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

source code of zlib/inftrees.c