245 lines
7.2 KiB
Go
245 lines
7.2 KiB
Go
package brotli
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/* NOLINT(build/header_guard) */
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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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/* 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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func HashTypeLengthH40() uint {
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return 4
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}
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func StoreLookaheadH40() 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 HashBytesH40(data []byte) uint {
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var h uint32 = BROTLI_UNALIGNED_LOAD32LE(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(h >> (32 - 15))
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}
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type SlotH40 struct {
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delta uint16
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next uint16
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}
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type BankH40 struct {
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slots [1 << 16]SlotH40
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}
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type H40 struct {
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HasherCommon
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addr [1 << 15]uint32
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head [1 << 15]uint16
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tiny_hash [65536]byte
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banks [1]BankH40
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free_slot_idx [1]uint16
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max_hops uint
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}
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func SelfH40(handle HasherHandle) *H40 {
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return handle.(*H40)
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}
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func (h *H40) Initialize(params *BrotliEncoderParams) {
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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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}
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func (h *H40) Prepare(one_shot bool, input_size uint, data []byte) {
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var partial_prepare_threshold uint = (1 << 15) >> 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 = HashBytesH40(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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var i int
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for i = 0; i < len(h.addr); i++ {
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h.addr[i] = 0xCCCCCCCC
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}
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h.head = [1 << 15]uint16{}
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}
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h.tiny_hash = [65536]byte{}
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h.free_slot_idx = [1]uint16{}
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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 StoreH40(handle HasherHandle, data []byte, mask uint, ix uint) {
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var self *H40 = SelfH40(handle)
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var key uint = HashBytesH40(data[ix&mask:])
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var bank uint = key & (1 - 1)
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var idx uint
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idx = uint(self.free_slot_idx[bank]) & ((1 << 16) - 1)
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self.free_slot_idx[bank]++
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var delta uint = ix - uint(self.addr[key])
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self.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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self.banks[bank].slots[idx].delta = uint16(delta)
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self.banks[bank].slots[idx].next = self.head[key]
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self.addr[key] = uint32(ix)
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self.head[key] = uint16(idx)
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}
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func StoreRangeH40(handle HasherHandle, 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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StoreH40(handle, data, mask, i)
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}
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}
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func (h *H40) StitchToPreviousBlock(num_bytes uint, position uint, ringbuffer []byte, ring_buffer_mask uint) {
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if num_bytes >= HashTypeLengthH40()-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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StoreH40(h, ringbuffer, ring_buffer_mask, position-3)
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StoreH40(h, ringbuffer, ring_buffer_mask, position-2)
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StoreH40(h, ringbuffer, ring_buffer_mask, position-1)
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}
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}
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func PrepareDistanceCacheH40(handle HasherHandle, distance_cache []int) {
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PrepareDistanceCache(distance_cache, 4)
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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: PrepareDistanceCacheH40 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 PrepareDistanceCacheH40 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 FindLongestMatchH40(handle HasherHandle, dictionary *BrotliEncoderDictionary, 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 self *H40 = SelfH40(handle)
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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 key uint = HashBytesH40(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 < 4; 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 && self.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(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 & (1 - 1)
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var backward uint = 0
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var hops uint = self.max_hops
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var delta uint = cur_ix - uint(self.addr[key])
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var slot uint = uint(self.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(self.banks[bank].slots[last].next)
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delta = uint(self.banks[bank].slots[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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StoreH40(handle, 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, handle, 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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