253 lines
7.6 KiB
Go
253 lines
7.6 KiB
Go
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package brotli
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import "encoding/binary"
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/* Copyright 2016 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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func (*hashForgetfulChain) HashTypeLength() uint {
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return 4
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}
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func (*hashForgetfulChain) StoreLookahead() uint {
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return 4
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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 (h *hashForgetfulChain) HashBytes(data []byte) uint {
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var hash uint32 = binary.LittleEndian.Uint32(data) * kHashMul32
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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 uint(hash >> (32 - h.bucketBits))
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}
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type slot struct {
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delta uint16
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next uint16
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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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Hashes are stored in chains which are bucketed to groups. Group of chains
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share a storage "bank". When more than "bank size" chain nodes are added,
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oldest nodes are replaced; this way several chains may share a tail. */
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type hashForgetfulChain struct {
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hasherCommon
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bucketBits uint
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numBanks uint
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bankBits uint
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numLastDistancesToCheck int
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addr []uint32
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head []uint16
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tiny_hash [65536]byte
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banks [][]slot
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free_slot_idx []uint16
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max_hops uint
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}
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func (h *hashForgetfulChain) Initialize(params *encoderParams) {
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var q uint
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if params.quality > 6 {
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q = 7
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} else {
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q = 8
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}
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h.max_hops = q << uint(params.quality-4)
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bankSize := 1 << h.bankBits
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bucketSize := 1 << h.bucketBits
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h.addr = make([]uint32, bucketSize)
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h.head = make([]uint16, bucketSize)
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h.banks = make([][]slot, h.numBanks)
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for i := range h.banks {
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h.banks[i] = make([]slot, bankSize)
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}
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h.free_slot_idx = make([]uint16, h.numBanks)
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}
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func (h *hashForgetfulChain) Prepare(one_shot bool, input_size uint, data []byte) {
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var partial_prepare_threshold uint = (1 << h.bucketBits) >> 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 bucket uint = h.HashBytes(data[i:])
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/* See InitEmpty comment. */
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h.addr[bucket] = 0xCCCCCCCC
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h.head[bucket] = 0xCCCC
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}
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} else {
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/* Fill |addr| array with 0xCCCCCCCC value. Because of wrapping, position
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processed by hasher never reaches 3GB + 64M; this makes all new chains
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to be terminated after the first node. */
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for i := range h.addr {
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h.addr[i] = 0xCCCCCCCC
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}
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for i := range h.head {
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h.head[i] = 0
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}
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}
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h.tiny_hash = [65536]byte{}
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for i := range h.free_slot_idx {
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h.free_slot_idx[i] = 0
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}
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}
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/* Look at 4 bytes at &data[ix & mask]. Compute a hash from these, and prepend
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node to corresponding chain; also update tiny_hash for current position. */
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func (h *hashForgetfulChain) Store(data []byte, mask uint, ix uint) {
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var key uint = h.HashBytes(data[ix&mask:])
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var bank uint = key & (h.numBanks - 1)
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idx := uint(h.free_slot_idx[bank]) & ((1 << h.bankBits) - 1)
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h.free_slot_idx[bank]++
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var delta uint = ix - uint(h.addr[key])
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h.tiny_hash[uint16(ix)] = byte(key)
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if delta > 0xFFFF {
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delta = 0xFFFF
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}
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h.banks[bank][idx].delta = uint16(delta)
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h.banks[bank][idx].next = h.head[key]
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h.addr[key] = uint32(ix)
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h.head[key] = uint16(idx)
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}
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func (h *hashForgetfulChain) 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 *hashForgetfulChain) StitchToPreviousBlock(num_bytes uint, position uint, ringbuffer []byte, ring_buffer_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, ring_buffer_mask, position-3)
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h.Store(ringbuffer, ring_buffer_mask, position-2)
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h.Store(ringbuffer, ring_buffer_mask, position-1)
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}
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}
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func (h *hashForgetfulChain) PrepareDistanceCache(distance_cache []int) {
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prepareDistanceCache(distance_cache, h.numLastDistancesToCheck)
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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: PrepareDistanceCachehashForgetfulChain 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 PrepareDistanceCachehashForgetfulChain 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 *hashForgetfulChain) 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 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 key uint = h.HashBytes(data[cur_ix_masked:])
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var tiny_hash byte = byte(key)
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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 < h.numLastDistancesToCheck; i++ {
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var backward uint = uint(distance_cache[i])
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var prev_ix uint = (cur_ix - backward)
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/* For distance code 0 we want to consider 2-byte matches. */
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if i > 0 && h.tiny_hash[uint16(prev_ix)] != tiny_hash {
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continue
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}
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if prev_ix >= cur_ix || 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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{
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var len uint = findMatchLengthWithLimit(data[prev_ix:], data[cur_ix_masked:], max_length)
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if len >= 2 {
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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(uint(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 bank uint = key & (h.numBanks - 1)
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var backward uint = 0
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var hops uint = h.max_hops
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var delta uint = cur_ix - uint(h.addr[key])
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var slot uint = uint(h.head[key])
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for {
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tmp6 := hops
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hops--
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if tmp6 == 0 {
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break
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}
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var prev_ix uint
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var last uint = slot
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backward += delta
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if backward > max_backward {
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break
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}
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prev_ix = (cur_ix - backward) & ring_buffer_mask
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slot = uint(h.banks[bank][last].next)
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delta = uint(h.banks[bank][last].delta)
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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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h.Store(data, ring_buffer_mask, cur_ix)
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}
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if out.score == min_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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