[wimlib] / src / decompress.c

/*
 * decompress.c
 *
 * Functions used for decompression.
 */

/*
 * Copyright (C) 2012 Eric Biggers
 *
 * This file is part of wimlib, a library for working with WIM files.
 *
 * wimlib is free software; you can redistribute it and/or modify it under the
 * terms of the GNU General Public License as published by the Free
 * Software Foundation; either version 3 of the License, or (at your option)
 * any later version.
 *
 * wimlib is distributed in the hope that it will be useful, but WITHOUT ANY
 * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
 * A PARTICULAR PURPOSE. See the GNU General Public License for more
 * details.
 *
 * You should have received a copy of the GNU General Public License
 * along with wimlib; if not, see http://www.gnu.org/licenses/.
 */

#include "decompress.h"
#include <string.h>

/*
 * Builds a fast huffman decoding table from an array that gives the length of
 * the codeword for each symbol in the alphabet.  Originally based on code
 * written by David Tritscher (taken the original LZX decompression code); also
 * heavily modified to add some optimizations used in the zlib code, as well as
 * more comments.
 *
 * @decode_table:       The array in which to create the fast huffman decoding
 *                      table.  It must have a length of at least
 *                      (2**table_bits) + 2 * num_syms to guarantee
 *                      that there is enough space.
 *
 * @num_syms:           Total number of symbols in the Huffman tree.
 *
 * @table_bits:         Any symbols with a code length of table_bits or less can
 *                      be decoded in one lookup of the table.  2**table_bits
 *                      must be greater than or equal to @num_syms if there are
 *                      any Huffman codes longer than @table_bits.
 *
 * @lens:               An array of length @num_syms, indexable by symbol, that
 *                      gives the length of the Huffman codeward for that
 *                      symbol.  Because the Huffman tree is in canonical form,
 *                      it can be reconstructed by only knowing the length of
 *                      the codeword for each symbol.  It is assumed, but not
 *                      checked, that every length is less than
 *                      @max_codeword_len.
 *
 * @max_codeword_len:   The longest codeword length allowed in the compression
 *                      format.
 *
 * Returns 0 on success; returns -1 if the length values do not correspond to a
 * valid Huffman tree.
 *
 * The format of the Huffamn decoding table is as follows.  The first (1 <<
 * table_bits) entries of the table are indexed by chunks of the input of size
 * @table_bits.  If the next Huffman codeword in the input happens to have a
 * length of exactly @table_bits, the symbol is simply read directly from the
 * decoding table.  Alternatively, if the next Huffman codeword has length _less
 * than_ @table_bits, the symbol is also read directly from the decode table;
 * this is possible because every entry in the table that is indexed by an
 * integer that has the shorter codeword as a binary prefix is filled in with
 * the appropriate symbol.  If a codeword has length n <= table_bits, it will
 * have 2**(table_bits - n) possible suffixes, and thus that many entries in the
 * decoding table.
 *
 * It's a bit more complicated if the next Huffman codeword has length of more
 * than @table_bits.  The table entry indexed by the first @table_bits of that
 * codeword cannot give the appropriate symbol directly, because that entry is
 * guaranteed to be referenced by the Huffman codewords of multiple symbols.
 * And while the LZX compression format does not allow codes longer than 16
 * bits, a table of size (2 ** 16) = 65536 entries would be too slow to create.
 *
 * There are several different ways to make it possible to look up the symbols
 * for codewords longer than @table_bits.  One way is to make the entries for
 * the prefixes of length @table_bits of those entries be pointers to additional
 * decoding tables that are indexed by some number of additional bits of the
 * codeword.  The technique used here is a bit simpler, however: just store the
 * needed subtrees of the Huffman tree in the decoding table after the lookup
 * entries, beginning at index (2**table_bits).  Real pointers are replaced by
 * indices into the decoding table, and symbol entries are distinguished from
 * pointers by the fact that values less than @num_syms must be symbol values.
 */
int make_huffman_decode_table(u16 decode_table[],  unsigned num_syms,
                              unsigned table_bits, const u8 lens[],
                              unsigned max_codeword_len)
{
        unsigned len_counts[max_codeword_len + 1];
        u16 sorted_syms[num_syms];
        unsigned offsets[max_codeword_len + 1];
        const unsigned table_num_entries = 1 << table_bits;

