1 /* inftrees.c -- generate Huffman trees for efficient decoding
2 * Copyright (C) 1995-2023 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 1.3 Copyright 1995-2023 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,
34 unsigned FAR *bits, unsigned short FAR *work) {
35 unsigned len; /* a code's length in bits */
36 unsigned sym; /* index of code symbols */
37 unsigned min, max; /* minimum and maximum code lengths */
38 unsigned root; /* number of index bits for root table */
39 unsigned curr; /* number of index bits for current table */
40 unsigned drop; /* code bits to drop for sub-table */
41 int left; /* number of prefix codes available */
42 unsigned used; /* code entries in table used */
43 unsigned huff; /* Huffman code */
44 unsigned incr; /* for incrementing code, index */
45 unsigned fill; /* index for replicating entries */
46 unsigned low; /* low bits for current root entry */
47 unsigned mask; /* mask for low root bits */
48 code here; /* table entry for duplication */
49 code FAR *next; /* next available space in table */
50 const unsigned short FAR *base; /* base value table to use */
51 const unsigned short FAR *extra; /* extra bits table to use */
52 unsigned match; /* use base and extra for symbol >= match */
53 unsigned short count[MAXBITS+1]; /* number of codes of each length */
54 unsigned short offs[MAXBITS+1]; /* offsets in table for each length */
55 static const unsigned short lbase[31] = { /* Length codes 257..285 base */
56 3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 15, 17, 19, 23, 27, 31,
57 35, 43, 51, 59, 67, 83, 99, 115, 131, 163, 195, 227, 258, 0, 0};
58 static const unsigned short lext[31] = { /* Length codes 257..285 extra */
59 16, 16, 16, 16, 16, 16, 16, 16, 17, 17, 17, 17, 18, 18, 18, 18,
60 19, 19, 19, 19, 20, 20, 20, 20, 21, 21, 21, 21, 16, 198, 203};
61 static const unsigned short dbase[32] = { /* Distance codes 0..29 base */
62 1, 2, 3, 4, 5, 7, 9, 13, 17, 25, 33, 49, 65, 97, 129, 193,
63 257, 385, 513, 769, 1025, 1537, 2049, 3073, 4097, 6145,
64 8193, 12289, 16385, 24577, 0, 0};
65 static const unsigned short dext[32] = { /* Distance codes 0..29 extra */
66 16, 16, 16, 16, 17, 17, 18, 18, 19, 19, 20, 20, 21, 21, 22, 22,
67 23, 23, 24, 24, 25, 25, 26, 26, 27, 27,
68 28, 28, 29, 29, 64, 64};
69
70 /*
71 Process a set of code lengths to create a canonical Huffman code. The
72 code lengths are lens[0..codes-1]. Each length corresponds to the
73 symbols 0..codes-1. The Huffman code is generated by first sorting the
74 symbols by length from short to long, and retaining the symbol order
75 for codes with equal lengths. Then the code starts with all zero bits
76 for the first code of the shortest length, and the codes are integer
77 increments for the same length, and zeros are appended as the length
78 increases. For the deflate format, these bits are stored backwards
79 from their more natural integer increment ordering, and so when the
80 decoding tables are built in the large loop below, the integer codes
81 are incremented backwards.
82
83 This routine assumes, but does not check, that all of the entries in
84 lens[] are in the range 0..MAXBITS. The caller must assure this.
85 1..MAXBITS is interpreted as that code length. zero means that that
86 symbol does not occur in this code.
87
88 The codes are sorted by computing a count of codes for each length,
89 creating from that a table of starting indices for each length in the
90 sorted table, and then entering the symbols in order in the sorted
91 table. The sorted table is work[], with that space being provided by
92 the caller.
93
94 The length counts are used for other purposes as well, i.e. finding
95 the minimum and maximum length codes, determining if there are any
96 codes at all, checking for a valid set of lengths, and looking ahead
97 at length counts to determine sub-table sizes when building the
98 decoding tables.
99 */
100
101 /* accumulate lengths for codes (assumes lens[] all in 0..MAXBITS) */
102 for (len = 0; len <= MAXBITS; len++)
103 count[len] = 0;
104 for (sym = 0; sym < codes; sym++)
105 count[lens[sym]]++;
106
107 /* bound code lengths, force root to be within code lengths */
108 root = *bits;
109 for (max = MAXBITS; max >= 1; max--)
110 if (count[max] != 0) break;
111 if (root > max) root = max;
112 if (max == 0) { /* no symbols to code at all */
113 here.op = (unsigned char)64; /* invalid code marker */
114 here.bits = (unsigned char)1;
115 here.val = (unsigned short)0;
116 *(*table)++ = here; /* make a table to force an error */
117 *(*table)++ = here;
118 *bits = 1;
119 return 0; /* no symbols, but wait for decoding to report error */
120 }
121 for (min = 1; min < max; min++)
122 if (count[min] != 0) break;
123 if (root < min) root = min;
124
125 /* check for an over-subscribed or incomplete set of lengths */
126 left = 1;
127 for (len = 1; len <= MAXBITS; len++) {
128 left <<= 1;
129 left -= count[len];
130 if (left < 0) return -1; /* over-subscribed */
131 }
132 if (left > 0 && (type == CODES || max != 1))
133 return -1; /* incomplete set */
134
135 /* generate offsets into symbol table for each length for sorting */
136 offs[1] = 0;
137 for (len = 1; len < MAXBITS; len++)
138 offs[len + 1] = offs[len] + count[len];
139
140 /* sort symbols by length, by symbol order within each length */
141 for (sym = 0; sym < codes; sym++)
142 if (lens[sym] != 0) work[offs[lens[sym]]++] = (unsigned short)sym;
143
144 /*
145 Create and fill in decoding tables. In this loop, the table being
146 filled is at next and has curr index bits. The code being used is huff
147 with length len. That code is converted to an index by dropping drop
148 bits off of the bottom. For codes where len is less than drop + curr,
149 those top drop + curr - len bits are incremented through all values to
150 fill the table with replicated entries.
