[wimlib] / include / wimlib / hc_matchfinder.h

/*
 * hc_matchfinder.h
 *
 * Author:      Eric Biggers
 * Year:        2014, 2015
 *
 * The author dedicates this file to the public domain.
 * You can do whatever you want with this file.
 *
 * ---------------------------------------------------------------------------
 *
 *                                 Algorithm
 *
 * This is a Hash Chains (hc) based matchfinder.
 *
 * The main data structure is a hash table where each hash bucket contains a
 * linked list (or "chain") of sequences whose first 4 bytes share the same hash
 * code.  Each sequence is identified by its starting position in the input
 * buffer.
 *
 * The algorithm processes the input buffer sequentially.  At each byte
 * position, the hash code of the first 4 bytes of the sequence beginning at
 * that position (the sequence being matched against) is computed.  This
 * identifies the hash bucket to use for that position.  Then, this hash
 * bucket's linked list is searched for matches.  Then, a new linked list node
 * is created to represent the current sequence and is prepended to the list.
 *
 * This algorithm has several useful properties:
 *
 * - It only finds true Lempel-Ziv matches; i.e., those where the matching
 *   sequence occurs prior to the sequence being matched against.
 *
 * - The sequences in each linked list are always sorted by decreasing starting
 *   position.  Therefore, the closest (smallest offset) matches are found
 *   first, which in many compression formats tend to be the cheapest to encode.
 *
 * - Although fast running time is not guaranteed due to the possibility of the
 *   lists getting very long, the worst degenerate behavior can be easily
 *   prevented by capping the number of nodes searched at each position.
 *
 * - If the compressor decides not to search for matches at a certain position,
 *   then that position can be quickly inserted without searching the list.
 *
 * - The algorithm is adaptable to sliding windows: just store the positions
 *   relative to a "base" value that is updated from time to time, and stop
 *   searching each list when the sequences get too far away.
 *
 * ---------------------------------------------------------------------------
 *
 *                              Notes on usage
 *
 * Before including this header, you must define 'mf_pos_t' to an integer type
 * that can represent all possible positions.  This can be a 16-bit or 32-bit
 * unsigned integer.  When possible, the former should be used due to the
 * reduced cache pressure.  This header can be included multiple times in a
 * single .c file with different 'mf_pos_t' definitions; however, you must
 * define a different MF_SUFFIX each time to generate different names for the
 * matchfinder structure and functions.
 *
 * The number of bytes that must be allocated for a given 'struct
 * hc_matchfinder' must be gotten by calling hc_matchfinder_size().
 *
 * ----------------------------------------------------------------------------
 *
 *                               Optimizations
 *
 * The main hash table and chains handle length 4+ matches.  Length 3 matches
 * are handled by a separate hash table with no chains.  This works well for
 * typical "greedy" or "lazy"-style compressors, where length 3 matches are
 * often only helpful if they have small offsets.  Instead of searching a full
 * chain for length 3+ matches, the algorithm just checks for one close length 3
 * match, then focuses on finding length 4+ matches.
 *
 * The longest_match() and skip_positions() functions are inlined into the
 * compressors that use them.  This isn't just about saving the overhead of a
 * function call.  These functions are intended to be called from the inner
 * loops of compressors, where giving the compiler more control over register
 * allocation is very helpful.  There is also significant benefit to be gained
 * from allowing the CPU to predict branches independently at each call site.
 * For example, "lazy"-style compressors can be written with two calls to
 * longest_match(), each of which starts with a different 'best_len' and
 * therefore has significantly different performance characteristics.
 *
 * Although any hash function can be used, a multiplicative hash is fast and
 * works well.
 *
 * On some processors, it is significantly faster to extend matches by whole
 * words (32 or 64 bits) instead of by individual bytes.  For this to be the
 * case, the processor must implement unaligned memory accesses efficiently and
 * must have either a fast "find first set bit" instruction or a fast "find last
 * set bit" instruction, depending on the processor's endianness.
 *
 * The code uses one loop for finding the first match and one loop for finding a
 * longer match.  Each of these loops is tuned for its respective task and in
 * combination are faster than a single generalized loop that handles both
 * tasks.
 *
 * The code also uses a tight inner loop that only compares the last and first
 * bytes of a potential match.  It is only when these bytes match that a full
 * match extension is attempted.
 *
 * ----------------------------------------------------------------------------
 */

#include <string.h>

#include "wimlib/lz_extend.h"
#include "wimlib/lz_hash.h"
#include "wimlib/unaligned.h"

#define HC_MATCHFINDER_HASH3_ORDER      14
#define HC_MATCHFINDER_HASH4_ORDER      15

