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217 lines
7.0 KiB
217 lines
7.0 KiB
package brotli
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import "encoding/binary"
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/* Copyright 2010 Google Inc. All Rights Reserved.
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Distributed under MIT license.
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See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
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*/
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/* A (forgetful) hash table to the data seen by the compressor, to
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help create backward references to previous data.
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This is a hash map of fixed size (bucket_size_) to a ring buffer of
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fixed size (block_size_). The ring buffer contains the last block_size_
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index positions of the given hash key in the compressed data. */
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func (*h6) HashTypeLength() uint {
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return 8
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}
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func (*h6) StoreLookahead() uint {
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return 8
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}
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/* HashBytes is the function that chooses the bucket to place the address in. */
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func hashBytesH6(data []byte, mask uint64, shift int) uint32 {
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var h uint64 = (binary.LittleEndian.Uint64(data) & mask) * kHashMul64Long
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/* The higher bits contain more mixture from the multiplication,
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so we take our results from there. */
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return uint32(h >> uint(shift))
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}
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type h6 struct {
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hasherCommon
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bucket_size_ uint
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block_size_ uint
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hash_shift_ int
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hash_mask_ uint64
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block_mask_ uint32
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num []uint16
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buckets []uint32
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}
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func (h *h6) Initialize(params *encoderParams) {
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h.hash_shift_ = 64 - h.params.bucket_bits
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h.hash_mask_ = (^(uint64(0))) >> uint(64-8*h.params.hash_len)
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h.bucket_size_ = uint(1) << uint(h.params.bucket_bits)
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h.block_size_ = uint(1) << uint(h.params.block_bits)
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h.block_mask_ = uint32(h.block_size_ - 1)
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h.num = make([]uint16, h.bucket_size_)
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h.buckets = make([]uint32, h.block_size_*h.bucket_size_)
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}
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func (h *h6) Prepare(one_shot bool, input_size uint, data []byte) {
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var num []uint16 = h.num
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var partial_prepare_threshold uint = h.bucket_size_ >> 6
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/* Partial preparation is 100 times slower (per socket). */
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if one_shot && input_size <= partial_prepare_threshold {
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var i uint
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for i = 0; i < input_size; i++ {
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var key uint32 = hashBytesH6(data[i:], h.hash_mask_, h.hash_shift_)
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num[key] = 0
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}
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} else {
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for i := 0; i < int(h.bucket_size_); i++ {
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num[i] = 0
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}
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}
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}
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/* Look at 4 bytes at &data[ix & mask].
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Compute a hash from these, and store the value of ix at that position. */
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func (h *h6) Store(data []byte, mask uint, ix uint) {
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var num []uint16 = h.num
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var key uint32 = hashBytesH6(data[ix&mask:], h.hash_mask_, h.hash_shift_)
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var minor_ix uint = uint(num[key]) & uint(h.block_mask_)
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var offset uint = minor_ix + uint(key<<uint(h.params.block_bits))
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h.buckets[offset] = uint32(ix)
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num[key]++
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}
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func (h *h6) StoreRange(data []byte, mask uint, ix_start uint, ix_end uint) {
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var i uint
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for i = ix_start; i < ix_end; i++ {
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h.Store(data, mask, i)
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}
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}
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func (h *h6) StitchToPreviousBlock(num_bytes uint, position uint, ringbuffer []byte, ringbuffer_mask uint) {
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if num_bytes >= h.HashTypeLength()-1 && position >= 3 {
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/* Prepare the hashes for three last bytes of the last write.
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These could not be calculated before, since they require knowledge
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of both the previous and the current block. */
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h.Store(ringbuffer, ringbuffer_mask, position-3)
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h.Store(ringbuffer, ringbuffer_mask, position-2)
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h.Store(ringbuffer, ringbuffer_mask, position-1)
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}
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}
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func (h *h6) PrepareDistanceCache(distance_cache []int) {
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prepareDistanceCache(distance_cache, h.params.num_last_distances_to_check)
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}
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/* Find a longest backward match of &data[cur_ix] up to the length of
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max_length and stores the position cur_ix in the hash table.
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REQUIRES: PrepareDistanceCacheH6 must be invoked for current distance cache
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values; if this method is invoked repeatedly with the same distance
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cache values, it is enough to invoke PrepareDistanceCacheH6 once.
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Does not look for matches longer than max_length.
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Does not look for matches further away than max_backward.
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Writes the best match into |out|.
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|out|->score is updated only if a better match is found. */
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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) {
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var num []uint16 = h.num
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var buckets []uint32 = h.buckets
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var cur_ix_masked uint = cur_ix & ring_buffer_mask
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var min_score uint = out.score
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var best_score uint = out.score
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var best_len uint = out.len
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var i uint
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var bucket []uint32
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/* Don't accept a short copy from far away. */
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out.len = 0
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out.len_code_delta = 0
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/* Try last distance first. */
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for i = 0; i < uint(h.params.num_last_distances_to_check); i++ {
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var backward uint = uint(distance_cache[i])
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var prev_ix uint = uint(cur_ix - backward)
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if prev_ix >= cur_ix {
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continue
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}
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if backward > max_backward {
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continue
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}
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prev_ix &= ring_buffer_mask
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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] {
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continue
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}
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{
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var len uint = findMatchLengthWithLimit(data[prev_ix:], data[cur_ix_masked:], max_length)
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if len >= 3 || (len == 2 && i < 2) {
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/* Comparing for >= 2 does not change the semantics, but just saves for
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a few unnecessary binary logarithms in backward reference score,
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since we are not interested in such short matches. */
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var score uint = backwardReferenceScoreUsingLastDistance(uint(len))
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if best_score < score {
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if i != 0 {
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score -= backwardReferencePenaltyUsingLastDistance(i)
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}
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if best_score < score {
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best_score = score
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best_len = uint(len)
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out.len = best_len
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out.distance = backward
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out.score = best_score
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}
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}
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}
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}
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}
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{
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var key uint32 = hashBytesH6(data[cur_ix_masked:], h.hash_mask_, h.hash_shift_)
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bucket = buckets[key<<uint(h.params.block_bits):]
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var down uint
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if uint(num[key]) > h.block_size_ {
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down = uint(num[key]) - h.block_size_
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} else {
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down = 0
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}
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for i = uint(num[key]); i > down; {
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var prev_ix uint
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i--
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prev_ix = uint(bucket[uint32(i)&h.block_mask_])
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var backward uint = cur_ix - prev_ix
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if backward > max_backward {
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break
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}
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prev_ix &= ring_buffer_mask
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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] {
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continue
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}
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{
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var len uint = findMatchLengthWithLimit(data[prev_ix:], data[cur_ix_masked:], max_length)
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if len >= 4 {
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/* Comparing for >= 3 does not change the semantics, but just saves
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for a few unnecessary binary logarithms in backward reference
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score, since we are not interested in such short matches. */
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var score uint = backwardReferenceScore(uint(len), backward)
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if best_score < score {
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best_score = score
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best_len = uint(len)
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out.len = best_len
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out.distance = backward
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out.score = best_score
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}
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}
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}
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}
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bucket[uint32(num[key])&h.block_mask_] = uint32(cur_ix)
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num[key]++
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}
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if min_score == out.score {
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searchInStaticDictionary(dictionary, h, data[cur_ix_masked:], max_length, max_backward+gap, max_distance, out, false)
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}
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}
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