package brotli /* Copyright 2018 Google Inc. All Rights Reserved. Distributed under MIT license. See file LICENSE for detail or copy at https://opensource.org/licenses/MIT */ /* NOTE: this hasher does not search in the dictionary. It is used as backup-hasher, the main hasher already searches in it. */ const kRollingHashMul32 uint32 = 69069 const kInvalidPosHashRolling uint32 = 0xffffffff /* This hasher uses a longer forward length, but returning a higher value here will hurt compression by the main hasher when combined with a composite hasher. The hasher tests for forward itself instead. */ func (*hashRolling) HashTypeLength() uint { return 4 } func (*hashRolling) StoreLookahead() uint { return 4 } /* Computes a code from a single byte. A lookup table of 256 values could be used, but simply adding 1 works about as good. */ func (*hashRolling) HashByte(b byte) uint32 { return uint32(b) + 1 } func (h *hashRolling) HashRollingFunctionInitial(state uint32, add byte, factor uint32) uint32 { return uint32(factor*state + h.HashByte(add)) } func (h *hashRolling) HashRollingFunction(state uint32, add byte, rem byte, factor uint32, factor_remove uint32) uint32 { return uint32(factor*state + h.HashByte(add) - factor_remove*h.HashByte(rem)) } /* Rolling hash for long distance long string matches. Stores one position per bucket, bucket key is computed over a long region. */ type hashRolling struct { hasherCommon jump int state uint32 table []uint32 next_ix uint chunk_len uint32 factor uint32 factor_remove uint32 } func (h *hashRolling) Initialize(params *encoderParams) { h.state = 0 h.next_ix = 0 h.factor = kRollingHashMul32 /* Compute the factor of the oldest byte to remove: factor**steps modulo 0xffffffff (the multiplications rely on 32-bit overflow) */ h.factor_remove = 1 for i := 0; i < 32; i += h.jump { h.factor_remove *= h.factor } h.table = make([]uint32, 16777216) for i := 0; i < 16777216; i++ { h.table[i] = kInvalidPosHashRolling } } func (h *hashRolling) Prepare(one_shot bool, input_size uint, data []byte) { /* Too small size, cannot use this hasher. */ if input_size < 32 { return } h.state = 0 for i := 0; i < 32; i += h.jump { h.state = h.HashRollingFunctionInitial(h.state, data[i], h.factor) } } func (*hashRolling) Store(data []byte, mask uint, ix uint) { } func (*hashRolling) StoreRange(data []byte, mask uint, ix_start uint, ix_end uint) { } func (h *hashRolling) StitchToPreviousBlock(num_bytes uint, position uint, ringbuffer []byte, ring_buffer_mask uint) { var position_masked uint /* In this case we must re-initialize the hasher from scratch from the current position. */ var available uint = num_bytes if position&uint(h.jump-1) != 0 { var diff uint = uint(h.jump) - (position & uint(h.jump-1)) if diff > available { available = 0 } else { available = available - diff } position += diff } position_masked = position & ring_buffer_mask /* wrapping around ringbuffer not handled. */ if available > ring_buffer_mask-position_masked { available = ring_buffer_mask - position_masked } h.Prepare(false, available, ringbuffer[position&ring_buffer_mask:]) h.next_ix = position } func (*hashRolling) PrepareDistanceCache(distance_cache []int) { } func (h *hashRolling) 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 pos uint = h.next_ix if cur_ix&uint(h.jump-1) != 0 { return } /* Not enough lookahead */ if max_length < 32 { return } for pos = h.next_ix; pos <= cur_ix; pos += uint(h.jump) { var code uint32 = h.state & ((16777216 * 64) - 1) var rem byte = data[pos&ring_buffer_mask] var add byte = data[(pos+32)&ring_buffer_mask] var found_ix uint = uint(kInvalidPosHashRolling) h.state = h.HashRollingFunction(h.state, add, rem, h.factor, h.factor_remove) if code < 16777216 { found_ix = uint(h.table[code]) h.table[code] = uint32(pos) if pos == cur_ix && uint32(found_ix) != kInvalidPosHashRolling { /* The cast to 32-bit makes backward distances up to 4GB work even if cur_ix is above 4GB, despite using 32-bit values in the table. */ var backward uint = uint(uint32(cur_ix - found_ix)) if backward <= max_backward { var found_ix_masked uint = found_ix & ring_buffer_mask var len uint = findMatchLengthWithLimit(data[found_ix_masked:], data[cur_ix_masked:], max_length) if len >= 4 && len > out.len { var score uint = backwardReferenceScore(uint(len), backward) if score > out.score { out.len = uint(len) out.distance = backward out.score = score out.len_code_delta = 0 } } } } } } h.next_ix = cur_ix + uint(h.jump) }