/* TEMPLATED functions and structures have MF_SUFFIX appended to their name.  */
#undef TEMPLATED
#define TEMPLATED(name)         CONCAT(name, MF_SUFFIX)

struct TEMPLATED(hc_matchfinder) {

        /* The hash table for finding length 3 matches  */
        mf_pos_t hash3_tab[1UL << HC_MATCHFINDER_HASH3_ORDER];

        /* The hash table which contains the first nodes of the linked lists for
         * finding length 4+ matches  */
        mf_pos_t hash4_tab[1UL << HC_MATCHFINDER_HASH4_ORDER];

        /* The "next node" references for the linked lists.  The "next node" of
         * the node for the sequence with position 'pos' is 'next_tab[pos]'.  */
        mf_pos_t next_tab[];
};

/* Return the number of bytes that must be allocated for a 'hc_matchfinder' that
 * can work with buffers up to the specified size.  */
static inline size_t
TEMPLATED(hc_matchfinder_size)(size_t max_bufsize)
{
        return sizeof(struct TEMPLATED(hc_matchfinder)) +
                (max_bufsize * sizeof(mf_pos_t));
}

/* Prepare the matchfinder for a new input buffer.  */
static inline void
TEMPLATED(hc_matchfinder_init)(struct TEMPLATED(hc_matchfinder) *mf)
{
        memset(mf, 0, sizeof(*mf));
}

/*
 * Find the longest match longer than 'best_len' bytes.
 *
 * @mf
 *      The matchfinder structure.
 * @in_begin
 *      Pointer to the beginning of the input buffer.
 * @cur_pos
 *      The current position in the input buffer (the position of the sequence
 *      being matched against).
 * @best_len
 *      Require a match longer than this length.
 * @max_len
 *      The maximum permissible match length at this position.
 * @nice_len
 *      Stop searching if a match of at least this length is found.
 *      Must be <= @max_len.
 * @max_search_depth
 *      Limit on the number of potential matches to consider.  Must be >= 1.
 * @next_hashes
 *      The precomputed hash codes for the sequence beginning at @in_next.
 *      These will be used and then updated with the precomputed hashcodes for
 *      the sequence beginning at @in_next + 1.
 * @offset_ret
 *      If a match is found, its offset is returned in this location.
 *
 * Return the length of the match found, or 'best_len' if no match longer than
 * 'best_len' was found.
 */
static inline u32
TEMPLATED(hc_matchfinder_longest_match)(struct TEMPLATED(hc_matchfinder) * const restrict mf,
                                        const u8 * const restrict in_begin,
                                        const ptrdiff_t cur_pos,
                                        u32 best_len,
                                        const u32 max_len,
                                        const u32 nice_len,
                                        const u32 max_search_depth,
                                        u32 next_hashes[const restrict static 2],
                                        u32 * const restrict offset_ret)
{
        const u8 *in_next = in_begin + cur_pos;
        u32 depth_remaining = max_search_depth;
        const u8 *best_matchptr = best_matchptr; /* uninitialized */
        mf_pos_t cur_node3, cur_node4;
        u32 hash3, hash4;
        u32 next_seq3, next_seq4;
        u32 seq4;
        const u8 *matchptr;
        u32 len;

        if (unlikely(max_len < 5)) /* can we read 4 bytes from 'in_next + 1'? */
                goto out;

        /* Get the precomputed hash codes.  */
        hash3 = next_hashes[0];
        hash4 = next_hashes[1];

        /* From the hash buckets, get the first node of each linked list.  */
        cur_node3 = mf->hash3_tab[hash3];
        cur_node4 = mf->hash4_tab[hash4];

        /* Update for length 3 matches.  This replaces the singleton node in the
         * 'hash3' bucket with the node for the current sequence.  */
        mf->hash3_tab[hash3] = cur_pos;

        /* Update for length 4 matches.  This prepends the node for the current
         * sequence to the linked list in the 'hash4' bucket.  */
        mf->hash4_tab[hash4] = cur_pos;
        mf->next_tab[cur_pos] = cur_node4;

        /* Compute the next hash codes.  */
        next_seq4 = load_u32_unaligned(in_next + 1);
        next_seq3 = loaded_u32_to_u24(next_seq4);
        next_hashes[0] = lz_hash(next_seq3, HC_MATCHFINDER_HASH3_ORDER);
        next_hashes[1] = lz_hash(next_seq4, HC_MATCHFINDER_HASH4_ORDER);
        prefetchw(&mf->hash3_tab[next_hashes[0]]);
        prefetchw(&mf->hash4_tab[next_hashes[1]]);

        if (best_len < 4) {  /* No match of length >= 4 found yet?  */

                /* Check for a length 3 match if needed.  */

                if (!cur_node3)
                        goto out;

                seq4 = load_u32_unaligned(in_next);

                if (best_len < 3) {
                        matchptr = &in_begin[cur_node3];
                        if (load_u24_unaligned(matchptr) == loaded_u32_to_u24(seq4)) {
                                best_len = 3;
                                best_matchptr = matchptr;
                        }
                }

