[wimlib] / include / wimlib / decompress_common.h

/*
 * decompress_common.h
 *
 * Header for decompression code shared by multiple compression formats.
 *
 * The following copying information applies to this specific source code file:
 *
 * 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
 * worldwide via the Creative Commons Zero 1.0 Universal Public Domain
 * Dedication (the "CC0").
 *
 * This software 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 CC0 for more details.
 *
 * You should have received a copy of the CC0 along with this software; if not
 * see <http://creativecommons.org/publicdomain/zero/1.0/>.
 */

#ifndef _WIMLIB_DECOMPRESS_COMMON_H
#define _WIMLIB_DECOMPRESS_COMMON_H

#include <string.h>

#include "wimlib/compiler.h"
#include "wimlib/types.h"
#include "wimlib/unaligned.h"

/* Structure that encapsulates a block of in-memory data being interpreted as a
 * stream of bits, optionally with interwoven literal bytes.  Bits are assumed
 * to be stored in little endian 16-bit coding units, with the bits ordered high
 * to low.  */
struct input_bitstream {

        /* Bits that have been read from the input buffer.  The bits are
         * left-justified; the next bit is always bit 31.  */
        u32 bitbuf;

        /* Number of bits currently held in @bitbuf.  */
        u32 bitsleft;

        /* Pointer to the next byte to be retrieved from the input buffer.  */
        const u8 *next;

        /* Pointer past the end of the input buffer.  */
        const u8 *end;
};

/* Initialize a bitstream to read from the specified input buffer.  */
static inline void
init_input_bitstream(struct input_bitstream *is, const void *buffer, u32 size)
{
        is->bitbuf = 0;
        is->bitsleft = 0;
        is->next = buffer;
        is->end = is->next + size;
}

/* Note: for performance reasons, the following methods don't return error codes
 * to the caller if the input buffer is overrun.  Instead, they just assume that
 * all overrun data is zeroes.  This has no effect on well-formed compressed
 * data.  The only disadvantage is that bad compressed data may go undetected,
 * but even this is irrelevant if higher level code checksums the uncompressed
 * data anyway.  */

/* Ensure the bit buffer variable for the bitstream contains at least @num_bits
 * bits.  Following this, bitstream_peek_bits() and/or bitstream_remove_bits()
 * may be called on the bitstream to peek or remove up to @num_bits bits.  */
static inline void
bitstream_ensure_bits(struct input_bitstream *is, const unsigned num_bits)
{
        /* This currently works for at most 17 bits.  */

        if (is->bitsleft >= num_bits)
                return;

        if (unlikely(is->end - is->next < 2))
                goto overflow;

        is->bitbuf |= (u32)get_unaligned_le16(is->next) << (16 - is->bitsleft);
        is->next += 2;
        is->bitsleft += 16;

        if (unlikely(num_bits == 17 && is->bitsleft == 16)) {
                if (unlikely(is->end - is->next < 2))
                        goto overflow;

                is->bitbuf |= (u32)get_unaligned_le16(is->next);
                is->next += 2;
                is->bitsleft = 32;
        }

        return;

overflow:
        is->bitsleft = 32;
}

/* Return the next @num_bits bits from the bitstream, without removing them.
 * There must be at least @num_bits remaining in the buffer variable, from a
 * previous call to bitstream_ensure_bits().  */
static inline u32
bitstream_peek_bits(const struct input_bitstream *is, const unsigned num_bits)
{
        return (is->bitbuf >> 1) >> (sizeof(is->bitbuf) * 8 - num_bits - 1);
}

/* Remove @num_bits from the bitstream.  There must be at least @num_bits
 * remaining in the buffer variable, from a previous call to
 * bitstream_ensure_bits().  */
static inline void
bitstream_remove_bits(struct input_bitstream *is, unsigned num_bits)
{
        is->bitbuf <<= num_bits;
        is->bitsleft -= num_bits;
}

/* Remove and return @num_bits bits from the bitstream.  There must be at least
 * @num_bits remaining in the buffer variable, from a previous call to
 * bitstream_ensure_bits().  */
static inline u32
bitstream_pop_bits(struct input_bitstream *is, unsigned num_bits)
{
        u32 bits = bitstream_peek_bits(is, num_bits);
        bitstream_remove_bits(is, num_bits);
        return bits;
}

