*
* The following copying information applies to this specific source code file:
*
- * Written in 2012-2015 by Eric Biggers <ebiggers3@gmail.com>
+ * Written in 2012-2016 by Eric Biggers <ebiggers3@gmail.com>
*
* To the extent possible under law, the author(s) have dedicated all copyright
* and related and neighboring rights to this software to the public domain
# include "config.h"
#endif
-#include "wimlib/decompress_common.h"
-
#include <string.h>
-#define USE_WORD_FILL
-
-#ifdef __GNUC__
-# ifdef __SSE2__
-# undef USE_WORD_FILL
-# define USE_SSE2_FILL
-# include <emmintrin.h>
-# endif
+#ifdef __SSE2__
+# include <emmintrin.h>
#endif
-/* Construct a direct mapping entry in the lookup table. */
-#define MAKE_DIRECT_ENTRY(symbol, length) ((symbol) | ((length) << 11))
+#include "wimlib/decompress_common.h"
/*
* make_huffman_decode_table() -
*
- * Build a decoding table for a canonical prefix code, or "Huffman code".
+ * Given an alphabet of symbols and the length of each symbol's codeword in a
+ * canonical prefix code, build a table for quickly decoding symbols that were
+ * encoded with that code.
*
- * This takes as input the length of the codeword for each symbol in the
- * alphabet and produces as output a table that can be used for fast
- * decoding of prefix-encoded symbols using read_huffsym().
+ * A _prefix code_ is an assignment of bitstrings called _codewords_ to symbols
+ * such that no whole codeword is a prefix of any other. A prefix code might be
+ * a _Huffman code_, which means that it is an optimum prefix code for a given
+ * list of symbol frequencies and was generated by the Huffman algorithm.
+ * Although the prefix codes processed here will ordinarily be "Huffman codes",
+ * strictly speaking the decoder cannot know whether a given code was actually
+ * generated by the Huffman algorithm or not.
*
- * Strictly speaking, a canonical prefix code might not be a Huffman
- * code. But this algorithm will work either way; and in fact, since
- * Huffman codes are defined in terms of symbol frequencies, there is no
- * way for the decompressor to know whether the code is a true Huffman
- * code or not until all symbols have been decoded.
+ * A prefix code is _canonical_ if and only if a longer codeword never
+ * lexicographically precedes a shorter codeword, and the lexicographic ordering
+ * of codewords of equal length is the same as the lexicographic ordering of the
+ * corresponding symbols. The advantage of using a canonical prefix code is
+ * that the codewords can be reconstructed from only the symbol => codeword
+ * length mapping. This eliminates the need to transmit the codewords
+ * explicitly. Instead, they can be enumerated in lexicographic order after
+ * sorting the symbols primarily by increasing codeword length and secondarily
+ * by increasing symbol value.
*
- * Because the prefix code is assumed to be "canonical", it can be
- * reconstructed directly from the codeword lengths. A prefix code is
- * canonical if and only if a longer codeword never lexicographically
- * precedes a shorter codeword, and the lexicographic ordering of
- * codewords of the same length is the same as the lexicographic ordering
- * of the corresponding symbols. Consequently, we can sort the symbols
- * primarily by codeword length and secondarily by symbol value, then
- * reconstruct the prefix code by generating codewords lexicographically
- * in that order.
+ * However, the decoder's real goal is to decode symbols with the code, not just
+ * generate the list of codewords. Consequently, this function directly builds
+ * a table for efficiently decoding symbols using the code. The basic idea is
+ * that given the next 'max_codeword_len' bits of input, the decoder can look up
+ * the next decoded symbol by indexing a table containing '2^max_codeword_len'
+ * entries. A codeword with length 'max_codeword_len' will have exactly one
+ * entry in this table, whereas a codeword shorter than 'max_codeword_len' will
+ * have multiple entries in this table. Precisely, a codeword of length 'n'
+ * will have '2^(max_codeword_len - n)' entries. The index of each such entry,
+ * considered as a bitstring of length 'max_codeword_len', will contain the
+ * corresponding codeword as a prefix.
*
- * This function does not, however, generate the prefix code explicitly.
- * Instead, it directly builds a table for decoding symbols using the
- * code. The basic idea is this: given the next 'max_codeword_len' bits
- * in the input, we can look up the decoded symbol by indexing a table
- * containing 2**max_codeword_len entries. A codeword with length
- * 'max_codeword_len' will have exactly one entry in this table, whereas
- * a codeword shorter than 'max_codeword_len' will have multiple entries
- * in this table. Precisely, a codeword of length n will be represented
- * by 2**(max_codeword_len - n) entries in this table. The 0-based index
- * of each such entry will contain the corresponding codeword as a prefix
- * when zero-padded on the left to 'max_codeword_len' binary digits.
