/* * jchuff.c * * Copyright (C) 1991-1998, Thomas G. Lane. * This file is part of the Independent JPEG Group's software. * For conditions of distribution and use, see the accompanying README file. * * This file contains Huffman entropy decoding routines which are shared * by the sequential, progressive and lossless decoders. */ #define JPEG_INTERNALS #include "jinclude.h" #include "jpeglib.h" #include "jchuff.h" /* Declarations shared with jc*huff.c */ /* * Compute the derived values for a Huffman table. * This routine also performs some validation checks on the table. */ GLOBAL(void) jpeg_make_c_derived_tbl (j_compress_ptr cinfo, boolean isDC, int tblno, c_derived_tbl ** pdtbl) { JHUFF_TBL *htbl; c_derived_tbl *dtbl; int p, i, l, lastp, si, maxsymbol; char huffsize[257]; unsigned int huffcode[257]; unsigned int code; /* Note that huffsize[] and huffcode[] are filled in code-length order, * paralleling the order of the symbols themselves in htbl->huffval[]. */ /* Find the input Huffman table */ if (tblno < 0 || tblno >= NUM_HUFF_TBLS) ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno); htbl = isDC ? cinfo->dc_huff_tbl_ptrs[tblno] : cinfo->ac_huff_tbl_ptrs[tblno]; if (htbl == NULL) ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno); /* Allocate a workspace if we haven't already done so. */ if (*pdtbl == NULL) *pdtbl = (c_derived_tbl *) (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE, SIZEOF(c_derived_tbl)); dtbl = *pdtbl; /* Figure C.1: make table of Huffman code length for each symbol */ p = 0; for (l = 1; l <= 16; l++) { i = (int) htbl->bits[l]; if (i < 0 || p + i > 256) /* protect against table overrun */ ERREXIT(cinfo, JERR_BAD_HUFF_TABLE); while (i--) huffsize[p++] = (char) l; } huffsize[p] = 0; lastp = p; /* Figure C.2: generate the codes themselves */ /* We also validate that the counts represent a legal Huffman code tree. */ code = 0; si = huffsize[0]; p = 0; while (huffsize[p]) { while (((int) huffsize[p]) == si) { huffcode[p++] = code; code++; } /* code is now 1 more than the last code used for codelength si; but * it must still fit in si bits, since no code is allowed to be all ones. * BUG FIX 2001-09-03: Comparison must be >, not >= */ if (((INT32) code) > (((INT32) 1) << si)) ERREXIT(cinfo, JERR_BAD_HUFF_TABLE); code <<= 1; si++; } /* Figure C.3: generate encoding tables */ /* These are code and size indexed by symbol value */ /* Set all codeless symbols to have code length 0; * this lets us detect duplicate VAL entries here, and later * allows emit_bits to detect any attempt to emit such symbols. */ MEMZERO(dtbl->ehufsi, SIZEOF(dtbl->ehufsi)); /* This is also a convenient place to check for out-of-range * and duplicated VAL entries. We allow 0..255 for AC symbols * but only 0..16 for DC. (We could constrain them further * based on data depth and mode, but this seems enough.) */ maxsymbol = isDC ? 16 : 255; for (p = 0; p < lastp; p++) { i = htbl->huffval[p]; if (i < 0 || i > maxsymbol || dtbl->ehufsi[i]) ERREXIT(cinfo, JERR_BAD_HUFF_TABLE); dtbl->ehufco[i] = huffcode[p]; dtbl->ehufsi[i] = huffsize[p]; } } /* * Generate the best Huffman code table for the given counts, fill htbl. * * The JPEG standard requires that no symbol be assigned a codeword of all * one bits (so that padding bits added at the end of a compressed segment * can't look like a valid code). Because of the canonical ordering of * codewords, this just means that there must be an unused slot in the * longest codeword length category. Section K.2 of the JPEG spec suggests * reserving such a slot by pretending that symbol 256 is a valid symbol * with count 1. In theory that's not optimal; giving it count zero but * including it in the symbol set anyway should give a better Huffman code. * But the theoretically better code actually seems to come out worse in * practice, because it produces more all-ones bytes (which incur stuffed * zero bytes in the final file). In any case the difference is tiny. * * The JPEG standard requires Huffman codes to be no more than 16 bits long. * If some symbols have a very small but nonzero probability, the Huffman tree * must be adjusted to meet the code length restriction. We currently use * the adjustment method suggested in JPEG section K.2. This method is *not* * optimal; it may not