--- /dev/null
+/*
+ * lz_hash.c
+ *
+ * This file provides the code to analyze a buffer of uncompressed data for
+ * matches, as per the LZ77 algorithm. It uses a hash table to accelerate the
+ * process. This is based on code from zlib (v. 1.2.5).
+ */
+
+/*
+ * Copyright (C) 2012, 2013 Eric Biggers
+ * Copyright (C) 1995-2010 Jean-loup Gailly and Mark Adler
+ *
+ * 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/.
+ */
+
+#ifdef HAVE_CONFIG_H
+# include <config.h>
+#endif
+
+#include "wimlib/lz_hash.h"
+#include "wimlib/util.h"
+
+#include <string.h>
+
+#define HASH_BITS 15
+#define HASH_SIZE (1 << HASH_BITS)
+#define HASH_MASK (HASH_SIZE - 1)
+#define HASH_SHIFT 5
+
+/* Hash function, based on code from zlib. This function will update and return
+ * the hash value @hash for the string ending on the additional input character
+ * @c. This function must be called for each consecutive character, because it
+ * uses a running hash value rather than computing it separately for each
+ * 3-character string.
+ *
+ * The AND operation guarantees that only 3 characters will affect the hash
+ * value, so every identical 3-character string will have the same hash value.
+ */
+static inline unsigned
+update_hash(unsigned hash, u8 c)
+{
+ return ((hash << HASH_SHIFT) ^ c) & HASH_MASK;
+}
+
+
+/* Insert a 3-character string at position @str_pos in @window and with hash
+ * code @hash into the hash table described by @hash_tab and @prev_tab. Based
+ * on code from zlib.
+ *
+ * The hash table uses chains (linked lists) for the hash buckets, but there are
+ * no real pointers involved. Indexing `hash_tab' by hash value gives the index
+ * within the window of the last string in the hash bucket. To find the index
+ * of the previous string in the hash chain, the `prev_tab' array is indexed by
+ * the string index. `prev_tab' can be indexed repeatedly by the string index
+ * to walk through the hash chain, until the special index `0' is reached,
+ * indicating the end of the hash chain.
+ */
+static inline unsigned
+insert_string(input_idx_t hash_tab[], input_idx_t prev_tab[],
+ const u8 window[], unsigned str_pos,
+ unsigned hash)
+{
+ hash = update_hash(hash, window[str_pos + 2]);
+ prev_tab[str_pos] = hash_tab[hash];
+ hash_tab[hash] = str_pos;
+ return hash;
+}
+
+
+/*
+ * Returns the longest match for a given input position.
+ *
+ * @window: The window of uncompressed data.
+ * @bytes_remaining: The number of bytes remaining in the window.
+ * @strstart: The index of the start of the string in the window that
+ * we are trying to find a match for.
+ * @prev_tab: The array of prev pointers for the hash table.
+ * @cur_match: The index of the head of the hash chain for matches
+ * having the hash value of the string beginning
+ * at index @strstart.
+ * @prev_len: The length of the match that was found for the string
+ * beginning at (@strstart - 1).
+ * @match_start_ret: A location into which the index of the start of the
+ * match will be returned.
+ * @params: Parameters that affect how long the search will proceed
+ * before going with the best that has been found
+ * so far.
+ * @min_start_pos: If the chain reaches a match starting before this
+ * position (including the end-of-chain 0), the search will
+ * be terminated.
+ *
+ * Returns the length of the match that was found.
+ */
+static unsigned
+longest_match(const u8 window[], unsigned bytes_remaining,
+ unsigned strstart, const input_idx_t prev_tab[],
+ unsigned cur_match, unsigned prev_len,
+ unsigned *match_start_ret,
+ const struct lz_params *params,
+ unsigned min_start_pos)
+{
+ unsigned chain_len = params->max_chain_len;
+
+ const u8 *scan = window + strstart;
+ const u8 *match;
+ unsigned len;
+ unsigned best_len = prev_len;
+ unsigned match_start = cur_match;
+
+ unsigned nice_match = min(params->nice_match, bytes_remaining);
+
+ const u8 *strend = scan + min(params->max_match, bytes_remaining);
+
+ u8 scan_end1 = scan[best_len - 1];
+ u8 scan_end = scan[best_len];
+
+
+ /* Do not waste too much time if we already have a good match: */
+ if (best_len >= params->good_match)
+ chain_len >>= 2;
+
+ do {
+ match = &window[cur_match];
+
+ /* Skip to next match if the match length cannot increase or if
+ * the match length is less than 2. Note that the checks below
+ * for insufficient lookahead only occur occasionally for
+ * performance reasons. Therefore uninitialized memory will be
+ * accessed, and conditional jumps will be made that depend on
+ * those values. However the length of the match is limited to
+ * the lookahead, so the output of lz_analyze_block() is not
+ * affected by the uninitialized values. */
+
+ if (match[best_len] != scan_end
+ || match[best_len - 1] != scan_end1
+ || *match != *scan
+ || *++match != scan[1])
+ continue;
+ scan++;
+
+ #if 0
+ do {
+ } while (scan < strend && *++match == *++scan);
+ #else
+
+ do {
+ } while (
+ *++match == *++scan && *++match == *++scan &&
+ *++match == *++scan && *++match == *++scan &&
+ *++match == *++scan && *++match == *++scan &&
+ *++match == *++scan && *++match == *++scan &&
+ scan < strend);
+ #endif
+ len = match - &window[cur_match];
+
+ scan = &window[strstart];
+
+ if (len > best_len) {
+ match_start = cur_match;
+ best_len = len;
+ if (len >= nice_match)
+ break;
+ scan_end1 = scan[best_len - 1];
+ scan_end = scan[best_len];
+ }
+ } while (--chain_len != 0 && (cur_match = prev_tab[cur_match]) >= min_start_pos);
+ *match_start_ret = match_start;
+ return min(min(best_len, bytes_remaining), params->max_match);
+}
+
+
+
+/*
+ * Determines the sequence of matches and literals that a block of data will be
+ * compressed to.