+ * That's the basic idea, but we extend it in two ways:
*
- * That's the basic idea, but we implement two optimizations regarding
- * the format of the decode table itself:
+ * - Often the maximum codeword length is too long for it to be efficient to
+ * build the full decode table whenever a new code is used. Instead, we build
+ * a "root" table using only '2^table_bits' entries, where 'table_bits <=
+ * max_codeword_len'. Then, a lookup of 'table_bits' bits produces either a
+ * symbol directly (for codewords not longer than 'table_bits'), or the index
+ * of a subtable which must be indexed with additional bits of input to fully
+ * decode the symbol (for codewords longer than 'table_bits').
*
- * - For many compression formats, the maximum codeword length is too
- * long for it to be efficient to build the full decoding table
- * whenever a new prefix code is used. Instead, we can build the table
- * using only 2**table_bits entries, where 'table_bits' is some number
- * less than or equal to 'max_codeword_len'. Then, only codewords of
- * length 'table_bits' and shorter can be directly looked up. For
- * longer codewords, the direct lookup instead produces the root of a
- * binary tree. Using this tree, the decoder can do traditional
- * bit-by-bit decoding of the remainder of the codeword. Child nodes
- * are allocated in extra entries at the end of the table; leaf nodes
- * contain symbols. Note that the long-codeword case is, in general,
- * not performance critical, since in Huffman codes the most frequently
- * used symbols are assigned the shortest codeword lengths.
+ * - Whenever the decoder decodes a symbol, it needs to know the codeword length
+ * so that it can remove the appropriate number of input bits. The obvious
+ * solution would be to simply retain the codeword lengths array and use the
+ * decoded symbol as an index into it. However, that would require two array
+ * accesses when decoding each symbol. Our strategy is to instead store the
+ * codeword length directly in the decode table entry along with the symbol.
*
- * - When we decode a symbol using a direct lookup of the table, we still
- * need to know its length so that the bitstream can be advanced by the
- * appropriate number of bits. The simple solution is to simply retain
- * the 'lens' array and use the decoded symbol as an index into it.
- * However, this requires two separate array accesses in the fast path.
- * The optimization is to store the length directly in the decode
- * table. We use the bottom 11 bits for the symbol and the top 5 bits
- * for the length. In addition, to combine this optimization with the
- * previous one, we introduce a special case where the top 2 bits of
- * the length are both set if the entry is actually the root of a
- * binary tree.
+ * See MAKE_DECODE_TABLE_ENTRY() for full details on the format of decode table
+ * entries, and see read_huffsym() for full details on how symbols are decoded.
*
* @decode_table:
- * The array in which to create the decoding table. This must be
- * 16-byte aligned and must have a length of at least
- * ((2**table_bits) + 2 * num_syms) entries. This is permitted to
- * alias @lens, since all information from @lens is consumed before
-* anything is written to @decode_table.
+ * The array in which to build the decode table. This must have been
+ * declared by the DECODE_TABLE() macro. This may alias @lens, since all
+ * @lens are consumed before the decode table is written to.
*
* @num_syms:
- * The number of symbols in the alphabet; also, the length of the
- * 'lens' array. Must be less than or equal to
- * DECODE_TABLE_MAX_SYMBOLS.
+ * The number of symbols in the alphabet.
*
* @table_bits:
- * The order of the decode table size, as explained above. Must be
- * less than or equal to DECODE_TABLE_MAX_TABLE_BITS.
+ * The log base 2 of the number of entries in the root table.
*
* @lens:
- * An array of length @num_syms, indexable by symbol, that gives the
- * length of the codeword, in bits, for that symbol. The length can
- * be 0, which means that the symbol does not have a codeword
- * assigned. This is permitted to alias @decode_table, since all
- * information from @lens is consumed before anything is written to
- * @decode_table.
+ * An array of length @num_syms, indexed by symbol, that gives the length
+ * of the codeword, in bits, for each symbol. The length can be 0, which
+ * means that the symbol does not have a codeword assigned. In addition,
+ * @lens may alias @decode_table, as noted above.
*
* @max_codeword_len:
- * The longest codeword length allowed in the compression format.
- * All entries in 'lens' must be less than or equal to this value.
