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- // Copyright 2016 The Snappy-Go Authors. All rights reserved.
- // Copyright (c) 2019 Klaus Post. All rights reserved.
- // Use of this source code is governed by a BSD-style
- // license that can be found in the LICENSE file.
- package s2
- import (
- "bytes"
- "encoding/binary"
- "math/bits"
- )
- func load32(b []byte, i int) uint32 {
- b = b[i:]
- b = b[:4]
- return uint32(b[0]) | uint32(b[1])<<8 | uint32(b[2])<<16 | uint32(b[3])<<24
- }
- func load64(b []byte, i int) uint64 {
- b = b[i:]
- b = b[:8]
- return uint64(b[0]) | uint64(b[1])<<8 | uint64(b[2])<<16 | uint64(b[3])<<24 |
- uint64(b[4])<<32 | uint64(b[5])<<40 | uint64(b[6])<<48 | uint64(b[7])<<56
- }
- // hash6 returns the hash of the lowest 6 bytes of u to fit in a hash table with h bits.
- // Preferably h should be a constant and should always be <64.
- func hash6(u uint64, h uint8) uint32 {
- const prime6bytes = 227718039650203
- return uint32(((u << (64 - 48)) * prime6bytes) >> ((64 - h) & 63))
- }
- func encodeGo(dst, src []byte) []byte {
- if n := MaxEncodedLen(len(src)); n < 0 {
- panic(ErrTooLarge)
- } else if len(dst) < n {
- dst = make([]byte, n)
- }
- // The block starts with the varint-encoded length of the decompressed bytes.
- d := binary.PutUvarint(dst, uint64(len(src)))
- if len(src) == 0 {
- return dst[:d]
- }
- if len(src) < minNonLiteralBlockSize {
- d += emitLiteral(dst[d:], src)
- return dst[:d]
- }
- n := encodeBlockGo(dst[d:], src)
- if n > 0 {
- d += n
- return dst[:d]
- }
- // Not compressible
- d += emitLiteral(dst[d:], src)
- return dst[:d]
- }
- // encodeBlockGo encodes a non-empty src to a guaranteed-large-enough dst. It
- // assumes that the varint-encoded length of the decompressed bytes has already
- // been written.
- //
- // It also assumes that:
- // len(dst) >= MaxEncodedLen(len(src)) &&
- // minNonLiteralBlockSize <= len(src) && len(src) <= maxBlockSize
- func encodeBlockGo(dst, src []byte) (d int) {
- // Initialize the hash table.
- const (
- tableBits = 14
- maxTableSize = 1 << tableBits
- )
- var table [maxTableSize]uint32
- // sLimit is when to stop looking for offset/length copies. The inputMargin
- // lets us use a fast path for emitLiteral in the main loop, while we are
- // looking for copies.
- sLimit := len(src) - inputMargin
- // Bail if we can't compress to at least this.
- dstLimit := len(src) - len(src)>>5 - 5
- // nextEmit is where in src the next emitLiteral should start from.
- nextEmit := 0
- // The encoded form must start with a literal, as there are no previous
- // bytes to copy, so we start looking for hash matches at s == 1.
- s := 1
- cv := load64(src, s)
- // We search for a repeat at -1, but don't output repeats when nextEmit == 0
- repeat := 1
- for {
- candidate := 0
- for {
- // Next src position to check
- nextS := s + (s-nextEmit)>>6 + 4
- if nextS > sLimit {
- goto emitRemainder
- }
- hash0 := hash6(cv, tableBits)
- hash1 := hash6(cv>>8, tableBits)
- candidate = int(table[hash0])
- candidate2 := int(table[hash1])
- table[hash0] = uint32(s)
- table[hash1] = uint32(s + 1)
- hash2 := hash6(cv>>16, tableBits)
- // Check repeat at offset checkRep.
