libdeflate/lib/bt_matchfinder.h

/*
 * bt_matchfinder.h - Lempel-Ziv matchfinding with a hash table of binary trees
 *
 * Written in 2014-2016 by Eric Biggers <ebiggers3@gmail.com>
 *
 * To the extent possible under law, the author(s) have dedicated all copyright
 * and related and neighboring rights to this software to the public domain
 * worldwide. This software is distributed without any warranty.
 *
 * You should have received a copy of the CC0 Public Domain Dedication along
 * with this software. If not, see
 * <http://creativecommons.org/publicdomain/zero/1.0/>.
 *
 * ----------------------------------------------------------------------------
 *
 * This is a Binary Trees (bt) based matchfinder.
 *
 * The main data structure is a hash table where each hash bucket contains a
 * binary tree of sequences whose first 4 bytes share the same hash code.  Each
 * sequence is identified by its starting position in the input buffer.  Each
 * binary tree is always sorted such that each left child represents a sequence
 * lexicographically lesser than its parent and each right child represents a
 * sequence lexicographically greater than its parent.
 *
 * 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, a new binary tree
 * node is created to represent the current sequence.  Then, in a single tree
 * traversal, the hash bucket's binary tree is searched for matches and is
 * re-rooted at the new node.
 *
 * Compared to the simpler algorithm that uses linked lists instead of binary
 * trees (see hc_matchfinder.h), the binary tree version gains more information
 * at each node visitation.  Ideally, the binary tree version will examine only
 * 'log(n)' nodes to find the same matches that the linked list version will
 * find by examining 'n' nodes.  In addition, the binary tree version can
 * examine fewer bytes at each node by taking advantage of the common prefixes
 * that result from the sort order, whereas the linked list version may have to
 * examine up to the full length of the match at each node.
 *
 * However, it is not always best to use the binary tree version.  It requires
 * nearly twice as much memory as the linked list version, and it takes time to
 * keep the binary trees sorted, even at positions where the compressor does not
 * need matches.  Generally, when doing fast compression on small buffers,
 * binary trees are the wrong approach.  They are best suited for thorough
 * compression and/or large buffers.
 *
 * ----------------------------------------------------------------------------
 */


#include "matchfinder_common.h"

#define BT_MATCHFINDER_HASH3_ORDER 16
#define BT_MATCHFINDER_HASH3_WAYS  2
#define BT_MATCHFINDER_HASH4_ORDER 16

#define BT_MATCHFINDER_TOTAL_HASH_LENGTH		\
	((1UL << BT_MATCHFINDER_HASH3_ORDER) * BT_MATCHFINDER_HASH3_WAYS + \
	 (1UL << BT_MATCHFINDER_HASH4_ORDER))

/* Representation of a match found by the bt_matchfinder  */
struct lz_match {

	/* The number of bytes matched.  */
	u16 length;

	/* The offset back from the current position that was matched.  */
	u16 offset;
};

struct bt_matchfinder {

	/* The hash table for finding length 3 matches  */
	mf_pos_t hash3_tab[1UL << BT_MATCHFINDER_HASH3_ORDER][BT_MATCHFINDER_HASH3_WAYS];

	/* The hash table which contains the roots of the binary trees for
	 * finding length 4+ matches  */
	mf_pos_t hash4_tab[1UL << BT_MATCHFINDER_HASH4_ORDER];

	/* The child node references for the binary trees.  The left and right
	 * children of the node for the sequence with position 'pos' are
	 * 'child_tab[pos * 2]' and 'child_tab[pos * 2 + 1]', respectively.  */
	mf_pos_t child_tab[2UL * MATCHFINDER_WINDOW_SIZE];

}
#ifdef _aligned_attribute
_aligned_attribute(MATCHFINDER_ALIGNMENT)
#endif
;

