404 lines
12 KiB
Go
404 lines
12 KiB
Go
// Copyright 2011 The Snappy-Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package snappy
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import (
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"encoding/binary"
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"errors"
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"io"
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)
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// maxOffset limits how far copy back-references can go, the same as the C++
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// code.
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const maxOffset = 1 << 15
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// emitLiteral writes a literal chunk and returns the number of bytes written.
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func emitLiteral(dst, lit []byte) int {
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i, n := 0, uint(len(lit)-1)
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switch {
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case n < 60:
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dst[0] = uint8(n)<<2 | tagLiteral
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i = 1
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case n < 1<<8:
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dst[0] = 60<<2 | tagLiteral
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dst[1] = uint8(n)
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i = 2
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case n < 1<<16:
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dst[0] = 61<<2 | tagLiteral
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dst[1] = uint8(n)
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dst[2] = uint8(n >> 8)
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i = 3
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case n < 1<<24:
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dst[0] = 62<<2 | tagLiteral
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dst[1] = uint8(n)
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dst[2] = uint8(n >> 8)
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dst[3] = uint8(n >> 16)
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i = 4
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case int64(n) < 1<<32:
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dst[0] = 63<<2 | tagLiteral
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dst[1] = uint8(n)
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dst[2] = uint8(n >> 8)
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dst[3] = uint8(n >> 16)
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dst[4] = uint8(n >> 24)
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i = 5
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default:
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panic("snappy: source buffer is too long")
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}
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if copy(dst[i:], lit) != len(lit) {
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panic("snappy: destination buffer is too short")
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}
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return i + len(lit)
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}
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// emitCopy writes a copy chunk and returns the number of bytes written.
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func emitCopy(dst []byte, offset, length int32) int {
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i := 0
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for length > 0 {
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x := length - 4
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if 0 <= x && x < 1<<3 && offset < 1<<11 {
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dst[i+0] = uint8(offset>>8)&0x07<<5 | uint8(x)<<2 | tagCopy1
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dst[i+1] = uint8(offset)
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i += 2
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break
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}
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x = length
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if x > 1<<6 {
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x = 1 << 6
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}
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dst[i+0] = uint8(x-1)<<2 | tagCopy2
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dst[i+1] = uint8(offset)
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dst[i+2] = uint8(offset >> 8)
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i += 3
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length -= x
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}
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return i
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}
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// Encode returns the encoded form of src. The returned slice may be a sub-
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// slice of dst if dst was large enough to hold the entire encoded block.
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// Otherwise, a newly allocated slice will be returned.
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//
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// It is valid to pass a nil dst.
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func Encode(dst, src []byte) []byte {
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if n := MaxEncodedLen(len(src)); n < 0 {
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panic(ErrTooLarge)
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} else if len(dst) < n {
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dst = make([]byte, n)
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}
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// The block starts with the varint-encoded length of the decompressed bytes.
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d := binary.PutUvarint(dst, uint64(len(src)))
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for len(src) > 0 {
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p := src
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src = nil
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if len(p) > maxBlockSize {
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p, src = p[:maxBlockSize], p[maxBlockSize:]
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}
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d += encodeBlock(dst[d:], p)
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}
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return dst[:d]
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}
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// encodeBlock encodes a non-empty src to a guaranteed-large-enough dst. It
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// assumes that the varint-encoded length of the decompressed bytes has already
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// been written.
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//
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// It also assumes that:
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// len(dst) >= MaxEncodedLen(len(src)) &&
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// 0 < len(src) && len(src) <= maxBlockSize
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func encodeBlock(dst, src []byte) (d int) {
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// Return early if src is short.
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if len(src) <= 4 {
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return emitLiteral(dst, src)
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}
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// Initialize the hash table. Its size ranges from 1<<8 to 1<<14 inclusive.
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const maxTableSize = 1 << 14
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shift, tableSize := uint(32-8), 1<<8
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for tableSize < maxTableSize && tableSize < len(src) {
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shift--
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tableSize *= 2
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}
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var table [maxTableSize]int32
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// Iterate over the source bytes.
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var (
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s int32 // The iterator position.
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t int32 // The last position with the same hash as s.
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lit int32 // The start position of any pending literal bytes.
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// Copied from the C++ snappy implementation:
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//
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// Heuristic match skipping: If 32 bytes are scanned with no matches
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// found, start looking only at every other byte. If 32 more bytes are
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// scanned, look at every third byte, etc.. When a match is found,
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// immediately go back to looking at every byte. This is a small loss
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// (~5% performance, ~0.1% density) for compressible data due to more
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// bookkeeping, but for non-compressible data (such as JPEG) it's a
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// huge win since the compressor quickly "realizes" the data is
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// incompressible and doesn't bother looking for matches everywhere.
