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package brotli
import "encoding/binary"
/* Copyright 2010 Google Inc. All Rights Reserved.
Distributed under MIT license.
See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
*/
/* 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. */
func (*h6) HashTypeLength() uint {
return 8
}
func (*h6) StoreLookahead() uint {
return 8
}
/* HashBytes is the function that chooses the bucket to place the address in. */
func hashBytesH6(data []byte, mask uint64, shift int) uint32 {
var h uint64 = (binary.LittleEndian.Uint64(data) & mask) * kHashMul64Long
/* The higher bits contain more mixture from the multiplication,
so we take our results from there. */
return uint32(h >> uint(shift))
}
type h6 struct {
hasherCommon
bucket_size_ uint
block_size_ uint
hash_shift_ int
hash_mask_ uint64
block_mask_ uint32
num []uint16
buckets []uint32
}
func (h *h6) Initialize(params *encoderParams) {
h.hash_shift_ = 64 - h.params.bucket_bits
h.hash_mask_ = (^(uint64(0))) >> uint(64-8*h.params.hash_len)
h.bucket_size_ = uint(1) << uint(h.params.bucket_bits)
h.block_size_ = uint(1) << uint(h.params.block_bits)
h.block_mask_ = uint32(h.block_size_ - 1)
h.num = make([]uint16, h.bucket_size_)
h.buckets = make([]uint32, h.block_size_*h.bucket_size_)
}
func (h *h6) Prepare(one_shot bool, input_size uint, data []byte) {
var num []uint16 = h.num
var partial_prepare_threshold uint = h.bucket_size_ >> 6
/* Partial preparation is 100 times slower (per socket). */
if one_shot && input_size <= partial_prepare_threshold {
var i uint
for i = 0; i < input_size; i++ {
var key uint32 = hashBytesH6(data[i:], h.hash_mask_, h.hash_shift_)
num[key] = 0
}
} else {
for i := 0; i < int(h.bucket_size_); i++ {
num[i] = 0
}
}
}
/* Look at 4 bytes at &data[ix & mask].
Compute a hash from these, and store the value of ix at that position. */
func (h *h6) Store(data []byte, mask uint, ix uint) {
var num []uint16 = h.num
var key uint32 = hashBytesH6(data[ix&mask:], h.hash_mask_, h.hash_shift_)
var minor_ix uint = uint(num[key]) & uint(h.block_mask_)
var offset uint = minor_ix + uint(key<<uint(h.params.block_bits))
h.buckets[offset] = uint32(ix)
num[key]++
}
func (h *h6) StoreRange(data []byte, mask uint, ix_start uint, ix_end uint) {
var i uint
for i = ix_start; i < ix_end; i++ {
h.Store(data, mask, i)
}
}
func (h *h6) StitchToPreviousBlock(num_bytes uint, position uint, ringbuffer []byte, ringbuffer_mask uint) {
if num_bytes >= h.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. */
h.Store(ringbuffer, ringbuffer_mask, position-3)
h.Store(ringbuffer, ringbuffer_mask, position-2)
h.Store(ringbuffer, ringbuffer_mask, position-1)
}
}
func (h *h6) PrepareDistanceCache(distance_cache []int) {
prepareDistanceCache(distance_cache, h.params.num_last_distances_to_check)
}
/* 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.
REQUIRES: PrepareDistanceCacheH6 must be invoked for current distance cache
values; if this method is invoked repeatedly with the same distance
cache values, it is enough to invoke PrepareDistanceCacheH6 once.
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|.
|out|->score is updated only if a better match is found. */
func (h *h6) FindLongestMatch(dictionary *encoderDictionary, data []byte, ring_buffer_mask uint, distance_cache []int, cur_ix uint, max_length uint, max_backward uint, gap uint, max_distance uint, out *hasherSearchResult) {
var num []uint16 = h.num
var buckets []uint32 = h.buckets
var cur_ix_masked uint = cur_ix & ring_buffer_mask
var min_score uint = out.score
var best_score uint = out.score
var best_len uint = out.len
var i uint
var bucket []uint32
/* Don't accept a short copy from far away. */
out.len = 0
out.len_code_delta = 0
/* Try last distance first. */
for i = 0; i < uint(h.params.num_last_distances_to_check); i++ {
var backward uint = uint(distance_cache[i])
var prev_ix uint = uint(cur_ix - backward)
if prev_ix >= cur_ix {
continue
}
if 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
}
{
var len uint = 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. */
var score uint = backwardReferenceScoreUsingLastDistance(uint(len))
if best_score < score {
if i != 0 {
score -= backwardReferencePenaltyUsingLastDistance(i)
}
if best_score < score {
best_score = score
best_len = uint(len)
out.len = best_len
out.distance = backward
out.score = best_score
}
}
}
}
}
{
var key uint32 = hashBytesH6(data[cur_ix_masked:], h.hash_mask_, h.hash_shift_)
bucket = buckets[key<<uint(h.params.block_bits):]
var down uint
if uint(num[key]) > h.block_size_ {
down = uint(num[key]) - h.block_size_
} else {
down = 0
}
for i = uint(num[key]); i > down; {
var prev_ix uint
i--
prev_ix = uint(bucket[uint32(i)&h.block_mask_])
var backward uint = cur_ix - prev_ix
if 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
}
{
var len uint = 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. */
var score uint = backwardReferenceScore(uint(len), backward)
if best_score < score {
best_score = score
best_len = uint(len)
out.len = best_len
out.distance = backward
out.score = best_score
}
}
}
}
bucket[uint32(num[key])&h.block_mask_] = uint32(cur_ix)
num[key]++
}
if min_score == out.score {
searchInStaticDictionary(dictionary, h, data[cur_ix_masked:], max_length, max_backward+gap, max_distance, out, false)
}
}
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