/* Read and return the next @num_bits bits from the bitstream.  */
static inline u32
bitstream_read_bits(struct input_bitstream *is, unsigned num_bits)
{
        bitstream_ensure_bits(is, num_bits);
        return bitstream_pop_bits(is, num_bits);
}

/* Read and return the next literal byte embedded in the bitstream.  */
static inline u8
bitstream_read_byte(struct input_bitstream *is)
{
        if (unlikely(is->end == is->next))
                return 0;
        return *is->next++;
}

/* Read and return the next 16-bit integer embedded in the bitstream.  */
static inline u16
bitstream_read_u16(struct input_bitstream *is)
{
        u16 v;

        if (unlikely(is->end - is->next < 2))
                return 0;
        v = get_unaligned_le16(is->next);
        is->next += 2;
        return v;
}

/* Read and return the next 32-bit integer embedded in the bitstream.  */
static inline u32
bitstream_read_u32(struct input_bitstream *is)
{
        u32 v;

        if (unlikely(is->end - is->next < 4))
                return 0;
        v = get_unaligned_le32(is->next);
        is->next += 4;
        return v;
}

/* Read into @dst_buffer an array of literal bytes embedded in the bitstream.
 * Return either a pointer to the byte past the last written, or NULL if the
 * read overflows the input buffer.  */
static inline void *
bitstream_read_bytes(struct input_bitstream *is, void *dst_buffer, size_t count)
{
        if (unlikely(is->end - is->next < count))
                return NULL;
        memcpy(dst_buffer, is->next, count);
        is->next += count;
        return (u8 *)dst_buffer + count;
}

/* Align the input bitstream on a coding-unit boundary.  */
static inline void
bitstream_align(struct input_bitstream *is)
{
        is->bitsleft = 0;
        is->bitbuf = 0;
}

/* Needed alignment of decode_table parameter to make_huffman_decode_table().
 *
 * Reason: We may fill the entries with SSE instructions without worrying
 * about dealing with the unaligned case.  */
#define DECODE_TABLE_ALIGNMENT 16

/* Maximum supported symbol count for make_huffman_decode_table().
 *
 * Reason: In direct mapping entries, we store the symbol in 11 bits.  */
#define DECODE_TABLE_MAX_SYMBOLS 2048

/* Maximum supported table bits for make_huffman_decode_table().
 *
 * Reason: In internal binary tree nodes, offsets are encoded in 14 bits.
 * But the real limit is 13, because we allocate entries past the end of
 * the direct lookup part of the table for binary tree nodes.  (Note: if
 * needed this limit could be removed by encoding the offsets relative to
 * &decode_table[1 << table_bits].)  */
#define DECODE_TABLE_MAX_TABLE_BITS 13

/* Maximum supported codeword length for make_huffman_decode_table().
 *
 * Reason: In direct mapping entries, we encode the codeword length in 5
 * bits, and the top 2 bits can't both be set because that has special
 * meaning.  */
#define DECODE_TABLE_MAX_CODEWORD_LEN 23

/* Reads and returns the next Huffman-encoded symbol from a bitstream.  If the
 * input data is exhausted, the Huffman symbol is decoded as if the missing bits
 * are all zeroes.
 *
 * XXX: This is mostly duplicated in lzms_decode_huffman_symbol() in
 * lzms_decompress.c.  */
static inline unsigned
read_huffsym(struct input_bitstream *istream, const u16 decode_table[],
             unsigned table_bits, unsigned max_codeword_len)
{
        unsigned entry;
        unsigned key_bits;

        bitstream_ensure_bits(istream, max_codeword_len);