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//
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// The "skip" variable keeps track of how many bytes there are since
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// the last match; dividing it by 32 (ie. right-shifting by five) gives
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// the number of bytes to move ahead for each iteration.
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skip uint32 = 32
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)
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for uint32(s+3) < uint32(len(src)) { // The uint32 conversions catch overflow from the +3.
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// Update the hash table.
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b0, b1, b2, b3 := src[s], src[s+1], src[s+2], src[s+3]
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h := uint32(b0) | uint32(b1)<<8 | uint32(b2)<<16 | uint32(b3)<<24
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p := &table[(h*0x1e35a7bd)>>shift]
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// We need to to store values in [-1, inf) in table. To save
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// some initialization time, (re)use the table's zero value
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// and shift the values against this zero: add 1 on writes,
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// subtract 1 on reads.
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t, *p = *p-1, s+1
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// If t is invalid or src[s:s+4] differs from src[t:t+4], accumulate a literal byte.
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if t < 0 || s-t >= maxOffset || b0 != src[t] || b1 != src[t+1] || b2 != src[t+2] || b3 != src[t+3] {
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s += int32(skip >> 5)
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skip++
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continue
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}
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skip = 32
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// Otherwise, we have a match. First, emit any pending literal bytes.
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if lit != s {
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d += emitLiteral(dst[d:], src[lit:s])
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}
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// Extend the match to be as long as possible.
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s0 := s
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s, t = s+4, t+4
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for int(s) < len(src) && src[s] == src[t] {
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s++
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t++
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}
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// Emit the copied bytes.
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d += emitCopy(dst[d:], s-t, s-s0)
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lit = s
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}
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// Emit any final pending literal bytes and return.
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if int(lit) != len(src) {
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d += emitLiteral(dst[d:], src[lit:])
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}
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return d
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}
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// MaxEncodedLen returns the maximum length of a snappy block, given its
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// uncompressed length.
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//
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// It will return a negative value if srcLen is too large to encode.
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func MaxEncodedLen(srcLen int) int {
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n := uint64(srcLen)
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if n > 0xffffffff {
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return -1
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}
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// Compressed data can be defined as:
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// compressed := item* literal*
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// item := literal* copy
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//
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// The trailing literal sequence has a space blowup of at most 62/60
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// since a literal of length 60 needs one tag byte + one extra byte
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// for length information.
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//
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// Item blowup is trickier to measure. Suppose the "copy" op copies
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// 4 bytes of data. Because of a special check in the encoding code,
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// we produce a 4-byte copy only if the offset is < 65536. Therefore
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// the copy op takes 3 bytes to encode, and this type of item leads
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// to at most the 62/60 blowup for representing literals.
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//
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// Suppose the "copy" op copies 5 bytes of data. If the offset is big
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// enough, it will take 5 bytes to encode the copy op. Therefore the
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// worst case here is a one-byte literal followed by a five-byte copy.
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// That is, 6 bytes of input turn into 7 bytes of "compressed" data.
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//
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// This last factor dominates the blowup, so the final estimate is:
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n = 32 + n + n/6
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if n > 0xffffffff {
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return -1
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}
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return int(n)
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}
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var errClosed = errors.New("snappy: Writer is closed")
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// NewWriter returns a new Writer that compresses to w.
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//
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// The Writer returned does not buffer writes. There is no need to Flush or
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// Close such a Writer.
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//
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// Deprecated: the Writer returned is not suitable for many small writes, only
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// for few large writes. Use NewBufferedWriter instead, which is efficient
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// regardless of the frequency and shape of the writes, and remember to Close
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// that Writer when done.
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func NewWriter(w io.Writer) *Writer {
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return &Writer{
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w: w,
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obuf: make([]byte, obufLen),
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}
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}
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// NewBufferedWriter returns a new Writer that compresses to w, using the
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// framing format described at
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// https://github.com/google/snappy/blob/master/framing_format.txt
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//
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// The Writer returned buffers writes. Users must call Close to guarantee all
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// data has been forwarded to the underlying io.Writer. They may also call
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// Flush zero or more times before calling Close.
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func NewBufferedWriter(w io.Writer) *Writer {
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return &Writer{
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w: w,
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ibuf: make([]byte, 0, maxBlockSize),
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obuf: make([]byte, obufLen),
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}
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}
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// Writer is an io.Writer than can write Snappy-compressed bytes.
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type Writer struct {
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w io.Writer
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err error
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// ibuf is a buffer for the incoming (uncompressed) bytes.
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//
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// Its use is optional. For backwards compatibility, Writers created by the
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// NewWriter function have ibuf == nil, do not buffer incoming bytes, and
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// therefore do not need to be Flush'ed or Close'd.
