1 | /* zran.c -- example of zlib/gzip stream indexing and random access |
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2 | * Copyright (C) 2005 Mark Adler |
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3 | * For conditions of distribution and use, see copyright notice in zlib.h |
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4 | Version 1.0 29 May 2005 Mark Adler */ |
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5 | |
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6 | /* Illustrate the use of Z_BLOCK, inflatePrime(), and inflateSetDictionary() |
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7 | for random access of a compressed file. A file containing a zlib or gzip |
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8 | stream is provided on the command line. The compressed stream is decoded in |
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9 | its entirety, and an index built with access points about every SPAN bytes |
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10 | in the uncompressed output. The compressed file is left open, and can then |
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11 | be read randomly, having to decompress on the average SPAN/2 uncompressed |
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12 | bytes before getting to the desired block of data. |
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13 | |
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14 | An access point can be created at the start of any deflate block, by saving |
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15 | the starting file offset and bit of that block, and the 32K bytes of |
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16 | uncompressed data that precede that block. Also the uncompressed offset of |
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17 | that block is saved to provide a referece for locating a desired starting |
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18 | point in the uncompressed stream. build_index() works by decompressing the |
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19 | input zlib or gzip stream a block at a time, and at the end of each block |
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20 | deciding if enough uncompressed data has gone by to justify the creation of |
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21 | a new access point. If so, that point is saved in a data structure that |
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22 | grows as needed to accommodate the points. |
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23 | |
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24 | To use the index, an offset in the uncompressed data is provided, for which |
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25 | the latest accees point at or preceding that offset is located in the index. |
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26 | The input file is positioned to the specified location in the index, and if |
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27 | necessary the first few bits of the compressed data is read from the file. |
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28 | inflate is initialized with those bits and the 32K of uncompressed data, and |
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29 | the decompression then proceeds until the desired offset in the file is |
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30 | reached. Then the decompression continues to read the desired uncompressed |
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31 | data from the file. |
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32 | |
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33 | Another approach would be to generate the index on demand. In that case, |
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34 | requests for random access reads from the compressed data would try to use |
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35 | the index, but if a read far enough past the end of the index is required, |
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36 | then further index entries would be generated and added. |
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37 | |
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38 | There is some fair bit of overhead to starting inflation for the random |
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39 | access, mainly copying the 32K byte dictionary. So if small pieces of the |
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40 | file are being accessed, it would make sense to implement a cache to hold |
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41 | some lookahead and avoid many calls to extract() for small lengths. |
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42 | |
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43 | Another way to build an index would be to use inflateCopy(). That would |
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44 | not be constrained to have access points at block boundaries, but requires |
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45 | more memory per access point, and also cannot be saved to file due to the |
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46 | use of pointers in the state. The approach here allows for storage of the |
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47 | index in a file. |
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48 | */ |
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49 | |
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50 | #include <stdio.h> |
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51 | #include <stdlib.h> |
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52 | #include <string.h> |
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53 | #include "zlib.h" |
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54 | |
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55 | #define local static |
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56 | |
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57 | #define SPAN 1048576L /* desired distance between access points */ |
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58 | #define WINSIZE 32768U /* sliding window size */ |
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59 | #define CHUNK 16384 /* file input buffer size */ |
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60 | |
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61 | /* access point entry */ |
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62 | struct point { |
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63 | off_t out; /* corresponding offset in uncompressed data */ |
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64 | off_t in; /* offset in input file of first full byte */ |
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65 | int bits; /* number of bits (1-7) from byte at in - 1, or 0 */ |
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66 | unsigned char window[WINSIZE]; /* preceding 32K of uncompressed data */ |
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67 | }; |
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68 | |
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69 | /* access point list */ |
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70 | struct access { |
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71 | int have; /* number of list entries filled in */ |
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72 | int size; /* number of list entries allocated */ |
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73 | struct point *list; /* allocated list */ |
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74 | }; |
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75 | |
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76 | /* Deallocate an index built by build_index() */ |
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77 | local void free_index(struct access *index) |
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78 | { |
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79 | if (index != NULL) { |
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80 | free(index->list); |
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81 | free(index); |
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82 | } |
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83 | } |
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84 | |
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85 | /* Add an entry to the access point list. If out of memory, deallocate the |
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86 | existing list and return NULL. */ |
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87 | local struct access *addpoint(struct access *index, int bits, |
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88 | off_t in, off_t out, unsigned left, unsigned char *window) |
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89 | { |
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90 | struct point *next; |
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91 | |
