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1 /* inftrees.c -- generate Huffman trees for efficient decoding

3 * For conditions of distribution and use, see copyright notice in zlib.h

4 */

6 #include "zutil.h"

7 #include "inftrees.h"

9 #define MAXBITS 15

11 const char inflate_copyright[] =

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 */

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 */

 int ZLIB_INTERNAL inflate_table(codetype type, unsigned short FAR *lens,

                              unsigned codes, code FAR * FAR *table,

                              unsigned FAR *bits, 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 int end; /* use base and extra for symbol > end */

     unsigned short count[MAXBITS+1];    /* number of codes of each length */

     unsigned short offs[MAXBITS+1];     /* offsets in table for each length */

     static const unsigned short lbase[31] = { /* Length codes 257..285 base */

         3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 15, 17, 19, 23, 27, 31,

         35, 43, 51, 59, 67, 83, 99, 115, 131, 163, 195, 227, 258, 0, 0};

     static const unsigned short lext[31] = { /* Length codes 257..285 extra */

         16, 16, 16, 16, 16, 16, 16, 16, 17, 17, 17, 17, 18, 18, 18, 18,

         19, 19, 19, 19, 20, 20, 20, 20, 21, 21, 21, 21, 16, 72, 78};

     static const unsigned short dbase[32] = { /* Distance codes 0..29 base */

         1, 2, 3, 4, 5, 7, 9, 13, 17, 25, 33, 49, 65, 97, 129, 193,

         257, 385, 513, 769, 1025, 1537, 2049, 3073, 4097, 6145,

         8193, 12289, 16385, 24577, 0, 0};

     static const unsigned short dext[32] = { /* Distance codes 0..29 extra */

         16, 16, 16, 16, 17, 17, 18, 18, 19, 19, 20, 20, 21, 21, 22, 22,

         23, 23, 24, 24, 25, 25, 26, 26, 27, 27,

         28, 28, 29, 29, 64, 64};

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.

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.

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.

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) */

     for (len = 0; len <= MAXBITS; len++)

104 count[len] = 0;

     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;

     for (max = MAXBITS; max >= 1; max--)

         if (count[max] != 0) break;

     if (root > max) root = max;

     if (max == 0) {                     /* no symbols to code at all */

         here.op = (unsigned char)64;    /* invalid code marker */

         here.bits = (unsigned char)1;

         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 }

     for (min = 1; min < max; min++)

         if (count[min] != 0) break;

     if (root < min) root = min;

125

126 /* check for an over-subscribed or incomplete set of lengths */

127 left = 1;

     for (len = 1; len <= MAXBITS; len++) {

129 left <<= 1;

130 left -= count[len];

         if (left < 0) return -1;        /* over-subscribed */

132 }

     if (left > 0 && (type == CODES || max != 1))

         return -1;                      /* incomplete set */

135

136 /* generate offsets into symbol table for each length for sorting */

     offs[1] = 0;

     for (len = 1; len < MAXBITS; len++)

         offs[len + 1] = offs[len] + count[len];

140

141 /* sort symbols by length, by symbol order within each length */

     for (sym = 0; sym < codes; sym++)

         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 end = 19;

181 break;

182 case LENS:

183 base = lbase;

184 base -= 257;

185 extra = lext;

186 extra -= 257;

187 end = 256;

188 break;

189 default: /* DISTS */

190 base = dbase;

191 extra = dext;

192 end = -1;

193 }

194

195 /* initialize state for loop */

196 huff = 0; /* starting code */

197 sym = 0; /* starting code symbol */

198 len = min; /* starting code length */

199 next = *table; /* current table to fill in */

200 curr = root; /* current table index bits */

201 drop = 0; /* current bits to drop from code for index */

     low = (unsigned)(-1);       /* trigger new sub-table when len > root */

     used = 1U << root;          /* use root table entries */

     mask = used - 1;            /* mask for comparing low */

205

206 /* check available table space */

     if ((type == LENS && used > ENOUGH_LENS) ||

         (type == DISTS && used > ENOUGH_DISTS))

209 return 1;

210

211 /* process all codes and make table entries */

212 for (;;) {

213 /* create table entry */

         here.bits = (unsigned char)(len - drop);

         if ((int)(work[sym]) < end) {

             here.op = (unsigned char)0;

217 here.val = work[sym];

218 }

         else if ((int)(work[sym]) > end) {

             here.op = (unsigned char)(extra[work[sym]]);

             here.val = base[work[sym]];

222 }

223 else {

             here.op = (unsigned char)(32 + 64);         /* end of block */

225 here.val = 0;

226 }

227

228 /* replicate for those indices with low len bits equal to huff */

         incr = 1U << (len - drop);

230 fill = 1U << curr;

231 min = fill; /* save offset to next table */

232 do {

233 fill -= incr;

             next[(huff >> drop) + fill] = here;

         } while (fill != 0);

236

237 /* backwards increment the len-bit code huff */

         incr = 1U << (len - 1);

239 while (huff & incr)

240 incr >>= 1;

         if (incr != 0) {

242 huff &= incr - 1;

243 huff += incr;

244 }

245 else

246 huff = 0;

247

248 /* go to next symbol, update count, len */

249 sym++;

         if (--(count[len]) == 0) {

             if (len == max) break;

252 len = lens[work[sym]];

253 }

254

255 /* create new sub-table if needed */

         if (len > root && (huff & mask) != low) {

257 /* if first time, transition to sub-tables */

             if (drop == 0)

259 drop = root;

260

261 /* increment past last table */

262 next += min; /* here min is 1 << curr */

263

264 /* determine length of next table */

265 curr = len - drop;

             left = (int)(1 << curr);

             while (curr + drop < max) {

268 left -= count[curr + drop];

                 if (left <= 0) break;

270 curr++;

271 left <<= 1;

272 }

273

274 /* check for enough space */

275 used += 1U << curr;

             if ((type == LENS && used > ENOUGH_LENS) ||

                 (type == DISTS && used > ENOUGH_DISTS))

278 return 1;

279

280 /* point entry in root table to sub-table */

281 low = huff & mask;

             (*table)[low].op = (unsigned char)curr;

             (*table)[low].bits = (unsigned char)root;

             (*table)[low].val = (unsigned short)(next - *table);

285 }

286 }

287

288 /* fill in remaining table entry if code is incomplete (guaranteed to have

289 at most one remaining entry, since if the code is incomplete, the

290 maximum code length that was allowed to get this far is one bit) */

     if (huff != 0) {

         here.op = (unsigned char)64;            /* invalid code marker */

         here.bits = (unsigned char)(len - drop);

         here.val = (unsigned short)0;

295 next[huff] = here;

296 }

297

298 /* set return parameters */

299 *table += used;

300 *bits = root;

301 return 0;

302 }