csrc/enc/hash.h
// 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 "./dictionary_hash.h"
#include "./fast_log.h"
#include "./find_match_length.h"
#include "./port.h"
#include "./prefix.h"
#include "./static_dict.h"
#include "./transform.h"
namespace brotli {
static const int kDistanceCacheIndex[] = {
0, 1, 2, 3, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1,
};
static const int kDistanceCacheOffset[] = {
0, 0, 0, 0, -1, 1, -2, 2, -3, 3, -1, 1, -2, 2, -3, 3
};
// 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,
int copy_length,
int backward_reference_offset) {
return (copy_length * average_cost -
1.20 * Log2Floor(backward_reference_offset));
}
inline double BackwardReferenceScoreUsingLastDistance(double average_cost,
int copy_length,
int distance_short_code) {
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
};
return (average_cost * copy_length
- kDistanceShortCodeBitCost[distance_short_code]);
}
// 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). Starting from the
// given index, kBucketSweep buckets are used to store values of a key.
template <int kBucketBits, int kBucketSweep>
class HashLongestMatchQuickly {
public:
HashLongestMatchQuickly() {
Reset();
}
void Reset() {
// It is not strictly necessary to fill this buffer here, but
// not filling will make the results of the compression stochastic
// (but correct). This is because random data would cause the
// system to find accidentally good backward references here and there.
std::fill(&buckets_[0],
&buckets_[sizeof(buckets_) / sizeof(buckets_[0])],
0);
num_dict_lookups_ = 0;
num_dict_matches_ = 0;
}
// Look at 4 bytes at data.
// Compute a hash from these, and store the value somewhere within
// [ix .. ix+3].
inline void Store(const uint8_t *data, const int ix) {
const uint32_t key = Hash<kBucketBits, 4>(data);
// Wiggle the value with the bucket sweep range.
const uint32_t off = (static_cast<uint32_t>(ix) >> 3) % kBucketSweep;
buckets_[key + off] = ix;
}
// 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);
}
}
bool HasStaticDictionary() const { return false; }
// Find a longest backward match of &ring_buffer[cur_ix & ring_buffer_mask]
// 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]) of the start of the best match into
// best_distance_out.
inline bool FindLongestMatch(const uint8_t * __restrict ring_buffer,
const size_t ring_buffer_mask,
const float* __restrict literal_cost,
const size_t literal_cost_mask,
const double average_cost,
const int* __restrict distance_cache,
const uint32_t cur_ix,
const uint32_t max_length,
const uint32_t max_backward,
int * __restrict best_len_out,
int * __restrict best_len_code_out,
int * __restrict best_distance_out,
double* __restrict best_score_out) {
const int best_len_in = *best_len_out;
const int cur_ix_masked = cur_ix & ring_buffer_mask;
int compare_char = ring_buffer[cur_ix_masked + best_len_in];
double best_score = *best_score_out;
int best_len = best_len_in;
int backward = distance_cache[0];
size_t prev_ix = cur_ix - backward;
bool match_found = false;
if (prev_ix < cur_ix) {
prev_ix &= ring_buffer_mask;
if (compare_char == ring_buffer[prev_ix + best_len]) {
int len = FindMatchLengthWithLimit(&ring_buffer[prev_ix],
&ring_buffer[cur_ix_masked],
max_length);
if (len >= 4) {
best_score = BackwardReferenceScoreUsingLastDistance(average_cost,
len, 0);
best_len = len;
*best_len_out = len;
*best_len_code_out = len;
*best_distance_out = backward;
*best_score_out = best_score;
compare_char = ring_buffer[cur_ix_masked + best_len];
if (kBucketSweep == 1) {
return true;
} else {
match_found = true;
}
}
}
}
const uint32_t key = Hash<kBucketBits, 4>(&ring_buffer[cur_ix_masked]);
if (kBucketSweep == 1) {
// Only one to look for, don't bother to prepare for a loop.
