xref: /illumos-gate/usr/src/contrib/zlib/inftrees.c (revision b8382935)
1 /* inftrees.c -- generate Huffman trees for efficient decoding
2  * Copyright (C) 1995-2017 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 const char inflate_copyright[] =
12    " inflate 1.2.11 Copyright 1995-2017 Mark Adler ";
13 /*
14   If you use the zlib library in a product, an acknowledgment is welcome
15   in the documentation of your product. If for some reason you cannot
16   include such an acknowledgment, I would appreciate that you keep this
17   copyright string in the executable of your product.
18  */
19 
20 /*
21    Build a set of tables to decode the provided canonical Huffman code.
22    The code lengths are lens[0..codes-1].  The result starts at *table,
23    whose indices are 0..2^bits-1.  work is a writable array of at least
24    lens shorts, which is used as a work area.  type is the type of code
25    to be generated, CODES, LENS, or DISTS.  On return, zero is success,
26    -1 is an invalid code, and +1 means that ENOUGH isn't enough.  table
27    on return points to the next available entry's address.  bits is the
28    requested root table index bits, and on return it is the actual root
29    table index bits.  It will differ if the request is greater than the
30    longest code or if it is less than the shortest code.
31  */
inflate_table(codetype type,unsigned short FAR * lens,unsigned codes,code FAR * FAR * table,unsigned FAR * bits,unsigned short FAR * work)32 int ZLIB_INTERNAL inflate_table(codetype type, unsigned short FAR *lens,
33     unsigned codes, code FAR * FAR *table, unsigned FAR *bits,
34     unsigned short FAR *work)
35 {
36     unsigned len;               /* a code's length in bits */
37     unsigned sym;               /* index of code symbols */
38     unsigned min, max;          /* minimum and maximum code lengths */
39     unsigned root;              /* number of index bits for root table */
40     unsigned curr;              /* number of index bits for current table */
41     unsigned drop;              /* code bits to drop for sub-table */
42     int left;                   /* number of prefix codes available */
43     unsigned used;              /* code entries in table used */
44     unsigned huff;              /* Huffman code */
45     unsigned incr;              /* for incrementing code, index */
46     unsigned fill;              /* index for replicating entries */
47     unsigned low;               /* low bits for current root entry */
48     unsigned mask;              /* mask for low root bits */
49     code here;                  /* table entry for duplication */
50     code FAR *next;             /* next available space in table */
51     const unsigned short FAR *base;     /* base value table to use */
52     const unsigned short FAR *extra;    /* extra bits table to use */
53     unsigned match;             /* use base and extra for symbol >= match */
54     unsigned short count[MAXBITS+1];    /* number of codes of each length */
55     unsigned short offs[MAXBITS+1];     /* offsets in table for each length */
56     static const unsigned short lbase[31] = { /* Length codes 257..285 base */
57         3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 15, 17, 19, 23, 27, 31,
58         35, 43, 51, 59, 67, 83, 99, 115, 131, 163, 195, 227, 258, 0, 0};
59     static const unsigned short lext[31] = { /* Length codes 257..285 extra */
60         16, 16, 16, 16, 16, 16, 16, 16, 17, 17, 17, 17, 18, 18, 18, 18,
61         19, 19, 19, 19, 20, 20, 20, 20, 21, 21, 21, 21, 16, 77, 202};
62     static const unsigned short dbase[32] = { /* Distance codes 0..29 base */
63         1, 2, 3, 4, 5, 7, 9, 13, 17, 25, 33, 49, 65, 97, 129, 193,
64         257, 385, 513, 769, 1025, 1537, 2049, 3073, 4097, 6145,
65         8193, 12289, 16385, 24577, 0, 0};
66     static const unsigned short dext[32] = { /* Distance codes 0..29 extra */
67         16, 16, 16, 16, 17, 17, 18, 18, 19, 19, 20, 20, 21, 21, 22, 22,
68         23, 23, 24, 24, 25, 25, 26, 26, 27, 27,
69         28, 28, 29, 29, 64, 64};
70 
71     /*
72        Process a set of code lengths to create a canonical Huffman code.  The
73        code lengths are lens[0..codes-1].  Each length corresponds to the
74        symbols 0..codes-1.  The Huffman code is generated by first sorting the
75        symbols by length from short to long, and retaining the symbol order
76        for codes with equal lengths.  Then the code starts with all zero bits
77        for the first code of the shortest length, and the codes are integer
78        increments for the same length, and zeros are appended as the length
79        increases.  For the deflate format, these bits are stored backwards
80        from their more natural integer increment ordering, and so when the
81        decoding tables are built in the large loop below, the integer codes
82        are incremented backwards.
