--- /dev/null
+/*
+ * lz_bt.c
+ *
+ * Binary tree match-finder for Lempel-Ziv compression.
+ *
+ * Author: Eric Biggers
+ * Year: 2014
+ *
+ * The author hereby releases this file into the public domain.
+ * You can do whatever you want with this file.
+ */
+
+/*
+ * Note: the binary tree search/update algorithm is based on code from the
+ * public domain LZMA SDK (authors: Igor Pavlov, Lasse Collin).
+ */
+
+#ifdef HAVE_CONFIG_H
+# include "config.h"
+#endif
+
+#include "wimlib/lz.h"
+#include "wimlib/lz_bt.h"
+#include "wimlib/util.h"
+#include <string.h>
+#include <pthread.h>
+
+#define LZ_BT_HASH_BITS 16
+#define LZ_BT_HASH_SIZE (1 << LZ_BT_HASH_BITS)
+#define LZ_BT_HASH_MASK (LZ_BT_HASH_SIZE - 1)
+#define LZ_BT_DIGRAM_TAB_SIZE (256 * 256)
+
+static u32 crc32_table[256];
+static pthread_once_t crc32_table_filled = PTHREAD_ONCE_INIT;
+
+static void
+crc32_init(void)
+{
+ for (u32 b = 0; b < 256; b++) {
+ u32 r = b;
+ for (int i = 0; i < 8; i++) {
+ if (r & 1)
+ r = (r >> 1) ^ 0xEDB88320;
+ else
+ r >>= 1;
+ }
+ crc32_table[b] = r;
+ }
+}
+
+/*
+ * Compute the hash code for the next 3-byte sequence in the window.
+ *
+ * @p
+ * A pointer to the next 3-byte sequence in the window.
+ *
+ * Returns the resulting hash code.
+ */
+static inline u32
+lz_bt_hash(const u8 *p)
+{
+ u32 hash = 0;
+
+ hash ^= crc32_table[p[0]];
+ hash ^= p[1];
+ hash ^= (u32)p[2] << 8;
+
+ return hash & LZ_BT_HASH_MASK;
+}
+
+/*
+ * Compute the number of bytes of memory that would be needed to initialize a
+ * binary tree match-finder with the specified maximum window size.
+ *
+ * @max_window_size
+ * The maximum window size, in bytes, to query.
+ *
+ * Returns the number of bytes that would be allocated by lz_bt_init(),
+ * excluding the size of the 'struct lz_bt' itself.
+ */
+u64
+lz_bt_get_needed_memory(lz_bt_pos_t max_window_size)
+{
+ u64 len;
+
+ len = LZ_BT_HASH_SIZE + LZ_BT_DIGRAM_TAB_SIZE;
+ len += 2 * (u64)max_window_size;
+
+ return len * sizeof(lz_bt_pos_t);
+}
+
+/*
+ * Initialize a binary tree match-finder.
+ *
+ * @mf
+ * The match-finder structure to initialize.
+ * @max_window_size
+ * The maximum window size that shall be supported by subsequent calls to
+ * lz_bt_load_window().
+ * @min_match_len
+ * The minimum length of matches that shall be produced by subsequent calls
+ * to lz_bt_get_matches(). This must be at least 2.
+ * @max_match_len
+ * The maximum length of matches that shall be produced by subsequent calls
+ * to lz_bt_get_matches(). This must be at least @min_match_len.
+ * @num_fast_bytes
+ * The maximum length of matches that shall be produced just using the
+ * binary tree search algorithm. If the longest match has this length,
+ * then lz_bt_get_matches() will extend it up to @max_match_len. This must
+ * be at least @min_match_len and no more than @max_match_len.
+ * @max_search_depth
+ * The maximum depth to descend into the binary search tree before halting
+ * the search.
+ *
+ * Returns %true if successful; %false if out of memory.
