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Source file src/compress/zlib/writer.go

Documentation: compress/zlib

     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  	"fmt"
    10  	"hash"
    11  	"hash/adler32"
    12  	"io"
    13  )
    14  
    15  // These constants are copied from the flate package, so that code that imports
    16  // "compress/zlib" does not also have to import "compress/flate".
    17  const (
    18  	NoCompression      = flate.NoCompression
    19  	BestSpeed          = flate.BestSpeed
    20  	BestCompression    = flate.BestCompression
    21  	DefaultCompression = flate.DefaultCompression
    22  	HuffmanOnly        = flate.HuffmanOnly
    23  )
    24  
    25  // A Writer takes data written to it and writes the compressed
    26  // form of that data to an underlying writer (see NewWriter).
    27  type Writer struct {
    28  	w           io.Writer
    29  	level       int
    30  	dict        []byte
    31  	compressor  *flate.Writer
    32  	digest      hash.Hash32
    33  	err         error
    34  	scratch     [4]byte
    35  	wroteHeader bool
    36  }
    37  
    38  // NewWriter creates a new Writer.
    39  // Writes to the returned Writer are compressed and written to w.
    40  //
    41  // It is the caller's responsibility to call Close on the Writer when done.
    42  // Writes may be buffered and not flushed until Close.
    43  func NewWriter(w io.Writer) *Writer {
    44  	z, _ := NewWriterLevelDict(w, DefaultCompression, nil)
    45  	return z
    46  }
    47  
    48  // NewWriterLevel is like NewWriter but specifies the compression level instead
    49  // of assuming DefaultCompression.
    50  //
    51  // The compression level can be DefaultCompression, NoCompression, HuffmanOnly
    52  // or any integer value between BestSpeed and BestCompression inclusive.
    53  // The error returned will be nil if the level is valid.
    54  func NewWriterLevel(w io.Writer, level int) (*Writer, error) {
    55  	return NewWriterLevelDict(w, level, nil)
    56  }
    57  
    58  // NewWriterLevelDict is like NewWriterLevel but specifies a dictionary to
    59  // compress with.
    60  //
    61  // The dictionary may be nil. If not, its contents should not be modified until
    62  // the Writer is closed.
    63  func NewWriterLevelDict(w io.Writer, level int, dict []byte) (*Writer, error) {
    64  	if level < HuffmanOnly || level > BestCompression {
    65  		return nil, fmt.Errorf("zlib: invalid compression level: %d", level)
    66  	}
    67  	return &Writer{
    68  		w:     w,
    69  		level: level,
    70  		dict:  dict,
    71  	}, nil
    72  }
    73  
    74  // Reset clears the state of the Writer z such that it is equivalent to its
    75  // initial state from NewWriterLevel or NewWriterLevelDict, but instead writing
    76  // to w.
    77  func (z *Writer) Reset(w io.Writer) {
    78  	z.w = w
    79  	// z.level and z.dict left unchanged.
    80  	if z.compressor != nil {
    81  		z.compressor.Reset(w)
    82  	}
    83  	if z.digest != nil {
    84  		z.digest.Reset()
    85  	}
    86  	z.err = nil
    87  	z.scratch = [4]byte{}
    88  	z.wroteHeader = false
    89  }
    90  
    91  // writeHeader writes the ZLIB header.
    92  func (z *Writer) writeHeader() (err error) {
    93  	z.wroteHeader = true
    94  	// ZLIB has a two-byte header (as documented in RFC 1950).
    95  	// The first four bits is the CINFO (compression info), which is 7 for the default deflate window size.