83 
84        This routine assumes, but does not check, that all of the entries in
85        lens[] are in the range 0..MAXBITS.  The caller must assure this.
86        1..MAXBITS is interpreted as that code length.  zero means that that
87        symbol does not occur in this code.
88 
89        The codes are sorted by computing a count of codes for each length,
90        creating from that a table of starting indices for each length in the
91        sorted table, and then entering the symbols in order in the sorted
92        table.  The sorted table is work[], with that space being provided by
93        the caller.
94 
95        The length counts are used for other purposes as well, i.e. finding
96        the minimum and maximum length codes, determining if there are any
97        codes at all, checking for a valid set of lengths, and looking ahead
98        at length counts to determine sub-table sizes when building the
99        decoding tables.
100      */
101 
102     /* accumulate lengths for codes (assumes lens[] all in 0..MAXBITS) */
103     for (len = 0; len <= MAXBITS; len++)
104         count[len] = 0;
105     for (sym = 0; sym < codes; sym++)
106         count[lens[sym]]++;
107 
108     /* bound code lengths, force root to be within code lengths */
109     root = *bits;
110     for (max = MAXBITS; max >= 1; max--)
111         if (count[max] != 0) break;
112     if (root > max) root = max;
113     if (max == 0) {                     /* no symbols to code at all */
114         here.op = (unsigned char)64;    /* invalid code marker */
115         here.bits = (unsigned char)1;
116         here.val = (unsigned short)0;
117         *(*table)++ = here;             /* make a table to force an error */
118         *(*table)++ = here;
119         *bits = 1;
120         return 0;     /* no symbols, but wait for decoding to report error */
121     }
122     for (min = 1; min < max; min++)
123         if (count[min] != 0) break;
124     if (root < min) root = min;
125 
126     /* check for an over-subscribed or incomplete set of lengths */
127     left = 1;
128     for (len = 1; len <= MAXBITS; len++) {
129         left <<= 1;
130         left -= count[len];
131         if (left < 0) return -1;        /* over-subscribed */
132     }
133     if (left > 0 && (type == CODES || max != 1))
134         return -1;                      /* incomplete set */
135 
136     /* generate offsets into symbol table for each length for sorting */
137     offs[1] = 0;
138     for (len = 1; len < MAXBITS; len++)
139         offs[len + 1] = offs[len] + count[len];
140 
141     /* sort symbols by length, by symbol order within each length */
142     for (sym = 0; sym < codes; sym++)
143         if (lens[sym] != 0) work[offs[lens[sym]]++] = (unsigned short)sym;
144 
145     /*
146        Create and fill in decoding tables.  In this loop, the table being
147        filled is at next and has curr index bits.  The code being used is huff
148        with length len.  That code is converted to an index by dropping drop
149        bits off of the bottom.  For codes where len is less than drop + curr,
150        those top drop + curr - len bits are incremented through all values to
151        fill the table with replicated entries.
152 
153        root is the number of index bits for the root table.  When len exceeds
154        root, sub-tables are created pointed to by the root entry with an index
155        of the low root bits of huff.  This is saved in low to check for when a
156        new sub-table should be started.  drop is zero when the root table is
157        being filled, and drop is root when sub-tables are being filled.
158 
159        When a new sub-table is needed, it is necessary to look ahead in the
160        code lengths to determine what size sub-table is needed.  The length
161        counts are used for this, and so count[] is decremented as codes are
162        entered in the tables.
163 
164        used keeps track of how many table entries have been allocated from the
165        provided *table space.  It is checked for LENS and DIST tables against
166        the constants ENOUGH_LENS and ENOUGH_DISTS to guard against changes in
167        the initial root table size constants.  See the comments in inftrees.h
168        for more information.