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92 | /* if list is empty, create it (start with eight points) */ |
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93 | if (index == NULL) { |
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94 | index = malloc(sizeof(struct access)); |
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95 | if (index == NULL) return NULL; |
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96 | index->list = malloc(sizeof(struct point) << 3); |
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97 | if (index->list == NULL) { |
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98 | free(index); |
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99 | return NULL; |
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100 | } |
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101 | index->size = 8; |
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102 | index->have = 0; |
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103 | } |
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104 | |
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105 | /* if list is full, make it bigger */ |
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106 | else if (index->have == index->size) { |
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107 | index->size <<= 1; |
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108 | next = realloc(index->list, sizeof(struct point) * index->size); |
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109 | if (next == NULL) { |
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110 | free_index(index); |
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111 | return NULL; |
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112 | } |
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113 | index->list = next; |
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114 | } |
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115 | |
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116 | /* fill in entry and increment how many we have */ |
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117 | next = index->list + index->have; |
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118 | next->bits = bits; |
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119 | next->in = in; |
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120 | next->out = out; |
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121 | if (left) |
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122 | memcpy(next->window, window + WINSIZE - left, left); |
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123 | if (left < WINSIZE) |
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124 | memcpy(next->window + left, window, WINSIZE - left); |
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125 | index->have++; |
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126 | |
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127 | /* return list, possibly reallocated */ |
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128 | return index; |
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129 | } |
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130 | |
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131 | /* Make one entire pass through the compressed stream and build an index, with |
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132 | access points about every span bytes of uncompressed output -- span is |
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133 | chosen to balance the speed of random access against the memory requirements |
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134 | of the list, about 32K bytes per access point. Note that data after the end |
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135 | of the first zlib or gzip stream in the file is ignored. build_index() |
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136 | returns the number of access points on success (>= 1), Z_MEM_ERROR for out |
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137 | of memory, Z_DATA_ERROR for an error in the input file, or Z_ERRNO for a |
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138 | file read error. On success, *built points to the resulting index. */ |
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139 | local int build_index(FILE *in, off_t span, struct access **built) |
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140 | { |
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141 | int ret; |
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142 | off_t totin, totout; /* our own total counters to avoid 4GB limit */ |
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143 | off_t last; /* totout value of last access point */ |
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144 | struct access *index; /* access points being generated */ |
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145 | z_stream strm; |
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146 | unsigned char input[CHUNK]; |
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147 | unsigned char window[WINSIZE]; |
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148 | |
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149 | /* initialize inflate */ |
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150 | strm.zalloc = Z_NULL; |
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151 | strm.zfree = Z_NULL; |
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152 | strm.opaque = Z_NULL; |
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153 | strm.avail_in = 0; |
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154 | strm.next_in = Z_NULL; |
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155 | ret = inflateInit2(&strm, 47); /* automatic zlib or gzip decoding */ |
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156 | if (ret != Z_OK) |
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157 | return ret; |
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158 | |
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159 | /* inflate the input, maintain a sliding window, and build an index -- this |
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160 | also validates the integrity of the compressed data using the check |
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161 | information at the end of the gzip or zlib stream */ |
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162 | totin = totout = last = 0; |
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163 | index = NULL; /* will be allocated by first addpoint() */ |
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164 | strm.avail_out = 0; |
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165 | do { |
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166 | /* get some compressed data from input file */ |
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167 | strm.avail_in = fread(input, 1, CHUNK, in); |
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168 | if (ferror(in)) { |
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169 | ret = Z_ERRNO; |
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170 | goto build_index_error; |
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171 | } |
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172 | if (strm.avail_in == 0) { |
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173 | ret = Z_DATA_ERROR; |
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174 | goto build_index_error; |
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175 | } |
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176 | strm.next_in = input; |
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177 | |
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178 | /* process all of that, or until end of stream */ |
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179 | do { |
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180 | /* reset sliding window if necessary */ |
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181 | if (strm.avail_out == 0) { |
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182 | strm.avail_out = WINSIZE; |
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183 | strm.next_out = window; |
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184 | } |
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185 | |
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186 | /* inflate until out of input, output, or at end of block -- |
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187 | update the total input and output counters */ |
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188 | totin += strm.avail_in; |
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189 | totout += strm.avail_out; |
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190 | ret = inflate(&strm, Z_BLOCK); /* return at end of block */ |
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191 | totin -= strm.avail_in; |
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192 | totout -= strm.avail_out; |
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193 | if (ret == Z_NEED_DICT) |
