lib/zlib_inflate/inftrees.c

   1 /* inftrees.c -- generate Huffman trees for efficient decoding
   2  * Copyright (C) 1995-2005 Mark Adler
   3  * For conditions of distribution and use, see copyright notice in zlib.h
   4  */
   5
   6 #include <linux/zutil.h>
   7 #include "inftrees.h"
   8
   9 #define MAXBITS 15
  10
  11 /*
  12    Build a set of tables to decode the provided canonical Huffman code.
  13    The code lengths are lens[0..codes-1].  The result starts at *table,
  14    whose indices are 0..2^bits-1.  work is a writable array of at least
  15    lens shorts, which is used as a work area.  type is the type of code
  16    to be generated, CODES, LENS, or DISTS.  On return, zero is success,
  17    -1 is an invalid code, and +1 means that ENOUGH isn't enough.  table
  18    on return points to the next available entry's address.  bits is the
  19    requested root table index bits, and on return it is the actual root
  20    table index bits.  It will differ if the request is greater than the
  21    longest code or if it is less than the shortest code.
  22  */
  23 int zlib_inflate_table(codetype type, unsigned short *lens, unsigned codes,
  24                         code **table, unsigned *bits, unsigned short *work)
  25 {
  26     unsigned len;               /* a code's length in bits */
  27     unsigned sym;               /* index of code symbols */
  28     unsigned min, max;          /* minimum and maximum code lengths */
  29     unsigned root;              /* number of index bits for root table */
  30     unsigned curr;              /* number of index bits for current table */
  31     unsigned drop;              /* code bits to drop for sub-table */
  32     int left;                   /* number of prefix codes available */
  33     unsigned used;              /* code entries in table used */
  34     unsigned huff;              /* Huffman code */
  35     unsigned incr;              /* for incrementing code, index */
  36     unsigned fill;              /* index for replicating entries */
  37     unsigned low;               /* low bits for current root entry */
  38     unsigned mask;              /* mask for low root bits */
  39     code this;                  /* table entry for duplication */
  40     code *next;             /* next available space in table */
  41     const unsigned short *base;     /* base value table to use */
  42     const unsigned short *extra;    /* extra bits table to use */
  43     int end;                    /* use base and extra for symbol > end */
  44     unsigned short count[MAXBITS+1];    /* number of codes of each length */
  45     unsigned short offs[MAXBITS+1];     /* offsets in table for each length */
  46     static const unsigned short lbase[31] = { /* Length codes 257..285 base */
  47         3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 15, 17, 19, 23, 27, 31,
  48         35, 43, 51, 59, 67, 83, 99, 115, 131, 163, 195, 227, 258, 0, 0};
  49     static const unsigned short lext[31] = { /* Length codes 257..285 extra */
  50         16, 16, 16, 16, 16, 16, 16, 16, 17, 17, 17, 17, 18, 18, 18, 18,
  51         19, 19, 19, 19, 20, 20, 20, 20, 21, 21, 21, 21, 16, 201, 196};
  52     static const unsigned short dbase[32] = { /* Distance codes 0..29 base */
  53         1, 2, 3, 4, 5, 7, 9, 13, 17, 25, 33, 49, 65, 97, 129, 193,
  54         257, 385, 513, 769, 1025, 1537, 2049, 3073, 4097, 6145,
  55         8193, 12289, 16385, 24577, 0, 0};
  56     static const unsigned short dext[32] = { /* Distance codes 0..29 extra */
  57         16, 16, 16, 16, 17, 17, 18, 18, 19, 19, 20, 20, 21, 21, 22, 22,
  58         23, 23, 24, 24, 25, 25, 26, 26, 27, 27,
  59         28, 28, 29, 29, 64, 64};
  60
  61     /*
  62        Process a set of code lengths to create a canonical Huffman code.  The
  63        code lengths are lens[0..codes-1].  Each length corresponds to the
  64        symbols 0..codes-1.  The Huffman code is generated by first sorting the
  65        symbols by length from short to long, and retaining the symbol order
  66        for codes with equal lengths.  Then the code starts with all zero bits
  67        for the first code of the shortest length, and the codes are integer
  68        increments for the same length, and zeros are appended as the length
  69        increases.  For the deflate format, these bits are stored backwards
  70        from their more natural integer increment ordering, and so when the
  71        decoding tables are built in the large loop below, the integer codes
  72        are incremented backwards.
  73
  74        This routine assumes, but does not check, that all of the entries in
  75        lens[] are in the range 0..MAXBITS.  The caller must assure this.