+ */
+bool
+lz_bt_init(struct lz_bt *mf,
+ lz_bt_pos_t max_window_size,
+ lz_bt_len_t min_match_len,
+ lz_bt_len_t max_match_len,
+ lz_bt_len_t num_fast_bytes,
+ u32 max_search_depth)
+{
+ u64 len;
+
+ /* Check and set parameters. */
+ LZ_ASSERT(min_match_len >= 2);
+ LZ_ASSERT(max_match_len >= min_match_len);
+ LZ_ASSERT(num_fast_bytes >= min_match_len);
+ LZ_ASSERT(num_fast_bytes <= max_match_len);
+
+ mf->max_window_size = max_window_size;
+ mf->min_match_len = min_match_len;
+ mf->max_match_len = max_match_len;
+ mf->num_fast_bytes = num_fast_bytes;
+ mf->max_search_depth = max_search_depth;
+
+ /* Allocate space for 'hash_tab', 'digram_tab', and 'child_tab'. */
+ len = LZ_BT_HASH_SIZE + (2 * (u64)max_window_size);
+ if (mf->min_match_len <= 2)
+ len += LZ_BT_DIGRAM_TAB_SIZE;
+ len *= sizeof(lz_bt_pos_t);
+ if ((size_t)len != len || !(mf->hash_tab = MALLOC(len)))
+ return false;
+ if (mf->min_match_len <= 2) {
+ mf->digram_tab = mf->hash_tab + LZ_BT_HASH_SIZE;
+ mf->child_tab = mf->digram_tab + LZ_BT_DIGRAM_TAB_SIZE;
+ } else {
+ mf->child_tab = mf->hash_tab + LZ_BT_HASH_SIZE;
+ }
+
+ /* Fill in the CRC32 table if not done already. */
+ pthread_once(&crc32_table_filled, crc32_init);
+
+ return true;
+}
+
+/*
+ * Destroy a binary tree match-finder.
+ *
+ * @mf
+ * The match-finder structure to destroy.
+ */
+void
+lz_bt_destroy(struct lz_bt *mf)
+{
+ FREE(mf->hash_tab);
+ /* mf->hash_tab shares storage with mf->digram_tab and mf->child_tab. */
+}
+
+/*
+ * Load a window into a binary tree match-finder.
+ *
+ * @mf
+ * The match-finder structure into which to load the window.
+ * @window
+ * Pointer to the window to load. This memory must remain available,
+ * unmodified, while the match-finder is being used.
+ * @window_size
+ * The size of the window, in bytes. This can't be larger than the
+ * @max_window_size with which lz_bt_init() was called.
+ */
+void
+lz_bt_load_window(struct lz_bt *mf, const u8 *window, lz_bt_pos_t window_size)
+{
+ LZ_ASSERT(window_size <= mf->max_window_size);
+ size_t clear_len;
+
+ mf->cur_window = window;
+ mf->cur_window_pos = 0;
+ mf->cur_window_size = window_size;
+
+ /* Clear the hash and digram tables.
+ * Note: The child table need not be cleared. */
+ clear_len = LZ_BT_HASH_SIZE;
+ if (mf->min_match_len <= 2)
+ clear_len += LZ_BT_DIGRAM_TAB_SIZE;
+ memset(mf->hash_tab, 0, clear_len * sizeof(lz_bt_pos_t));
+}
+
+/*
+ * Search the binary tree of the current hash code for matches. At the same
+ * time, update this tree to add the current position in the window.
+ *
+ * @window
+ * The window being searched.
+ * @cur_window_pos
+ * The current position in the window.
+ * @min_len
+ * Ignore matches shorter than this length. This must be at least 1.
+ * @max_len
+ * Don't produce any matches longer than this length. If we find a match
+ * this long, terminate the search and return.
+ * @max_depth
+ * Stop if we reach this depth in the binary tree.
+ * @child_tab
+ * Table of child pointers for the binary tree. The children of the node
+ * for position 'i' in the window are child_tab[i * 2] and child_tab[i*2 +
+ * 1]. Zero is reserved for the 'null' value (no child). Consequently, we
+ * don't recognize matches beginning at position 0. In fact, the node for
+ * position 0 in the window will not be used at all, which is just as well
+ * because we use 0-based indices which don't work for position 0.
