL ? 10 :
literal_cost[cur_ix_masked] +
literal_cost[(cur_ix + 1) & ring_buffer_mask] + 1.2;
bool match_found = false;
// Don't accept a short copy from far away.
double best_score = 8.115;
if (insert_length_ < 4) {
double cost_diff[4] = { 0.10, 0.04, 0.02, 0.01 };
best_score += cost_diff[insert_length_];
}
size_t best_len = *best_len_out;
*best_len_out = 0;
size_t best_ix = 1;
// Try last distance first.
for (int i = 0; i < 16; ++i) {
size_t prev_ix = cur_ix;
switch(i) {
case 0: prev_ix -= last_distance1_; break;
case 1: prev_ix -= last_distance2_; break;
case 2: prev_ix -= last_distance3_; break;
case 3: prev_ix -= last_distance4_; break;
case 4: prev_ix -= last_distance1_ - 1; break;
case 5: prev_ix -= last_distance1_ + 1; break;
case 6: prev_ix -= last_distance1_ - 2; break;
case 7: prev_ix -= last_distance1_ + 2; break;
case 8: prev_ix -= last_distance1_ - 3; break;
case 9: prev_ix -= last_distance1_ + 3; break;
case 10: prev_ix -= last_distance2_ - 1; break;
case 11: prev_ix -= last_distance2_ + 1; break;
case 12: prev_ix -= last_distance2_ - 2; break;
case 13: prev_ix -= last_distance2_ + 2; break;
case 14: prev_ix -= last_distance2_ - 3; break;
case 15: prev_ix -= last_distance2_ + 3; break;
}
if (prev_ix >= cur_ix) {
continue;
}
const size_t backward = cur_ix - prev_ix;
if (PREDICT_FALSE(backward > max_backward)) {
continue;
}
prev_ix &= ring_buffer_mask;
if (cur_ix_masked + best_len > ring_buffer_mask ||
prev_ix + best_len > ring_buffer_mask ||
data[cur_ix_masked + best_len] != data[prev_ix + best_len]) {
continue;
}
const size_t len =
FindMatchLengthWithLimit(&data[prev_ix], &data[cur_ix_masked],
max_length);
if (len >= std::max(kMinLength, 3) ||
(kMinLength == 2 && len == 2 && i < 2)) {
// Comparing for >= 2 does not change the semantics, but just saves for
// a few unnecessary binary logarithms in backward reference score,
// since we are not interested in such short matches.
const double score = BackwardReferenceScoreUsingLastDistance(
average_cost_,
start_cost4,
start_cost3,
start_cost2,
len, i);
if (best_score < score) {
best_score = score;
best_len = len;
best_ix = backward;
*best_len_out = best_len;
*best_len_code_out = best_len;
*best_distance_out = best_ix;
*best_score_out = best_score;
match_found = true;
*in_dictionary = backward > max_backward;
}
}
}
if (kMinLength == 2) {
int stop = int(cur_ix) - 64;
if (stop < 0) { stop = 0; }
start_cost2 -= 1.0;
for (int i = cur_ix - 1; i > stop; --i) {
size_t prev_ix = i;
const size_t backward = cur_ix - prev_ix;
if (PREDICT_FALSE(backward > max_backward)) {
break;
}
prev_ix &= ring_buffer_mask;
if (data[cur_ix_masked] != data[prev_ix] ||
data[cur_ix_masked + 1] != data[prev_ix + 1]) {
continue;
}
int len = 2;
const double score = start_cost2 - 2.3 * Log2Floor(backward);
if (best_score < score) {
best_score = score;
best_len = len;
best_ix = backward;
*best_len_out = best_len;
*best_len_code_out = best_len;
*best_distance_out = best_ix;
match_found = true;
}
}
}
const uint32_t key = Hash<kBucketBits, kMinLength>(&data[cur_ix_masked]);
const int * __restrict const bucket = &buckets_[key][0];
const int down = (num_[key] > kBlockSize) ? (num_[key] - kBlockSize) : 0;
for (int i = num_[key] - 1; i >= down; --i) {
int prev_ix = bucket[i & kBlockMask];
if (prev_ix >= 0) {
const size_t backward = cur_ix - prev_ix;
if (PREDICT_FALSE(backward > max_backward)) {
break;
}
prev_ix &= ring_buffer_mask;
if (cur_ix_masked + best_len > ring_buffer_mask ||
prev_ix + best_len > ring_buffer_mask ||
data[cur_ix_masked + best_len] != data[prev_ix + best_len]) {
continue;
}
const size_t len =
FindMatchLengthWithLimit(&data[prev_ix], &data[cur_ix_masked],
max_length);
if (len >= std::max(kMinLength, 3)) {
// Comparing for >= 3 does not change the semantics, but just saves
// for a few unnecessary binary logarithms in backward reference
// score, since we are not interested in such short matches.
