writer.go

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package zlib
     6  
     7  import (
     8  	"compress/flate"
     9  	"encoding/binary"
    10  	"fmt"
    11  	"hash"
    12  	"hash/adler32"
    13  	"io"
    14  )
    15  
    16  // These constants are copied from the flate package, so that code that imports
    17  // "compress/zlib" does not also have to import "compress/flate".
    18  const (
    19  	NoCompression      = flate.NoCompression
    20  	BestSpeed          = flate.BestSpeed
    21  	BestCompression    = flate.BestCompression
    22  	DefaultCompression = flate.DefaultCompression
    23  	HuffmanOnly        = flate.HuffmanOnly
    24  )
    25  
    26  // A Writer takes data written to it and writes the compressed
    27  // form of that data to an underlying writer (see NewWriter).
    28  type Writer struct {
    29  	w           io.Writer
    30  	level       int
    31  	dict        []byte
    32  	compressor  *flate.Writer
    33  	digest      hash.Hash32
    34  	err         error
    35  	scratch     [4]byte
    36  	wroteHeader bool
    37  }
    38  
    39  // NewWriter creates a new Writer.
    40  // Writes to the returned Writer are compressed and written to w.
    41  //
    42  // It is the caller's responsibility to call Close on the Writer when done.
    43  // Writes may be buffered and not flushed until Close.
    44  func NewWriter(w io.Writer) *Writer {
    45  	z, _ := NewWriterLevelDict(w, DefaultCompression, nil)
    46  	return z
    47  }
    48  
    49  // NewWriterLevel is like NewWriter but specifies the compression level instead
    50  // of assuming DefaultCompression.
    51  //
    52  // The compression level can be DefaultCompression, NoCompression, HuffmanOnly
    53  // or any integer value between BestSpeed and BestCompression inclusive.
    54  // The error returned will be nil if the level is valid.
    55  func NewWriterLevel(w io.Writer, level int) (*Writer, error) {
    56  	return NewWriterLevelDict(w, level, nil)
    57  }
    58  
    59  // NewWriterLevelDict is like NewWriterLevel but specifies a dictionary to
    60  // compress with.
    61  //
    62  // The dictionary may be nil. If not, its contents should not be modified until
    63  // the Writer is closed.
    64  func NewWriterLevelDict(w io.Writer, level int, dict []byte) (*Writer, error) {
    65  	if level < HuffmanOnly || level > BestCompression {
    66  		return nil, fmt.Errorf("zlib: invalid compression level: %d", level)
    67  	}
    68  	return &Writer{
    69  		w:     w,
    70  		level: level,
    71  		dict:  dict,
    72  	}, nil
    73  }
    74  
    75  // Reset clears the state of the Writer z such that it is equivalent to its
    76  // initial state from NewWriterLevel or NewWriterLevelDict, but instead writing
    77  // to w.
    78  func (z *Writer) Reset(w io.Writer) {
    79  	z.w = w
    80  	// z.level and z.dict left unchanged.
    81  	if z.compressor != nil {
    82  		z.compressor.Reset(w)
    83  	}
    84  	if z.digest != nil {
    85  		z.digest.Reset()
    86  	}
    87  	z.err = nil
    88  	z.scratch = [4]byte{}
    89  	z.wroteHeader = false
    90  }
    91  
    92  // writeHeader writes the ZLIB header.
    93  func (z *Writer) writeHeader() (err error) {
    94  	z.wroteHeader = true
    95  	// ZLIB has a two-byte header (as documented in RFC 1950).
    96  	// The first four bits is the CINFO (compression info), which is 7 for the default deflate window size.
    97  	// The next four bits is the CM (compression method), which is 8 for deflate.
    98  	z.scratch[0] = 0x78
    99  	// The next two bits is the FLEVEL (compression level). The four values are:
   100  	// 0=fastest, 1=fast, 2=default, 3=best.
   101  	// The next bit, FDICT, is set if a dictionary is given.
   102  	// The final five FCHECK bits form a mod-31 checksum.
   103  	switch z.level {
   104  	case -2, 0, 1:
   105  		z.scratch[1] = 0 << 6
   106  	case 2, 3, 4, 5:
   107  		z.scratch[1] = 1 << 6
   108  	case 6, -1:
   109  		z.scratch[1] = 2 << 6
   110  	case 7, 8, 9:
   111  		z.scratch[1] = 3 << 6
   112  	default:
   113  		panic("unreachable")
   114  	}
   115  	if z.dict != nil {
   116  		z.scratch[1] |= 1 << 5
   117  	}
   118  	z.scratch[1] += uint8(31 - (uint16(z.scratch[0])<<8+uint16(z.scratch[1]))%31)
   119  	if _, err = z.w.Write(z.scratch[0:2]); err != nil {
   120  		return err
   121  	}
   122  	if z.dict != nil {
   123  		// The next four bytes are the Adler-32 checksum of the dictionary.
   124  		binary.BigEndian.PutUint32(z.scratch[:], adler32.Checksum(z.dict))
   125  		if _, err = z.w.Write(z.scratch[0:4]); err != nil {
   126  			return err
   127  		}
   128  	}
   129  	if z.compressor == nil {
   130  		// Initialize deflater unless the Writer is being reused
   131  		// after a Reset call.
   132  		z.compressor, err = flate.NewWriterDict(z.w, z.level, z.dict)
   133  		if err != nil {
   134  			return err
   135  		}
   136  		z.digest = adler32.New()
   137  	}
   138  	return nil
   139  }
   140  
   141  // Write writes a compressed form of p to the underlying io.Writer. The
   142  // compressed bytes are not necessarily flushed until the Writer is closed or
   143  // explicitly flushed.
   144  func (z *Writer) Write(p []byte) (n int, err error) {
   145  	if !z.wroteHeader {
   146  		z.err = z.writeHeader()
   147  	}
   148  	if z.err != nil {
   149  		return 0, z.err
   150  	}
   151  	if len(p) == 0 {
   152  		return 0, nil
   153  	}
   154  	n, err = z.compressor.Write(p)
   155  	if err != nil {
   156  		z.err = err
   157  		return
   158  	}
   159  	z.digest.Write(p)
   160  	return
   161  }
   162  
   163  // Flush flushes the Writer to its underlying io.Writer.
   164  func (z *Writer) Flush() error {
   165  	if !z.wroteHeader {
   166  		z.err = z.writeHeader()
   167  	}
   168  	if z.err != nil {
   169  		return z.err
   170  	}
   171  	z.err = z.compressor.Flush()
   172  	return z.err
   173  }
   174  
   175  // Close closes the Writer, flushing any unwritten data to the underlying
   176  // io.Writer, but does not close the underlying io.Writer.
   177  func (z *Writer) Close() error {
   178  	if !z.wroteHeader {
   179  		z.err = z.writeHeader()
   180  	}
   181  	if z.err != nil {
   182  		return z.err
   183  	}
   184  	z.err = z.compressor.Close()
   185  	if z.err != nil {
   186  		return z.err
   187  	}
   188  	checksum := z.digest.Sum32()
   189  	// ZLIB (RFC 1950) is big-endian, unlike GZIP (RFC 1952).
   190  	binary.BigEndian.PutUint32(z.scratch[:], checksum)
   191  	_, z.err = z.w.Write(z.scratch[0:4])
   192  	return z.err
   193  }
   194
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