Module: Zlib (Ruby 2.3.4)

Zlib

This module provides access to the zlib library. Zlib is designed to be a portable, free, general-purpose, legally unencumbered – that is, not covered by any patents – lossless data-compression library for use on virtually any computer hardware and operating system.

The zlib compression library provides in-memory compression and decompression functions, including integrity checks of the uncompressed data.

The zlib compressed data format is described in RFC 1950, which is a wrapper around a deflate stream which is described in RFC 1951.

The library also supports reading and writing files in gzip (.gz) format with an interface similar to that of IO. The gzip format is described in RFC 1952 which is also a wrapper around a deflate stream.

The zlib format was designed to be compact and fast for use in memory and on communications channels. The gzip format was designed for single-file compression on file systems, has a larger header than zlib to maintain directory information, and uses a different, slower check method than zlib.

See your system’s zlib.h for further information about zlib

Sample usage ↑

Using the wrapper to compress strings with default parameters is quite simple:

require "zlib"

data_to_compress = File.read("don_quixote.txt")

puts "Input size: #{data_to_compress.size}"
#=> Input size: 2347740

data_compressed = Zlib::Deflate.deflate(data_to_compress)

puts "Compressed size: #{data_compressed.size}"
#=> Compressed size: 887238

uncompressed_data = Zlib::Inflate.inflate(data_compressed)

puts "Uncompressed data is: #{uncompressed_data}"
#=> Uncompressed data is: The Project Gutenberg EBook of Don Quixote...

Class tree ↑

(if you have GZIP_SUPPORT)

Constants

ASCII

Represents text data as guessed by deflate.

NOTE: The underlying constant Z_ASCII was deprecated in favor of Z_TEXT in zlib 1.2.2. New applications should not use this constant.

See Zlib::ZStream#data_type.

BEST_COMPRESSION

Slowest compression level, but with the best space savings.

BEST_SPEED

Fastest compression level, but with the lowest space savings.

BINARY

Represents binary data as guessed by deflate.

See Zlib::ZStream#data_type.

DEFAULT_COMPRESSION

Default compression level which is a good trade-off between space and time

DEFAULT_STRATEGY

Default deflate strategy which is used for normal data.

DEF_MEM_LEVEL

The default memory level for allocating zlib deflate compression state.

FILTERED

Deflate strategy for data produced by a filter (or predictor). The effect of FILTERED is to force more Huffman codes and less string matching; it is somewhat intermediate between DEFAULT_STRATEGY and HUFFMAN_ONLY. Filtered data consists mostly of small values with a somewhat random distribution.

FINISH

Processes all pending input and flushes pending output.

FIXED

Deflate strategy which prevents the use of dynamic Huffman codes, allowing for a simpler decoder for specialized applications.

FULL_FLUSH

Flushes all output as with SYNC_FLUSH, and the compression state is reset so that decompression can restart from this point if previous compressed data has been damaged or if random access is desired. Like SYNC_FLUSH, using FULL_FLUSH too often can seriously degrade compression.

HUFFMAN_ONLY

Deflate strategy which uses Huffman codes only (no string matching).

MAX_MEM_LEVEL

The maximum memory level for allocating zlib deflate compression state.

MAX_WBITS

The maximum size of the zlib history buffer. Note that zlib allows larger values to enable different inflate modes. See Zlib::Inflate.new for details.

NO_COMPRESSION

No compression, passes through data untouched. Use this for appending pre-compressed data to a deflate stream.

NO_FLUSH

NO_FLUSH is the default flush method and allows deflate to decide how much data to accumulate before producing output in order to maximize compression.

OS_AMIGA

OS code for Amiga hosts

OS_ATARI

OS code for Atari hosts

OS_CODE

The OS code of current host

OS_CPM

OS code for CP/M hosts

OS_MACOS

OS code for Mac OS hosts

OS_MSDOS

OS code for MSDOS hosts

OS_OS2

OS code for OS2 hosts

OS_QDOS

OS code for QDOS hosts

OS_RISCOS

OS code for RISC OS hosts

OS_TOPS20

OS code for TOPS-20 hosts

OS_UNIX

OS code for UNIX hosts

OS_UNKNOWN

OS code for unknown hosts

OS_VMCMS

OS code for VM OS hosts

OS_VMS

OS code for VMS hosts

OS_WIN32

OS code for Win32 hosts

OS_ZSYSTEM

OS code for Z-System hosts

RLE

Deflate compression strategy designed to be almost as fast as HUFFMAN_ONLY, but give better compression for PNG image data.

SYNC_FLUSH

The SYNC_FLUSH method flushes all pending output to the output buffer and the output is aligned on a byte boundary. Flushing may degrade compression so it should be used only when necessary, such as at a request or response boundary for a network stream.

TEXT

Represents text data as guessed by deflate.

See Zlib::ZStream#data_type.

UNKNOWN

Represents an unknown data type as guessed by deflate.

See Zlib::ZStream#data_type.

VERSION

The Ruby/zlib version string.

