reader.go

Documentation: github.com/klauspost/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  /*
     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/zlib"
    29  	"hash"
    30  	"hash/adler32"
    31  	"io"
    32  
    33  	"github.com/klauspost/compress/flate"
    34  )
    35  
    36  const zlibDeflate = 8
    37  
    38  var (
    39  	// ErrChecksum is returned when reading ZLIB data that has an invalid checksum.
    40  	ErrChecksum = zlib.ErrChecksum
    41  	// ErrDictionary is returned when reading ZLIB data that has an invalid dictionary.
    42  	ErrDictionary = zlib.ErrDictionary
    43  	// ErrHeader is returned when reading ZLIB data that has an invalid header.
    44  	ErrHeader = zlib.ErrHeader
    45  )
    46  
    47  type reader struct {
    48  	r            flate.Reader
    49  	decompressor io.ReadCloser
    50  	digest       hash.Hash32
    51  	err          error
    52  	scratch      [4]byte
    53  }
    54  
    55  // Resetter resets a ReadCloser returned by NewReader or NewReaderDict to
    56  // to switch to a new underlying Reader. This permits reusing a ReadCloser
    57  // instead of allocating a new one.
    58  type Resetter interface {
    59  	// Reset discards any buffered data and resets the Resetter as if it was
    60  	// newly initialized with the given reader.
    61  	Reset(r io.Reader, dict []byte) error
    62  }
    63  
    64  // NewReader creates a new ReadCloser.
    65  // Reads from the returned ReadCloser read and decompress data from r.
    66  // If r does not implement io.ByteReader, the decompressor may read more
    67  // data than necessary from r.
    68  // It is the caller's responsibility to call Close on the ReadCloser when done.
    69  //
    70  // The ReadCloser returned by NewReader also implements Resetter.
    71  func NewReader(r io.Reader) (io.ReadCloser, error) {
    72  	return NewReaderDict(r, nil)
    73  }
    74  
    75  // NewReaderDict is like NewReader but uses a preset dictionary.
    76  // NewReaderDict ignores the dictionary if the compressed data does not refer to it.
    77  // If the compressed data refers to a different dictionary, NewReaderDict returns ErrDictionary.
    78  //
    79  // The ReadCloser returned by NewReaderDict also implements Resetter.
    80  func NewReaderDict(r io.Reader, dict []byte) (io.ReadCloser, error) {
    81  	z := new(reader)
    82  	err := z.Reset(r, dict)
    83  	if err != nil {
    84  		return nil, err
    85  	}
    86  	return z, nil
    87  }
    88  
    89  func (z *reader) Read(p []byte) (int, error) {
    90  	if z.err != nil {
    91  		return 0, z.err
    92  	}
    93  
    94  	var n int
    95  	n, z.err = z.decompressor.Read(p)
    96  	z.digest.Write(p[0:n])
    97  	if z.err != io.EOF {
    98  		// In the normal case we return here.
    99  		return n, z.err
   100  	}
   101  
   102  	// Finished file; check checksum.
   103  	if _, err := io.ReadFull(z.r, z.scratch[0:4]); err != nil {
   104  		if err == io.EOF {
   105  			err = io.ErrUnexpectedEOF
   106  		}
   107  		z.err = err
   108  		return n, z.err
   109  	}
   110  	// ZLIB (RFC 1950) is big-endian, unlike GZIP (RFC 1952).
   111  	checksum := uint32(z.scratch[0])<<24 | uint32(z.scratch[1])<<16 | uint32(z.scratch[2])<<8 | uint32(z.scratch[3])
   112  	if checksum != z.digest.Sum32() {
   113  		z.err = ErrChecksum
   114  		return n, z.err
   115  	}
   116  	return n, io.EOF
   117  }
   118  
   119  // Calling Close does not close the wrapped io.Reader originally passed to NewReader.
   120  // In order for the ZLIB checksum to be verified, the reader must be
   121  // fully consumed until the io.EOF.
   122  func (z *reader) Close() error {
   123  	if z.err != nil && z.err != io.EOF {
   124  		return z.err
   125  	}
   126  	z.err = z.decompressor.Close()
   127  	return z.err
   128  }
   129  
   130  func (z *reader) Reset(r io.Reader, dict []byte) error {
   131  	*z = reader{decompressor: z.decompressor, digest: z.digest}
   132  	if fr, ok := r.(flate.Reader); ok {
   133  		z.r = fr
   134  	} else {
   135  		z.r = bufio.NewReader(r)
   136  	}
   137  
   138  	// Read the header (RFC 1950 section 2.2.).
   139  	_, z.err = io.ReadFull(z.r, z.scratch[0:2])
   140  	if z.err != nil {
   141  		if z.err == io.EOF {
   142  			z.err = io.ErrUnexpectedEOF
   143  		}
   144  		return z.err
   145  	}
   146  	h := uint(z.scratch[0])<<8 | uint(z.scratch[1])
   147  	if (z.scratch[0]&0x0f != zlibDeflate) || (h%31 != 0) {
   148  		z.err = ErrHeader
   149  		return z.err
   150  	}
   151  	haveDict := z.scratch[1]&0x20 != 0
   152  	if haveDict {
   153  		_, z.err = io.ReadFull(z.r, z.scratch[0:4])
   154  		if z.err != nil {
   155  			if z.err == io.EOF {
   156  				z.err = io.ErrUnexpectedEOF
   157  			}
   158  			return z.err
   159  		}
   160  		checksum := uint32(z.scratch[0])<<24 | uint32(z.scratch[1])<<16 | uint32(z.scratch[2])<<8 | uint32(z.scratch[3])
   161  		if checksum != adler32.Checksum(dict) {
   162  			z.err = ErrDictionary
   163  			return z.err
   164  		}
   165  	}
   166  
   167  	if z.decompressor == nil {
   168  		if haveDict {
   169  			z.decompressor = flate.NewReaderDict(z.r, dict)
   170  		} else {
   171  			z.decompressor = flate.NewReader(z.r)
   172  		}
   173  	} else {
   174  		z.decompressor.(flate.Resetter).Reset(z.r, dict)
   175  	}
   176  
   177  	if z.digest != nil {
   178  		z.digest.Reset()
   179  	} else {
   180  		z.digest = adler32.New()
   181  	}
   182  	return nil
   183  }
   184
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