/* NOLINT(build/header_guard) */ | |
/* Copyright 2010 Google Inc. All Rights Reserved. | |
Distributed under MIT license. | |
See file LICENSE for detail or copy at https://opensource.org/licenses/MIT | |
*/ | |
/* template parameters: FN, BUCKET_BITS, BLOCK_BITS, | |
NUM_LAST_DISTANCES_TO_CHECK */ | |
/* 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 (BUCKET_SIZE) to a ring buffer of | |
fixed size (BLOCK_SIZE). The ring buffer contains the last BLOCK_SIZE | |
index positions of the given hash key in the compressed data. */ | |
#define HashLongestMatch HASHER() | |
/* Number of hash buckets. */ | |
#define BUCKET_SIZE (1 << BUCKET_BITS) | |
/* Only BLOCK_SIZE newest backward references are kept, | |
and the older are forgotten. */ | |
#define BLOCK_SIZE (1u << BLOCK_BITS) | |
/* Mask for accessing entries in a block (in a ringbuffer manner). */ | |
#define BLOCK_MASK ((1 << BLOCK_BITS) - 1) | |
#define HASH_MAP_SIZE (2 << BUCKET_BITS) | |
static BROTLI_INLINE size_t FN(HashTypeLength)(void) { return 4; } | |
static BROTLI_INLINE size_t FN(StoreLookahead)(void) { return 4; } | |
/* HashBytes is the function that chooses the bucket to place | |
the address in. The HashLongestMatch and HashLongestMatchQuickly | |
classes have separate, different implementations of hashing. */ | |
static uint32_t FN(HashBytes)(const uint8_t *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 - BUCKET_BITS); | |
} | |
typedef struct HashLongestMatch { | |
/* Number of entries in a particular bucket. */ | |
uint16_t num_[BUCKET_SIZE]; | |
/* Buckets containing BLOCK_SIZE of backward references. */ | |
uint32_t buckets_[BLOCK_SIZE << BUCKET_BITS]; | |
/* True if num_ array needs to be initialized. */ | |
BROTLI_BOOL is_dirty_; | |
DictionarySearchStatictics dict_search_stats_; | |
} HashLongestMatch; | |
static void FN(Reset)(HashLongestMatch* self) { | |
self->is_dirty_ = BROTLI_TRUE; | |
DictionarySearchStaticticsReset(&self->dict_search_stats_); | |
} | |
static void FN(InitEmpty)(HashLongestMatch* self) { | |
if (self->is_dirty_) { | |
memset(self->num_, 0, sizeof(self->num_)); | |
self->is_dirty_ = BROTLI_FALSE; | |
} | |
} | |
static void FN(InitForData)(HashLongestMatch* self, const uint8_t* data, | |
size_t num) { | |
size_t i; | |
for (i = 0; i < num; ++i) { | |
const uint32_t key = FN(HashBytes)(&data[i]); | |
self->num_[key] = 0; | |
} | |
if (num != 0) { | |
self->is_dirty_ = BROTLI_FALSE; | |
} | |
} | |
static void FN(Init)( | |
MemoryManager* m, HashLongestMatch* self, const uint8_t* data, | |
const BrotliEncoderParams* params, size_t position, size_t bytes, | |
BROTLI_BOOL is_last) { | |
/* Choose which init method is faster. | |
Init() is about 100 times faster than InitForData(). */ | |
const size_t kMaxBytesForPartialHashInit = HASH_MAP_SIZE >> 7; | |
BROTLI_UNUSED(m); | |
BROTLI_UNUSED(params); | |
if (position == 0 && is_last && bytes <= kMaxBytesForPartialHashInit) { | |
FN(InitForData)(self, data, bytes); | |
} else { | |
FN(InitEmpty)(self); | |
} | |
} | |
/* Look at 4 bytes at &data[ix & mask]. | |
Compute a hash from these, and store the value of ix at that position. */ | |
static BROTLI_INLINE void FN(Store)(HashLongestMatch* self, const uint8_t *data, | |
const size_t mask, const size_t ix) { | |
const uint32_t key = FN(HashBytes)(&data[ix & mask]); | |
const size_t minor_ix = self->num_[key] & BLOCK_MASK; | |
self->buckets_[minor_ix + (key << BLOCK_BITS)] = (uint32_t)ix; | |
++self->num_[key]; | |
} | |