+ * @cur_match
+ * The position in the window at which the binary tree for the current hash
+ * code is rooted. This can be 0, which indicates that the binary tree for
+ * the current hash code is empty.
+ * @matches
+ * The array in which to produce the matches. The matches will be produced
+ * in order of increasing length and increasing offset. No more than one
+ * match shall have any given length, nor shall any match be shorter than
+ * @min_len, nor shall any match be longer than @max_len, nor shall any two
+ * matches have the same offset.
+ *
+ * Returns the number of matches found and written to @matches.
+ */
+static lz_bt_len_t
+do_search(const u8 window[restrict],
+ const lz_bt_pos_t cur_window_pos,
+ const lz_bt_len_t min_len,
+ const lz_bt_len_t max_len,
+ const u32 max_depth,
+ lz_bt_pos_t child_tab[restrict],
+ lz_bt_pos_t cur_match,
+ struct raw_match matches[restrict])
+{
+ /*
+ * Here's my explanation of how this code actually works. Beware: this
+ * algorithm is a *lot* trickier than searching for matches via hash
+ * chains. But it can be significantly better, especially when doing
+ * "optimal" parsing, which is why it gets used, e.g. in LZMA as well as
+ * here.
+ *
+ * ---------------------------------------------------------------------
+ *
+ * Data structure
+ *
+ * Basically, there is not just one binary tree, but rather one binary
+ * tree per hash code. For a given hash code, the binary tree indexes
+ * previous positions in the window that have that same hash code. The
+ * key for each node is the "string", or byte sequence, beginning at the
+ * corresponding position in the window.
+ *
+ * Each tree maintains the invariant that if node C is a child of node
+ * P, then the window position represented by node C is smaller than
+ * ("left of") the window position represented by node P. Equivalently,
+ * while descending into a tree, the match distances ("offsets") from
+ * the current position are non-decreasing --- actually strictly
+ * increasing, because each node represents a unique position.
+ *
+ * In addition, not all previous positions sharing the same hash code
+ * will necessarily be represented in each binary tree; see the
+ * "Updating" section.
+ *
+ * ---------------------------------------------------------------------
+ *
+ * Searching
+ *
+ * Suppose we want to search for LZ77-style matches with the string
+ * beginning at the current window position and extending for @max_len
+ * bytes. To do this, we can search for this string in the binary tree
+ * for this string's hash code. Each node visited during the search is
+ * a potential match. This method will find the matches efficiently
+ * because they will converge on the current string, due to the nature
+ * of the binary search.
+ *
+ * Naively, when visiting a node that represents a match of length N, we
+ * must compare N + 1 bytes in order to determine the length of that
+ * match and the lexicographic ordering of that match relative to the
+ * current string (which determines whether we need to step left or
+ * right into the next level of the tree, as per the standard binary
+ * tree search algorithm). However, as an optimization, we need not
+ * explicitly examine the full length of the match at each node. To see
+ * that this is true, suppose that we examine a node during the search,
+ * and we find that the corresponding match is less (alt. greater) than
+ * the current string. Then, because of how binary tree search
+ * operates, the match must be lexicographically greater (alt. lesser)
+ * than any ancestor node that corresponded to a match lexicographically
+ * lesser (alt. greater) than the current string. Therefore, the match
+ * must be at least as long as the match for any such ancestor node.
+ * Therefore, the lengths of lexicographically-lesser (alt. greater)
+ * matches must be non-decreasing as they are encountered by the tree
+ * search.
+ *
+ * Using this observation, we can maintain two variables,
+ * 'longest_lt_match_len' and 'longest_gt_match_len', that represent the
+ * length of the longest lexicographically lesser and greater,
+ * respectively, match that has been examined so far. Then, when
+ * examining a new match, we need only start comparing at the index
+ * min(longest_lt_match_len, longest_gt_match_len) byte. Note that we
+ * cannot know beforehand whether the match will be lexicographically
+ * lesser or greater, hence the need for taking the minimum of these two
+ * lengths.
