286 lines
8.9 KiB
Go
286 lines
8.9 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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// 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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// The dst and src must not overlap. 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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if len(p) < minNonLiteralBlockSize {
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d += emitLiteral(dst[d:], p)
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} else {
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d += encodeBlock(dst[d:], p)
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}
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}
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return dst[:d]
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}
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// inputMargin is the minimum number of extra input bytes to keep, inside
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// encodeBlock's inner loop. On some architectures, this margin lets us
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// implement a fast path for emitLiteral, where the copy of short (<= 16 byte)
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// literals can be implemented as a single load to and store from a 16-byte
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// register. That literal's actual length can be as short as 1 byte, so this
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// can copy up to 15 bytes too much, but that's OK as subsequent iterations of
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// the encoding loop will fix up the copy overrun, and this inputMargin ensures
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// that we don't overrun the dst and src buffers.
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const inputMargin = 16 - 1
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// minNonLiteralBlockSize is the minimum size of the input to encodeBlock that
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// could be encoded with a copy tag. This is the minimum with respect to the
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// algorithm used by encodeBlock, not a minimum enforced by the file format.
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//
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// The encoded output must start with at least a 1 byte literal, as there are
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// no previous bytes to copy. A minimal (1 byte) copy after that, generated
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// from an emitCopy call in encodeBlock's main loop, would require at least
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// another inputMargin bytes, for the reason above: we want any emitLiteral
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// calls inside encodeBlock's main loop to use the fast path if possible, which
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// requires being able to overrun by inputMargin bytes. Thus,
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// minNonLiteralBlockSize equals 1 + 1 + inputMargin.
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//
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// The C++ code doesn't use this exact threshold, but it could, as discussed at
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// https://groups.google.com/d/topic/snappy-compression/oGbhsdIJSJ8/discussion
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// The difference between Go (2+inputMargin) and C++ (inputMargin) is purely an
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// optimization. It should not affect the encoded form. This is tested by
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// TestSameEncodingAsCppShortCopies.
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const minNonLiteralBlockSize = 1 + 1 + inputMargin
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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 that 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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