  76        1..MAXBITS is interpreted as that code length.  zero means that that
  77        symbol does not occur in this code.
  78
  79        The codes are sorted by computing a count of codes for each length,
  80        creating from that a table of starting indices for each length in the
  81        sorted table, and then entering the symbols in order in the sorted
  82        table.  The sorted table is work[], with that space being provided by
  83        the caller.
  84
  85        The length counts are used for other purposes as well, i.e. finding
  86        the minimum and maximum length codes, determining if there are any
  87        codes at all, checking for a valid set of lengths, and looking ahead
  88        at length counts to determine sub-table sizes when building the
  89        decoding tables.
  90      */
  91
  92     /* accumulate lengths for codes (assumes lens[] all in 0..MAXBITS) */
  93     for (len = 0; len <= MAXBITS; len++)
  94         count[len] = 0;
  95     for (sym = 0; sym < codes; sym++)
  96         count[lens[sym]]++;
  97
  98     /* bound code lengths, force root to be within code lengths */
  99     root = *bits;
 100     for (max = MAXBITS; max >= 1; max--)
 101         if (count[max] != 0) break;
 102     if (root > max) root = max;
 103     if (max == 0) {                     /* no symbols to code at all */
 104         this.op = (unsigned char)64;    /* invalid code marker */
 105         this.bits = (unsigned char)1;
 106         this.val = (unsigned short)0;
 107         *(*table)++ = this;             /* make a table to force an error */
 108         *(*table)++ = this;
 109         *bits = 1;
 110         return 0;     /* no symbols, but wait for decoding to report error */
 111     }
 112     for (min = 1; min <= MAXBITS; min++)
 113         if (count[min] != 0) break;
 114     if (root < min) root = min;
 115
 116     /* check for an over-subscribed or incomplete set of lengths */
 117     left = 1;
 118     for (len = 1; len <= MAXBITS; len++) {
 119         left <<= 1;
 120         left -= count[len];
 121         if (left < 0) return -1;        /* over-subscribed */
 122     }
 123     if (left > 0 && (type == CODES || max != 1))
 124         return -1;                      /* incomplete set */
 125
 126     /* generate offsets into symbol table for each length for sorting */
 127     offs[1] = 0;
 128     for (len = 1; len < MAXBITS; len++)
 129         offs[len + 1] = offs[len] + count[len];
 130
 131     /* sort symbols by length, by symbol order within each length */
 132     for (sym = 0; sym < codes; sym++)
 133         if (lens[sym] != 0) work[offs[lens[sym]]++] = (unsigned short)sym;
 134
 135     /*
 136        Create and fill in decoding tables.  In this loop, the table being
 137        filled is at next and has curr index bits.  The code being used is huff
 138        with length len.  That code is converted to an index by dropping drop
 139        bits off of the bottom.  For codes where len is less than drop + curr,
 140        those top drop + curr - len bits are incremented through all values to
 141        fill the table with replicated entries.
 142
 143        root is the number of index bits for the root table.  When len exceeds
 144        root, sub-tables are created pointed to by the root entry with an index
 145        of the low root bits of huff.  This is saved in low to check for when a
 146        new sub-table should be started.  drop is zero when the root table is
 147        being filled, and drop is root when sub-tables are being filled.
 148
 149        When a new sub-table is needed, it is necessary to look ahead in the
 150        code lengths to determine what size sub-table is needed.  The length
 151        counts are used for this, and so count[] is decremented as codes are
 152        entered in the tables.
 153
 154        used keeps track of how many table entries have been allocated from the
 155        provided *table space.  It is checked when a LENS table is being made
 156        against the space in *table, ENOUGH, minus the maximum space needed by
 157        the worst case distance code, MAXD.  This should never happen, but the
 158        sufficiency of ENOUGH has not been proven exhaustively, hence the check.
 159        This assumes that when type == LENS, bits == 9.
 160
 161        sym increments through all symbols, and the loop terminates when
 162        all codes of length max, i.e. all codes, have been processed.  This
 163        routine permits incomplete codes, so another loop after this one fills
 164        in the rest of the decoding tables with invalid code markers.