ZLIB_VERSION

The string which represents the version of zlib.h

Public Class Methods

adler32(string, adler)

Calculates Adler-32 checksum for string, and returns updated value of adler. If string is omitted, it returns the Adler-32 initial value. If adler is omitted, it assumes that the initial value is given to adler.

Example usage:

require "zlib"

data = "foo"
puts "Adler32 checksum: #{Zlib.adler32(data).to_s(16)}"
#=> Adler32 checksum: 2820145

 
               static VALUE
rb_zlib_adler32(int argc, VALUE *argv, VALUE klass)
{
    return do_checksum(argc, argv, adler32);
}
adler32_combine(adler1, adler2, len2)

Combine two Adler-32 check values in to one. alder1 is the first Adler-32 value, adler2 is the second Adler-32 value. len2 is the length of the string used to generate adler2.

 
               static VALUE
rb_zlib_adler32_combine(VALUE klass, VALUE adler1, VALUE adler2, VALUE len2)
{
  return ULONG2NUM(
        adler32_combine(NUM2ULONG(adler1), NUM2ULONG(adler2), NUM2LONG(len2)));
}
crc32(string, crc)

Calculates CRC checksum for string, and returns updated value of crc. If string is omitted, it returns the CRC initial value. If crc is omitted, it assumes that the initial value is given to crc.

FIXME: expression.

 
               static VALUE
rb_zlib_crc32(int argc, VALUE *argv, VALUE klass)
{
    return do_checksum(argc, argv, crc32);
}
crc32_combine(crc1, crc2, len2)

Combine two CRC-32 check values in to one. crc1 is the first CRC-32 value, crc2 is the second CRC-32 value. len2 is the length of the string used to generate crc2.

 
               static VALUE
rb_zlib_crc32_combine(VALUE klass, VALUE crc1, VALUE crc2, VALUE len2)
{
  return ULONG2NUM(
        crc32_combine(NUM2ULONG(crc1), NUM2ULONG(crc2), NUM2LONG(len2)));
}
crc_table()

Returns the table for calculating CRC checksum as an array.

 
               static VALUE
rb_zlib_crc_table(VALUE obj)
{
#if !defined(HAVE_TYPE_Z_CRC_T)
    /* z_crc_t is defined since zlib-1.2.7. */
    typedef unsigned long z_crc_t;
#endif
    const z_crc_t *crctbl;
    VALUE dst;
    int i;

    crctbl = get_crc_table();
    dst = rb_ary_new2(256);

    for (i = 0; i < 256; i++) {
        rb_ary_push(dst, rb_uint2inum(crctbl[i]));
    }
    return dst;
}
deflate(string[, level])
Zlib::Deflate.deflate(string[, level])

Compresses the given string. Valid values of level are Zlib::NO_COMPRESSION, Zlib::BEST_SPEED, Zlib::BEST_COMPRESSION, Zlib::DEFAULT_COMPRESSION, or an integer from 0 to 9.

This method is almost equivalent to the following code:

def deflate(string, level)
  z = Zlib::Deflate.new(level)
  dst = z.deflate(string, Zlib::FINISH)
  z.close
  dst
end

See also ::inflate

 
               static VALUE
rb_deflate_s_deflate(int argc, VALUE *argv, VALUE klass)
{
    struct zstream z;
    VALUE src, level, dst, args[2];
    int err, lev;

    rb_scan_args(argc, argv, "11", &src, &level);

    lev = ARG_LEVEL(level);
    StringValue(src);
    zstream_init_deflate(&z);
    err = deflateInit(&z.stream, lev);
    if (err != Z_OK) {
        raise_zlib_error(err, z.stream.msg);
    }
    ZSTREAM_READY(&z);

    args[0] = (VALUE)&z;
    args[1] = src;
    dst = rb_ensure(deflate_run, (VALUE)args, zstream_end, (VALUE)&z);

    OBJ_INFECT(dst, src);
    return dst;
}
inflate(string)
Zlib::Inflate.inflate(string)

Decompresses string. Raises a Zlib::NeedDict exception if a preset dictionary is needed for decompression.

This method is almost equivalent to the following code:

def inflate(string)
  zstream = Zlib::Inflate.new
  buf = zstream.inflate(string)
  zstream.finish
  zstream.close
  buf
end

See also ::deflate

 
               static VALUE
rb_inflate_s_inflate(VALUE obj, VALUE src)
{
    struct zstream z;
    VALUE dst, args[2];
    int err;

    StringValue(src);
    zstream_init_inflate(&z);
    err = inflateInit(&z.stream);
    if (err != Z_OK) {
        raise_zlib_error(err, z.stream.msg);
    }
    ZSTREAM_READY(&z);

    args[0] = (VALUE)&z;
    args[1] = src;
    dst = rb_ensure(inflate_run, (VALUE)args, zstream_end, (VALUE)&z);

    OBJ_INFECT(dst, src);
    return dst;
}
zlib_version()

Returns the string which represents the version of zlib library.

 
               static VALUE
rb_zlib_version(VALUE klass)
{
    VALUE str;

    str = rb_str_new2(zlibVersion());
    OBJ_TAINT(str);  /* for safe */
    return str;
}