lz_sarray: Performance and memory usage optimizations
[wimlib] / include / wimlib / lz_sarray.h
index 6e06e42..6f878dd 100644 (file)
@@ -51,8 +51,16 @@ typedef u16 lz_sarray_len_t;
 /* Cost type, for the user-provided match cost evaluation function.  */
 typedef lz_sarray_pos_t lz_sarray_cost_t;
 
+/* Type of distances in suffix array links.  A larger type would allow skipping
+ * irrelevant suffixes more quickly, which is especially helpful towards the
+ * start of the window.  However, even a single byte allows skipping 255 at a
+ * time, which where it matters is already a big improvement over the
+ * alternative of searching the suffixes consecutively.  */
+typedef u8 lz_sarray_delta_t;
+
 #define LZ_SARRAY_LEN_MAX      ((lz_sarray_len_t)~0UL)
 #define LZ_SARRAY_POS_MAX      ((lz_sarray_pos_t)~0UL)
+#define LZ_SARRAY_DELTA_MAX    ((lz_sarray_delta_t)~0UL)
 #define LZ_SARRAY_INFINITE_COST        ((lz_sarray_cost_t)~0UL)
 
 /* State of the suffix array LZ (Lempel-Ziv) match-finder.
@@ -103,48 +111,80 @@ struct lz_sarray {
         * used to keep track of which rank suffixes in the suffix array appear
         * before the current position.  Instead of searching in the original
         * suffix array, scans for matches at a given position traverse a linked
-        * list containing only suffixes that appear before that position.  */
+        * list containing (usually) only suffixes that appear before that
+        * position.  */
        struct salink *salink;
 };
 
-/* Suffix array link; one of these exists for each position in the suffix array.
- */
+/* Suffix array link.  An array of these structures, one per suffix rank, is
+ * used as a replacement for the raw LCP (Longest Common Prefix) array to allow
+ * skipping over suffixes that appear later in the window and hence cannot be
+ * used as LZ77 matches.  */
 struct salink {
-       /* Rank of highest ranked suffix that has rank lower than the suffix
-        * corresponding to this structure and either has a lower position
-        * (initially) or has a position lower than the highest position at
-        * which matches have been searched for so far, or LZ_SARRAY_POS_MAX if
-        * there is no such suffix.
-        *
-        * Think of this as a pointer to the closest position in the suffix
-        * array to the left that corresponds to a suffix that begins at a
-        * position in the current dictionary (i.e. before the current position
-        * in the window).  */
-       lz_sarray_pos_t prev;
-
-       /* Rank of lowest ranked suffix that has rank greater than the suffix
-        * corresponding to this structure and either has a lower position
-        * (intially) or has a position lower than the highest position at which
-        * matches have been searched for so far, or LZ_SARRAY_POS_MAX if there
-        * is no such suffix.
-
-        * Think of this as a pointer to the closest position in the suffix
-        * array to the right that corresponds to a suffix that begins at a
-        * position in the current dictionary (i.e. before the current position
-        * in the window).  */
-       lz_sarray_pos_t next;
-
-       /* Length of longest common prefix between the suffix corresponding to
-        * this structure and the suffix with rank @prev, or 0 if @prev is
-        * LZ_SARRAY_POS_MAX.  Capped to the maximum match length.  */
-       lz_sarray_len_t lcpprev;
-
-       /* Length of longest common prefix between the suffix corresponding to
-        * this structure and the suffix with rank @next, or 0 if @next is
-        * LZ_SARRAY_POS_MAX.  Capped to the maximum match length.  */
-       lz_sarray_len_t lcpnext;
+       union {
+               /* Temporary fields used while this structure is being
+                * initialized.
+                *
+                * Note: we want the entire `struct salink' to be only 6 bytes,
+                * even though this makes "next_initial" unaligned.  */
+               struct {
+                       lz_sarray_pos_t next_initial;
+                       lz_sarray_len_t lcpnext_initial;
+               } _packed_attribute;
+
+               struct {
+                       /* Intially, the length, in bytes, of the longest common
+                        * prefix (LCP) between the suffix having this rank and
+                        * the suffix with the the smallest larger rank that
+                        * starts earlier in the window than the suffix having
+                        * this rank.
+                        *
+                        * Later, during match-finding, after the corresponding
+                        * suffix has entered the LZ77 dictionary, this value
+                        * may be updated by lz_sarray_update_salink() to refer
+                        * instead to a lexicographically closer (but still
+                        * larger) suffix that begins at a later position that
+                        * has entered the LZ77 dictionary.  */
+                       lz_sarray_len_t   lcpnext;
+
+                       /* Initially, the length, in bytes, of the longest
+                        * common prefix (LCP) between the suffix having this
+                        * rank and the suffix with the the largest smaller rank
+                        * that starts earlier in the window than the suffix
+                        * having this rank.
+                        *
+                        * Later, during match-finding, after the corresponding
+                        * suffix has entered the LZ77 dictionary, this value
+                        * may be updated by lz_sarray_update_salink() to refer
+                        * instead to a lexicographically closer (but still
+                        * smaller) suffix that begins at a later position that
+                        * has entered the LZ77 dictionary.  */
+                       lz_sarray_len_t   lcpprev;
+
+                       /* Distance to the suffix referred to in the description
+                        * of "lcpnext" above, but capped to a maximum value to
+                        * save memory.  If the true distance was truncated,
+                        * this will give the distance to the rank of a suffix
+                        * that is lexicographically closer to the current
+                        * suffix than the desired suffix, but appears *later*
+                        * in the window and hence cannot be used as the basis
+                        * for a LZ77 match.  */
+                       lz_sarray_delta_t dist_to_next;
+
+                       /* Distance to the suffix referred to in the description
+                        * of "lcpprev" above, but capped to a maximum value to
+                        * save memory.  If the true distance was truncated,
+                        * this will give the distance to the rank of a suffix
+                        * that is lexicographically closer to the current
+                        * suffix than the desired suffix, but appears *later*
+                        * in the window and hence cannot be used as the basis
+                        * for a LZ77 match.  */
+                       lz_sarray_delta_t dist_to_prev;
+               };
+       };
 };
 
