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1
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2 NAME
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3 bzip2, bunzip2 - a block-sorting file compressor, v1.0.6
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4 bzcat - decompresses files to stdout
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5 bzip2recover - recovers data from damaged bzip2 files
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6
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7
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8 SYNOPSIS
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9 bzip2 [ -cdfkqstvzVL123456789 ] [ filenames ... ]
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10 bunzip2 [ -fkvsVL ] [ filenames ... ]
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11 bzcat [ -s ] [ filenames ... ]
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12 bzip2recover filename
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13
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14
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15 DESCRIPTION
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16 bzip2 compresses files using the Burrows-Wheeler block
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17 sorting text compression algorithm, and Huffman coding.
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18 Compression is generally considerably better than that
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19 achieved by more conventional LZ77/LZ78-based compressors,
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20 and approaches the performance of the PPM family of sta-
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21 tistical compressors.
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22
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23 The command-line options are deliberately very similar to
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24 those of GNU gzip, but they are not identical.
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25
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26 bzip2 expects a list of file names to accompany the com-
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27 mand-line flags. Each file is replaced by a compressed
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28 version of itself, with the name "original_name.bz2".
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29 Each compressed file has the same modification date, per-
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30 missions, and, when possible, ownership as the correspond-
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31 ing original, so that these properties can be correctly
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32 restored at decompression time. File name handling is
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33 naive in the sense that there is no mechanism for preserv-
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34 ing original file names, permissions, ownerships or dates
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35 in filesystems which lack these concepts, or have serious
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36 file name length restrictions, such as MS-DOS.
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37
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38 bzip2 and bunzip2 will by default not overwrite existing
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39 files. If you want this to happen, specify the -f flag.
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40
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41 If no file names are specified, bzip2 compresses from
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42 standard input to standard output. In this case, bzip2
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43 will decline to write compressed output to a terminal, as
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44 this would be entirely incomprehensible and therefore
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45 pointless.
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46
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47 bunzip2 (or bzip2 -d) decompresses all specified files.
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48 Files which were not created by bzip2 will be detected and
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49 ignored, and a warning issued. bzip2 attempts to guess
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50 the filename for the decompressed file from that of the
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51 compressed file as follows:
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52
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53 filename.bz2 becomes filename
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54 filename.bz becomes filename
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55 filename.tbz2 becomes filename.tar
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56 filename.tbz becomes filename.tar
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57 anyothername becomes anyothername.out
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58
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59 If the file does not end in one of the recognised endings,
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60 .bz2, .bz, .tbz2 or .tbz, bzip2 complains that it cannot
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61 guess the name of the original file, and uses the original
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62 name with .out appended.
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63
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64 As with compression, supplying no filenames causes decom-
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65 pression from standard input to standard output.
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66
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67 bunzip2 will correctly decompress a file which is the con-
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68 catenation of two or more compressed files. The result is
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69 the concatenation of the corresponding uncompressed files.
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70 Integrity testing (-t) of concatenated compressed files is
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71 also supported.
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72
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73 You can also compress or decompress files to the standard
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74 output by giving the -c flag. Multiple files may be com-
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75 pressed and decompressed like this. The resulting outputs
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76 are fed sequentially to stdout. Compression of multiple
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77 files in this manner generates a stream containing multi-
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78 ple compressed file representations. Such a stream can be
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79 decompressed correctly only by bzip2 version 0.9.0 or
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80 later. Earlier versions of bzip2 will stop after decom-
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81 pressing the first file in the stream.
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82
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83 bzcat (or bzip2 -dc) decompresses all specified files to
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84 the standard output.
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85
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86 bzip2 will read arguments from the environment variables
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87 BZIP2 and BZIP, in that order, and will process them
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88 before any arguments read from the command line. This
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89 gives a convenient way to supply default arguments.
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90
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91 Compression is always performed, even if the compressed
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92 file is slightly larger than the original. Files of less
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93 than about one hundred bytes tend to get larger, since the
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94 compression mechanism has a constant overhead in the
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95 region of 50 bytes. Random data (including the output of
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96 most file compressors) is coded at about 8.05 bits per
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97 byte, giving an expansion of around 0.5%.
