#### /Modules/zlib/inftrees.c

C | 329 lines | 189 code | 27 blank | 113 comment | 60 complexity | a6a36b368661bdabf81b1fe66f3e1831 MD5 | raw file

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 "zutil.h" 7#include "inftrees.h" 8 9#define MAXBITS 15 10 11const char inflate_copyright[] = 12 " inflate 1.2.3 Copyright 1995-2005 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 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 this; /* 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 int end; /* use base and extra for symbol > end */ 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, 201, 196}; 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 this.op = (unsigned char)64; /* invalid code marker */ 119 this.bits = (unsigned char)1; 120 this.val = (unsigned short)0; 121 *(*table)++ = this; /* make a table to force an error */ 122 *(*table)++ = this; 123 *bits = 1; 124 return 0; /* no symbols, but wait for decoding to report error */ 125 } 126 for (min = 1; min <= MAXBITS; 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 when a LENS table is being made 170 against the space in *table, ENOUGH, minus the maximum space needed by 171 the worst case distance code, MAXD. This should never happen, but the 172 sufficiency of ENOUGH has not been proven exhaustively, hence the check. 173 This assumes that when type == LENS, bits == 9. 174 175 sym increments through all symbols, and the loop terminates when 176 all codes of length max, i.e. all codes, have been processed. This 177 routine permits incomplete codes, so another loop after this one fills 178 in the rest of the decoding tables with invalid code markers. 179 */ 180 181 /* set up for code type */ 182 switch (type) { 183 case CODES: 184 base = extra = work; /* dummy value--not used */ 185 end = 19; 186 break; 187 case LENS: 188 base = lbase; 189 base -= 257; 190 extra = lext; 191 extra -= 257; 192 end = 256; 193 break; 194 default: /* DISTS */ 195 base = dbase; 196 extra = dext; 197 end = -1; 198 } 199 200 /* initialize state for loop */ 201 huff = 0; /* starting code */ 202 sym = 0; /* starting code symbol */ 203 len = min; /* starting code length */ 204 next = *table; /* current table to fill in */ 205 curr = root; /* current table index bits */ 206 drop = 0; /* current bits to drop from code for index */ 207 low = (unsigned)(-1); /* trigger new sub-table when len > root */ 208 used = 1U << root; /* use root table entries */ 209 mask = used - 1; /* mask for comparing low */ 210 211 /* check available table space */ 212 if (type == LENS && used >= ENOUGH - MAXD) 213 return 1; 214 215 /* process all codes and make table entries */ 216 for (;;) { 217 /* create table entry */ 218 this.bits = (unsigned char)(len - drop); 219 if ((int)(work[sym]) < end) { 220 this.op = (unsigned char)0; 221 this.val = work[sym]; 222 } 223 else if ((int)(work[sym]) > end) { 224 this.op = (unsigned char)(extra[work[sym]]); 225 this.val = base[work[sym]]; 226 } 227 else { 228 this.op = (unsigned char)(32 + 64); /* end of block */ 229 this.val = 0; 230 } 231 232 /* replicate for those indices with low len bits equal to huff */ 233 incr = 1U << (len - drop); 234 fill = 1U << curr; 235 min = fill; /* save offset to next table */ 236 do { 237 fill -= incr; 238 next[(huff >> drop) + fill] = this; 239 } while (fill != 0); 240 241 /* backwards increment the len-bit code huff */ 242 incr = 1U << (len - 1); 243 while (huff & incr) 244 incr >>= 1; 245 if (incr != 0) { 246 huff &= incr - 1; 247 huff += incr; 248 } 249 else 250 huff = 0; 251 252 /* go to next symbol, update count, len */ 253 sym++; 254 if (--(count[len]) == 0) { 255 if (len == max) break; 256 len = lens[work[sym]]; 257 } 258 259 /* create new sub-table if needed */ 260 if (len > root && (huff & mask) != low) { 261 /* if first time, transition to sub-tables */ 262 if (drop == 0) 263 drop = root; 264 265 /* increment past last table */ 266 next += min; /* here min is 1 << curr */ 267 268 /* determine length of next table */ 269 curr = len - drop; 270 left = (int)(1 << curr); 271 while (curr + drop < max) { 272 left -= count[curr + drop]; 273 if (left <= 0) break; 274 curr++; 275 left <<= 1; 276 } 277 278 /* check for enough space */ 279 used += 1U << curr; 280 if (type == LENS && used >= ENOUGH - MAXD) 281 return 1; 282 283 /* point entry in root table to sub-table */ 284 low = huff & mask; 285 (*table)[low].op = (unsigned char)curr; 286 (*table)[low].bits = (unsigned char)root; 287 (*table)[low].val = (unsigned short)(next - *table); 288 } 289 } 290 291 /* 292 Fill in rest of table for incomplete codes. This loop is similar to the 293 loop above in incrementing huff for table indices. It is assumed that 294 len is equal to curr + drop, so there is no loop needed to increment 295 through high index bits. When the current sub-table is filled, the loop 296 drops back to the root table to fill in any remaining entries there. 297 */ 298 this.op = (unsigned char)64; /* invalid code marker */ 299 this.bits = (unsigned char)(len - drop); 300 this.val = (unsigned short)0; 301 while (huff != 0) { 302 /* when done with sub-table, drop back to root table */ 303 if (drop != 0 && (huff & mask) != low) { 304 drop = 0; 305 len = root; 306 next = *table; 307 this.bits = (unsigned char)len; 308 } 309 310 /* put invalid code marker in table */ 311 next[huff >> drop] = this; 312 313 /* backwards increment the len-bit code huff */ 314 incr = 1U << (len - 1); 315 while (huff & incr) 316 incr >>= 1; 317 if (incr != 0) { 318 huff &= incr - 1; 319 huff += incr; 320 } 321 else 322 huff = 0; 323 } 324 325 /* set return parameters */ 326 *table += used; 327 *bits = root; 328 return 0; 329}