+ *
+ * As noted earlier, as we descend into the tree, the potential matches
+ * will have strictly increasing offsets. To make things faster for
+ * higher-level parsing / match-choosing code, we do not want to return
+ * a shorter match that has a larger offset than a longer match. This
+ * is because a longer match can always be truncated to a shorter match
+ * if needed, and smaller offsets usually (depending on the compression
+ * format) take fewer bits to encode than larger offsets.
+ * Consequently, we keep a potential match only if it is longer than the
+ * previous longest match that has been found. This has the added
+ * advantage of producing the array of matches sorted by strictly
+ * increasing lengths as well as strictly decreasing offsets.
+ *
+ * In degenerate cases, the binary tree might become severely
+ * unbalanced. To prevent excessive running times, we stop immediately
+ * (and return any matches that happen to have been found so far) if the
+ * current depth exceeds @max_depth. Note that this cutoff can occur
+ * before the longest match has been found, which is usually bad for the
+ * compression ratio.
+ *
+ * ---------------------------------------------------------------------
+ *
+ * Updating
+ *
+ * I've explained how to find matches by searching the binary tree of
+ * the current hash code. But how do we get the binary tree in the
+ * first place? Since the tree is built incrementally, the real
+ * question is how do we update the tree to "add" the current window
+ * position.
+ *
+ * The tree maintains the invariant that a node's parent always has a
+ * larger position (a.k.a. smaller match offset) than itself.
+ * Therefore, the root node must always have the largest position; and
+ * since the current position is larger than any previous position, the
+ * current position must become the root of the tree.
+ *
+ * A correct, but silly, approach is to simply add the previous root as
+ * a child of the new root, using either the left or right child pointer
+ * depending on the lexicographic ordering of the strings. This works,
+ * but it really just produces a linked list, so it's not sufficient.
+ *
+ * Instead, we can initially mark the new root's left child pointer as
+ * "pending (less than)" and its right child pointer as "pending
+ * (greater than)". Then, during the search, when we examine a match
+ * that is lexicographically less than the current string, we link the
+ * "pending (less than)" pointer to the node of that match, then set the
+ * right child pointer of *that* node as "pending (less than)".
+ * Similarly, when we examine a match that is lexicographically greater
+ * than the current string, we link the "pending (greater than)" pointer
+ * to the node of that match, then set the left child pointer of *that*
+ * node as "pending (greater than)".
+ *
+ * If the search terminates before the current string is found (up to a
+ * precision of @max_len bytes), then we set "pending (less than)" and
+ * "pending (greater than)" to point to nothing. Alternatively, if the
+ * search terminates due to finding the current string (up to a
+ * precision of @max_len bytes), then we set "pending (less than)" and
+ * "pending (greater than)" to point to the appropriate children of that
+ * match.
+ *
+ * Why does this work? Well, we can think of it this way: the "pending
+ * (less than)" pointer is reserved for the next match we find that is
+ * lexicographically *less than* the current string, and the "pending
+ * (greater than)" pointer is reserved for the next match we find that
+ * is lexicographically *greater than* the current string. This
+ * explains why when we find a match that is lexicographically less than
+ * the current string, we set the "pending (less than)" pointer to point
+ * to that match. And the reason we change "pending (less than)" to the
+ * right pointer of the match in that case is because we're walking down
+ * into that subtree, and the next match lexicographically *less than*
+ * the current string is guaranteed to be lexicographically *greater
+ * than* that match, so it should be set as the right subtree of that
+ * match. But the next match in that subtree that is lexicographically
+ * *greater than* the current string will need to be moved to the
+ * "pending (greater than)" pointer farther up the tree.
+ *
+ * It's complicated, but it should make sense if you think about it.
+ * The algorithm basically just moves subtrees into the correct
+ * locations as it walks down the tree for the search. But also, if the
+ * algorithm actually finds a match of length @max_len with the current
+ * string, it no longer needs that match node and can discard it. The
+ * algorithm also will discard nodes if the search terminates due to the
+ * depth limit. For these reasons, the binary tree might not, in fact,
+ * contain all valid positions.