169 
170        sym increments through all symbols, and the loop terminates when
171        all codes of length max, i.e. all codes, have been processed.  This
172        routine permits incomplete codes, so another loop after this one fills
173        in the rest of the decoding tables with invalid code markers.
174      */
175 
176     /* set up for code type */
177     switch (type) {
178     case CODES:
179         base = extra = work;    /* dummy value--not used */
180         match = 20;
181         break;
182     case LENS:
183         base = lbase;
184         extra = lext;
185         match = 257;
186         break;
187     default:    /* DISTS */
188         base = dbase;
189         extra = dext;
190         match = 0;
191     }
192 
193     /* initialize state for loop */
194     huff = 0;                   /* starting code */
195     sym = 0;                    /* starting code symbol */
196     len = min;                  /* starting code length */
197     next = *table;              /* current table to fill in */
198     curr = root;                /* current table index bits */
199     drop = 0;                   /* current bits to drop from code for index */
200     low = (unsigned)(-1);       /* trigger new sub-table when len > root */
201     used = 1U << root;          /* use root table entries */
202     mask = used - 1;            /* mask for comparing low */
203 
204     /* check available table space */
205     if ((type == LENS && used > ENOUGH_LENS) ||
206         (type == DISTS && used > ENOUGH_DISTS))
207         return 1;
208 
209     /* process all codes and make table entries */
210     for (;;) {
211         /* create table entry */
212         here.bits = (unsigned char)(len - drop);
213         if (work[sym] + 1U < match) {
214             here.op = (unsigned char)0;
215             here.val = work[sym];
216         }
217         else if (work[sym] >= match) {
218             here.op = (unsigned char)(extra[work[sym] - match]);
219             here.val = base[work[sym] - match];
220         }
221         else {
222             here.op = (unsigned char)(32 + 64);         /* end of block */
223             here.val = 0;
224         }
225 
226         /* replicate for those indices with low len bits equal to huff */
227         incr = 1U << (len - drop);
228         fill = 1U << curr;
229         min = fill;                 /* save offset to next table */
230         do {
231             fill -= incr;
232             next[(huff >> drop) + fill] = here;
233         } while (fill != 0);
234 
235         /* backwards increment the len-bit code huff */
236         incr = 1U << (len - 1);
237         while (huff & incr)
238             incr >>= 1;
239         if (incr != 0) {
240             huff &= incr - 1;
241             huff += incr;
242         }
243         else
244             huff = 0;
245 
246         /* go to next symbol, update count, len */
247         sym++;
248         if (--(count[len]) == 0) {
249             if (len == max) break;
250             len = lens[work[sym]];
251         }
252 
253         /* create new sub-table if needed */
254         if (len > root && (huff & mask) != low) {
255             /* if first time, transition to sub-tables */
256             if (drop == 0)
257                 drop = root;
258 
259             /* increment past last table */
260             next += min;            /* here min is 1 << curr */
261 
262             /* determine length of next table */
263             curr = len - drop;
264             left = (int)(1 << curr);
265             while (curr + drop < max) {
266                 left -= count[curr + drop];
267                 if (left <= 0) break;
268                 curr++;
269                 left <<= 1;
270             }
271 
272             /* check for enough space */
273             used += 1U << curr;
274             if ((type == LENS && used > ENOUGH_LENS) ||
275                 (type == DISTS && used > ENOUGH_DISTS))
276                 return 1;
277 
278             /* point entry in root table to sub-table */
279             low = huff & mask;
280             (*table)[low].op = (unsigned char)curr;
281             (*table)[low].bits = (unsigned char)root;
282             (*table)[low].val = (unsigned short)(next - *table);
283         }
284     }
285 
286     /* fill in remaining table entry if code is incomplete (guaranteed to have
287        at most one remaining entry, since if the code is incomplete, the
288        maximum code length that was allowed to get this far is one bit) */
289     if (huff != 0) {
290         here.op = (unsigned char)64;            /* invalid code marker */
291         here.bits = (unsigned char)(len - drop);
292         here.val = (unsigned short)0;
293         next[huff] = here;
294     }
295 
296     /* set return parameters */
297     *table += used;
298     *bits = root;
299     return 0;
300 }
301