src/zlib/inftrees.c

   1 /* inftrees.c -- generate Huffman trees for efficient decoding
   2  * Copyright (C) 1995-2012 Mark Adler
   3  * For conditions of distribution and use, see copyright notice in zlib.h
   4  */
   5
   6 #include "zutil.h"
   7 #include "inftrees.h"
   8
   9 #define MAXBITS 15
  10
  11 #ifdef ZLIB_DISABLED
  12 const char inflate_copyright[] =
  13    " inflate 1.2.7 Copyright 1995-2012 Mark Adler ";
  14 /*
  15   If you use the zlib library in a product, an acknowledgment is welcome
  16   in the documentation of your product. If for some reason you cannot
  17   include such an acknowledgment, I would appreciate that you keep this
  18   copyright string in the executable of your product.
  19  */
  20 #endif
  21
  22 /*
  23    Build a set of tables to decode the provided canonical Huffman code.
  24    The code lengths are lens[0..codes-1].  The result starts at *table,
  25    whose indices are 0..2^bits-1.  work is a writable array of at least
  26    lens shorts, which is used as a work area.  type is the type of code
  27    to be generated, CODES, LENS, or DISTS.  On return, zero is success,
  28    -1 is an invalid code, and +1 means that ENOUGH isn't enough.  table
  29    on return points to the next available entry's address.  bits is the
  30    requested root table index bits, and on return it is the actual root
  31    table index bits.  It will differ if the request is greater than the
  32    longest code or if it is less than the shortest code.
  33  */
  34 int ZLIB_INTERNAL inflate_table(type, lens, codes, table, bits, work)
  35 codetype type;
  36 unsigned short FAR *lens;
  37 unsigned codes;
  38 code FAR * FAR *table;
  39 unsigned FAR *bits;
  40 unsigned short FAR *work;
  41 {
  42     unsigned len;               /* a code's length in bits */
  43     unsigned sym;               /* index of code symbols */
  44     unsigned min, max;          /* minimum and maximum code lengths */
  45     unsigned root;              /* number of index bits for root table */
  46     unsigned curr;              /* number of index bits for current table */
  47     unsigned drop;              /* code bits to drop for sub-table */
  48     int left;                   /* number of prefix codes available */
  49     unsigned used;              /* code entries in table used */
  50     unsigned huff;              /* Huffman code */
  51     unsigned incr;              /* for incrementing code, index */
  52     unsigned fill;              /* index for replicating entries */
  53     unsigned low;               /* low bits for current root entry */
  54     unsigned mask;              /* mask for low root bits */
  55     code here;                  /* table entry for duplication */
  56     code FAR *next;             /* next available space in table */
  57     const unsigned short FAR *base;     /* base value table to use */
  58     const unsigned short FAR *extra;    /* extra bits table to use */
  59     int end;                    /* use base and extra for symbol > end */
  60     unsigned short count[MAXBITS+1];    /* number of codes of each length */
  61     unsigned short offs[MAXBITS+1];     /* offsets in table for each length */
  62     static const unsigned short lbase[31] = { /* Length codes 257..285 base */
  63         3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 15, 17, 19, 23, 27, 31,
  64         35, 43, 51, 59, 67, 83, 99, 115, 131, 163, 195, 227, 258, 0, 0};
  65     static const unsigned short lext[31] = { /* Length codes 257..285 extra */
  66         16, 16, 16, 16, 16, 16, 16, 16, 17, 17, 17, 17, 18, 18, 18, 18,
  67         19, 19, 19, 19, 20, 20, 20, 20, 21, 21, 21, 21, 16, 78, 68};
  68     static const unsigned short dbase[32] = { /* Distance codes 0..29 base */
  69         1, 2, 3, 4, 5, 7, 9, 13, 17, 25, 33, 49, 65, 97, 129, 193,
  70         257, 385, 513, 769, 1025, 1537, 2049, 3073, 4097, 6145,
  71         8193, 12289, 16385, 24577, 0, 0};
  72     static const unsigned short dext[32] = { /* Distance codes 0..29 extra */
  73         16, 16, 16, 16, 17, 17, 18, 18, 19, 19, 20, 20, 21, 21, 22, 22,
  74         23, 23, 24, 24, 25, 25, 26, 26, 27, 27,
  75         28, 28, 29, 29, 64, 64};
  76
  77     /*
  78        Process a set of code lengths to create a canonical Huffman code.  The
  79        code lengths are lens[0..codes-1].  Each length corresponds to the
  80        symbols 0..codes-1.  The Huffman code is generated by first sorting the
  81        symbols by length from short to long, and retaining the symbol order
  82        for codes with equal lengths.  Then the code starts with all zero bits
  83        for the first code of the shortest length, and the codes are integer
  84        increments for the same length, and zeros are appended as the length
  85        increases.  For the deflate format, these bits are stored backwards
  86        from their more natural integer increment ordering, and so when the
  87        decoding tables are built in the large loop below, the integer codes
  88        are incremented backwards.