+ */
+
+ lz_bt_len_t num_matches = 0;
+ lz_bt_len_t longest_lt_match_len = 0;
+ lz_bt_len_t longest_gt_match_len = 0;
+ lz_bt_len_t longest_match_len = min_len - 1;
+ lz_bt_pos_t *pending_lt_ptr = &child_tab[cur_window_pos * 2 + 0];
+ lz_bt_pos_t *pending_gt_ptr = &child_tab[cur_window_pos * 2 + 1];
+ const u8 *strptr = &window[cur_window_pos];
+ u32 depth_remaining = max_depth;
+ for (;;) {
+ const u8 *matchptr;
+ lz_bt_len_t len;
+
+ if (depth_remaining-- == 0 || cur_match == 0) {
+ *pending_lt_ptr = 0;
+ *pending_gt_ptr = 0;
+ return num_matches;
+ }
+
+ matchptr = &window[cur_match];
+ len = min(longest_lt_match_len, longest_gt_match_len);
+
+ if (matchptr[len] == strptr[len]) {
+
+ while (++len != max_len)
+ if (matchptr[len] != strptr[len])
+ break;
+
+ if (len > longest_match_len) {
+ longest_match_len = len;
+
+ matches[num_matches++] = (struct raw_match) {
+ .len = len,
+ .offset = cur_window_pos - cur_match,
+ };
+
+ if (len == max_len) {
+ *pending_lt_ptr = child_tab[cur_match * 2 + 0];
+ *pending_gt_ptr = child_tab[cur_match * 2 + 1];
+ return num_matches;
+ }
+ }
+ }
+
+ if (matchptr[len] < strptr[len]) {
+ *pending_lt_ptr = cur_match;
+ pending_lt_ptr = &child_tab[cur_match * 2 + 1];
+ cur_match = *pending_lt_ptr;
+ longest_lt_match_len = len;
+ } else {
+ *pending_gt_ptr = cur_match;
+ pending_gt_ptr = &child_tab[cur_match * 2 + 0];
+ cur_match = *pending_gt_ptr;
+ longest_gt_match_len = len;
+ }
+ }
+}
+
+/*
+ * Retrieve a list of matches at the next position in the window.
+ *
+ * @mf
+ * The binary tree match-finder structure into which a window has been
+ * loaded using lz_bt_load_window().
+ * @matches
+ * The array into which the matches will be returned. The length of this
+ * array must be at least (@mf->num_fast_bytes - @mf->min_match_len + 1).
+ *
+ * The return value is the number of matches that were found and stored in the
+ * 'matches' array. The matches will be ordered by strictly increasing length
+ * and strictly increasing offset. No match shall have length less than
+ * @min_match_len, and no match shall have length greater than @max_match_len.
+ * The return value may be 0, which indicates that no matches were found.
+ *
+ * On completion, the binary tree match-finder is advanced to the next position
+ * in the window.
