unpack.c

   1 /* unpack.c -- decompress files in pack format.
   2
   3    Copyright (C) 1997, 1999, 2006, 2009-2011 Free Software Foundation, Inc.
   4    Copyright (C) 1992-1993 Jean-loup Gailly
   5
   6    This program is free software; you can redistribute it and/or modify
   7    it under the terms of the GNU General Public License as published by
   8    the Free Software Foundation; either version 3, or (at your option)
   9    any later version.
  10
  11    This program is distributed in the hope that it will be useful,
  12    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14    GNU General Public License for more details.
  15
  16    You should have received a copy of the GNU General Public License
  17    along with this program; if not, write to the Free Software Foundation,
  18    Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
  19
  20 #include <config.h>
  21 #include "tailor.h"
  22 #include "gzip.h"
  23
  24 #define MIN(a,b) ((a) <= (b) ? (a) : (b))
  25 /* The arguments must not have side effects. */
  26
  27 #define MAX_BITLEN 25
  28 /* Maximum length of Huffman codes. (Minor modifications to the code
  29  * would be needed to support 32 bits codes, but pack never generates
  30  * more than 24 bits anyway.)
  31  */
  32
  33 #define LITERALS 256
  34 /* Number of literals, excluding the End of Block (EOB) code */
  35
  36 #define MAX_PEEK 12
  37 /* Maximum number of 'peek' bits used to optimize traversal of the
  38  * Huffman tree.
  39  */
  40
  41 local ulg orig_len;       /* original uncompressed length */
  42 local int max_len;        /* maximum bit length of Huffman codes */
  43
  44 local uch literal[LITERALS];
  45 /* The literal bytes present in the Huffman tree. The EOB code is not
  46  * represented.
  47  */
  48
  49 local int lit_base[MAX_BITLEN+1];
  50 /* All literals of a given bit length are contiguous in literal[] and
  51  * have contiguous codes. literal[code+lit_base[len]] is the literal
  52  * for a code of len bits.
  53  */
  54
  55 local int leaves [MAX_BITLEN+1]; /* Number of leaves for each bit length */
  56 local int parents[MAX_BITLEN+1]; /* Number of parents for each bit length */
  57
  58 local int peek_bits; /* Number of peek bits currently used */
  59
  60 /* local uch prefix_len[1 << MAX_PEEK]; */
  61 #define prefix_len outbuf
  62 /* For each bit pattern b of peek_bits bits, prefix_len[b] is the length
  63  * of the Huffman code starting with a prefix of b (upper bits), or 0
  64  * if all codes of prefix b have more than peek_bits bits. It is not
  65  * necessary to have a huge table (large MAX_PEEK) because most of the
  66  * codes encountered in the input stream are short codes (by construction).
  67  * So for most codes a single lookup will be necessary.
  68  */
  69 #if (1<<MAX_PEEK) > OUTBUFSIZ
  70     error cannot overlay prefix_len and outbuf
  71 #endif
  72
  73 local ulg bitbuf;
  74 /* Bits are added on the low part of bitbuf and read from the high part. */
  75
  76 local int valid;                  /* number of valid bits in bitbuf */
  77 /* all bits above the last valid bit are always zero */
  78
  79 /* Set code to the next 'bits' input bits without skipping them. code
  80  * must be the name of a simple variable and bits must not have side effects.
  81  * IN assertions: bits <= 25 (so that we still have room for an extra byte
  82  * when valid is only 24), and mask = (1<<bits)-1.
  83  */
  84 #define look_bits(code,bits,mask) \
  85 { \
  86   while (valid < (bits)) bitbuf = (bitbuf<<8) | (ulg)get_byte(), valid += 8; \
  87   code = (bitbuf >> (valid-(bits))) & (mask); \
  88 }
  89
  90 /* Skip the given number of bits (after having peeked at them): */
  91 #define skip_bits(bits)  (valid -= (bits))
  92
  93 #define clear_bitbuf() (valid = 0, bitbuf = 0)
  94
  95 /* Local functions */
  96
  97 local void read_tree  (void);
  98 local void build_tree (void);
  99
 100 /* ===========================================================================
 101  * Read the Huffman tree.
 102  */
 103 local void read_tree()
 104 {
 105     int len;  /* bit length */
 106     int base; /* base offset for a sequence of leaves */
 107     int n;
 108     int max_leaves = 1;
 109
 110     /* Read the original input size, MSB first */
 111     orig_len = 0;
 112     for (n = 1; n <= 4; n++) orig_len = (orig_len << 8) | (ulg)get_byte();
 113
 114     max_len = (int)get_byte(); /* maximum bit length of Huffman codes */
 115     if (max_len > MAX_BITLEN) {
 116         gzip_error ("invalid compressed data -- Huffman code > 32 bits");
 117     }
 118
 119     /* Get the number of leaves at each bit length */
 120     n = 0;
 121     for (len = 1; len <= max_len; len++) {
 122         leaves[len] = (int)get_byte();
 123         if (max_leaves - (len == max_len) < leaves[len])
 124           gzip_error ("too many leaves in Huffman tree");
 125         max_leaves = (max_leaves - leaves[len] + 1) * 2 - 1;
 126         n += leaves[len];
 127     }
 128     if (LITERALS <= n) {
 129         gzip_error ("too many leaves in Huffman tree");
 130     }
 131     Trace((stderr, "orig_len %lu, max_len %d, leaves %d\n",
 132            orig_len, max_len, n));
 133     /* There are at least 2 and at most 256 leaves of length max_len.