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98
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99 As a self-check for your protection, bzip2 uses 32-bit
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100 CRCs to make sure that the decompressed version of a file
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101 is identical to the original. This guards against corrup-
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102 tion of the compressed data, and against undetected bugs
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103 in bzip2 (hopefully very unlikely). The chances of data
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104 corruption going undetected is microscopic, about one
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105 chance in four billion for each file processed. Be aware,
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106 though, that the check occurs upon decompression, so it
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107 can only tell you that something is wrong. It can't help
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108 you recover the original uncompressed data. You can use
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109 bzip2recover to try to recover data from damaged files.
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110
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111 Return values: 0 for a normal exit, 1 for environmental
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112 problems (file not found, invalid flags, I/O errors, &c),
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113 2 to indicate a corrupt compressed file, 3 for an internal
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114 consistency error (eg, bug) which caused bzip2 to panic.
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115
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116
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117 OPTIONS
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118 -c --stdout
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119 Compress or decompress to standard output.
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120
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121 -d --decompress
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122 Force decompression. bzip2, bunzip2 and bzcat are
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123 really the same program, and the decision about
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124 what actions to take is done on the basis of which
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125 name is used. This flag overrides that mechanism,
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126 and forces bzip2 to decompress.
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127
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128 -z --compress
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129 The complement to -d: forces compression,
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130 regardless of the invocation name.
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131
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132 -t --test
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133 Check integrity of the specified file(s), but don't
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134 decompress them. This really performs a trial
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135 decompression and throws away the result.
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136
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137 -f --force
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138 Force overwrite of output files. Normally, bzip2
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139 will not overwrite existing output files. Also
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140 forces bzip2 to break hard links to files, which it
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141 otherwise wouldn't do.
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142
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143 bzip2 normally declines to decompress files which
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144 don't have the correct magic header bytes. If
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145 forced (-f), however, it will pass such files
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146 through unmodified. This is how GNU gzip behaves.
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147
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148 -k --keep
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149 Keep (don't delete) input files during compression
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150 or decompression.
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151
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152 -s --small
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153 Reduce memory usage, for compression, decompression
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154 and testing. Files are decompressed and tested
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155 using a modified algorithm which only requires 2.5
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156 bytes per block byte. This means any file can be
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157 decompressed in 2300k of memory, albeit at about
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158 half the normal speed.
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159
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160 During compression, -s selects a block size of
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161 200k, which limits memory use to around the same
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162 figure, at the expense of your compression ratio.
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163 In short, if your machine is low on memory (8
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164 megabytes or less), use -s for everything. See
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165 MEMORY MANAGEMENT below.
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166
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167 -q --quiet
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168 Suppress non-essential warning messages. Messages
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169 pertaining to I/O errors and other critical events
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170 will not be suppressed.
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171
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172 -v --verbose
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173 Verbose mode -- show the compression ratio for each
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174 file processed. Further -v's increase the ver-
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175 bosity level, spewing out lots of information which
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176 is primarily of interest for diagnostic purposes.
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177
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178 -L --license -V --version
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179 Display the software version, license terms and
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180 conditions.
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181
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182 -1 (or --fast) to -9 (or --best)
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183 Set the block size to 100 k, 200 k .. 900 k when
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184 compressing. Has no effect when decompressing.
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185 See MEMORY MANAGEMENT below. The --fast and --best
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186 aliases are primarily for GNU gzip compatibility.
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187 In particular, --fast doesn't make things signifi-
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188 cantly faster. And --best merely selects the
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189 default behaviour.
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190
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191 -- Treats all subsequent arguments as file names, even
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192 if they start with a dash. This is so you can han-
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193 dle files with names beginning with a dash, for
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194 example: bzip2 -- -myfilename.
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195
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196 --repetitive-fast --repetitive-best
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197 These flags are redundant in versions 0.9.5 and
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198 above. They provided some coarse control over the
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199 behaviour of the sorting algorithm in earlier ver-
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200 sions, which was sometimes useful. 0.9.5 and above
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201 have an improved algorithm which renders these
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202 flags irrelevant.
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203
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204
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205 MEMORY MANAGEMENT
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206 bzip2 compresses large files in blocks. The block size
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207 affects both the compression ratio achieved, and the
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208 amount of memory needed for compression and decompression.
