/*
 * bt_matchfinder.h
 *
 * Author:	Eric Biggers
 * Year:	2014, 2015
 *
 * The author dedicates this file to the public domain.
 * You can do whatever you want with this file.
 *
 * ----------------------------------------------------------------------------
 *
 * 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 3 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 3 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 <string.h>

#include "wimlib/lz_extend.h"
#include "wimlib/lz_hash.h"

#define BT_MATCHFINDER_HASH3_ORDER 16

/* TEMPLATED functions and structures have MF_SUFFIX appended to their name.  */
#undef TEMPLATED
#define TEMPLATED(name)		CONCAT(name, MF_SUFFIX)

#ifndef _WIMLIB_BT_MATCHFINDER_H
#define _WIMLIB_BT_MATCHFINDER_H

/* Non-templated definitions  */

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

	/* The number of bytes matched.  */
	u32 length;

	/* The offset back from the current position that was matched.  */
	u32 offset;
};

#endif /* _WIMLIB_BT_MATCHFINDER_H */

struct TEMPLATED(bt_matchfinder) {

	/* The hash table for finding length 2 matches, if enabled  */
#ifdef BT_MATCHFINDER_HASH2_ORDER
	mf_pos_t hash2_tab[1UL << BT_MATCHFINDER_HASH2_ORDER];
#endif

	/* The hash table which contains the roots of the binary trees for
	 * finding length 3 matches  */
	mf_pos_t hash3_tab[1UL << BT_MATCHFINDER_HASH3_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[];
};

/* Return the number of bytes that must be allocated for a 'bt_matchfinder' that
 * can work with buffers up to the specified size.  */
static inline size_t
TEMPLATED(bt_matchfinder_size)(size_t max_bufsize)
{
	return sizeof(struct TEMPLATED(bt_matchfinder)) +
		(2 * max_bufsize * sizeof(mf_pos_t));
}

/* Prepare the matchfinder for a new input buffer.  */
static inline void
TEMPLATED(bt_matchfinder_init)(struct TEMPLATED(bt_matchfinder) *mf)
{
	memset(mf, 0, sizeof(*mf));
}

static inline mf_pos_t *
TEMPLATED(bt_left_child)(struct TEMPLATED(bt_matchfinder) *mf, u32 node)
{
	return &mf->child_tab[(node << 1) + 0];
}

static inline mf_pos_t *
TEMPLATED(bt_right_child)(struct TEMPLATED(bt_matchfinder) *mf, u32 node)
{
	return &mf->child_tab[(node << 1) + 1];
}

/* Advance the binary tree matchfinder by one byte, optionally recording
 * matches.  @record_matches should be a compile-time constant.  */
static inline struct lz_match *
TEMPLATED(bt_matchfinder_advance_one_byte)(struct TEMPLATED(bt_matchfinder) * const restrict mf,
					   const u8 * const restrict in_begin,
					   const ptrdiff_t cur_pos,
					   const u32 max_len,
					   const u32 nice_len,
					   const u32 max_search_depth,
					   u32 * const restrict next_hash,
					   u32 * const restrict best_len_ret,
					   struct lz_match * restrict lz_matchptr,
					   const bool record_matches)
{
	const u8 *in_next = in_begin + cur_pos;
	u32 depth_remaining = max_search_depth;
#ifdef BT_MATCHFINDER_HASH2_ORDER
	u16 seq2;
	u32 hash2;
#endif
	u32 hash3;
	u32 cur_node;
	const u8 *matchptr;
	mf_pos_t *pending_lt_ptr, *pending_gt_ptr;
	u32 best_lt_len, best_gt_len;
	u32 len;
	u32 best_len = 2;

	if (unlikely(max_len < LZ_HASH3_REQUIRED_NBYTES + 1)) {
		*best_len_ret = best_len;
		return lz_matchptr;
	}

	hash3 = *next_hash;
	*next_hash = lz_hash(load_u24_unaligned(in_next + 1), BT_MATCHFINDER_HASH3_ORDER);
	prefetchw(&mf->hash3_tab[*next_hash]);