static BROTLI_INLINE void FN(StoreRange)(HashLongestMatch* self, | |
const uint8_t *data, const size_t mask, const size_t ix_start, | |
const size_t ix_end) { | |
size_t i; | |
for (i = ix_start; i < ix_end; ++i) { | |
FN(Store)(self, data, mask, i); | |
} | |
} | |
static BROTLI_INLINE void FN(StitchToPreviousBlock)(HashLongestMatch* self, | |
size_t num_bytes, size_t position, const uint8_t* ringbuffer, | |
size_t ringbuffer_mask) { | |
if (num_bytes >= FN(HashTypeLength)() - 1 && 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. */ | |
FN(Store)(self, ringbuffer, ringbuffer_mask, position - 3); | |
FN(Store)(self, ringbuffer, ringbuffer_mask, position - 2); | |
FN(Store)(self, ringbuffer, ringbuffer_mask, position - 1); | |
} | |
} | |
/* Find a longest backward match of &data[cur_ix] up to the length of | |
max_length and stores the position cur_ix in the hash table. | |
Does not look for matches longer than max_length. | |
Does not look for matches further away than max_backward. | |
Writes the best match into |out|. | |
Returns true when match is found, otherwise false. */ | |
static BROTLI_INLINE BROTLI_BOOL FN(FindLongestMatch)(HashLongestMatch* self, | |
const uint8_t* BROTLI_RESTRICT data, const size_t ring_buffer_mask, | |
const int* BROTLI_RESTRICT distance_cache, const size_t cur_ix, | |
const size_t max_length, const size_t max_backward, | |
HasherSearchResult* BROTLI_RESTRICT out) { | |
const size_t cur_ix_masked = cur_ix & ring_buffer_mask; | |
BROTLI_BOOL is_match_found = BROTLI_FALSE; | |
/* Don't accept a short copy from far away. */ | |
score_t best_score = out->score; | |
size_t best_len = out->len; | |
size_t i; | |
out->len = 0; | |
out->len_x_code = 0; | |
/* Try last distance first. */ | |
for (i = 0; i < NUM_LAST_DISTANCES_TO_CHECK; ++i) { | |
const size_t idx = kDistanceCacheIndex[i]; | |
const size_t backward = | |
(size_t)(distance_cache[idx] + kDistanceCacheOffset[i]); | |
size_t prev_ix = (size_t)(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 >= 3 || (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. */ | |
score_t score = BackwardReferenceScoreUsingLastDistance(len, i); | |
if (best_score < score) { | |
best_score = score; | |
best_len = len; | |
out->len = best_len; | |
out->distance = backward; | |
out->score = best_score; | |
is_match_found = BROTLI_TRUE; | |
} | |
} | |
} | |
} | |
{ | |
const uint32_t key = FN(HashBytes)(&data[cur_ix_masked]); | |
uint32_t* BROTLI_RESTRICT bucket = &self->buckets_[key << BLOCK_BITS]; | |
const size_t down = | |
(self->num_[key] > BLOCK_SIZE) ? (self->num_[key] - BLOCK_SIZE) : 0u; | |
for (i = self->num_[key]; i > down;) { | |
size_t prev_ix = bucket[--i & BLOCK_MASK]; | |
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 >= 4) { | |
/* 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. */ | |
score_t score = BackwardReferenceScore(len, backward); | |
if (best_score < score) { | |
best_score = score; | |
best_len = len; | |
out->len = best_len; | |
out->distance = backward; | |
out->score = best_score; | |
is_match_found = BROTLI_TRUE; | |
} | |
} | |
} | |
} | |
bucket[self->num_[key] & BLOCK_MASK] = (uint32_t)cur_ix; | |
++self->num_[key]; | |
} | |
if (!is_match_found) { | |
is_match_found = SearchInStaticDictionary(&self->dict_search_stats_, | |
&data[cur_ix_masked], max_length, max_backward, out, BROTLI_FALSE); | |
} | |
return is_match_found; | |
} | |
#undef HASH_MAP_SIZE | |
#undef BLOCK_MASK | |
#undef BLOCK_SIZE | |
#undef BUCKET_SIZE | |
#undef HashLongestMatch |