- * This must be less than or equal to DECODE_TABLE_MAX_CODEWORD_LEN.
+ * The maximum codeword length permitted for this code. All entries in
+ * 'lens' must be less than or equal to this value.
*
- * Returns 0 on success, or -1 if the lengths do not form a valid prefix
- * code.
+ * Returns 0 on success, or -1 if the lengths do not form a valid prefix code.
*/
int
-make_huffman_decode_table(u16 decode_table[const],
- const unsigned num_syms,
- const unsigned table_bits,
- const u8 lens[const],
- const unsigned max_codeword_len)
+make_huffman_decode_table(u16 decode_table[], unsigned num_syms,
+ unsigned table_bits, const u8 lens[],
+ unsigned max_codeword_len)
{
- const unsigned table_num_entries = 1 << table_bits;
- unsigned len_counts[max_codeword_len + 1];
+ u16 len_counts[max_codeword_len + 1];
+ u16 offsets[max_codeword_len + 1];
u16 sorted_syms[num_syms];
- int left;
- void *decode_table_ptr;
+ s32 remainder = 1;
+ void *entry_ptr = decode_table;
+ unsigned codeword_len = 1;
unsigned sym_idx;
- unsigned codeword_len;
- unsigned stores_per_loop;
- unsigned decode_table_pos;
-
-#ifdef USE_WORD_FILL
- const unsigned entries_per_word = WORDSIZE / sizeof(decode_table[0]);
-#endif
-
-#ifdef USE_SSE2_FILL
- const unsigned entries_per_xmm = sizeof(__m128i) / sizeof(decode_table[0]);
-#endif
+ unsigned codeword;
+ unsigned subtable_pos;
+ unsigned subtable_bits;
+ unsigned subtable_prefix;
- /* Count how many symbols have each possible codeword length.
- * Note that a length of 0 indicates the corresponding symbol is not
- * used in the code and therefore does not have a codeword. */
+ /* Count how many codewords have each length, including 0. */
for (unsigned len = 0; len <= max_codeword_len; len++)
len_counts[len] = 0;
for (unsigned sym = 0; sym < num_syms; sym++)
len_counts[lens[sym]]++;
- /* We can assume all lengths are <= max_codeword_len, but we
- * cannot assume they form a valid prefix code. A codeword of
- * length n should require a proportion of the codespace equaling
- * (1/2)^n. The code is valid if and only if the codespace is
- * exactly filled by the lengths, by this measure. */
- left = 1;
+ /* It is already guaranteed that all lengths are <= max_codeword_len,
+ * but it cannot be assumed they form a complete prefix code. A
+ * codeword of length n should require a proportion of the codespace
+ * equaling (1/2)^n. The code is complete if and only if, by this
+ * measure, the codespace is exactly filled by the lengths. */
for (unsigned len = 1; len <= max_codeword_len; len++) {
- left <<= 1;
- left -= len_counts[len];
- if (unlikely(left < 0)) {
- /* The lengths overflow the codespace; that is, the code
- * is over-subscribed. */
+ remainder = (remainder << 1) - len_counts[len];
+ /* Do the lengths overflow the codespace? */
+ if (unlikely(remainder < 0))
return -1;
- }
}
- if (unlikely(left != 0)) {
+ if (remainder != 0) {
/* The lengths do not fill the codespace; that is, they form an
- * incomplete set. */
- if (left == (1 << max_codeword_len)) {
- /* The code is completely empty. This is arguably
- * invalid, but in fact it is valid in LZX and XPRESS,
- * so we must allow it. By definition, no symbols can
- * be decoded with an empty code. Consequently, we
- * technically don't even need to fill in the decode
- * table. However, to avoid accessing uninitialized
- * memory if the algorithm nevertheless attempts to
- * decode symbols using such a code, we zero out the
- * decode table. */
- memset(decode_table, 0,
- table_num_entries * sizeof(decode_table[0]));
- return 0;
- }
- return -1;
+ * incomplete code. This is permitted only if the code is empty
+ * (contains no symbols). */
+
+ if (unlikely(remainder != 1U << max_codeword_len))
+ return -1;
+
+ /* The code is empty. When processing a well-formed stream, the
+ * decode table need not be initialized in this case. However,
+ * we cannot assume the stream is well-formed, so we must
+ * initialize the decode table anyway. Setting all entries to 0
+ * makes the decode table always produce symbol '0' without
+ * consuming any bits, which is good enough. */
+ memset(decode_table, 0, sizeof(decode_table[0]) << table_bits);
+ return 0;
}
- /* Sort the symbols primarily by length and secondarily by symbol order.