#ifdef BT_MATCHFINDER_HASH2_ORDER
	seq2 = load_u16_unaligned(in_next);
	hash2 = lz_hash(seq2, BT_MATCHFINDER_HASH2_ORDER);
	cur_node = mf->hash2_tab[hash2];
	mf->hash2_tab[hash2] = cur_pos;
	if (record_matches &&
	    seq2 == load_u16_unaligned(&in_begin[cur_node]) &&
	    likely(in_next != in_begin))
	{
		lz_matchptr->length = 2;
		lz_matchptr->offset = in_next - &in_begin[cur_node];
		lz_matchptr++;
	}
#endif

	cur_node = mf->hash3_tab[hash3];
	mf->hash3_tab[hash3] = cur_pos;

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

	if (!cur_node) {
		*pending_lt_ptr = 0;
		*pending_gt_ptr = 0;
		*best_len_ret = best_len;
		return lz_matchptr;
	}

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

	for (;;) {
		matchptr = &in_begin[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 = *TEMPLATED(bt_left_child)(mf, cur_node);
					*pending_gt_ptr = *TEMPLATED(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 = TEMPLATED(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 = TEMPLATED(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 || !--depth_remaining) {
			*pending_lt_ptr = 0;
			*pending_gt_ptr = 0;
			*best_len_ret = best_len;
			return lz_matchptr;
		}
	}
}

/*
 * Retrieve a list of matches with the current position.
 *
 * @mf
 *	The matchfinder structure.
 * @in_begin
 *	Pointer to the beginning of the input buffer.
 * @cur_pos
 *	The current position in the input buffer (the position of the sequence
 *	being matched against).
 * @max_len
 *	The maximum permissible match length at this position.
 * @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_hash
 *	Pointer to the hash code for the current sequence, which was computed
 *	one position in advance so that the binary tree root could be
 *	prefetched.  This is an input/output parameter.
 * @best_len_ret
 *	If a match of length >= 3 was found, then the length of the longest such
 *	match is written here; otherwise 2 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 increasing
 *	offset.  The maximum number of matches that may be found is
 *	'min(nice_len, max_len) - 2 + 1', or one less if length 2 matches are
 *	disabled.
 *
 * 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 inline struct lz_match *
TEMPLATED(bt_matchfinder_get_matches)(struct TEMPLATED(bt_matchfinder) *mf,
				      const u8 *in_begin,
				      ptrdiff_t cur_pos,
				      u32 max_len,
				      u32 nice_len,
				      u32 max_search_depth,
				      u32 *next_hash,
				      u32 *best_len_ret,
				      struct lz_match *lz_matchptr)
{
	return TEMPLATED(bt_matchfinder_advance_one_byte)(mf,
							  in_begin,
							  cur_pos,
							  max_len,
							  nice_len,
							  max_search_depth,
							  next_hash,
							  best_len_ret,
							  lz_matchptr,
							  true);
}

/*
 * Advance the matchfinder, but don't record any matches.
 *
 * @mf
 *	The matchfinder structure.
 * @in_begin
 *	Pointer to the beginning of the input buffer.
 * @cur_pos
 *	The current position in the input buffer.
 * @max_len
 *	The maximum permissible match length at this position.
 * @nice_len
 *	Stop searching if a match of at least this length is found.
 * @max_search_depth
 *	Limit on the number of potential matches to consider.
 * @next_hash
 *	Pointer to the hash code for the current sequence, which was computed
 *	one position in advance so that the binary tree root could be
 *	prefetched.  This is an input/output parameter.
 *
 * Note: this is very similar to bt_matchfinder_get_matches() because both
 * functions must do hashing and tree re-rooting.  This version just doesn't
 * actually record any matches.
 */
static inline void
TEMPLATED(bt_matchfinder_skip_position)(struct TEMPLATED(bt_matchfinder) *mf,
					const u8 *in_begin,
					ptrdiff_t cur_pos,
					u32 max_len,
					u32 nice_len,
					u32 max_search_depth,
					u32 *next_hash)
{
	u32 best_len;
	TEMPLATED(bt_matchfinder_advance_one_byte)(mf,
						   in_begin,
						   cur_pos,
						   max_len,
						   nice_len,
						   max_search_depth,
						   next_hash,
						   &best_len,
						   NULL,
						   false);
}