/* Prepare the matchfinder for a new input buffer.  */
static forceinline void
bt_matchfinder_init(struct bt_matchfinder *mf)
{
	matchfinder_init((mf_pos_t *)mf, BT_MATCHFINDER_TOTAL_HASH_LENGTH);
}

static forceinline void
bt_matchfinder_slide_window(struct bt_matchfinder *mf)
{
	matchfinder_rebase((mf_pos_t *)mf,
			   sizeof(struct bt_matchfinder) / sizeof(mf_pos_t));
}

static forceinline mf_pos_t *
bt_left_child(struct bt_matchfinder *mf, s32 node)
{
	return &mf->child_tab[2 * (node & (MATCHFINDER_WINDOW_SIZE - 1)) + 0];
}

static forceinline mf_pos_t *
bt_right_child(struct bt_matchfinder *mf, s32 node)
{
	return &mf->child_tab[2 * (node & (MATCHFINDER_WINDOW_SIZE - 1)) + 1];
}

/* The minimum permissible value of 'max_len' for bt_matchfinder_get_matches()
 * and bt_matchfinder_skip_position().  There must be sufficiently many bytes
 * remaining to load a 32-bit integer from the *next* position.  */
#define BT_MATCHFINDER_REQUIRED_NBYTES	5

/* Advance the binary tree matchfinder by one byte, optionally recording
 * matches.  @record_matches should be a compile-time constant.  */
static forceinline struct lz_match *
bt_matchfinder_advance_one_byte(struct bt_matchfinder * const restrict mf,
				const u8 * const restrict in_base,
				const ptrdiff_t cur_pos,
				const u32 max_len,
				const u32 nice_len,
				const u32 max_search_depth,
				u32 * const restrict next_hashes,
				u32 * const restrict best_len_ret,
				struct lz_match * restrict lz_matchptr,
				const bool record_matches)
{
	const u8 *in_next = in_base + cur_pos;
	u32 depth_remaining = max_search_depth;
	const s32 cutoff = cur_pos - MATCHFINDER_WINDOW_SIZE;
	u32 next_seq4;
	u32 next_seq3;
	u32 hash3;
	u32 hash4;
	s32 cur_node;
#if BT_MATCHFINDER_HASH3_WAYS >= 2
	s32 cur_node_2;
#endif
	const u8 *matchptr;
	mf_pos_t *pending_lt_ptr, *pending_gt_ptr;
	u32 best_lt_len, best_gt_len;
	u32 len;
	u32 best_len = 3;

	STATIC_ASSERT(BT_MATCHFINDER_HASH3_WAYS >= 1 &&
		      BT_MATCHFINDER_HASH3_WAYS <= 2);

	next_seq4 = load_u32_unaligned(in_next + 1);
	next_seq3 = loaded_u32_to_u24(next_seq4);

	hash3 = next_hashes[0];
	hash4 = next_hashes[1];

	next_hashes[0] = lz_hash(next_seq3, BT_MATCHFINDER_HASH3_ORDER);
	next_hashes[1] = lz_hash(next_seq4, BT_MATCHFINDER_HASH4_ORDER);
	prefetchw(&mf->hash3_tab[next_hashes[0]]);
	prefetchw(&mf->hash4_tab[next_hashes[1]]);

	cur_node = mf->hash3_tab[hash3][0];
	mf->hash3_tab[hash3][0] = cur_pos;
#if BT_MATCHFINDER_HASH3_WAYS >= 2
	cur_node_2 = mf->hash3_tab[hash3][1];
	mf->hash3_tab[hash3][1] = cur_node;
#endif
	if (record_matches && cur_node > cutoff) {
		u32 seq3 = load_u24_unaligned(in_next);
		if (seq3 == load_u24_unaligned(&in_base[cur_node])) {
			lz_matchptr->length = 3;
			lz_matchptr->offset = in_next - &in_base[cur_node];
			lz_matchptr++;
		}
	#if BT_MATCHFINDER_HASH3_WAYS >= 2
		else if (cur_node_2 > cutoff &&
			seq3 == load_u24_unaligned(&in_base[cur_node_2]))
		{
			lz_matchptr->length = 3;
			lz_matchptr->offset = in_next - &in_base[cur_node_2];
			lz_matchptr++;
		}
	#endif
	}

	cur_node = mf->hash4_tab[hash4];
	mf->hash4_tab[hash4] = cur_pos;

	pending_lt_ptr = bt_left_child(mf, cur_pos);
	pending_gt_ptr = bt_right_child(mf, cur_pos);

	if (cur_node <= cutoff) {
		*pending_lt_ptr = MATCHFINDER_INITVAL;
		*pending_gt_ptr = MATCHFINDER_INITVAL;
		*best_len_ret = best_len;
		return lz_matchptr;
	}