+ */
+lz_bt_len_t
+lz_bt_get_matches(struct lz_bt *mf, struct raw_match matches[])
+{
+ lz_bt_pos_t bytes_remaining;
+ lz_bt_len_t num_matches;
+ lz_bt_pos_t cur_match;
+ u32 hash;
+
+ LZ_ASSERT(mf->cur_window_pos < mf->cur_window_size);
+
+ bytes_remaining = lz_bt_get_remaining_size(mf);
+
+ /* If there are fewer than 3 bytes remaining, we can't even compute a
+ * hash to look up a binary tree root. If there are exactly 2 bytes
+ * remaining we could still search for a length-2 match using the digram
+ * table, but it's not worth bothering. (Note: this is also useful for
+ * LZX, since this excludes the length 2 match having the maximum
+ * offset, which isn't allowed.) */
+ if (bytes_remaining < 3) {
+ mf->cur_window_pos++;
+ return 0;
+ }
+
+ num_matches = 0;
+
+ /* Search the digram table for a length 2 match. */
+ if (mf->min_match_len <= 2) {
+ u8 c1, c2;
+ u16 digram;
+
+ c1 = mf->cur_window[mf->cur_window_pos];
+ c2 = mf->cur_window[mf->cur_window_pos + 1];
+ digram = (u16)c1 | ((u16)c2 << 8);
+ cur_match = mf->digram_tab[digram];
+ mf->digram_tab[digram] = mf->cur_window_pos;
+
+ /* We're only interested in matches of length exactly 2, since
+ * those won't be found during the binary tree search. */
+ if (cur_match != 0 && mf->cur_window[cur_match + 2] !=
+ mf->cur_window[mf->cur_window_pos + 2])
+ {
+ matches[num_matches++] = (struct raw_match) {
+ .len = 2,
+ .offset = mf->cur_window_pos - cur_match,
+ };
+ }
+ }
+
+ /* Hash the length-3 byte sequence beginning at the current position in
+ * the window. */
+ hash = lz_bt_hash(&mf->cur_window[mf->cur_window_pos]);
+
+ /* The corresponding hash bucket in 'hash_tab' contains the root of the
+ * binary tree of previous window positions that have the same hash
+ * code. */
+ cur_match = mf->hash_tab[hash];
+
+ /* Update the hash bucket to point to the binary tree rooted at the
+ * current position, which we will construct in do_search(). */
+ mf->hash_tab[hash] = mf->cur_window_pos;
+
+ /* Search the binary tree for matches. At the same time, build the
+ * binary tree rooted at the current position, which replaces the one we
+ * search. */
+ num_matches += do_search(mf->cur_window,
+ mf->cur_window_pos,
+ max(3, mf->min_match_len),
+ min(bytes_remaining, mf->num_fast_bytes),
+ mf->max_search_depth,
+ mf->child_tab,
+ cur_match,
+ &matches[num_matches]);
+
+ /* If the longest match is @num_fast_bytes in length, it may have been
+ * truncated. Try extending it up to the maximum match length. */
+ if (num_matches != 0 && matches[num_matches - 1].len == mf->num_fast_bytes) {
+ lz_bt_pos_t limit;
+ const u8 *strptr, *matchptr;
+ lz_bt_len_t len;
+
+ limit = min(bytes_remaining, mf->max_match_len);
+ strptr = &mf->cur_window[mf->cur_window_pos];
+ matchptr = strptr - matches[num_matches - 1].offset;
+ len = matches[num_matches - 1].len;
+ while (len < limit && strptr[len] == matchptr[len])
+ len++;
+ matches[num_matches - 1].len = len;
+ }
+
+#ifdef ENABLE_LZ_DEBUG
+ /* Check the matches. */
+ for (lz_bt_len_t i = 0; i < num_matches; i++) {
+ const u8 *matchptr, *strptr;
+
+ /* Length valid? */
+ LZ_ASSERT(matches[i].len >= mf->min_match_len);
+ LZ_ASSERT(matches[i].len <= min(mf->max_match_len, bytes_remaining));
+
+ /* Offset valid? */
+ LZ_ASSERT(matches[i].offset >= 1);
+ LZ_ASSERT(matches[i].offset <= lz_bt_get_position(mf));
+
+ /* Lengths and offsets strictly increasing? */
+ if (i > 0) {
+ LZ_ASSERT(matches[i].len > matches[i - 1].len);
+ LZ_ASSERT(matches[i].offset > matches[i - 1].offset);
+ }
+
+ /* Actually a match? */
+ strptr = lz_bt_get_window_ptr(mf);
+ matchptr = strptr - matches[i].offset;
+ LZ_ASSERT(!memcmp(strptr, matchptr, matches[i].len));
+
+ /* Match can't be extended further? */
+ LZ_ASSERT(matches[i].len == min(mf->max_match_len, bytes_remaining) ||
+ strptr[matches[i].len] != matchptr[matches[i].len]);
+ }
+#endif /* ENABLE_LZ_DEBUG */
+
+ /* Advance to the next position in the window. */
+ mf->cur_window_pos++;
+
+ /* Return the number of matches found. */
+ return num_matches;
+}
+
+/* This is the same as do_search(), but it does not save any matches.