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209 The flags -1 through -9 specify the block size to be
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210 100,000 bytes through 900,000 bytes (the default) respec-
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211 tively. At decompression time, the block size used for
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212 compression is read from the header of the compressed
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213 file, and bunzip2 then allocates itself just enough memory
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214 to decompress the file. Since block sizes are stored in
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215 compressed files, it follows that the flags -1 to -9 are
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216 irrelevant to and so ignored during decompression.
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217
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218 Compression and decompression requirements, in bytes, can
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219 be estimated as:
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220
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221 Compression: 400k + ( 8 x block size )
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222
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223 Decompression: 100k + ( 4 x block size ), or
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224 100k + ( 2.5 x block size )
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225
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226 Larger block sizes give rapidly diminishing marginal
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227 returns. Most of the compression comes from the first two
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228 or three hundred k of block size, a fact worth bearing in
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229 mind when using bzip2 on small machines. It is also
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230 important to appreciate that the decompression memory
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231 requirement is set at compression time by the choice of
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232 block size.
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233
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234 For files compressed with the default 900k block size,
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235 bunzip2 will require about 3700 kbytes to decompress. To
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236 support decompression of any file on a 4 megabyte machine,
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237 bunzip2 has an option to decompress using approximately
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238 half this amount of memory, about 2300 kbytes. Decompres-
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239 sion speed is also halved, so you should use this option
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240 only where necessary. The relevant flag is -s.
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241
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242 In general, try and use the largest block size memory con-
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243 straints allow, since that maximises the compression
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244 achieved. Compression and decompression speed are virtu-
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245 ally unaffected by block size.
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246
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247 Another significant point applies to files which fit in a
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248 single block -- that means most files you'd encounter
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249 using a large block size. The amount of real memory
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250 touched is proportional to the size of the file, since the
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251 file is smaller than a block. For example, compressing a
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252 file 20,000 bytes long with the flag -9 will cause the
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253 compressor to allocate around 7600k of memory, but only
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254 touch 400k + 20000 * 8 = 560 kbytes of it. Similarly, the
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255 decompressor will allocate 3700k but only touch 100k +
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256 20000 * 4 = 180 kbytes.
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257
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258 Here is a table which summarises the maximum memory usage
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259 for different block sizes. Also recorded is the total
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260 compressed size for 14 files of the Calgary Text Compres-
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261 sion Corpus totalling 3,141,622 bytes. This column gives
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262 some feel for how compression varies with block size.
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263 These figures tend to understate the advantage of larger
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264 block sizes for larger files, since the Corpus is domi-
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265 nated by smaller files.
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266
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267 Compress Decompress Decompress Corpus
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268 Flag usage usage -s usage Size
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269
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270 -1 1200k 500k 350k 914704
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271 -2 2000k 900k 600k 877703
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272 -3 2800k 1300k 850k 860338
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273 -4 3600k 1700k 1100k 846899
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274 -5 4400k 2100k 1350k 845160
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275 -6 5200k 2500k 1600k 838626
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276 -7 6100k 2900k 1850k 834096
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277 -8 6800k 3300k 2100k 828642
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278 -9 7600k 3700k 2350k 828642
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279
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280
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281 RECOVERING DATA FROM DAMAGED FILES
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282 bzip2 compresses files in blocks, usually 900kbytes long.
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283 Each block is handled independently. If a media or trans-
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284 mission error causes a multi-block .bz2 file to become
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285 damaged, it may be possible to recover data from the
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286 undamaged blocks in the file.
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287
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288 The compressed representation of each block is delimited
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289 by a 48-bit pattern, which makes it possible to find the
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290 block boundaries with reasonable certainty. Each block
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291 also carries its own 32-bit CRC, so damaged blocks can be
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292 distinguished from undamaged ones.
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293
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294 bzip2recover is a simple program whose purpose is to
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295 search for blocks in .bz2 files, and write each block out
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296 into its own .bz2 file. You can then use bzip2 -t to test
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297 the integrity of the resulting files, and decompress those
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298 which are undamaged.