151
152 root is the number of index bits for the root table. When len exceeds
153 root, sub-tables are created pointed to by the root entry with an index
154 of the low root bits of huff. This is saved in low to check for when a
155 new sub-table should be started. drop is zero when the root table is
156 being filled, and drop is root when sub-tables are being filled.
157
158 When a new sub-table is needed, it is necessary to look ahead in the
159 code lengths to determine what size sub-table is needed. The length
160 counts are used for this, and so count[] is decremented as codes are
161 entered in the tables.
162
163 used keeps track of how many table entries have been allocated from the
164 provided *table space. It is checked for LENS and DIST tables against
165 the constants ENOUGH_LENS and ENOUGH_DISTS to guard against changes in
166 the initial root table size constants. See the comments in inftrees.h
167 for more information.
168
169 sym increments through all symbols, and the loop terminates when
170 all codes of length max, i.e. all codes, have been processed. This
171 routine permits incomplete codes, so another loop after this one fills
172 in the rest of the decoding tables with invalid code markers.
173 */
174
175 /* set up for code type */
176 switch (type) {
177 case CODES:
178 base = extra = work; /* dummy value--not used */
179 match = 20;
180 break;
181 case LENS:
182 base = lbase;
183 extra = lext;
184 match = 257;
185 break;
186 default: /* DISTS */
187 base = dbase;
188 extra = dext;
189 match = 0;
190 }
191
192 /* initialize state for loop */
193 huff = 0; /* starting code */
194 sym = 0; /* starting code symbol */
195 len = min; /* starting code length */
196 next = *table; /* current table to fill in */
197 curr = root; /* current table index bits */
198 drop = 0; /* current bits to drop from code for index */
199 low = (unsigned)(-1); /* trigger new sub-table when len > root */
200 used = 1U << root; /* use root table entries */
201 mask = used - 1; /* mask for comparing low */
202
203 /* check available table space */
204 if ((type == LENS && used > ENOUGH_LENS) ||
205 (type == DISTS && used > ENOUGH_DISTS))
206 return 1;
207
208 /* process all codes and make table entries */
209 for (;;) {
210 /* create table entry */
211 here.bits = (unsigned char)(len - drop);
212 if (work[sym] + 1U < match) {
213 here.op = (unsigned char)0;
214 here.val = work[sym];
215 }
216 else if (work[sym] >= match) {
217 here.op = (unsigned char)(extra[work[sym] - match]);
218 here.val = base[work[sym] - match];
219 }
220 else {
221 here.op = (unsigned char)(32 + 64); /* end of block */
222 here.val = 0;
223 }
224
225 /* replicate for those indices with low len bits equal to huff */
226 incr = 1U << (len - drop);
227 fill = 1U << curr;
228 min = fill; /* save offset to next table */
229 do {
230 fill -= incr;
231 next[(huff >> drop) + fill] = here;
232 } while (fill != 0);
233
234 /* backwards increment the len-bit code huff */
235 incr = 1U << (len - 1);
236 while (huff & incr)
237 incr >>= 1;
238 if (incr != 0) {
239 huff &= incr - 1;
240 huff += incr;
241 }
242 else
243 huff = 0;
244
245 /* go to next symbol, update count, len */
246 sym++;
247 if (--(count[len]) == 0) {
248 if (len == max) break;
249 len = lens[work[sym]];
250 }
251
252 /* create new sub-table if needed */
253 if (len > root && (huff & mask) != low) {
254 /* if first time, transition to sub-tables */
255 if (drop == 0)
256 drop = root;
257
258 /* increment past last table */
259 next += min; /* here min is 1 << curr */
260
261 /* determine length of next table */
262 curr = len - drop;
263 left = (int)(1 << curr);
264 while (curr + drop < max) {
265 left -= count[curr + drop];
266 if (left <= 0) break;
267 curr++;
268 left <<= 1;
269 }
270
271 /* check for enough space */
272 used += 1U << curr;
273 if ((type == LENS && used > ENOUGH_LENS) ||
274 (type == DISTS && used > ENOUGH_DISTS))
275 return 1;
276
277 /* point entry in root table to sub-table */
278 low = huff & mask;
279 (*table)[low].op = (unsigned char)curr;
280 (*table)[low].bits = (unsigned char)root;
281 (*table)[low].val = (unsigned short)(next - *table);
282 }
283 }
284
285 /* fill in remaining table entry if code is incomplete (guaranteed to have
286 at most one remaining entry, since if the code is incomplete, the
287 maximum code length that was allowed to get this far is one bit) */
288 if (huff != 0) {
289 here.op = (unsigned char)64; /* invalid code marker */
290 here.bits = (unsigned char)(len - drop);
291 here.val = (unsigned short)0;
292 next[huff] = here;
293 }
294
295 /* set return parameters */
296 *table += used;
297 *bits = root;
298 return 0;
299 }
300