- */
- {
- unsigned offsets[max_codeword_len + 1];
+ /* Sort the symbols primarily by increasing codeword length and
+ * secondarily by increasing symbol value. */
- /* Initialize 'offsets' so that offsets[len] for 1 <= len <=
- * max_codeword_len is the number of codewords shorter than
- * 'len' bits. */
- offsets[1] = 0;
- for (unsigned len = 1; len < max_codeword_len; len++)
- offsets[len + 1] = offsets[len] + len_counts[len];
+ /* Initialize 'offsets' so that 'offsets[len]' is the number of
+ * codewords shorter than 'len' bits, including length 0. */
+ offsets[0] = 0;
+ for (unsigned len = 0; len < max_codeword_len; len++)
+ offsets[len + 1] = offsets[len] + len_counts[len];
- /* Use the 'offsets' array to sort the symbols.
- * Note that we do not include symbols that are not used in the
- * code. Consequently, fewer than 'num_syms' entries in
- * 'sorted_syms' may be filled. */
- for (unsigned sym = 0; sym < num_syms; sym++)
- if (lens[sym] != 0)
- sorted_syms[offsets[lens[sym]]++] = sym;
- }
+ /* Use the 'offsets' array to sort the symbols. */
+ for (unsigned sym = 0; sym < num_syms; sym++)
+ sorted_syms[offsets[lens[sym]]++] = sym;
- /* Fill entries for codewords with length <= table_bits
- * --- that is, those short enough for a direct mapping.
+ /*
+ * Fill the root table entries for codewords no longer than table_bits.
*
* The table will start with entries for the shortest codeword(s), which
- * have the most entries. From there, the number of entries per
+ * will have the most entries. From there, the number of entries per
* codeword will decrease. As an optimization, we may begin filling
* entries with SSE2 vector accesses (8 entries/store), then change to
- * 'machine_word_t' accesses (2 or 4 entries/store), then change to
- * 16-bit accesses (1 entry/store). */
- decode_table_ptr = decode_table;
- sym_idx = 0;
- codeword_len = 1;
-#ifdef USE_SSE2_FILL
- /* Fill the entries one 128-bit vector at a time.
- * This is 8 entries per store. */
- stores_per_loop = (1 << (table_bits - codeword_len)) / entries_per_xmm;
- for (; stores_per_loop != 0; codeword_len++, stores_per_loop >>= 1) {
+ * word accesses (2 or 4 entries/store), then change to 16-bit accesses
+ * (1 entry/store).
+ */
+ sym_idx = offsets[0];
+
+#ifdef __SSE2__
+ /* Fill entries one 128-bit vector (8 entries) at a time. */
+ for (unsigned stores_per_loop = (1U << (table_bits - codeword_len)) /
+ (sizeof(__m128i) / sizeof(decode_table[0]));
+ stores_per_loop != 0; codeword_len++, stores_per_loop >>= 1)
+ {
unsigned end_sym_idx = sym_idx + len_counts[codeword_len];
for (; sym_idx < end_sym_idx; sym_idx++) {
- /* Note: unlike in the machine_word_t version below, the
- * __m128i type already has __attribute__((may_alias)),
- * so using it to access the decode table, which is an
- * array of unsigned shorts, will not violate strict
+ /* Note: unlike in the "word" version below, the __m128i
+ * type already has __attribute__((may_alias)), so using
+ * it to access an array of u16 will not violate strict
* aliasing. */
- u16 entry;
- __m128i v;
- __m128i *p;
- unsigned n;
-
- entry = MAKE_DIRECT_ENTRY(sorted_syms[sym_idx], codeword_len);
-
- v = _mm_set1_epi16(entry);
- p = (__m128i*)decode_table_ptr;
- n = stores_per_loop;
+ __m128i v = _mm_set1_epi16(
+ MAKE_DECODE_TABLE_ENTRY(sorted_syms[sym_idx],
+ codeword_len));
+ unsigned n = stores_per_loop;
do {
- *p++ = v;
+ *(__m128i *)entry_ptr = v;
+ entry_ptr += sizeof(v);
} while (--n);
- decode_table_ptr = p;
}
}
-#endif /* USE_SSE2_FILL */
+#endif /* __SSE2__ */
-#ifdef USE_WORD_FILL
- /* Fill the entries one machine word at a time.