+
 /*-----------------------------------*/
 /* Functions defined in lz_sarray.c  */
 /*-----------------------------------*/
@@ -180,26 +220,16 @@ lz_sarray_get_pos(const struct lz_sarray *mf)
 static _always_inline_attribute void
 lz_sarray_update_salink(const lz_sarray_pos_t r, struct salink link[])
 {
-       /* next = rank of LOWEST ranked suffix that is ranked HIGHER than the
-        * current suffix AND has a LOWER position, or LZ_SARRAY_POS_MAX if none
-        * exists.  */
-       const lz_sarray_pos_t next = link[r].next;
-
-       /* prev = rank of HIGHEST ranked suffix that is ranked LOWER than the
-        * current suffix AND has a LOWER position, or LZ_SARRAY_POS_MAX if none
-        * exists.  */
-       const lz_sarray_pos_t prev = link[r].prev;
-
-       /* Link the suffix at the current position into the linked list that
-        * contains all suffixes referenced by the suffix array that appear at
-        * or before the current position, sorted by rank.  */
-       if (next != LZ_SARRAY_POS_MAX) {
-               link[next].prev = r;
+       const lz_sarray_pos_t next = r + link[r].dist_to_next;
+       const lz_sarray_pos_t prev = r - link[r].dist_to_prev;
+
+       if (next != r && link[r].dist_to_next < link[next].dist_to_prev) {
+               link[next].dist_to_prev = link[r].dist_to_next;
                link[next].lcpprev = link[r].lcpnext;
        }
 
-       if (prev != LZ_SARRAY_POS_MAX) {
-               link[prev].next = r;
+       if (prev != r && link[r].dist_to_prev < link[prev].dist_to_next) {
+               link[prev].dist_to_next = link[r].dist_to_prev;
                link[prev].lcpnext = link[r].lcpprev;
        }
 }
@@ -243,7 +273,6 @@ lz_sarray_get_matches(struct lz_sarray *mf,
        const lz_sarray_pos_t * const restrict SA = mf->SA;
        const lz_sarray_pos_t * const restrict ISA = mf->ISA;
        struct salink * const restrict link = mf->salink;
-       const lz_sarray_pos_t min_match_len = mf->min_match_len;
        const u32 max_matches_to_consider = mf->max_matches_to_consider;
        const u32 max_matches_to_return = mf->max_matches_to_return;
 
@@ -266,8 +295,8 @@ lz_sarray_get_matches(struct lz_sarray *mf,
         * length on both sides in sync in order to choose the lowest-cost match
         * of each length.
         */
-       lz_sarray_pos_t L = link[r].prev;
-       lz_sarray_pos_t R = link[r].next;
+       lz_sarray_pos_t L = r - link[r].dist_to_prev;
+       lz_sarray_pos_t R = r + link[r].dist_to_next;
        lz_sarray_pos_t lenL = link[r].lcpprev;
        lz_sarray_pos_t lenR = link[r].lcpnext;
 
@@ -276,121 +305,197 @@ lz_sarray_get_matches(struct lz_sarray *mf,
 
        /* best_cost = cost of lowest-cost match found so far.
         *
-        * We keep track of this so that we can ignore shorter matches that do
-        * not have lower costs than longer matches already found.  */
+        * Shorter matches that do not have a lower cost than this are
+        * discarded, since presumably it would be cheaper to output the bytes
+        * from the longer match instead.  */
        lz_sarray_cost_t best_cost = LZ_SARRAY_INFINITE_COST;
 
        /* count_remaining = maximum number of possible matches remaining to be
         * considered.  */
        u32 count_remaining = max_matches_to_consider;
 