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ibuf []byte
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// obuf is a buffer for the outgoing (compressed) bytes.
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obuf []byte
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// wroteStreamHeader is whether we have written the stream header.
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wroteStreamHeader bool
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}
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// Reset discards the writer's state and switches the Snappy writer to write to
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// w. This permits reusing a Writer rather than allocating a new one.
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func (w *Writer) Reset(writer io.Writer) {
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w.w = writer
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w.err = nil
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if w.ibuf != nil {
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w.ibuf = w.ibuf[:0]
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}
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w.wroteStreamHeader = false
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}
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// Write satisfies the io.Writer interface.
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func (w *Writer) Write(p []byte) (nRet int, errRet error) {
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if w.ibuf == nil {
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// Do not buffer incoming bytes. This does not perform or compress well
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// if the caller of Writer.Write writes many small slices. This
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// behavior is therefore deprecated, but still supported for backwards
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// compatibility with code that doesn't explicitly Flush or Close.
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return w.write(p)
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}
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// The remainder of this method is based on bufio.Writer.Write from the
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// standard library.
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for len(p) > (cap(w.ibuf)-len(w.ibuf)) && w.err == nil {
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var n int
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if len(w.ibuf) == 0 {
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// Large write, empty buffer.
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// Write directly from p to avoid copy.
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n, _ = w.write(p)
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} else {
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n = copy(w.ibuf[len(w.ibuf):cap(w.ibuf)], p)
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w.ibuf = w.ibuf[:len(w.ibuf)+n]
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w.Flush()
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}
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nRet += n
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p = p[n:]
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}
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if w.err != nil {
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return nRet, w.err
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}
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n := copy(w.ibuf[len(w.ibuf):cap(w.ibuf)], p)
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w.ibuf = w.ibuf[:len(w.ibuf)+n]
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nRet += n
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return nRet, nil
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}
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func (w *Writer) write(p []byte) (nRet int, errRet error) {
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if w.err != nil {
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return 0, w.err
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}
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for len(p) > 0 {
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obufStart := len(magicChunk)
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if !w.wroteStreamHeader {
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w.wroteStreamHeader = true
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copy(w.obuf, magicChunk)
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obufStart = 0
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}
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var uncompressed []byte
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if len(p) > maxBlockSize {
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uncompressed, p = p[:maxBlockSize], p[maxBlockSize:]
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} else {
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uncompressed, p = p, nil
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}
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checksum := crc(uncompressed)
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// Compress the buffer, discarding the result if the improvement
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// isn't at least 12.5%.
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compressed := Encode(w.obuf[obufHeaderLen:], uncompressed)
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chunkType := uint8(chunkTypeCompressedData)
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chunkLen := 4 + len(compressed)
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obufEnd := obufHeaderLen + len(compressed)
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if len(compressed) >= len(uncompressed)-len(uncompressed)/8 {
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chunkType = chunkTypeUncompressedData
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chunkLen = 4 + len(uncompressed)
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obufEnd = obufHeaderLen
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}
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// Fill in the per-chunk header that comes before the body.
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w.obuf[len(magicChunk)+0] = chunkType
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w.obuf[len(magicChunk)+1] = uint8(chunkLen >> 0)
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w.obuf[len(magicChunk)+2] = uint8(chunkLen >> 8)
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w.obuf[len(magicChunk)+3] = uint8(chunkLen >> 16)
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w.obuf[len(magicChunk)+4] = uint8(checksum >> 0)
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w.obuf[len(magicChunk)+5] = uint8(checksum >> 8)
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w.obuf[len(magicChunk)+6] = uint8(checksum >> 16)
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w.obuf[len(magicChunk)+7] = uint8(checksum >> 24)
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if _, err := w.w.Write(w.obuf[obufStart:obufEnd]); err != nil {
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w.err = err
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return nRet, err
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}
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if chunkType == chunkTypeUncompressedData {
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if _, err := w.w.Write(uncompressed); err != nil {
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w.err = err
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return nRet, err
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}
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}
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nRet += len(uncompressed)
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}
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return nRet, nil
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}
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// Flush flushes the Writer to its underlying io.Writer.
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func (w *Writer) Flush() error {
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if w.err != nil {
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return w.err
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}
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if len(w.ibuf) == 0 {
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return nil
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}
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w.write(w.ibuf)
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w.ibuf = w.ibuf[:0]
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return w.err
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}
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// Close calls Flush and then closes the Writer.
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func (w *Writer) Close() error {
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w.Flush()
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ret := w.err
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if w.err == nil {
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w.err = errClosed
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}
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return ret
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}
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