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194 | ret = Z_DATA_ERROR; |
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195 | if (ret == Z_MEM_ERROR || ret == Z_DATA_ERROR) |
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196 | goto build_index_error; |
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197 | if (ret == Z_STREAM_END) |
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198 | break; |
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199 | |
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200 | /* if at end of block, consider adding an index entry (note that if |
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201 | data_type indicates an end-of-block, then all of the |
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202 | uncompressed data from that block has been delivered, and none |
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203 | of the compressed data after that block has been consumed, |
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204 | except for up to seven bits) -- the totout == 0 provides an |
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205 | entry point after the zlib or gzip header, and assures that the |
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206 | index always has at least one access point; we avoid creating an |
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207 | access point after the last block by checking bit 6 of data_type |
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208 | */ |
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209 | if ((strm.data_type & 128) && !(strm.data_type & 64) && |
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210 | (totout == 0 || totout - last > span)) { |
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211 | index = addpoint(index, strm.data_type & 7, totin, |
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212 | totout, strm.avail_out, window); |
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213 | if (index == NULL) { |
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214 | ret = Z_MEM_ERROR; |
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215 | goto build_index_error; |
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216 | } |
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217 | last = totout; |
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218 | } |
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219 | } while (strm.avail_in != 0); |
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220 | } while (ret != Z_STREAM_END); |
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221 | |
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222 | /* clean up and return index (release unused entries in list) */ |
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223 | (void)inflateEnd(&strm); |
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224 | index = realloc(index, sizeof(struct point) * index->have); |
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225 | index->size = index->have; |
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226 | *built = index; |
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227 | return index->size; |
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228 | |
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229 | /* return error */ |
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230 | build_index_error: |
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231 | (void)inflateEnd(&strm); |
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232 | if (index != NULL) |
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233 | free_index(index); |
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234 | return ret; |
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235 | } |
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236 | |
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237 | /* Use the index to read len bytes from offset into buf, return bytes read or |
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238 | negative for error (Z_DATA_ERROR or Z_MEM_ERROR). If data is requested past |
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239 | the end of the uncompressed data, then extract() will return a value less |
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240 | than len, indicating how much as actually read into buf. This function |
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241 | should not return a data error unless the file was modified since the index |
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242 | was generated. extract() may also return Z_ERRNO if there is an error on |
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243 | reading or seeking the input file. */ |
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244 | local int extract(FILE *in, struct access *index, off_t offset, |
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245 | unsigned char *buf, int len) |
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246 | { |
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247 | int ret, skip; |
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248 | z_stream strm; |
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249 | struct point *here; |
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250 | unsigned char input[CHUNK]; |
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251 | unsigned char discard[WINSIZE]; |
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252 | |
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253 | /* proceed only if something reasonable to do */ |
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254 | if (len < 0) |
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255 | return 0; |
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256 | |
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257 | /* find where in stream to start */ |
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258 | here = index->list; |
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259 | ret = index->have; |
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260 | while (--ret && here[1].out <= offset) |
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261 | here++; |
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262 | |
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263 | /* initialize file and inflate state to start there */ |
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264 | strm.zalloc = Z_NULL; |
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265 | strm.zfree = Z_NULL; |
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266 | strm.opaque = Z_NULL; |
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267 | strm.avail_in = 0; |
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268 | strm.next_in = Z_NULL; |
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269 | ret = inflateInit2(&strm, -15); /* raw inflate */ |
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270 | if (ret != Z_OK) |
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271 | return ret; |
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272 | ret = fseeko(in, here->in - (here->bits ? 1 : 0), SEEK_SET); |
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273 | if (ret == -1) |
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274 | goto extract_ret; |
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275 | if (here->bits) { |
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276 | ret = getc(in); |
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277 | if (ret == -1) { |
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278 | ret = ferror(in) ? Z_ERRNO : Z_DATA_ERROR; |
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279 | goto extract_ret; |
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280 | } |
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281 | (void)inflatePrime(&strm, here->bits, ret >> (8 - here->bits)); |
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282 | } |
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283 | (void)inflateSetDictionary(&strm, here->window, WINSIZE); |
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284 | |
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285 | /* skip uncompressed bytes until offset reached, then satisfy request */ |
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286 | offset -= here->out; |
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287 | strm.avail_in = 0; |
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288 | skip = 1; /* while skipping to offset */ |
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289 | do { |
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290 | /* define where to put uncompressed data, and how much */ |
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291 | if (offset == 0 && skip) { /* at offset now */ |
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292 | strm.avail_out = len; |
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293 | strm.next_out = buf; |
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294 | skip = 0; /* only do this once */ |
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295 | } |