 165      */
 166
 167     /* set up for code type */
 168     switch (type) {
 169     case CODES:
 170         base = extra = work;    /* dummy value--not used */
 171         end = 19;
 172         break;
 173     case LENS:
 174         base = lbase;
 175         base -= 257;
 176         extra = lext;
 177         extra -= 257;
 178         end = 256;
 179         break;
 180     default:            /* DISTS */
 181         base = dbase;
 182         extra = dext;
 183         end = -1;
 184     }
 185
 186     /* initialize state for loop */
 187     huff = 0;                   /* starting code */
 188     sym = 0;                    /* starting code symbol */
 189     len = min;                  /* starting code length */
 190     next = *table;              /* current table to fill in */
 191     curr = root;                /* current table index bits */
 192     drop = 0;                   /* current bits to drop from code for index */
 193     low = (unsigned)(-1);       /* trigger new sub-table when len > root */
 194     used = 1U << root;          /* use root table entries */
 195     mask = used - 1;            /* mask for comparing low */
 196
 197     /* check available table space */
 198     if (type == LENS && used >= ENOUGH - MAXD)
 199         return 1;
 200
 201     /* process all codes and make table entries */
 202     for (;;) {
 203         /* create table entry */
 204         this.bits = (unsigned char)(len - drop);
 205         if ((int)(work[sym]) < end) {
 206             this.op = (unsigned char)0;
 207             this.val = work[sym];
 208         }
 209         else if ((int)(work[sym]) > end) {
 210             this.op = (unsigned char)(extra[work[sym]]);
 211             this.val = base[work[sym]];
 212         }
 213         else {
 214             this.op = (unsigned char)(32 + 64);         /* end of block */
 215             this.val = 0;
 216         }
 217
 218         /* replicate for those indices with low len bits equal to huff */
 219         incr = 1U << (len - drop);
 220         fill = 1U << curr;
 221         min = fill;                 /* save offset to next table */
 222         do {
 223             fill -= incr;
 224             next[(huff >> drop) + fill] = this;
 225         } while (fill != 0);
 226
 227         /* backwards increment the len-bit code huff */
 228         incr = 1U << (len - 1);
 229         while (huff & incr)
 230             incr >>= 1;
 231         if (incr != 0) {
 232             huff &= incr - 1;
 233             huff += incr;
 234         }
 235         else
 236             huff = 0;
 237
 238         /* go to next symbol, update count, len */
 239         sym++;
 240         if (--(count[len]) == 0) {
 241             if (len == max) break;
 242             len = lens[work[sym]];
 243         }
 244
 245         /* create new sub-table if needed */
 246         if (len > root && (huff & mask) != low) {
 247             /* if first time, transition to sub-tables */
 248             if (drop == 0)
 249                 drop = root;
 250
 251             /* increment past last table */
 252             next += min;            /* here min is 1 << curr */
 253
 254             /* determine length of next table */
 255             curr = len - drop;
 256             left = (int)(1 << curr);
 257             while (curr + drop < max) {
 258                 left -= count[curr + drop];
 259                 if (left <= 0) break;
 260                 curr++;
 261                 left <<= 1;
 262             }
 263
 264             /* check for enough space */
 265             used += 1U << curr;
 266             if (type == LENS && used >= ENOUGH - MAXD)
 267                 return 1;
 268
 269             /* point entry in root table to sub-table */
 270             low = huff & mask;
 271             (*table)[low].op = (unsigned char)curr;
 272             (*table)[low].bits = (unsigned char)root;
 273             (*table)[low].val = (unsigned short)(next - *table);
 274         }
 275     }
 276
 277     /*
 278        Fill in rest of table for incomplete codes.  This loop is similar to the
 279        loop above in incrementing huff for table indices.  It is assumed that
 280        len is equal to curr + drop, so there is no loop needed to increment
 281        through high index bits.  When the current sub-table is filled, the loop
 282        drops back to the root table to fill in any remaining entries there.
 283      */
 284     this.op = (unsigned char)64;                /* invalid code marker */
 285     this.bits = (unsigned char)(len - drop);
 286     this.val = (unsigned short)0;
 287     while (huff != 0) {
 288         /* when done with sub-table, drop back to root table */
 289         if (drop != 0 && (huff & mask) != low) {
 290             drop = 0;
 291             len = root;
 292             next = *table;
 293             this.bits = (unsigned char)len;
 294         }
 295
 296         /* put invalid code marker in table */
 297         next[huff >> drop] = this;
 298
 299         /* backwards increment the len-bit code huff */
 300         incr = 1U << (len - 1);
 301         while (huff & incr)
 302             incr >>= 1;
 303         if (incr != 0) {
 304             huff &= incr - 1;
 305             huff += incr;
 306         }
 307         else
 308             huff = 0;
 309     }
 310
 311     /* set return parameters */
 312     *table += used;
 313     *bits = root;
 314     return 0;
 315 }