+ * See do_search() for explanatory comments. */
+static void
+do_skip(const u8 window[restrict],
+ const lz_bt_pos_t cur_window_pos,
+ const lz_bt_len_t max_len,
+ u32 depth_remaining,
+ lz_bt_pos_t child_tab[restrict],
+ lz_bt_pos_t cur_match)
+{
+ lz_bt_len_t longest_lt_match_len = 0;
+ lz_bt_len_t longest_gt_match_len = 0;
+ lz_bt_pos_t *pending_lt_ptr = &child_tab[cur_window_pos * 2 + 0];
+ lz_bt_pos_t *pending_gt_ptr = &child_tab[cur_window_pos * 2 + 1];
+ const u8 * const strptr = &window[cur_window_pos];
+ for (;;) {
+ const u8 *matchptr;
+ lz_bt_len_t len;
+
+ if (depth_remaining-- == 0 || cur_match == 0) {
+ *pending_lt_ptr = 0;
+ *pending_gt_ptr = 0;
+ return;
+ }
+
+ matchptr = &window[cur_match];
+ len = min(longest_lt_match_len, longest_gt_match_len);
+
+ if (matchptr[len] == strptr[len]) {
+ do {
+ if (++len == max_len) {
+ *pending_lt_ptr = child_tab[cur_match * 2 + 0];
+ *pending_gt_ptr = child_tab[cur_match * 2 + 1];
+ return;
+ }
+ } while (matchptr[len] == strptr[len]);
+ }
+ if (matchptr[len] < strptr[len]) {
+ *pending_lt_ptr = cur_match;
+ pending_lt_ptr = &child_tab[cur_match * 2 + 1];
+ cur_match = *pending_lt_ptr;
+ longest_lt_match_len = len;
+ } else {
+ *pending_gt_ptr = cur_match;
+ pending_gt_ptr = &child_tab[cur_match * 2 + 0];
+ cur_match = *pending_gt_ptr;
+ longest_gt_match_len = len;
+ }
+ }
+}
+
+/* Skip the current position in the binary tree match-finder. */
+static void
+lz_bt_skip_position(struct lz_bt *mf)
+{
+ lz_bt_pos_t bytes_remaining;
+ u32 hash;
+ lz_bt_pos_t cur_match;
+
+ LZ_ASSERT(mf->cur_window_pos < mf->cur_window_size);
+
+ bytes_remaining = lz_bt_get_remaining_size(mf);
+
+ /* As explained in lz_bt_get_matches(), we don't search for matches if
+ * there are fewer than 3 bytes remaining in the window. */
+ if (bytes_remaining < 3) {
+ mf->cur_window_pos++;
+ return;
+ }
+
+ /* Update the digram table. */
+ if (mf->min_match_len <= 2) {
+ u8 c1, c2;
+ u16 digram;
+
+ c1 = mf->cur_window[mf->cur_window_pos];
+ c2 = mf->cur_window[mf->cur_window_pos + 1];
+ digram = (u16)c1 | ((u16)c2 << 8);
+ mf->digram_tab[digram] = mf->cur_window_pos;
+ }
+
+ /* Update the hash table. */
+ hash = lz_bt_hash(&mf->cur_window[mf->cur_window_pos]);
+ cur_match = mf->hash_tab[hash];
+ mf->hash_tab[hash] = mf->cur_window_pos;
+
+ /* Update the binary tree for the appropriate hash code. */
+ do_skip(mf->cur_window,
+ mf->cur_window_pos,
+ min(bytes_remaining, mf->num_fast_bytes),
+ mf->max_search_depth,
+ mf->child_tab,
+ cur_match);
+
+ /* Advance to the next position. */
+ mf->cur_window_pos++;
+}
+
+/* Skip 'n' positions in the binary tree match-finder. */
+void
+lz_bt_skip_positions(struct lz_bt *mf, unsigned n)
+{
+ while (n--)
+ lz_bt_skip_position(mf);
+}
projects / wimlib / blobdiff