	best_lt_len = 0;
	best_gt_len = 0;
	len = 0;

	for (;;) {
		matchptr = &in_base[cur_node];

		if (matchptr[len] == in_next[len]) {
			len = lz_extend(in_next, matchptr, len + 1,
					(record_matches ? max_len : nice_len));
			if (!record_matches || len > best_len) {
				if (record_matches) {
					best_len = len;
					lz_matchptr->length = len;
					lz_matchptr->offset = in_next - matchptr;
					lz_matchptr++;
				}
				if (len >= nice_len) {
					*pending_lt_ptr = *bt_left_child(mf, cur_node);
					*pending_gt_ptr = *bt_right_child(mf, cur_node);
					*best_len_ret = best_len;
					return lz_matchptr;
				}
			}
		}

		if (matchptr[len] < in_next[len]) {
			*pending_lt_ptr = cur_node;
			pending_lt_ptr = bt_right_child(mf, cur_node);
			cur_node = *pending_lt_ptr;
			best_lt_len = len;
			if (best_gt_len < len)
				len = best_gt_len;
		} else {
			*pending_gt_ptr = cur_node;
			pending_gt_ptr = bt_left_child(mf, cur_node);
			cur_node = *pending_gt_ptr;
			best_gt_len = len;
			if (best_lt_len < len)
				len = best_lt_len;
		}

		if (cur_node <= cutoff || !--depth_remaining) {
			*pending_lt_ptr = MATCHFINDER_INITVAL;
			*pending_gt_ptr = MATCHFINDER_INITVAL;
			*best_len_ret = best_len;
			return lz_matchptr;
		}
	}
}

/*
 * Retrieve a list of matches with the current position.
 *
 * @mf
 *	The matchfinder structure.
 * @in_base
 *	Pointer to the next byte in the input buffer to process _at the last
 *	time bt_matchfinder_init() or bt_matchfinder_slide_window() was called_.
 * @cur_pos
 *	The current position in the input buffer relative to @in_base (the
 *	position of the sequence being matched against).
 * @max_len
 *	The maximum permissible match length at this position.  Must be >=
 *	BT_MATCHFINDER_REQUIRED_NBYTES.
 * @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.
 * @best_len_ret
 *	If a match of length >= 4 was found, then the length of the longest such
 *	match is written here; otherwise 3 is written here.  (Note: this is
 *	redundant with the 'struct lz_match' array, but this is easier for the
 *	compiler to optimize when inlined and the caller immediately does a
 *	check against 'best_len'.)
 * @lz_matchptr
 *	An array in which this function will record the matches.  The recorded
 *	matches will be sorted by strictly increasing length and (non-strictly)
 *	increasing offset.  The maximum number of matches that may be found is
 *	'nice_len - 2'.
 *
 * The return value is a pointer to the next available slot in the @lz_matchptr
 * array.  (If no matches were found, this will be the same as @lz_matchptr.)
 */
static forceinline struct lz_match *
bt_matchfinder_get_matches(struct bt_matchfinder *mf,
			   const u8 *in_base,
			   ptrdiff_t cur_pos,
			   u32 max_len,
			   u32 nice_len,
			   u32 max_search_depth,
			   u32 next_hashes[2],
			   u32 *best_len_ret,
			   struct lz_match *lz_matchptr)
{
	return bt_matchfinder_advance_one_byte(mf,
					       in_base,
					       cur_pos,
					       max_len,
					       nice_len,
					       max_search_depth,
					       next_hashes,
					       best_len_ret,
					       lz_matchptr,
					       true);
}

/*
 * Advance the matchfinder, but don't record any matches.
 *
 * This is very similar to bt_matchfinder_get_matches() because both functions
 * must do hashing and tree re-rooting.
 */
static forceinline void
bt_matchfinder_skip_position(struct bt_matchfinder *mf,
			     const u8 *in_base,
			     ptrdiff_t cur_pos,
			     u32 max_len,
			     u32 nice_len,
			     u32 max_search_depth,
			     u32 next_hashes[2])
{
	u32 best_len;
	bt_matchfinder_advance_one_byte(mf,
					in_base,
					cur_pos,
					max_len,
					nice_len,
					max_search_depth,
					next_hashes,
					&best_len,
					NULL,
					false);
}