prev_ix = buckets_[key];
backward = cur_ix - prev_ix;
prev_ix &= ring_buffer_mask;
if (compare_char != ring_buffer[prev_ix + best_len_in]) {
return false;
}
if (PREDICT_FALSE(backward == 0 || backward > max_backward)) {
return false;
}
const int len = FindMatchLengthWithLimit(&ring_buffer[prev_ix],
&ring_buffer[cur_ix_masked],
max_length);
if (len >= 4) {
*best_len_out = len;
*best_len_code_out = len;
*best_distance_out = backward;
*best_score_out = BackwardReferenceScore(average_cost, len, backward);
return true;
} else {
return false;
}
} else {
uint32_t *bucket = buckets_ + key;
prev_ix = *bucket++;
for (int i = 0; i < kBucketSweep; ++i, prev_ix = *bucket++) {
const int backward = cur_ix - prev_ix;
prev_ix &= ring_buffer_mask;
if (compare_char != ring_buffer[prev_ix + best_len]) {
continue;
}
if (PREDICT_FALSE(backward == 0 || backward > max_backward)) {
continue;
}
const int len =
FindMatchLengthWithLimit(&ring_buffer[prev_ix],
&ring_buffer[cur_ix_masked],
max_length);
if (len >= 4) {
const double score = BackwardReferenceScore(average_cost,
len, backward);
if (best_score < score) {
best_score = score;
best_len = len;
*best_len_out = best_len;
*best_len_code_out = best_len;
*best_distance_out = backward;
*best_score_out = score;
compare_char = ring_buffer[cur_ix_masked + best_len];
match_found = true;
}
}
}
if (!match_found && num_dict_matches_ >= (num_dict_lookups_ >> 7)) {
++num_dict_lookups_;
const uint32_t key = Hash<14, 4>(&ring_buffer[cur_ix_masked]) << 1;
const uint16_t v = kStaticDictionaryHash[key];
if (v > 0) {
const int len = v & 31;
const int dist = v >> 5;
const int offset = kBrotliDictionaryOffsetsByLength[len] + len * dist;
if (len <= max_length) {
const int matchlen =
FindMatchLengthWithLimit(&ring_buffer[cur_ix_masked],
&kBrotliDictionary[offset], len);
if (matchlen == len) {
const size_t backward = max_backward + dist + 1;
const double score = BackwardReferenceScore(average_cost,
len, backward);
if (best_score < score) {
++num_dict_matches_;
best_score = score;
best_len = len;
*best_len_out = best_len;
*best_len_code_out = best_len;
*best_distance_out = backward;
*best_score_out = best_score;
return true;
}
}
}
}
}
return match_found;
}
}
private:
static const uint32_t kBucketSize = 1 << kBucketBits;
uint32_t buckets_[kBucketSize + kBucketSweep];
size_t num_dict_lookups_;
size_t num_dict_matches_;
};
// 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,
int kNumLastDistancesToCheck,
bool kUseCostModel,
bool kUseDictionary>
class HashLongestMatch {
public:
HashLongestMatch() : static_dict_(NULL) {
Reset();
}
void Reset() {
std::fill(&num_[0], &num_[sizeof(num_) / sizeof(num_[0])], 0);
num_dict_lookups_ = 0;
num_dict_matches_ = 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 size_t ring_buffer_mask,
const float * __restrict literal_cost,
const size_t literal_cost_mask,
const double average_cost,
const int* __restrict distance_cache,
const uint32_t cur_ix,
uint32_t max_length,
const uint32_t max_backward,
int * __restrict best_len_out,
int * __restrict best_len_code_out,
int * __restrict best_distance_out,
double * __restrict best_score_out) {
*best_len_code_out = 0;
const size_t cur_ix_masked = cur_ix & ring_buffer_mask;
double start_cost_diff4 = 0.0;
double start_cost_diff3 = 0.0;
double start_cost_diff2 = 0.0;
if (kUseCostModel) {
start_cost_diff4 = literal_cost == NULL ? 0 :
literal_cost[cur_ix & literal_cost_mask] +
literal_cost[(cur_ix + 1) & literal_cost_mask] +
literal_cost[(cur_ix + 2) & literal_cost_mask] +
literal_cost[(cur_ix + 3) & literal_cost_mask] -
4 * average_cost;
start_cost_diff3 = literal_cost == NULL ? 0 :
literal_cost[cur_ix & literal_cost_mask] +
literal_cost[(cur_ix + 1) & literal_cost_mask] +
literal_cost[(cur_ix + 2) & literal_cost_mask] -
3 * average_cost + 0.3;
start_cost_diff2 = literal_cost == NULL ? 0 :
literal_cost[cur_ix & literal_cost_mask] +
literal_cost[(cur_ix + 1) & literal_cost_mask] -
2 * average_cost + 1.2;
}
bool match_found = false;
// Don't accept a short copy from far away.
double best_score = *best_score_out;
int best_len = *best_len_out;
*best_len_out = 0;
// Try last distance first.
for (int i = 0; i < kNumLastDistancesToCheck; ++i) {
const int idx = kDistanceCacheIndex[i];
const int backward = distance_cache[idx] + kDistanceCacheOffset[i];
size_t prev_ix = cur_ix - backward;
if (prev_ix >= cur_ix) {
continue;
}
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.