- * On 32-bit systems this is 2 entries per store, while on 64-bit
- * systems this is 4 entries per store. */
- stores_per_loop = (1 << (table_bits - codeword_len)) / entries_per_word;
- for (; stores_per_loop != 0; codeword_len++, stores_per_loop >>= 1) {
+#ifdef __GNUC__
+ /* Fill entries one word (2 or 4 entries) at a time. */
+ for (unsigned stores_per_loop = (1U << (table_bits - codeword_len)) /
+ (WORDBYTES / sizeof(decode_table[0]));
+ stores_per_loop != 0; codeword_len++, stores_per_loop >>= 1)
+ {
unsigned end_sym_idx = sym_idx + len_counts[codeword_len];
for (; sym_idx < end_sym_idx; sym_idx++) {
* the code with -fno-strict-aliasing to guarantee
* correctness. To work around this problem, use the
* gcc 'may_alias' extension. */
- typedef machine_word_t _may_alias_attribute aliased_word_t;
-
- machine_word_t v;
- aliased_word_t *p;
- unsigned n;
-
- STATIC_ASSERT(WORDSIZE == 4 || WORDSIZE == 8);
-
- v = MAKE_DIRECT_ENTRY(sorted_syms[sym_idx], codeword_len);
- v |= v << 16;
- v |= v << (WORDSIZE == 8 ? 32 : 0);
-
- p = (aliased_word_t *)decode_table_ptr;
- n = stores_per_loop;
-
+ typedef machine_word_t
+ __attribute__((may_alias)) aliased_word_t;
+ aliased_word_t v = repeat_u16(
+ MAKE_DECODE_TABLE_ENTRY(sorted_syms[sym_idx],
+ codeword_len));
+ unsigned n = stores_per_loop;
do {
- *p++ = v;
+ *(aliased_word_t *)entry_ptr = v;
+ entry_ptr += sizeof(v);
} while (--n);
- decode_table_ptr = p;
}
}
-#endif /* USE_WORD_FILL */
+#endif /* __GNUC__ */
- /* Fill the entries one 16-bit integer at a time. */
- stores_per_loop = (1 << (table_bits - codeword_len));
- for (; stores_per_loop != 0; codeword_len++, stores_per_loop >>= 1) {
+ /* Fill entries one at a time. */
+ for (unsigned stores_per_loop = (1U << (table_bits - codeword_len));
+ stores_per_loop != 0; codeword_len++, stores_per_loop >>= 1)
+ {
unsigned end_sym_idx = sym_idx + len_counts[codeword_len];
for (; sym_idx < end_sym_idx; sym_idx++) {
- u16 entry;
- u16 *p;
- unsigned n;
-
- entry = MAKE_DIRECT_ENTRY(sorted_syms[sym_idx], codeword_len);
-
- p = (u16*)decode_table_ptr;
- n = stores_per_loop;
-
+ u16 v = MAKE_DECODE_TABLE_ENTRY(sorted_syms[sym_idx],
+ codeword_len);
+ unsigned n = stores_per_loop;
do {
- *p++ = entry;
+ *(u16 *)entry_ptr = v;
+ entry_ptr += sizeof(v);
} while (--n);
-
- decode_table_ptr = p;
}
}
- /* If we've filled in the entire table, we are done. Otherwise,
- * there are codewords longer than table_bits for which we must
- * generate binary trees. */
-
- decode_table_pos = (u16*)decode_table_ptr - decode_table;
- if (decode_table_pos != table_num_entries) {
- unsigned j;
- unsigned next_free_tree_slot;
- unsigned cur_codeword;
-
- /* First, zero out the remaining entries. This is
- * necessary so that these entries appear as
- * "unallocated" in the next part. Each of these entries
- * will eventually be filled with the representation of
- * the root node of a binary tree. */
- j = decode_table_pos;
- do {
- decode_table[j] = 0;
- } while (++j != table_num_entries);
-
- /* We allocate child nodes starting at the end of the
- * direct lookup table. Note that there should be
- * 2*num_syms extra entries for this purpose, although
- * fewer than this may actually be needed. */
- next_free_tree_slot = table_num_entries;
-
- /* Iterate through each codeword with length greater than
- * 'table_bits', primarily in order of codeword length
- * and secondarily in order of symbol. */
- for (cur_codeword = decode_table_pos << 1;
- codeword_len <= max_codeword_len;
- codeword_len++, cur_codeword <<= 1)
- {
- unsigned end_sym_idx = sym_idx + len_counts[codeword_len];
- for (; sym_idx < end_sym_idx; sym_idx++, cur_codeword++)
- {
- /* 'sym' is the symbol represented by the
- * codeword. */
- unsigned sym = sorted_syms[sym_idx];
-
- unsigned extra_bits = codeword_len - table_bits;
