ref: 994d01dff2f65cbabc5abbd56ff12428e29490b8
dir: /3rd/brieflz/brieflz_hashchain.h/
//
// BriefLZ - small fast Lempel-Ziv
//
// Lazy parsing with chains of previous positions
//
// Copyright (c) 2016-2020 Joergen Ibsen
//
// This software is provided 'as-is', without any express or implied
// warranty. In no event will the authors be held liable for any damages
// arising from the use of this software.
//
// Permission is granted to anyone to use this software for any purpose,
// including commercial applications, and to alter it and redistribute it
// freely, subject to the following restrictions:
//
// 1. The origin of this software must not be misrepresented; you must
// not claim that you wrote the original software. If you use this
// software in a product, an acknowledgment in the product
// documentation would be appreciated but is not required.
//
// 2. Altered source versions must be plainly marked as such, and must
// not be misrepresented as being the original software.
//
// 3. This notice may not be removed or altered from any source
// distribution.
//
#ifndef BRIEFLZ_HASHCHAIN_H_INCLUDED
#define BRIEFLZ_HASHCHAIN_H_INCLUDED
static size_t
blz_hashchain_workmem_size(size_t src_size)
{
return (LOOKUP_SIZE + src_size) * sizeof(blz_word);
}
// Lazy parsing with chains of previous positions.
//
// Use the lookup table to create chains of previous positions for a given
// hash. This version uses an array of previous positions the size of the
// input.
//
// Since the same four bytes may occur many times, it is often required to
// limit the number of positions checked. The search is limited to following
// max_depth positions down each chain.
//
static unsigned long
blz_pack_hashchain(const void *src, void *dst, unsigned long src_size, void *workmem,
const unsigned long max_depth, const unsigned long accept_len)
{
struct blz_state bs;
blz_word *const lookup = (blz_word *) workmem;
blz_word *const prev = lookup + LOOKUP_SIZE;
const unsigned char *const in = (const unsigned char *) src;
const unsigned long last_match_pos = src_size > 4 ? src_size - 4 : 0;
unsigned long cur = 0;
assert(src_size < BLZ_WORD_MAX);
// Check for empty input
if (src_size == 0) {
return 0;
}
bs.next_out = (unsigned char *) dst;
// First byte verbatim
*bs.next_out++ = in[0];
// Check for 1 byte input
if (src_size == 1) {
return 1;
}
// Initialize first tag
bs.tag_out = bs.next_out;
bs.next_out += 2;
bs.tag = 0;
bs.bits_left = 16;
// Initialize lookup
for (unsigned long i = 0; i < LOOKUP_SIZE; ++i) {
lookup[i] = NO_MATCH_POS;
}
// Build hash chains in prev
if (last_match_pos > 0) {
for (unsigned long i = 0; i <= last_match_pos; ++i) {
const unsigned long hash = blz_hash4(&in[i]);
prev[i] = lookup[hash];
lookup[hash] = i;
}
}
// Main compression loop
for (cur = 1; cur <= last_match_pos; ) {
unsigned long best_pos = NO_MATCH_POS;
unsigned long best_len = 0;
// Look up first match for current position
unsigned long pos = prev[cur];
const unsigned long len_limit = src_size - cur;
unsigned long num_chain = max_depth;
// Check matches
while (pos != NO_MATCH_POS && num_chain--) {
unsigned long len = 0;
// If next byte matches, so this has a chance to be a longer match
if (best_len < len_limit
&& in[pos + best_len] == in[cur + best_len]) {
// Find match len
while (len < len_limit && in[pos + len] == in[cur + len]) {
++len;
}
}
// Update best match
if (blz_match_better(cur, pos, len, best_pos, best_len)) {
best_pos = pos;
best_len = len;
if (best_len >= accept_len) {
break;
}
}
// Go to previous match
pos = prev[pos];
}
// Check if match at next position is better
if (best_len > 3 && best_len < accept_len && cur < last_match_pos) {
// Look up first match for next position
unsigned long next_pos = prev[cur + 1];
const unsigned long next_len_limit = src_size - (cur + 1);
num_chain = max_depth;
// Check matches
while (next_pos != NO_MATCH_POS && num_chain--) {
unsigned long next_len = 0;
// Check match
if (best_len - 1 < next_len_limit
&& in[next_pos + best_len - 1] == in[cur + 1 + best_len - 1]) {
while (next_len < next_len_limit
&& in[next_pos + next_len] == in[cur + 1 + next_len]) {
++next_len;
}
}
if (next_len >= best_len) {
// Replace with next match if it extends backwards
if (next_pos > 0 && in[next_pos - 1] == in[cur]) {
if (blz_match_better(cur, next_pos - 1, next_len + 1, best_pos, best_len)) {
best_pos = next_pos - 1;
best_len = next_len + 1;
}
}
else {
// Drop current match if next match is better
if (blz_next_match_better(cur, next_pos, next_len, best_pos, best_len)) {
best_len = 0;
break;
}
}
}
// Go to previous match
next_pos = prev[next_pos];
}
}
// Output match or literal
if (best_len > 4 || (best_len == 4 && cur - best_pos - 1 < 0x3FE00UL)) {
const unsigned long offs = cur - best_pos - 1;
// Output match tag
blz_putbit(&bs, 1);
// Output match length
blz_putgamma(&bs, best_len - 2);
// Output match offset
blz_putgamma(&bs, (offs >> 8) + 2);
*bs.next_out++ = offs & 0x00FF;
cur += best_len;
}
else {
// Output literal tag
blz_putbit(&bs, 0);
// Copy literal
*bs.next_out++ = in[cur++];
}
}
// Output any remaining literals
while (cur < src_size) {
// Output literal tag
blz_putbit(&bs, 0);
// Copy literal
*bs.next_out++ = in[cur++];
}
// Trailing one bit to delimit any literal tags
blz_putbit(&bs, 1);
// Shift last tag into position and store
bs.tag <<= bs.bits_left;
bs.tag_out[0] = bs.tag & 0x00FF;
bs.tag_out[1] = (bs.tag >> 8) & 0x00FF;
// Return compressed size
return (unsigned long) (bs.next_out - (unsigned char *) dst);
}
#endif /* BRIEFLZ_HASHCHAIN_H_INCLUDED */