reader.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  /*
     6  Package zlib implements reading and writing of zlib format compressed data,
     7  as specified in RFC 1950.
     8  
     9  The implementation provides filters that uncompress during reading
    10  and compress during writing.  For example, to write compressed data
    11  to a buffer:
    12  
    13  	var b bytes.Buffer
    14  	w := zlib.NewWriter(&b)
    15  	w.Write([]byte("hello, world\n"))
    16  	w.Close()
    17  
    18  and to read that data back:
    19  
    20  	r, err := zlib.NewReader(&b)
    21  	io.Copy(os.Stdout, r)
    22  	r.Close()
    23  */
    24  package zlib
    25  
    26  import (
    27  	"bufio"
    28  	"compress/flate"
    29  	"errors"
    30  	"hash"
    31  	"hash/adler32"
    32  	"io"
    33  )
    34  
    35  const zlibDeflate = 8
    36  
    37  var (
    38  	// ErrChecksum is returned when reading ZLIB data that has an invalid checksum.
    39  	ErrChecksum = errors.New("zlib: invalid checksum")
    40  	// ErrDictionary is returned when reading ZLIB data that has an invalid dictionary.
    41  	ErrDictionary = errors.New("zlib: invalid dictionary")
    42  	// ErrHeader is returned when reading ZLIB data that has an invalid header.
    43  	ErrHeader = errors.New("zlib: invalid header")
    44  )
    45  
    46  type reader struct {
    47  	r            flate.Reader
    48  	decompressor io.ReadCloser
    49  	digest       hash.Hash32
    50  	err          error
    51  	scratch      [4]byte
    52  }
    53  
    54  // Resetter resets a ReadCloser returned by NewReader or NewReaderDict
    55  // to switch to a new underlying Reader. This permits reusing a ReadCloser
    56  // instead of allocating a new one.
    57  type Resetter interface {
    58  	// Reset discards any buffered data and resets the Resetter as if it was
    59  	// newly initialized with the given reader.
    60  	Reset(r io.Reader, dict []byte) error
    61  }
    62  
    63  // NewReader creates a new ReadCloser.
    64  // Reads from the returned ReadCloser read and decompress data from r.
    65  // If r does not implement io.ByteReader, the decompressor may read more
    66  // data than necessary from r.
    67  // It is the caller's responsibility to call Close on the ReadCloser when done.
    68  //
    69  // The ReadCloser returned by NewReader also implements Resetter.
    70  func NewReader(r io.Reader) (io.ReadCloser, error) {
    71  	return NewReaderDict(r, nil)
    72  }
    73  
    74  // NewReaderDict is like NewReader but uses a preset dictionary.
    75  // NewReaderDict ignores the dictionary if the compressed data does not refer to it.
    76  // If the compressed data refers to a different dictionary, NewReaderDict returns ErrDictionary.
    77  //
    78  // The ReadCloser returned by NewReaderDict also implements Resetter.
    79  func NewReaderDict(r io.Reader, dict []byte) (io.ReadCloser, error) {
    80  	z := new(reader)
    81  	err := z.Reset(r, dict)
    82  	if err != nil {
    83  		return nil, err
    84  	}
    85  	return z, nil
    86  }
    87  
    88  func (z *reader) Read(p []byte) (int, error) {
    89  	if z.err != nil {
    90  		return 0, z.err
    91  	}
    92  
    93  	var n int
    94  	n, z.err = z.decompressor.Read(p)
    95  	z.digest.Write(p[0:n])
    96  	if z.err != io.EOF {
    97  		// In the normal case we return here.
    98  		return n, z.err
    99  	}
   100  
   101  	// Finished file; check checksum.
   102  	if _, err := io.ReadFull(z.r, z.scratch[0:4]); err != nil {
   103  		if err == io.EOF {
   104  			err = io.ErrUnexpectedEOF
   105  		}
   106  		z.err = err
   107  		return n, z.err
   108  	}
   109  	// ZLIB (RFC 1950) is big-endian, unlike GZIP (RFC 1952).
   110  	checksum := uint32(z.scratch[0])<<24 | uint32(z.scratch[1])<<16 | uint32(z.scratch[2])<<8 | uint32(z.scratch[3])
   111  	if checksum != z.digest.Sum32() {
   112  		z.err = ErrChecksum
   113  		return n, z.err
   114  	}
   115  	return n, io.EOF
   116  }
   117  
   118  // Calling Close does not close the wrapped io.Reader originally passed to NewReader.
   119  // In order for the ZLIB checksum to be verified, the reader must be
   120  // fully consumed until the io.EOF.
   121  func (z *reader) Close() error {
   122  	if z.err != nil && z.err != io.EOF {
   123  		return z.err
   124  	}
   125  	z.err = z.decompressor.Close()
   126  	return z.err
   127  }
   128  
   129  func (z *reader) Reset(r io.Reader, dict []byte) error {
   130  	*z = reader{decompressor: z.decompressor}
   131  	if fr, ok := r.(flate.Reader); ok {
   132  		z.r = fr
   133  	} else {
   134  		z.r = bufio.NewReader(r)
   135  	}
   136  
   137  	// Read the header (RFC 1950 section 2.2.).
   138  	_, z.err = io.ReadFull(z.r, z.scratch[0:2])
   139  	if z.err != nil {
   140  		if z.err == io.EOF {
   141  			z.err = io.ErrUnexpectedEOF
   142  		}
   143  		return z.err
   144  	}
   145  	h := uint(z.scratch[0])<<8 | uint(z.scratch[1])
   146  	if (z.scratch[0]&0x0f != zlibDeflate) || (h%31 != 0) {
   147  		z.err = ErrHeader
   148  		return z.err
   149  	}
   150  	haveDict := z.scratch[1]&0x20 != 0
   151  	if haveDict {
   152  		_, z.err = io.ReadFull(z.r, z.scratch[0:4])
   153  		if z.err != nil {
   154  			if z.err == io.EOF {
   155  				z.err = io.ErrUnexpectedEOF
   156  			}
   157  			return z.err
   158  		}
   159  		checksum := uint32(z.scratch[0])<<24 | uint32(z.scratch[1])<<16 | uint32(z.scratch[2])<<8 | uint32(z.scratch[3])
   160  		if checksum != adler32.Checksum(dict) {
   161  			z.err = ErrDictionary
   162  			return z.err
   163  		}
   164  	}
   165  
   166  	if z.decompressor == nil {
   167  		if haveDict {
   168  			z.decompressor = flate.NewReaderDict(z.r, dict)
   169  		} else {
   170  			z.decompressor = flate.NewReader(z.r)
   171  		}
   172  	} else {
   173  		z.decompressor.(flate.Resetter).Reset(z.r, dict)
   174  	}
   175  	z.digest = adler32.New()
   176  	return nil
   177  }
   178
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