        /* accumulate lengths for codes */
        for (unsigned i = 0; i <= max_codeword_len; i++)
                len_counts[i] = 0;

        for (unsigned sym = 0; sym < num_syms; sym++) {
                wimlib_assert2(lens[sym] <= max_codeword_len);
                len_counts[lens[sym]]++;
        }

        /* check for an over-subscribed or incomplete set of lengths */
        int left = 1;
        for (unsigned len = 1; len <= max_codeword_len; len++) {
                left <<= 1;
                left -= len_counts[len];
                if (left < 0) { /* over-subscribed */
                        ERROR("Invalid Huffman code (over-subscribed)");
                        return -1;
                }
        }
        if (left != 0) /* incomplete set */{
                if (left == 1 << max_codeword_len) {
                        /* Empty code--- okay in XPRESS and LZX */
                        memset(decode_table, 0,
                               table_num_entries * sizeof(decode_table[0]));
                        return 0;
                } else {
                        ERROR("Invalid Huffman code (incomplete set)");
                        return -1;
                }
        }

        /* Generate offsets into symbol table for each length for sorting */
        offsets[1] = 0;
        for (unsigned len = 1; len < max_codeword_len; len++)
                offsets[len + 1] = offsets[len] + len_counts[len];

        /* Sort symbols primarily by length and secondarily by symbol order.
         * This is basically a count-sort over the codeword lengths.
         * In the process, calculate the number of symbols that have nonzero
         * length and are therefore used in the symbol stream. */
        unsigned num_used_syms = 0;
        for (unsigned sym = 0; sym < num_syms; sym++) {
                if (lens[sym] != 0) {
                        sorted_syms[offsets[lens[sym]]++] = sym;
                        num_used_syms++;
                }
        }

        /* Fill entries for codewords short enough for a direct mapping.  We can
         * take advantage of the ordering of the codewords, since the Huffman
         * code is canonical.  It must be the case that all the codewords of
         * some length L numerically precede all the codewords of length L + 1.
         * Furthermore, if we have 2 symbols A and B with the same codeword
         * length but symbol A is sorted before symbol B, then then we know that
         * the codeword for A numerically precedes the codeword for B. */
        unsigned decode_table_pos = 0;
        unsigned i = 0;

        wimlib_assert2(num_used_syms != 0);
        while (1) {
                unsigned sym = sorted_syms[i];
                unsigned codeword_len = lens[sym];
                if (codeword_len > table_bits)
                        break;

                unsigned num_entries = 1 << (table_bits - codeword_len);
                if (num_entries >=
                        (sizeof(unsigned long) / sizeof(decode_table[0])))
                {
                        wimlib_assert2(decode_table_pos % 4 == 0);
                        BUILD_BUG_ON(sizeof(unsigned long) != 4 &&
                                     sizeof(unsigned long) != 8);

                        unsigned long *p = (unsigned long *)&decode_table[decode_table_pos];
                        unsigned long n = num_entries /
                                                (sizeof(unsigned long) /
                                                        sizeof(decode_table[0]));
                        unsigned long v = sym;
                        if (sizeof(unsigned long) >= 4)
                                v |= v << 16;
                        if (sizeof(unsigned long) >= 8)
                                v |= v << 32;
                        do {
                                *p++ = v;
                        } while (--n);

                        decode_table_pos += num_entries;
                } else {
                        do {
                                decode_table[decode_table_pos++] = sym;
                        } while (--num_entries);
                }
                wimlib_assert2(decode_table_pos <= table_num_entries);
                if (++i == num_used_syms) {
                        wimlib_assert2(decode_table_pos == table_num_entries);
                        /* No codewords were longer than @table_bits, so the
                         * table is now entirely filled with the codewords. */
                        return 0;
                }
        }

        wimlib_assert2(i < num_used_syms);
        wimlib_assert2(decode_table_pos < table_num_entries);

        /* Fill in the remaining entries, which correspond to codes longer than
         * @table_bits.
         *
         * First, zero out the rest of the entries.  This is necessary so that
         * the entries appear as "unallocated" in the next part. */
        {
                unsigned j = decode_table_pos;
                do {
                        decode_table[j] = 0;
                } while (++j != table_num_entries);
        }

