lzma_encoder_optimum_fast.c

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//
//
//  Author:     Igor Pavlov
//
//  This file has been put into the public domain.
//  You can do whatever you want with this file.
//
 
#include "lzma_encoder_private.h"
#include "memcmplen.h"
 
 
#define change_pair(small_dist, big_dist) \
    (((big_dist) >> 7) > (small_dist))
 
 
extern void
lzma_lzma_optimum_fast(lzma_lzma1_encoder *restrict coder,
        lzma_mf *restrict mf,
        uint32_t *restrict back_res, uint32_t *restrict len_res)
{
    const uint32_t nice_len = mf->nice_len;
 
    uint32_t len_main;
    uint32_t matches_count;
    if (mf->read_ahead == 0) {
        len_main = mf_find(mf, &matches_count, coder->matches);
    } else {
        assert(mf->read_ahead == 1);
        len_main = coder->longest_match_length;
        matches_count = coder->matches_count;
    }
 
    const uint8_t *buf = mf_ptr(mf) - 1;
    const uint32_t buf_avail = my_min(mf_avail(mf) + 1, MATCH_LEN_MAX);
 
    if (buf_avail < 2) {
        // There's not enough input left to encode a match.
        *back_res = UINT32_MAX;
        *len_res = 1;
        return;
    }
 
    // Look for repeated matches; scan the previous four match distances
    uint32_t rep_len = 0;
    uint32_t rep_index = 0;
 
    for (uint32_t i = 0; i < REPS; ++i) {
        // Pointer to the beginning of the match candidate
        const uint8_t *const buf_back = buf - coder->reps[i] - 1;
 
        // If the first two bytes (2 == MATCH_LEN_MIN) do not match,
        // this rep is not useful.
        if (not_equal_16(buf, buf_back))
            continue;
 
        // The first two bytes matched.
        // Calculate the length of the match.
        const uint32_t len = lzma_memcmplen(
                buf, buf_back, 2, buf_avail);
 
        // If we have found a repeated match that is at least
        // nice_len long, return it immediately.
        if (len >= nice_len) {
            *back_res = i;
            *len_res = len;
            mf_skip(mf, len - 1);
            return;
        }
 
        if (len > rep_len) {
            rep_index = i;
            rep_len = len;
        }
    }
 
    // We didn't find a long enough repeated match. Encode it as a normal
    // match if the match length is at least nice_len.
    if (len_main >= nice_len) {
        *back_res = coder->matches[matches_count - 1].dist + REPS;
        *len_res = len_main;
        mf_skip(mf, len_main - 1);
        return;
    }
 
    uint32_t back_main = 0;
    if (len_main >= 2) {
        back_main = coder->matches[matches_count - 1].dist;
 
        while (matches_count > 1 && len_main ==
                coder->matches[matches_count - 2].len + 1) {
            if (!change_pair(coder->matches[
                        matches_count - 2].dist,
                    back_main))
                break;
 
            --matches_count;
            len_main = coder->matches[matches_count - 1].len;
            back_main = coder->matches[matches_count - 1].dist;
        }
 
        if (len_main == 2 && back_main >= 0x80)
            len_main = 1;
    }
 
    if (rep_len >= 2) {
        if (rep_len + 1 >= len_main
                || (rep_len + 2 >= len_main
                    && back_main > (UINT32_C(1) << 9))
                || (rep_len + 3 >= len_main
                    && back_main > (UINT32_C(1) << 15))) {
            *back_res = rep_index;
            *len_res = rep_len;
            mf_skip(mf, rep_len - 1);
            return;
        }
    }
 
    if (len_main < 2 || buf_avail <= 2) {
        *back_res = UINT32_MAX;
        *len_res = 1;
        return;
    }
 
    // Get the matches for the next byte. If we find a better match,
    // the current byte is encoded as a literal.
    coder->longest_match_length = mf_find(mf,
            &coder->matches_count, coder->matches);
 
    if (coder->longest_match_length >= 2) {
        const uint32_t new_dist = coder->matches[
                coder->matches_count - 1].dist;
 
        if ((coder->longest_match_length >= len_main
                    && new_dist < back_main)
                || (coder->longest_match_length == len_main + 1
                    && !change_pair(back_main, new_dist))
                || (coder->longest_match_length > len_main + 1)
                || (coder->longest_match_length + 1 >= len_main
                    && len_main >= 3
                    && change_pair(new_dist, back_main))) {
            *back_res = UINT32_MAX;
            *len_res = 1;
            return;
        }
    }
 
    // In contrast to LZMA SDK, dictionary could not have been moved
    // between mf_find() calls, thus it is safe to just increment
    // the old buf pointer instead of recalculating it with mf_ptr().
    ++buf;
 
    const uint32_t limit = my_max(2, len_main - 1);
 
    for (uint32_t i = 0; i < REPS; ++i) {
        if (memcmp(buf, buf - coder->reps[i] - 1, limit) == 0) {
            *back_res = UINT32_MAX;
            *len_res = 1;
            return;
        }
    }
 
    *back_res = back_main + REPS;
    *len_res = len_main;
    mf_skip(mf, len_main - 2);
    return;
}