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299
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300 bzip2recover takes a single argument, the name of the dam-
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301 aged file, and writes a number of files
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302 "rec00001file.bz2", "rec00002file.bz2", etc, containing
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303 the extracted blocks. The output filenames are
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304 designed so that the use of wildcards in subsequent pro-
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305 cessing -- for example, "bzip2 -dc rec*file.bz2 > recov-
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306 ered_data" -- processes the files in the correct order.
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307
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308 bzip2recover should be of most use dealing with large .bz2
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309 files, as these will contain many blocks. It is clearly
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310 futile to use it on damaged single-block files, since a
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311 damaged block cannot be recovered. If you wish to min-
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312 imise any potential data loss through media or transmis-
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313 sion errors, you might consider compressing with a smaller
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314 block size.
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315
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316
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317 PERFORMANCE NOTES
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318 The sorting phase of compression gathers together similar
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319 strings in the file. Because of this, files containing
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320 very long runs of repeated symbols, like "aabaabaabaab
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321 ..." (repeated several hundred times) may compress more
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322 slowly than normal. Versions 0.9.5 and above fare much
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323 better than previous versions in this respect. The ratio
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324 between worst-case and average-case compression time is in
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325 the region of 10:1. For previous versions, this figure
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326 was more like 100:1. You can use the -vvvv option to mon-
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327 itor progress in great detail, if you want.
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328
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329 Decompression speed is unaffected by these phenomena.
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330
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331 bzip2 usually allocates several megabytes of memory to
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332 operate in, and then charges all over it in a fairly ran-
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333 dom fashion. This means that performance, both for com-
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334 pressing and decompressing, is largely determined by the
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335 speed at which your machine can service cache misses.
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336 Because of this, small changes to the code to reduce the
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337 miss rate have been observed to give disproportionately
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338 large performance improvements. I imagine bzip2 will per-
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339 form best on machines with very large caches.
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340
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341
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342 CAVEATS
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343 I/O error messages are not as helpful as they could be.
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344 bzip2 tries hard to detect I/O errors and exit cleanly,
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345 but the details of what the problem is sometimes seem
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346 rather misleading.
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347
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348 This manual page pertains to version 1.0.6 of bzip2. Com-
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349 pressed data created by this version is entirely forwards
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350 and backwards compatible with the previous public
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351 releases, versions 0.1pl2, 0.9.0, 0.9.5, 1.0.0, 1.0.1,
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352 1.0.2 and above, but with the following exception: 0.9.0
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353 and above can correctly decompress multiple concatenated
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354 compressed files. 0.1pl2 cannot do this; it will stop
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355 after decompressing just the first file in the stream.
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356
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357 bzip2recover versions prior to 1.0.2 used 32-bit integers
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358 to represent bit positions in compressed files, so they
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359 could not handle compressed files more than 512 megabytes
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360 long. Versions 1.0.2 and above use 64-bit ints on some
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361 platforms which support them (GNU supported targets, and
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362 Windows). To establish whether or not bzip2recover was
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363 built with such a limitation, run it without arguments.
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364 In any event you can build yourself an unlimited version
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365 if you can recompile it with MaybeUInt64 set to be an
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366 unsigned 64-bit integer.
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367
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368
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369 AUTHOR
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370 Julian Seward, jsewardbzip.org.
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371
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372 http://www.bzip.org
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373
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374 The ideas embodied in bzip2 are due to (at least) the fol-
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375 lowing people: Michael Burrows and David Wheeler (for the
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376 block sorting transformation), David Wheeler (again, for
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377 the Huffman coder), Peter Fenwick (for the structured cod-
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378 ing model in the original bzip, and many refinements), and
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379 Alistair Moffat, Radford Neal and Ian Witten (for the
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380 arithmetic coder in the original bzip). I am much
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381 indebted for their help, support and advice. See the man-
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382 ual in the source distribution for pointers to sources of
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383 documentation. Christian von Roques encouraged me to look
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384 for faster sorting algorithms, so as to speed up compres-
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385 sion. Bela Lubkin encouraged me to improve the worst-case
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386 compression performance. Donna Robinson XMLised the docu-
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387 mentation. The bz* scripts are derived from those of GNU
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388 gzip. Many people sent patches, helped with portability
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389 problems, lent machines, gave advice and were generally
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390 helpful.
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391
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