  89
  90        This routine assumes, but does not check, that all of the entries in
  91        lens[] are in the range 0..MAXBITS.  The caller must assure this.
  92        1..MAXBITS is interpreted as that code length.  zero means that that
  93        symbol does not occur in this code.
  94
  95        The codes are sorted by computing a count of codes for each length,
  96        creating from that a table of starting indices for each length in the
  97        sorted table, and then entering the symbols in order in the sorted
  98        table.  The sorted table is work[], with that space being provided by
  99        the caller.
 100
 101        The length counts are used for other purposes as well, i.e. finding
 102        the minimum and maximum length codes, determining if there are any
 103        codes at all, checking for a valid set of lengths, and looking ahead
 104        at length counts to determine sub-table sizes when building the
 105        decoding tables.
 106      */
 107
 108     /* accumulate lengths for codes (assumes lens[] all in 0..MAXBITS) */
 109     for (len = 0; len <= MAXBITS; len++)
 110         count[len] = 0;
 111     for (sym = 0; sym < codes; sym++)
 112         count[lens[sym]]++;
 113
 114     /* bound code lengths, force root to be within code lengths */
 115     root = *bits;
 116     for (max = MAXBITS; max >= 1; max--)
 117         if (count[max] != 0) break;
 118     if (root > max) root = max;
 119     if (max == 0) {                     /* no symbols to code at all */
 120         here.op = (unsigned char)64;    /* invalid code marker */
 121         here.bits = (unsigned char)1;
 122         here.val = (unsigned short)0;
 123         *(*table)++ = here;             /* make a table to force an error */
 124         *(*table)++ = here;
 125         *bits = 1;
 126         return 0;     /* no symbols, but wait for decoding to report error */
 127     }
 128     for (min = 1; min < max; min++)
 129         if (count[min] != 0) break;
 130     if (root < min) root = min;
 131
 132     /* check for an over-subscribed or incomplete set of lengths */
 133     left = 1;
 134     for (len = 1; len <= MAXBITS; len++) {
 135         left <<= 1;
 136         left -= count[len];
 137         if (left < 0) return -1;        /* over-subscribed */
 138     }
 139     if (left > 0 && (type == CODES || max != 1))
 140         return -1;                      /* incomplete set */
 141
 142     /* generate offsets into symbol table for each length for sorting */
 143     offs[1] = 0;
 144     for (len = 1; len < MAXBITS; len++)
 145         offs[len + 1] = offs[len] + count[len];
 146
 147     /* sort symbols by length, by symbol order within each length */
 148     for (sym = 0; sym < codes; sym++)
 149         if (lens[sym] != 0) work[offs[lens[sym]]++] = (unsigned short)sym;
 150
 151     /*
 152        Create and fill in decoding tables.  In this loop, the table being
 153        filled is at next and has curr index bits.  The code being used is huff
 154        with length len.  That code is converted to an index by dropping drop
 155        bits off of the bottom.  For codes where len is less than drop + curr,
 156        those top drop + curr - len bits are incremented through all values to
 157        fill the table with replicated entries.
 158
 159        root is the number of index bits for the root table.  When len exceeds
 160        root, sub-tables are created pointed to by the root entry with an index
 161        of the low root bits of huff.  This is saved in low to check for when a
 162        new sub-table should be started.  drop is zero when the root table is
 163        being filled, and drop is root when sub-tables are being filled.
 164
 165        When a new sub-table is needed, it is necessary to look ahead in the
 166        code lengths to determine what size sub-table is needed.  The length
 167        counts are used for this, and so count[] is decremented as codes are
 168        entered in the tables.
 169
 170        used keeps track of how many table entries have been allocated from the
 171        provided *table space.  It is checked for LENS and DIST tables against
 172        the constants ENOUGH_LENS and ENOUGH_DISTS to guard against changes in
 173        the initial root table size constants.  See the comments in inftrees.h
 174        for more information.