        /* Assert that 2**table_bits is at least num_syms.  If this wasn't the
         * case, we wouldn't be able to distinguish pointer entries from symbol
         * entries. */
        wimlib_assert2(table_num_entries >= num_syms);

        /* The current Huffman codeword  */
        unsigned cur_codeword = decode_table_pos;

        /* The tree nodes are allocated starting at decode_table[1 <<
         * table_bits].  Remember that the full size of the table, including the
         * extra space for the tree nodes, is actually 2**table_bits + 2 *
         * num_syms slots, while table_num_entries is only 2**table_Bits. */
        unsigned next_free_tree_slot = table_num_entries;

        /* Go through every codeword of length greater than @table_bits,
         * primarily in order of codeword length and secondarily in order of
         * symbol. */
        unsigned prev_codeword_len = table_bits;
        do {
                unsigned sym = sorted_syms[i];
                unsigned codeword_len = lens[sym];
                unsigned extra_bits = codeword_len - table_bits;
                unsigned extra_mask;

                cur_codeword <<= (codeword_len - prev_codeword_len);
                prev_codeword_len = codeword_len;

                /* index of the current node; find it from the prefix of the
                 * current Huffman codeword. */
                unsigned node_idx = cur_codeword >> extra_bits;
                wimlib_assert2(node_idx < table_num_entries);

                /* Go through each bit of the current Huffman codeword beyond
                 * the prefix of length @table_bits and walk the tree,
                 * allocating any slots that have not yet been allocated. */
                do {

                        /* If the current tree node points to nowhere
                         * but we need to follow it, allocate a new node
                         * for it to point to. */
                        if (decode_table[node_idx] == 0) {
                                decode_table[node_idx] = next_free_tree_slot;
                                decode_table[next_free_tree_slot++] = 0;
                                decode_table[next_free_tree_slot++] = 0;
                                wimlib_assert2(next_free_tree_slot <=
                                               table_num_entries + 2 * num_syms);
                        }

                        /* Set node_idx to left child */
                        node_idx = decode_table[node_idx];

                        /* Is the next bit 0 or 1? If 0, go left (already done).
                         * If 1, go right by incrementing node_idx. */
                        --extra_bits;
                        node_idx += (cur_codeword >> extra_bits) & 1;
                } while (extra_bits != 0);

                /* node_idx is now the index of the leaf entry into which the
                 * actual symbol will go. */
                decode_table[node_idx] = sym;

                /* cur_codeword is always incremented because this is
                 * how canonical Huffman codes are generated (add 1 for
                 * each code, then left shift whenever the code length
                 * increases) */
                cur_codeword++;
        } while (++i != num_used_syms);
        return 0;
}

/* Reads a Huffman-encoded symbol when it is known there are less than
 * MAX_CODE_LEN bits remaining in the bitstream. */
int read_huffsym_near_end_of_input(struct input_bitstream *istream,
                                   const u16 decode_table[],
                                   const u8 lens[],
                                   unsigned num_syms,
                                   unsigned table_bits,
                                   unsigned *n)
{
        unsigned bitsleft = istream->bitsleft;
        unsigned key_size;
        u16 sym;
        u16 key_bits;

        if (table_bits > bitsleft) {
                key_size = bitsleft;
                bitsleft = 0;
                key_bits = bitstream_peek_bits(istream, key_size) <<
                                                (table_bits - key_size);
        } else {
                key_size = table_bits;
                bitsleft -= table_bits;
                key_bits = bitstream_peek_bits(istream, table_bits);
        }

        sym = decode_table[key_bits];
        if (sym >= num_syms) {
                bitstream_remove_bits(istream, key_size);
                do {
                        if (bitsleft == 0) {
                                ERROR("Input stream exhausted");
                                return -1;
                        }
                        key_bits = sym + bitstream_peek_bits(istream, 1);
                        bitstream_remove_bits(istream, 1);
                        bitsleft--;
                } while ((sym = decode_table[key_bits]) >= num_syms);
        } else {
                bitstream_remove_bits(istream, lens[sym]);
        }
        *n = sym;
        return 0;
}