    96  	// The next four bits is the CM (compression method), which is 8 for deflate.
    97  	z.scratch[0] = 0x78
    98  	// The next two bits is the FLEVEL (compression level). The four values are:
    99  	// 0=fastest, 1=fast, 2=default, 3=best.
   100  	// The next bit, FDICT, is set if a dictionary is given.
   101  	// The final five FCHECK bits form a mod-31 checksum.
   102  	switch z.level {
   103  	case -2, 0, 1:
   104  		z.scratch[1] = 0 << 6
   105  	case 2, 3, 4, 5:
   106  		z.scratch[1] = 1 << 6
   107  	case 6, -1:
   108  		z.scratch[1] = 2 << 6
   109  	case 7, 8, 9:
   110  		z.scratch[1] = 3 << 6
   111  	default:
   112  		panic("unreachable")
   113  	}
   114  	if z.dict != nil {
   115  		z.scratch[1] |= 1 << 5
   116  	}
   117  	z.scratch[1] += uint8(31 - (uint16(z.scratch[0])<<8+uint16(z.scratch[1]))%31)
   118  	if _, err = z.w.Write(z.scratch[0:2]); err != nil {
   119  		return err
   120  	}
   121  	if z.dict != nil {
   122  		// The next four bytes are the Adler-32 checksum of the dictionary.
   123  		checksum := adler32.Checksum(z.dict)
   124  		z.scratch[0] = uint8(checksum >> 24)
   125  		z.scratch[1] = uint8(checksum >> 16)
   126  		z.scratch[2] = uint8(checksum >> 8)
   127  		z.scratch[3] = uint8(checksum >> 0)
   128  		if _, err = z.w.Write(z.scratch[0:4]); err != nil {
   129  			return err
   130  		}
   131  	}
   132  	if z.compressor == nil {
   133  		// Initialize deflater unless the Writer is being reused
   134  		// after a Reset call.
   135  		z.compressor, err = flate.NewWriterDict(z.w, z.level, z.dict)
   136  		if err != nil {
   137  			return err
   138  		}
   139  		z.digest = adler32.New()
   140  	}
   141  	return nil
   142  }
   143  
   144  // Write writes a compressed form of p to the underlying io.Writer. The
   145  // compressed bytes are not necessarily flushed until the Writer is closed or
   146  // explicitly flushed.
   147  func (z *Writer) Write(p []byte) (n int, err error) {
   148  	if !z.wroteHeader {
   149  		z.err = z.writeHeader()
   150  	}
   151  	if z.err != nil {
   152  		return 0, z.err
   153  	}
   154  	if len(p) == 0 {
   155  		return 0, nil
   156  	}
   157  	n, err = z.compressor.Write(p)
   158  	if err != nil {
   159  		z.err = err
   160  		return
   161  	}
   162  	z.digest.Write(p)
   163  	return
   164  }
   165  
   166  // Flush flushes the Writer to its underlying io.Writer.
   167  func (z *Writer) Flush() error {
   168  	if !z.wroteHeader {
   169  		z.err = z.writeHeader()
   170  	}
   171  	if z.err != nil {
   172  		return z.err
   173  	}
   174  	z.err = z.compressor.Flush()
   175  	return z.err
   176  }
   177  
   178  // Close closes the Writer, flushing any unwritten data to the underlying
   179  // io.Writer, but does not close the underlying io.Writer.
   180  func (z *Writer) Close() error {
   181  	if !z.wroteHeader {
   182  		z.err = z.writeHeader()
   183  	}
   184  	if z.err != nil {
   185  		return z.err
   186  	}
   187  	z.err = z.compressor.Close()
   188  	if z.err != nil {
   189  		return z.err
   190  	}
   191  	checksum := z.digest.Sum32()
   192  	// ZLIB (RFC 1950) is big-endian, unlike GZIP (RFC 1952).
   193  	z.scratch[0] = uint8(checksum >> 24)
   194  	z.scratch[1] = uint8(checksum >> 16)
   195  	z.scratch[2] = uint8(checksum >> 8)
   196  	z.scratch[3] = uint8(checksum >> 0)
   197  	_, z.err = z.w.Write(z.scratch[0:4])
   198  	return z.err
   199  }
   200  

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