        /* Index the decode table by the next table_bits bits of the input.  */
        key_bits = bitstream_peek_bits(istream, table_bits);
        entry = decode_table[key_bits];
        if (likely(entry < 0xC000)) {
                /* Fast case: The decode table directly provided the
                 * symbol and codeword length.  The low 11 bits are the
                 * symbol, and the high 5 bits are the codeword length.  */
                bitstream_remove_bits(istream, entry >> 11);
                return entry & 0x7FF;
        } else {
                /* Slow case: The codeword for the symbol is longer than
                 * table_bits, so the symbol does not have an entry
                 * directly in the first (1 << table_bits) entries of the
                 * decode table.  Traverse the appropriate binary tree
                 * bit-by-bit to decode the symbol.  */
                bitstream_remove_bits(istream, table_bits);
                do {
                        key_bits = (entry & 0x3FFF) + bitstream_pop_bits(istream, 1);
                } while ((entry = decode_table[key_bits]) >= 0xC000);
                return entry;
        }
}

extern int
make_huffman_decode_table(u16 decode_table[], unsigned num_syms,
                          unsigned num_bits, const u8 lens[],
                          unsigned max_codeword_len);

static inline void
copy_word_unaligned(const void *src, void *dst)
{
        store_word_unaligned(load_word_unaligned(src), dst);
}

static inline machine_word_t
repeat_byte(u8 b)
{
        machine_word_t v;

        STATIC_ASSERT(WORDSIZE == 4 || WORDSIZE == 8);

        v = b;
        v |= v << 8;
        v |= v << 16;
        v |= v << ((WORDSIZE == 8) ? 32 : 0);
        return v;
}

/*
 * Copy an LZ77 match at (dst - offset) to dst.
 *
 * The length and offset must be already validated --- that is, (dst - offset)
 * can't underrun the output buffer, and (dst + length) can't overrun the output
 * buffer.  Also, the length cannot be 0.
 *
 * @winend points to the byte past the end of the output buffer.
 * This function won't write any data beyond this position.
 */
static inline void
lz_copy(u8 *dst, u32 length, u32 offset, const u8 *winend, u32 min_length)
{
        const u8 *src = dst - offset;
        const u8 * const end = dst + length;

        /*
         * Try to copy one machine word at a time.  On i386 and x86_64 this is
         * faster than copying one byte at a time, unless the data is
         * near-random and all the matches have very short lengths.  Note that
         * since this requires unaligned memory accesses, it won't necessarily
         * be faster on every architecture.
         *
         * Also note that we might copy more than the length of the match.  For
         * example, if a word is 8 bytes and the match is of length 5, then
         * we'll simply copy 8 bytes.  This is okay as long as we don't write
         * beyond the end of the output buffer, hence the check for (winend -
         * end >= WORDSIZE - 1).
         */
        if (UNALIGNED_ACCESS_IS_FAST &&
            likely(winend - end >= WORDSIZE - 1))
        {

                if (offset >= WORDSIZE) {
                        /* The source and destination words don't overlap.  */

                        /* To improve branch prediction, one iteration of this
                         * loop is unrolled.  Most matches are short and will
                         * fail the first check.  But if that check passes, then
                         * it becomes increasing likely that the match is long
                         * and we'll need to continue copying.  */

                        copy_word_unaligned(src, dst);
                        src += WORDSIZE;
                        dst += WORDSIZE;

                        if (dst < end) {
                                do {
                                        copy_word_unaligned(src, dst);
                                        src += WORDSIZE;
                                        dst += WORDSIZE;
                                } while (dst < end);
                        }
                        return;
                } else if (offset == 1) {

                        /* Offset 1 matches are equivalent to run-length
                         * encoding of the previous byte.  This case is common
                         * if the data contains many repeated bytes.  */

                        machine_word_t v = repeat_byte(*(dst - 1));
                        do {
                                store_word_unaligned(v, dst);
                                src += WORDSIZE;
                                dst += WORDSIZE;
                        } while (dst < end);
                        return;
                }
                /*
                 * We don't bother with special cases for other 'offset <
                 * WORDSIZE', which are usually rarer than 'offset == 1'.  Extra
                 * checks will just slow things down.  Actually, it's possible
                 * to handle all the 'offset < WORDSIZE' cases using the same
                 * code, but it still becomes more complicated doesn't seem any
                 * faster overall; it definitely slows down the more common
                 * 'offset == 1' case.
                 */
        }

        /* Fall back to a bytewise copy.  */

        if (min_length >= 2) {
                *dst++ = *src++;
                length--;
        }
        if (min_length >= 3) {
                *dst++ = *src++;
                length--;
        }
        if (min_length >= 4) {
                *dst++ = *src++;
                length--;
        }
        do {
                *dst++ = *src++;
        } while (--length);
}

#endif /* _WIMLIB_DECOMPRESS_COMMON_H */