const double score = BackwardReferenceScore(average_cost_,
start_cost4,
start_cost3,
start_cost2,
len, backward);
if (best_score < score) {
best_score = score;
best_len = len;
best_ix = backward;
*best_len_out = best_len;
*best_len_code_out = best_len;
*best_distance_out = best_ix;
*best_score_out = best_score;
match_found = true;
*in_dictionary = false;
}
}
}
}
if (static_dict_ != NULL) {
// We decide based on first 4 bytes how many bytes to test for.
int prefix = BROTLI_UNALIGNED_LOAD32(&data[cur_ix_masked]);
int maxlen = static_dict_->GetLength(prefix);
for (int len = std::min<size_t>(maxlen, max_length);
len > best_len && len >= 4; --len) {
std::string snippet((const char *)&data[cur_ix_masked], len);
int copy_len_code;
int word_id;
if (static_dict_->Get(snippet, &copy_len_code, &word_id)) {
const size_t backward = max_backward + word_id + 1;
const double score = BackwardReferenceScore(average_cost_,
start_cost4,
start_cost3,
start_cost2,
len, backward);
if (best_score < score) {
best_score = score;
best_len = len;
best_ix = backward;
*best_len_out = best_len;
*best_len_code_out = copy_len_code;
*best_distance_out = best_ix;
*best_score_out = best_score;
match_found = true;
*in_dictionary = true;
}
}
}
}
return match_found;
}
void set_last_distance(int v) {
if (last_distance1_ != v) {
last_distance4_ = last_distance3_;
last_distance3_ = last_distance2_;
last_distance2_ = last_distance1_;
last_distance1_ = v;
}
}
int last_distance() const { return last_distance1_; }
void set_insert_length(int v) { insert_length_ = v; }
void set_average_cost(double v) { average_cost_ = v; }
private:
// Number of hash buckets.
static const uint32_t kBucketSize = 1 << kBucketBits;
// Only kBlockSize newest backward references are kept,
// and the older are forgotten.
static const uint32_t kBlockSize = 1 << kBlockBits;
// Mask for accessing entries in a block (in a ringbuffer manner).
static const uint32_t kBlockMask = (1 << kBlockBits) - 1;
// Number of entries in a particular bucket.
uint16_t num_[kBucketSize];
// Buckets containing kBlockSize of backward references.
int buckets_[kBucketSize][kBlockSize];
int last_distance1_;
int last_distance2_;
int last_distance3_;
int last_distance4_;
// Cost adjustment for how many literals we are planning to insert
// anyway.
int insert_length_;
double average_cost_;
const StaticDictionary *static_dict_;
};
struct Hashers {
enum Type {
HASH_15_8_4 = 0,
HASH_15_8_2 = 1,
};
void Init(Type type) {
switch (type) {
case HASH_15_8_4:
hash_15_8_4.reset(new HashLongestMatch<15, 8, 4>());
break;
case HASH_15_8_2:
hash_15_8_2.reset(new HashLongestMatch<15, 8, 2>());
break;
default:
break;
}
}
void SetStaticDictionary(const StaticDictionary *dict) {
if (hash_15_8_4.get() != NULL) hash_15_8_4->SetStaticDictionary(dict);
if (hash_15_8_2.get() != NULL) hash_15_8_2->SetStaticDictionary(dict);
}
std::unique_ptr<HashLongestMatch<15, 8, 4> > hash_15_8_4;
std::unique_ptr<HashLongestMatch<15, 8, 2> > hash_15_8_2;
};
} // namespace brotli
#endif // BROTLI_ENC_HASH_H_