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296 | if (offset > WINSIZE) { /* skip WINSIZE bytes */ |
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297 | strm.avail_out = WINSIZE; |
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298 | strm.next_out = discard; |
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299 | offset -= WINSIZE; |
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300 | } |
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301 | else if (offset != 0) { /* last skip */ |
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302 | strm.avail_out = (unsigned)offset; |
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303 | strm.next_out = discard; |
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304 | offset = 0; |
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305 | } |
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306 | |
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307 | /* uncompress until avail_out filled, or end of stream */ |
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308 | do { |
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309 | if (strm.avail_in == 0) { |
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310 | strm.avail_in = fread(input, 1, CHUNK, in); |
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311 | if (ferror(in)) { |
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312 | ret = Z_ERRNO; |
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313 | goto extract_ret; |
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314 | } |
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315 | if (strm.avail_in == 0) { |
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316 | ret = Z_DATA_ERROR; |
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317 | goto extract_ret; |
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318 | } |
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319 | strm.next_in = input; |
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320 | } |
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321 | ret = inflate(&strm, Z_NO_FLUSH); /* normal inflate */ |
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322 | if (ret == Z_NEED_DICT) |
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323 | ret = Z_DATA_ERROR; |
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324 | if (ret == Z_MEM_ERROR || ret == Z_DATA_ERROR) |
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325 | goto extract_ret; |
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326 | if (ret == Z_STREAM_END) |
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327 | break; |
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328 | } while (strm.avail_out != 0); |
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329 | |
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330 | /* if reach end of stream, then don't keep trying to get more */ |
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331 | if (ret == Z_STREAM_END) |
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332 | break; |
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333 | |
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334 | /* do until offset reached and requested data read, or stream ends */ |
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335 | } while (skip); |
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336 | |
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337 | /* compute number of uncompressed bytes read after offset */ |
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338 | ret = skip ? 0 : len - strm.avail_out; |
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339 | |
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340 | /* clean up and return bytes read or error */ |
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341 | extract_ret: |
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342 | (void)inflateEnd(&strm); |
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343 | return ret; |
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344 | } |
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345 | |
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346 | /* Demonstrate the use of build_index() and extract() by processing the file |
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347 | provided on the command line, and the extracting 16K from about 2/3rds of |
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348 | the way through the uncompressed output, and writing that to stdout. */ |
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349 | int main(int argc, char **argv) |
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350 | { |
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351 | int len; |
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352 | off_t offset; |
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353 | FILE *in; |
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354 | struct access *index = NULL; |
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355 | unsigned char buf[CHUNK]; |
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356 | |
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357 | /* open input file */ |
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358 | if (argc != 2) { |
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359 | fprintf(stderr, "usage: zran file.gz\n"); |
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360 | return 1; |
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361 | } |
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362 | in = fopen(argv[1], "rb"); |
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363 | if (in == NULL) { |
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364 | fprintf(stderr, "zran: could not open %s for reading\n", argv[1]); |
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365 | return 1; |
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366 | } |
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367 | |
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368 | /* build index */ |
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369 | len = build_index(in, SPAN, &index); |
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370 | if (len < 0) { |
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371 | fclose(in); |
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372 | switch (len) { |
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373 | case Z_MEM_ERROR: |
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374 | fprintf(stderr, "zran: out of memory\n"); |
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375 | break; |
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376 | case Z_DATA_ERROR: |
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377 | fprintf(stderr, "zran: compressed data error in %s\n", argv[1]); |
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378 | break; |
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379 | case Z_ERRNO: |
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380 | fprintf(stderr, "zran: read error on %s\n", argv[1]); |
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381 | break; |
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382 | default: |
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383 | fprintf(stderr, "zran: error %d while building index\n", len); |
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384 | } |
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385 | return 1; |
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386 | } |
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387 | fprintf(stderr, "zran: built index with %d access points\n", len); |
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388 | |
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389 | /* use index by reading some bytes from an arbitrary offset */ |
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390 | offset = (index->list[index->have - 1].out << 1) / 3; |
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391 | len = extract(in, index, offset, buf, CHUNK); |
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392 | if (len < 0) |
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393 | fprintf(stderr, "zran: extraction failed: %s error\n", |
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394 | len == Z_MEM_ERROR ? "out of memory" : "input corrupted"); |
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395 | else { |
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396 | fwrite(buf, 1, len, stdout); |
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397 | fprintf(stderr, "zran: extracted %d bytes at %llu\n", len, offset); |
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398 | } |
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399 | |
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400 | /* clean up and exit */ |
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401 | free_index(index); |
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402 | fclose(in); |
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403 | return 0; |
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404 | } |
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