choose the best possible limited-length code. But * typically only very-low-frequency symbols will be given less-than-optimal * lengths, so the code is almost optimal. Experimental comparisons against * an optimal limited-length-code algorithm indicate that the difference is * microscopic --- usually less than a hundredth of a percent of total size. * So the extra complexity of an optimal algorithm doesn't seem worthwhile. */ GLOBAL(void) jpeg_gen_optimal_table (j_compress_ptr cinfo, JHUFF_TBL * htbl, long freq[]) { #define MAX_CLEN 32 /* assumed maximum initial code length */ UINT8 bits[MAX_CLEN+1]; /* bits[k] = # of symbols with code length k */ int codesize[257]; /* codesize[k] = code length of symbol k */ int others[257]; /* next symbol in current branch of tree */ int c1, c2; int p, i, j; long v; /* This algorithm is explained in section K.2 of the JPEG standard */ MEMZERO(bits, SIZEOF(bits)); MEMZERO(codesize, SIZEOF(codesize)); for (i = 0; i < 257; i++) others[i] = -1; /* init links to empty */ freq[256] = 1; /* make sure 256 has a nonzero count */ /* Including the pseudo-symbol 256 in the Huffman procedure guarantees * that no real symbol is given code-value of all ones, because 256 * will be placed last in the largest codeword category. */ /* Huffman's basic algorithm to assign optimal code lengths to symbols */ for (;;) { /* Find the smallest nonzero frequency, set c1 = its symbol */ /* In case of ties, take the larger symbol number */ c1 = -1; v = 1000000000L; for (i = 0; i <= 256; i++) { if (freq[i] && freq[i] <= v) { v = freq[i]; c1 = i; } } /* Find the next smallest nonzero frequency, set c2 = its symbol */ /* In case of ties, take the larger symbol number */ c2 = -1; v = 1000000000L; for (i = 0; i <= 256; i++) { if (freq[i] && freq[i] <= v && i != c1) { v = freq[i]; c2 = i; } } /* Done if we've merged everything into one frequency */ if (c2 < 0) break; /* Else merge the two counts/trees */ freq[c1] += freq[c2]; freq[c2] = 0; /* Increment the codesize of everything in c1's tree branch */ codesize[c1]++; while (others[c1] >= 0) { c1 = others[c1]; codesize[c1]++; } others[c1] = c2; /* chain c2 onto c1's tree branch */ /* Increment the codesize of everything in c2's tree branch */ codesize[c2]++; while (others[c2] >= 0) { c2 = others[c2]; codesize[c2]++; } } /* Now count the number of symbols of each code length */ for (i = 0; i <= 256; i++) { if (codesize[i]) { /* The JPEG standard seems to think that this can't happen, */ /* but I'm paranoid... */ if (codesize[i] > MAX_CLEN) ERREXIT(cinfo, JERR_HUFF_CLEN_OVERFLOW); bits[codesize[i]]++; } } /* JPEG doesn't allow symbols with code lengths over 16 bits, so if the pure * Huffman procedure assigned any such lengths, we must adjust the coding. * Here is what the JPEG spec says about how this next bit works: * Since symbols are paired for the longest Huffman code, the symbols are * removed from this length category two at a time. The prefix for the pair * (which is one bit shorter) is allocated to one of the pair; then, * skipping the BITS entry for that prefix length, a code word from the next * shortest nonzero BITS entry is converted into a prefix for two code words * one bit longer. */ for (i = MAX_CLEN; i > 16; i--) { while (bits[i] > 0) { j = i - 2; /* find length of new prefix to be used */ while (bits[j] == 0) j--; bits[i] -= 2; /* remove two symbols */ bits[i-1]++; /* one goes in this length */ bits[j+1] += 2; /* two new symbols in this length */ bits[j]--; /* symbol of this length is now a prefix */ } } /* Remove the count for the pseudo-symbol 256 from the largest codelength */ while (bits[i] == 0) /* find largest codelength still in use */ i--; bits[i]--; /* Return final symbol counts (only for lengths 0..16) */ MEMCOPY(htbl->bits, bits, SIZEOF(htbl->bits)); /* Return a list of the symbols sorted by code length */ /* It's not real clear to me why we don't need to consider the codelength * changes made above, but the JPEG spec seems to think this works. */ p = 0; for (i = 1; i <= MAX_CLEN; i++) { for (j = 0; j <= 255; j++) { if (codesize[j] == i) { htbl->huffval[p] = (UINT8) j; p++; } } } /* Set sent_table FALSE so updated table will be written to JPEG file. */ htbl->sent_table = FALSE; }

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