                /* Check for a length 4 match.  */

                if (!cur_node4)
                        goto out;

                for (;;) {
                        /* No length 4 match found yet.  Check the first 4 bytes.  */
                        matchptr = &in_begin[cur_node4];

                        if (load_u32_unaligned(matchptr) == seq4)
                                break;

                        /* The first 4 bytes did not match.  Keep trying.  */
                        cur_node4 = mf->next_tab[cur_node4];
                        if (!cur_node4 || !--depth_remaining)
                                goto out;
                }

                /* Found a match of length >= 4.  Extend it to its full length.  */
                best_matchptr = matchptr;
                best_len = lz_extend(in_next, best_matchptr, 4, max_len);
                if (best_len >= nice_len)
                        goto out;
                cur_node4 = mf->next_tab[cur_node4];
                if (!cur_node4 || !--depth_remaining)
                        goto out;
        } else {
                if (!cur_node4 || best_len >= nice_len)
                        goto out;
        }

        /* Check for matches of length >= 5.  */

        for (;;) {
                for (;;) {
                        matchptr = &in_begin[cur_node4];

                        /* Already found a length 4 match.  Try for a longer
                         * match; start by checking either the last 4 bytes and
                         * the first 4 bytes, or the last byte.  (The last byte,
                         * the one which would extend the match length by 1, is
                         * the most important.)  */
                #if UNALIGNED_ACCESS_IS_FAST
                        if ((load_u32_unaligned(matchptr + best_len - 3) ==
                             load_u32_unaligned(in_next + best_len - 3)) &&
                            (load_u32_unaligned(matchptr) ==
                             load_u32_unaligned(in_next)))
                #else
                        if (matchptr[best_len] == in_next[best_len])
                #endif
                                break;

                        /* Continue to the next node in the list.  */
                        cur_node4 = mf->next_tab[cur_node4];
                        if (!cur_node4 || !--depth_remaining)
                                goto out;
                }

        #if UNALIGNED_ACCESS_IS_FAST
                len = 4;
        #else
                len = 0;
        #endif
                len = lz_extend(in_next, matchptr, len, max_len);
                if (len > best_len) {
                        /* This is the new longest match.  */
                        best_len = len;
                        best_matchptr = matchptr;
                        if (best_len >= nice_len)
                                goto out;
                }

                /* Continue to the next node in the list.  */
                cur_node4 = mf->next_tab[cur_node4];
                if (!cur_node4 || !--depth_remaining)
                        goto out;
        }
out:
        *offset_ret = in_next - best_matchptr;
        return best_len;
}

/*
 * Advance the matchfinder, but don't search for matches.
 *
 * @mf
 *      The matchfinder structure.
 * @in_begin
 *      Pointer to the beginning of the input buffer.
 * @cur_pos
 *      The current position in the input buffer (the position of the sequence
 *      being matched against).
 * @end_pos
 *      The length of the input buffer.
 * @next_hashes
 *      The precomputed hash codes for the sequence beginning at @in_next.
 *      These will be used and then updated with the precomputed hashcodes for
 *      the sequence beginning at @in_next + @count.
 * @count
 *      The number of bytes to advance.  Must be > 0.
 *
 * Returns @in_next + @count.
 */
static inline const u8 *
TEMPLATED(hc_matchfinder_skip_positions)(struct TEMPLATED(hc_matchfinder) * const restrict mf,
                                         const u8 * const restrict in_begin,
                                         const ptrdiff_t cur_pos,
                                         const ptrdiff_t end_pos,
                                         const u32 count,
                                         u32 next_hashes[const restrict static 2])
{
        const u8 *in_next = in_begin + cur_pos;
        const u8 * const stop_ptr = in_next + count;

        if (likely(count + 5 <= end_pos - cur_pos)) {
                u32 hash3, hash4;
                u32 next_seq3, next_seq4;

                hash3 = next_hashes[0];
                hash4 = next_hashes[1];
                do {
                        mf->hash3_tab[hash3] = in_next - in_begin;
                        mf->next_tab[in_next - in_begin] = mf->hash4_tab[hash4];
                        mf->hash4_tab[hash4] = in_next - in_begin;

                        next_seq4 = load_u32_unaligned(++in_next);
                        next_seq3 = loaded_u32_to_u24(next_seq4);
                        hash3 = lz_hash(next_seq3, HC_MATCHFINDER_HASH3_ORDER);
                        hash4 = lz_hash(next_seq4, HC_MATCHFINDER_HASH4_ORDER);

                } while (in_next != stop_ptr);

                prefetchw(&mf->hash3_tab[hash3]);
                prefetchw(&mf->hash4_tab[hash4]);
                next_hashes[0] = hash3;
                next_hashes[1] = hash4;
        }

        return stop_ptr;
}