+ /* If all symbols were processed, then no subtables are required. */
+ if (sym_idx == num_syms)
+ return 0;
+
+ /* At least one subtable is required. Process the remaining symbols. */
+ codeword = ((u16 *)entry_ptr - decode_table) << 1;
+ subtable_pos = 1U << table_bits;
+ subtable_bits = table_bits;
+ subtable_prefix = -1;
+ do {
+ while (len_counts[codeword_len] == 0) {
+ codeword_len++;
+ codeword <<= 1;
+ }
- unsigned node_idx = cur_codeword >> extra_bits;
+ unsigned prefix = codeword >> (codeword_len - table_bits);
+
+ /* Start a new subtable if the first 'table_bits' bits of the
+ * codeword don't match the prefix for the previous subtable, or
+ * if this will be the first subtable. */
+ if (prefix != subtable_prefix) {
+
+ subtable_prefix = prefix;
+
+ /*
+ * Calculate the subtable length. If the codeword
+ * length exceeds 'table_bits' by n, then the subtable
+ * needs at least 2^n entries. But it may need more; if
+ * there are fewer than 2^n codewords of length
+ * 'table_bits + n' remaining, then n will need to be
+ * incremented to bring in longer codewords until the
+ * subtable can be filled completely. Note that it
+ * always will, eventually, be possible to fill the
+ * subtable, since it was previously verified that the
+ * code is complete.
+ */
+ subtable_bits = codeword_len - table_bits;
+ remainder = (s32)1 << subtable_bits;
+ for (;;) {
+ remainder -= len_counts[table_bits +
+ subtable_bits];
+ if (remainder <= 0)
+ break;
+ subtable_bits++;
+ remainder <<= 1;
+ }
- /* Go through each bit of the current codeword
- * beyond the prefix of length @table_bits and
- * walk the appropriate binary tree, allocating
- * any slots that have not yet been allocated.
- *
- * Note that the 'pointer' entry to the binary
- * tree, which is stored in the direct lookup
- * portion of the table, is represented
- * identically to other internal (non-leaf)
- * nodes of the binary tree; it can be thought
- * of as simply the root of the tree. The
- * representation of these internal nodes is
- * simply the index of the left child combined
- * with the special bits 0xC000 to distinguish
- * the entry from direct mapping and leaf node
- * entries. */
- do {
+ /* Create the entry that points from the root table to
+ * the subtable. This entry contains the index of the
+ * start of the subtable and the number of bits with
+ * which the subtable is indexed (the log base 2 of the
+ * number of entries it contains). */
+ decode_table[subtable_prefix] =
+ MAKE_DECODE_TABLE_ENTRY(subtable_pos,
+ subtable_bits);
+ }
- /* At least one bit remains in the
- * codeword, but the current node is an
- * unallocated leaf. Change it to an
- * internal node. */
- if (decode_table[node_idx] == 0) {
- decode_table[node_idx] =
- next_free_tree_slot | 0xC000;
- decode_table[next_free_tree_slot++] = 0;
- decode_table[next_free_tree_slot++] = 0;
- }
+ /* Fill the subtable entries for this symbol. */
+ u16 entry = MAKE_DECODE_TABLE_ENTRY(sorted_syms[sym_idx],
+ codeword_len - table_bits);
+ unsigned n = 1U << (subtable_bits - (codeword_len -
+ table_bits));
+ do {
+ decode_table[subtable_pos++] = entry;
+ } while (--n);
- /* Go to the left child if the next bit
- * in the codeword is 0; otherwise go to
- * the right child. */
- node_idx = decode_table[node_idx] & 0x3FFF;
- --extra_bits;
- node_idx += (cur_codeword >> extra_bits) & 1;
- } while (extra_bits != 0);
+ len_counts[codeword_len]--;
+ codeword++;
+ } while (++sym_idx < num_syms);
- /* We've traversed the tree using the entire
- * codeword, and we're now at the entry where
- * the actual symbol will be stored. This is
- * distinguished from internal nodes by not
- * having its high two bits set. */
- decode_table[node_idx] = sym;
- }
- }
- }
return 0;
}
git://wimlib.net / wimlib / blobdiff