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package brotli
import "encoding/binary"
/* Copyright 2016 Google Inc. All Rights Reserved.
Distributed under MIT license.
See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
*/
func (*hashForgetfulChain) HashTypeLength() uint {
return 4
}
func (*hashForgetfulChain) StoreLookahead() uint {
return 4
}
/* HashBytes is the function that chooses the bucket to place the address in.*/
func (h *hashForgetfulChain) HashBytes(data []byte) uint {
var hash uint32 = binary.LittleEndian.Uint32(data) * kHashMul32
/* The higher bits contain more mixture from the multiplication,
so we take our results from there. */
return uint(hash >> (32 - h.bucketBits))
}
type slot struct {
delta uint16
next uint16
}
/* A (forgetful) hash table to the data seen by the compressor, to
help create backward references to previous data.
Hashes are stored in chains which are bucketed to groups. Group of chains
share a storage "bank". When more than "bank size" chain nodes are added,
oldest nodes are replaced; this way several chains may share a tail. */
type hashForgetfulChain struct {
hasherCommon
bucketBits uint
numBanks uint
bankBits uint
numLastDistancesToCheck int
addr []uint32
head []uint16
tiny_hash [65536]byte
banks [][]slot
free_slot_idx []uint16
max_hops uint
}
func (h *hashForgetfulChain) Initialize(params *encoderParams) {
var q uint
if params.quality > 6 {
q = 7
} else {
q = 8
}
h.max_hops = q << uint(params.quality-4)
bankSize := 1 << h.bankBits
bucketSize := 1 << h.bucketBits
h.addr = make([]uint32, bucketSize)
h.head = make([]uint16, bucketSize)
h.banks = make([][]slot, h.numBanks)
for i := range h.banks {
h.banks[i] = make([]slot, bankSize)
}
h.free_slot_idx = make([]uint16, h.numBanks)
}
func (h *hashForgetfulChain) Prepare(one_shot bool, input_size uint, data []byte) {
var partial_prepare_threshold uint = (1 << h.bucketBits) >> 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 bucket uint = h.HashBytes(data[i:])
/* See InitEmpty comment. */
h.addr[bucket] = 0xCCCCCCCC
h.head[bucket] = 0xCCCC
}
} else {
/* Fill |addr| array with 0xCCCCCCCC value. Because of wrapping, position
processed by hasher never reaches 3GB + 64M; this makes all new chains
to be terminated after the first node. */
for i := range h.addr {
h.addr[i] = 0xCCCCCCCC
}
for i := range h.head {
h.head[i] = 0
}
}
h.tiny_hash = [65536]byte{}
for i := range h.free_slot_idx {
h.free_slot_idx[i] = 0
}
}
/* Look at 4 bytes at &data[ix & mask]. Compute a hash from these, and prepend
node to corresponding chain; also update tiny_hash for current position. */
func (h *hashForgetfulChain) Store(data []byte, mask uint, ix uint) {
var key uint = h.HashBytes(data[ix&mask:])
var bank uint = key & (h.numBanks - 1)
idx := uint(h.free_slot_idx[bank]) & ((1 << h.bankBits) - 1)
h.free_slot_idx[bank]++
var delta uint = ix - uint(h.addr[key])
h.tiny_hash[uint16(ix)] = byte(key)
if delta > 0xFFFF {
delta = 0xFFFF
}
h.banks[bank][idx].delta = uint16(delta)
h.banks[bank][idx].next = h.head[key]
h.addr[key] = uint32(ix)
h.head[key] = uint16(idx)
}
func (h *hashForgetfulChain) 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 *hashForgetfulChain) StitchToPreviousBlock(num_bytes uint, position uint, ringbuffer []byte, ring_buffer_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, ring_buffer_mask, position-3)
h.Store(ringbuffer, ring_buffer_mask, position-2)
h.Store(ringbuffer, ring_buffer_mask, position-1)
}
}
func (h *hashForgetfulChain) PrepareDistanceCache(distance_cache []int) {
prepareDistanceCache(distance_cache, h.numLastDistancesToCheck)
}
/* 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: PrepareDistanceCachehashForgetfulChain 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 PrepareDistanceCachehashForgetfulChain 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 *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) {
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 key uint = h.HashBytes(data[cur_ix_masked:])
var tiny_hash byte = byte(key)
/* 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 < h.numLastDistancesToCheck; i++ {
var backward uint = uint(distance_cache[i])
var prev_ix uint = (cur_ix - backward)
/* For distance code 0 we want to consider 2-byte matches. */
if i > 0 && h.tiny_hash[uint16(prev_ix)] != tiny_hash {
continue
}
if prev_ix >= cur_ix || backward > max_backward {
continue
}
prev_ix &= ring_buffer_mask
{
var len uint = findMatchLengthWithLimit(data[prev_ix:], data[cur_ix_masked:], max_length)
if len >= 2 {
var score uint = backwardReferenceScoreUsingLastDistance(uint(len))
if best_score < score {
if i != 0 {
score -= backwardReferencePenaltyUsingLastDistance(uint(i))
}
if best_score < score {
best_score = score
best_len = uint(len)
out.len = best_len
out.distance = backward
out.score = best_score
}
}
}
}
}
{
var bank uint = key & (h.numBanks - 1)
var backward uint = 0
var hops uint = h.max_hops
var delta uint = cur_ix - uint(h.addr[key])
var slot uint = uint(h.head[key])
for {
tmp6 := hops
hops--
if tmp6 == 0 {
break
}
var prev_ix uint
var last uint = slot
backward += delta
if backward > max_backward {
break
}
prev_ix = (cur_ix - backward) & ring_buffer_mask
slot = uint(h.banks[bank][last].next)
delta = uint(h.banks[bank][last].delta)
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
}
}
}
}
h.Store(data, ring_buffer_mask, cur_ix)
}
if out.score == min_score {
searchInStaticDictionary(dictionary, h, data[cur_ix_masked:], max_length, max_backward+gap, max_distance, out, false)
}
}
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