+ *
+ * @window: The data that is to be compressed.
+ * @window_size: The length of @window, in bytes.
+ * @record_match: Consumer for matches.
+ * @record_literal: Consumer for literals.
+ * @record_ctx: Context passed to @record_match and @record_literal.
+ * @params: Structure that contains parameters that affect how the
+ * analysis proceeds (mainly how good the matches
+ * have to be).
+ * @prev_tab: Temporary space containing least @window_size elements.
+ */
+void
+lz_analyze_block(const u8 window[restrict],
+ input_idx_t window_size,
+ lz_record_match_t record_match,
+ lz_record_literal_t record_literal,
+ void *record_ctx,
+ const struct lz_params *params,
+ input_idx_t prev_tab[restrict])
+{
+ unsigned cur_input_pos = 0;
+ unsigned hash = 0;
+ unsigned hash_head = 0;
+ unsigned prev_len = params->min_match - 1;
+ unsigned prev_start;
+ unsigned match_len = params->min_match - 1;
+ unsigned match_start = 0;
+ bool match_available = false;
+ input_idx_t hash_tab[HASH_SIZE];
+ unsigned min_start_pos = 1;
+
+ ZERO_ARRAY(hash_tab);
+
+ do {
+ /* If there are at least 3 characters remaining in the input,
+ * insert the 3-character string beginning at
+ * window[cur_input_pos] into the hash table.
+ *
+ * hash_head is set to the index of the previous string in the
+ * hash bucket, or 0 if there is no such string */
+ if (window_size - cur_input_pos >= params->min_match) {
+ hash = insert_string(hash_tab, prev_tab,
+ window,
+ cur_input_pos, hash);
+ hash_head = prev_tab[cur_input_pos];
+ } else {
+ hash_head = 0;
+ }
+
+
+ /* Find the longest match, discarding those <= prev_len. */
+ prev_len = match_len;
+ prev_start = match_start;
+ match_len = params->min_match - 1;
+
+ if (cur_input_pos > params->max_offset)
+ min_start_pos = cur_input_pos - params->max_offset;
+ else
+ min_start_pos = 1;
+
+ if (hash_head >= min_start_pos &&
+ prev_len < params->max_lazy_match)
+ {
+ /* To simplify the code, we prevent matches with the
+ * string of window index 0 (in particular we have to
+ * avoid a match of the string with itself at the start
+ * of the input file). */
+ match_len = longest_match(window,
+ window_size - cur_input_pos,
+ cur_input_pos, prev_tab,
+ hash_head, prev_len,
+ &match_start, params,
+ min_start_pos);
+
+ if (match_len == params->min_match &&
+ cur_input_pos - match_start > params->too_far)
+ match_len = params->min_match - 1;
+ }
+
+ /* If there was a match at the previous step and the current
+ * match is not better, output the previous match:
+ */
+ if (prev_len >= params->min_match && match_len <= prev_len) {
+
+
+ (*record_match)(prev_len,
+ cur_input_pos - 1 - prev_start,
+ record_ctx);
+
+ /* Insert in hash table all strings up to the end of the
+ * match. strstart-1 and strstart are already inserted.
+ * If there is not enough lookahead, the last two
+ * strings are not inserted in the hash table. */
+
+ /* Do not insert strings in hash table beyond this. */
+ unsigned max_insert = window_size - params->min_match;
+
+ prev_len -= 2;
+
+ do {
+ if (++cur_input_pos <= max_insert) {
+ hash = insert_string(hash_tab, prev_tab,
+ window,
+ cur_input_pos,
+ hash);
+ }
+ } while (--prev_len != 0);
+ match_available = false;
+ match_len = params->min_match - 1;
+ } else if (match_available) {
+ /* If there was no match at the previous position,
+ * output a single literal. If there was a match but the
+ * current match is longer, truncate the previous match
+ * to a single literal. */
+ (*record_literal)(window[cur_input_pos - 1], record_ctx);
+ } else {
+ /* There is no previous match to compare with, wait for
+ * the next step to decide. */
+ match_available = true;
+ }
+ } while (++cur_input_pos < window_size);
+
+ if (match_available)
+ (*record_literal)(window[cur_input_pos - 1], record_ctx);
+}
git://wimlib.net / wimlib / blobdiff