 134      * (Pack arbitrarily rejects empty files and files consisting of
 135      * a single byte even repeated.) To fit the last leaf count in a
 136      * byte, it is offset by 2. However, the last literal is the EOB
 137      * code, and is not transmitted explicitly in the tree, so we must
 138      * adjust here by one only.
 139      */
 140     leaves[max_len]++;
 141
 142     /* Now read the leaves themselves */
 143     base = 0;
 144     for (len = 1; len <= max_len; len++) {
 145         /* Remember where the literals of this length start in literal[] : */
 146         lit_base[len] = base;
 147         /* And read the literals: */
 148         for (n = leaves[len]; n > 0; n--) {
 149             literal[base++] = (uch)get_byte();
 150         }
 151     }
 152     leaves[max_len]++; /* Now include the EOB code in the Huffman tree */
 153 }
 154
 155 /* ===========================================================================
 156  * Build the Huffman tree and the prefix table.
 157  */
 158 local void build_tree()
 159 {
 160     int nodes = 0; /* number of nodes (parents+leaves) at current bit length */
 161     int len;       /* current bit length */
 162     uch *prefixp;  /* pointer in prefix_len */
 163
 164     for (len = max_len; len >= 1; len--) {
 165         /* The number of parent nodes at this level is half the total
 166          * number of nodes at parent level:
 167          */
 168         nodes >>= 1;
 169         parents[len] = nodes;
 170         /* Update lit_base by the appropriate bias to skip the parent nodes
 171          * (which are not represented in the literal array):
 172          */
 173         lit_base[len] -= nodes;
 174         /* Restore nodes to be parents+leaves: */
 175         nodes += leaves[len];
 176     }
 177     /* Construct the prefix table, from shortest leaves to longest ones.
 178      * The shortest code is all ones, so we start at the end of the table.
 179      */
 180     peek_bits = MIN(max_len, MAX_PEEK);
 181     prefixp = &prefix_len[1<<peek_bits];
 182     for (len = 1; len <= peek_bits; len++) {
 183         int prefixes = leaves[len] << (peek_bits-len); /* may be 0 */
 184         while (prefixes--) *--prefixp = (uch)len;
 185     }
 186     /* The length of all other codes is unknown: */
 187     while (prefixp > prefix_len) *--prefixp = 0;
 188 }
 189
 190 /* ===========================================================================
 191  * Unpack in to out.  This routine does not support the old pack format
 192  * with magic header \037\037.
 193  *
 194  * IN assertions: the buffer inbuf contains already the beginning of
 195  *   the compressed data, from offsets inptr to insize-1 included.
 196  *   The magic header has already been checked. The output buffer is cleared.
 197  */
 198 int unpack(in, out)
 199     int in, out;            /* input and output file descriptors */
 200 {
 201     int len;                /* Bit length of current code */
 202     unsigned eob;           /* End Of Block code */
 203     register unsigned peek; /* lookahead bits */
 204     unsigned peek_mask;     /* Mask for peek_bits bits */
 205
 206     ifd = in;
 207     ofd = out;
 208
 209     read_tree();     /* Read the Huffman tree */
 210     build_tree();    /* Build the prefix table */
 211     clear_bitbuf();  /* Initialize bit input */
 212     peek_mask = (1<<peek_bits)-1;
 213
 214     /* The eob code is the largest code among all leaves of maximal length: */
 215     eob = leaves[max_len]-1;
 216     Trace((stderr, "eob %d %x\n", max_len, eob));
 217
 218     /* Decode the input data: */
 219     for (;;) {
 220         /* Since eob is the longest code and not shorter than max_len,
 221          * we can peek at max_len bits without having the risk of reading
 222          * beyond the end of file.
 223          */
 224         look_bits(peek, peek_bits, peek_mask);
 225         len = prefix_len[peek];
 226         if (len > 0) {
 227             peek >>= peek_bits - len; /* discard the extra bits */
 228         } else {
 229             /* Code of more than peek_bits bits, we must traverse the tree */
 230             ulg mask = peek_mask;
 231             len = peek_bits;
 232             do {
 233                 len++, mask = (mask<<1)+1;
 234                 look_bits(peek, len, mask);
 235             } while (peek < (unsigned)parents[len]);
 236             /* loop as long as peek is a parent node */
 237         }
 238         /* At this point, peek is the next complete code, of len bits */
 239         if (peek == eob && len == max_len) break; /* end of file? */
 240         put_ubyte(literal[peek+lit_base[len]]);
 241         Tracev((stderr,"%02d %04x %c\n", len, peek,
 242                 literal[peek+lit_base[len]]));
 243         skip_bits(len);
 244     } /* for (;;) */
 245
 246     flush_window();
 247     if (orig_len != (ulg)(bytes_out & 0xffffffff)) {
 248         gzip_error ("invalid compressed data--length error");
 249     }
 250     return OK;
 251 }