 175
 176        sym increments through all symbols, and the loop terminates when
 177        all codes of length max, i.e. all codes, have been processed.  This
 178        routine permits incomplete codes, so another loop after this one fills
 179        in the rest of the decoding tables with invalid code markers.
 180      */
 181
 182     /* set up for code type */
 183     switch (type) {
 184     case CODES:
 185         base = extra = work;    /* dummy value--not used */
 186         end = 19;
 187         break;
 188     case LENS:
 189         base = lbase;
 190         base -= 257;
 191         extra = lext;
 192         extra -= 257;
 193         end = 256;
 194         break;
 195     default:            /* DISTS */
 196         base = dbase;
 197         extra = dext;
 198         end = -1;
 199     }
 200
 201     /* initialize state for loop */
 202     huff = 0;                   /* starting code */
 203     sym = 0;                    /* starting code symbol */
 204     len = min;                  /* starting code length */
 205     next = *table;              /* current table to fill in */
 206     curr = root;                /* current table index bits */
 207     drop = 0;                   /* current bits to drop from code for index */
 208     low = (unsigned)(-1);       /* trigger new sub-table when len > root */
 209     used = 1U << root;          /* use root table entries */
 210     mask = used - 1;            /* mask for comparing low */
 211
 212     /* check available table space */
 213     if ((type == LENS && used >= ENOUGH_LENS) ||
 214         (type == DISTS && used >= ENOUGH_DISTS))
 215         return 1;
 216
 217     /* process all codes and make table entries */
 218     for (;;) {
 219         /* create table entry */
 220         here.bits = (unsigned char)(len - drop);
 221         if ((int)(work[sym]) < end) {
 222             here.op = (unsigned char)0;
 223             here.val = work[sym];
 224         }
 225         else if ((int)(work[sym]) > end) {
 226             here.op = (unsigned char)(extra[work[sym]]);
 227             here.val = base[work[sym]];
 228         }
 229         else {
 230             here.op = (unsigned char)(32 + 64);         /* end of block */
 231             here.val = 0;
 232         }
 233
 234         /* replicate for those indices with low len bits equal to huff */
 235         incr = 1U << (len - drop);
 236         fill = 1U << curr;
 237         min = fill;                 /* save offset to next table */
 238         do {
 239             fill -= incr;
 240             next[(huff >> drop) + fill] = here;
 241         } while (fill != 0);
 242
 243         /* backwards increment the len-bit code huff */
 244         incr = 1U << (len - 1);
 245         while (huff & incr)
 246             incr >>= 1;
 247         if (incr != 0) {
 248             huff &= incr - 1;
 249             huff += incr;
 250         }
 251         else
 252             huff = 0;
 253
 254         /* go to next symbol, update count, len */
 255         sym++;
 256         if (--(count[len]) == 0) {
 257             if (len == max) break;
 258             len = lens[work[sym]];
 259         }
 260
 261         /* create new sub-table if needed */
 262         if (len > root && (huff & mask) != low) {
 263             /* if first time, transition to sub-tables */
 264             if (drop == 0)
 265                 drop = root;
 266
 267             /* increment past last table */
 268             next += min;            /* here min is 1 << curr */
 269
 270             /* determine length of next table */
 271             curr = len - drop;
 272             left = (int)(1 << curr);
 273             while (curr + drop < max) {
 274                 left -= count[curr + drop];
 275                 if (left <= 0) break;
 276                 curr++;
 277                 left <<= 1;
 278             }
 279
 280             /* check for enough space */
 281             used += 1U << curr;
 282             if ((type == LENS && used >= ENOUGH_LENS) ||
 283                 (type == DISTS && used >= ENOUGH_DISTS))
 284                 return 1;
 285
 286             /* point entry in root table to sub-table */
 287             low = huff & mask;
 288             (*table)[low].op = (unsigned char)curr;
 289             (*table)[low].bits = (unsigned char)root;
 290             (*table)[low].val = (unsigned short)(next - *table);
 291         }
 292     }
 293
 294     /* fill in remaining table entry if code is incomplete (guaranteed to have
 295        at most one remaining entry, since if the code is incomplete, the
 296        maximum code length that was allowed to get this far is one bit) */
 297     if (huff != 0) {
 298         here.op = (unsigned char)64;            /* invalid code marker */
 299         here.bits = (unsigned char)(len - drop);
 300         here.val = (unsigned short)0;
 301         next[huff] = here;
 302     }
 303
 304     /* set return parameters */
 305     *table += used;
 306     *bits = root;
 307     return 0;
 308 }