double score = BackwardReferenceScoreUsingLastDistance(
average_cost, len, i);
if (kUseCostModel) {
switch (len) {
case 2: score += start_cost_diff2; break;
case 3: score += start_cost_diff3; break;
default: score += start_cost_diff4;
}
}
if (best_score < score) {
best_score = score;
best_len = len;
*best_len_out = best_len;
*best_len_code_out = best_len;
*best_distance_out = backward;
*best_score_out = best_score;
match_found = true;
}
}
}
if (kMinLength == 2) {
int stop = int(cur_ix) - 64;
if (stop < 0) { stop = 0; }
start_cost_diff2 -= 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 =
average_cost * 2 - 2.3 * Log2Floor(backward) + start_cost_diff2;
if (best_score < score) {
best_score = score;
best_len = len;
*best_len_out = best_len;
*best_len_code_out = best_len;
*best_distance_out = backward;
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.
double score = BackwardReferenceScore(average_cost,
len, backward);
if (kUseCostModel) {
score += (len >= 4) ? start_cost_diff4 : start_cost_diff3;
}
if (best_score < score) {
best_score = score;
best_len = len;
*best_len_out = best_len;
*best_len_code_out = best_len;
*best_distance_out = backward;
*best_score_out = best_score;
match_found = true;
}
}
}
}
if (!match_found && num_dict_matches_ >= (num_dict_lookups_ >> 7)) {
uint32_t key = Hash<14, 4>(&data[cur_ix_masked]) << 1;
for (int k = 0; k < 2; ++k, ++key) {
++num_dict_lookups_;
const uint16_t v = kStaticDictionaryHash[key];
if (v > 0) {
const int len = v & 31;
const int dist = v >> 5;
const int offset = kBrotliDictionaryOffsetsByLength[len] + len * dist;
if (len <= max_length) {
const int matchlen =
FindMatchLengthWithLimit(&data[cur_ix_masked],
&kBrotliDictionary[offset], len);
if (matchlen == len) {
const size_t backward = max_backward + dist + 1;
double score = BackwardReferenceScore(average_cost,
len, backward);
if (kUseCostModel) {
score += start_cost_diff4;
}
if (best_score < score) {
++num_dict_matches_;
best_score = score;
best_len = len;
*best_len_out = best_len;
*best_len_code_out = best_len;
*best_distance_out = backward;
*best_score_out = best_score;
match_found = true;
break;
}
}
}
}
}
}
if (kUseDictionary && static_dict_ != NULL) {
// We decide based on first 4 bytes how many bytes to test for.
uint32_t 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, ©_len_code, &word_id)) {
const size_t backward = max_backward + word_id + 1;
const double score = (BackwardReferenceScore(average_cost,
len, backward) +
start_cost_diff4);
if (best_score < score) {
best_score = score;
best_len = len;
*best_len_out = best_len;
*best_len_code_out = copy_len_code;
*best_distance_out = backward;
*best_score_out = best_score;
match_found = true;
}
}
}
}
return match_found;
}
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];
size_t num_dict_lookups_;
size_t num_dict_matches_;
const StaticDictionary *static_dict_;
};
struct Hashers {
typedef HashLongestMatchQuickly<16, 1> H1;
typedef HashLongestMatchQuickly<17, 4> H2;
typedef HashLongestMatch<14, 4, 4, 4, false, false> H3;
typedef HashLongestMatch<14, 5, 4, 4, false, false> H4;
typedef HashLongestMatch<15, 6, 4, 10, false, false> H5;
typedef HashLongestMatch<15, 7, 4, 10, false, false> H6;
typedef HashLongestMatch<15, 8, 4, 16, false, false> H7;
typedef HashLongestMatch<15, 8, 4, 16, true, true> H8;
typedef HashLongestMatch<15, 8, 2, 16, true, false> H9;
void Init(int type) {
switch (type) {
case 1: hash_h1.reset(new H1); break;
case 2: hash_h2.reset(new H2); break;
case 3: hash_h3.reset(new H3); break;
case 4: hash_h4.reset(new H4); break;
case 5: hash_h5.reset(new H5); break;
case 6: hash_h6.reset(new H6); break;
case 7: hash_h7.reset(new H7); break;
case 8: hash_h8.reset(new H8); break;
case 9: hash_h9.reset(new H9); break;
default: break;
}
}
void SetStaticDictionary(const StaticDictionary *dict) {
if (hash_h8.get() != NULL) hash_h8->SetStaticDictionary(dict);
}
std::unique_ptr<H1> hash_h1;
std::unique_ptr<H2> hash_h2;
std::unique_ptr<H3> hash_h3;
std::unique_ptr<H4> hash_h4;
std::unique_ptr<H5> hash_h5;
std::unique_ptr<H6> hash_h6;
std::unique_ptr<H7> hash_h7;
std::unique_ptr<H8> hash_h8;
std::unique_ptr<H9> hash_h9;
};
} // namespace brotli
#endif // BROTLI_ENC_HASH_H_