- const checkRep = 1
- if uint32(cv>>(checkRep*8)) == load32(src, s-repeat+checkRep) {
- base := s + checkRep
- // Extend back
- for i := base - repeat; base > nextEmit && i > 0 && src[i-1] == src[base-1]; {
- i--
- base--
- }
- d += emitLiteral(dst[d:], src[nextEmit:base])
- // Extend forward
- candidate := s - repeat + 4 + checkRep
- s += 4 + checkRep
- for s <= sLimit {
- if diff := load64(src, s) ^ load64(src, candidate); diff != 0 {
- s += bits.TrailingZeros64(diff) >> 3
- break
- }
- s += 8
- candidate += 8
- }
- if nextEmit > 0 {
- // same as `add := emitCopy(dst[d:], repeat, s-base)` but skips storing offset.
- d += emitRepeat(dst[d:], repeat, s-base)
- } else {
- // First match, cannot be repeat.
- d += emitCopy(dst[d:], repeat, s-base)
- }
- nextEmit = s
- if s >= sLimit {
- goto emitRemainder
- }
- cv = load64(src, s)
- continue
- }
- if uint32(cv) == load32(src, candidate) {
- break
- }
- candidate = int(table[hash2])
- if uint32(cv>>8) == load32(src, candidate2) {
- table[hash2] = uint32(s + 2)
- candidate = candidate2
- s++
- break
- }
- table[hash2] = uint32(s + 2)
- if uint32(cv>>16) == load32(src, candidate) {
- s += 2
- break
- }
- cv = load64(src, nextS)
- s = nextS
- }
- // Extend backwards
- for candidate > 0 && s > nextEmit && src[candidate-1] == src[s-1] {
- candidate--
- s--
- }
- // Bail if we exceed the maximum size.
- if d+(s-nextEmit) > dstLimit {
- return 0
- }
- // A 4-byte match has been found. We'll later see if more than 4 bytes
- // match. But, prior to the match, src[nextEmit:s] are unmatched. Emit
- // them as literal bytes.
- d += emitLiteral(dst[d:], src[nextEmit:s])
- // Call emitCopy, and then see if another emitCopy could be our next
- // move. Repeat until we find no match for the input immediately after
- // what was consumed by the last emitCopy call.
- //
- // If we exit this loop normally then we need to call emitLiteral next,
- // though we don't yet know how big the literal will be. We handle that
- // by proceeding to the next iteration of the main loop. We also can
- // exit this loop via goto if we get close to exhausting the input.
- for {
- // Invariant: we have a 4-byte match at s, and no need to emit any
- // literal bytes prior to s.
- base := s
- repeat = base - candidate
- // Extend the 4-byte match as long as possible.
- s += 4
- candidate += 4
- for s <= len(src)-8 {
- if diff := load64(src, s) ^ load64(src, candidate); diff != 0 {
- s += bits.TrailingZeros64(diff) >> 3
- break
- }
- s += 8
- candidate += 8
- }
- d += emitCopy(dst[d:], repeat, s-base)
- if false {
- // Validate match.
- a := src[base:s]
- b := src[base-repeat : base-repeat+(s-base)]
- if !bytes.Equal(a, b) {
- panic("mismatch")
- }
- }
- nextEmit = s
- if s >= sLimit {
- goto emitRemainder
- }
- if d > dstLimit {
- // Do we have space for more, if not bail.
- return 0
- }
- // Check for an immediate match, otherwise start search at s+1
- x := load64(src, s-2)
- m2Hash := hash6(x, tableBits)
- currHash := hash6(x>>16, tableBits)
- candidate = int(table[currHash])
- table[m2Hash] = uint32(s - 2)
- table[currHash] = uint32(s)
- if uint32(x>>16) != load32(src, candidate) {
- cv = load64(src, s+1)
- s++
- break
- }
- }
- }
- emitRemainder:
- if nextEmit < len(src) {
- // Bail if we exceed the maximum size.
- if d+len(src)-nextEmit > dstLimit {
- return 0
- }
- d += emitLiteral(dst[d:], src[nextEmit:])
- }
- return d
- }
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