-       /* pending = match currently being considered for a specific length.  */
-       struct raw_match pending;
-       lz_sarray_cost_t pending_cost;
-
-       while (lenL >= min_match_len || lenR >= min_match_len)
-       {
-               pending.len = lenL;
-               pending_cost = LZ_SARRAY_INFINITE_COST;
+       /* pending_offset = offset of lowest-cost match found for the current
+        * length, or 0 if no match has been found yet.  */
+       lz_sarray_pos_t pending_offset = 0;
+
+       /* Note: some 'goto' statements are used in the remainder of this
+        * function to remove unnecessary checks and create branches that the
+        * CPU may predict better.  (This function is performance critical.)  */
+
+       if (lenL != 0 && lenL >= lenR)
+               goto extend_left;
+       else if (lenR != 0)
+               goto extend_right;
+       else
+               return 0;
+
+extend_left:
+       /* Search suffixes on the left until the match length has decreased
+        * below the next match length on the right or to below the minimum
+        * match length.  */
+       for (;;) {
+               lz_sarray_pos_t offset;
                lz_sarray_cost_t cost;
+               lz_sarray_pos_t old_L;
+               lz_sarray_pos_t old_lenL;
+
+               /* Check for hard cutoff on amount of work done.  */
+               if (count_remaining-- == 0) {
+                       if (pending_offset != 0) {
+                               /* Save pending match.  */
+                               matches[nmatches++] = (struct raw_match){
+                                       .len = lenL,
+                                       .offset = pending_offset,
+                               };
+                       }
+                       return nmatches;
+               }
+
+               if (SA[L] < i) {
+                       /* Suffix is in LZ77 dictionary.  (Check was needed
+                        * because the salink array caps distances to save
+                        * memory.)  */
+
+                       offset = i - SA[L];
+
+                       /* Save match offset if it results in lower cost.  */
+                       cost = (*eval_match_cost)(lenL, offset,
+                                                 eval_match_cost_ctx);
+                       if (cost < best_cost) {
+                               best_cost = cost;
+                               pending_offset = offset;
+                       }
+               }
+
+               /* Advance left to next suffix.  */
+
+               old_L = L;
+               old_lenL = lenL;
 
-               /* Extend left.  */
-               if (lenL >= min_match_len && lenL >= lenR) {
-                       for (;;) {
+               L -= link[L].dist_to_prev;
 
-                               if (--count_remaining == 0)
-                                       goto out_save_pending;
+               if (link[old_L].lcpprev < old_lenL) {
+                       /* Match length decreased.  */
 
-                               lz_sarray_pos_t offset = i - SA[L];
+                       lenL = link[old_L].lcpprev;
 
-                               /* Save match if it has smaller cost.  */
-                               cost = (*eval_match_cost)(lenL, offset,
-                                                         eval_match_cost_ctx);
-                               if (cost < pending_cost) {
-                                       pending.offset = offset;
-                                       pending_cost = cost;
+                       if (old_lenL > lenR) {
+                               /* Neither the right side nor the left size has
+                                * any more matches of length @old_lenL.  If a
+                                * pending match exists, save it.  */
+                               if (pending_offset != 0) {
+                                       matches[nmatches++] = (struct raw_match){
+                                               .len = old_lenL,
+                                               .offset = pending_offset,
+                                       };
+                                       if (nmatches == max_matches_to_return)
+                                               return nmatches;
+
+                                       pending_offset = 0;
                                }
 
-                               if (link[L].lcpprev < lenL) {
-                                       /* Match length decreased.  */
-
-                                       lenL = link[L].lcpprev;
-
-                                       /* Save the pending match unless the
-                                        * right side still may have matches of
-                                        * this length to be scanned, or if a
-                                        * previous (longer) match had lower
-                                        * cost.  */
-                                       if (pending.len > lenR) {
-                                               if (pending_cost < best_cost) {
-                                                       best_cost = pending_cost;
-                                                       matches[nmatches++] = pending;
-                                                       if (nmatches == max_matches_to_return)
-                                                               return nmatches;
-                                               }
-                                               pending.len = lenL;
-                                               pending_cost = LZ_SARRAY_INFINITE_COST;
-                                       }
-                                       if (lenL < min_match_len || lenL < lenR)
-                                               break;
+                               if (lenL >= lenR) {
+                                       /* New match length on left is still at
+                                        * least as large as the next match
+                                        * length on the right:  Keep extending
+                                        * left, unless the minimum match length
+                                        * would be underrun.  */
+                                       if (lenL == 0)
+                                               return nmatches;
+                                       goto extend_left;
                                }
-                               L = link[L].prev;
                        }
+
+                       /* Here we have lenL < lenR.  Extend right, unless the
+                        * minimum match length would be underrun.  */
+                       if (lenR == 0)
+                               return nmatches;
+                       goto extend_right;
                }
+       }
 
-               pending.len = lenR;
+extend_right:
+       /* Search suffixes on the right until the match length has decreased to
+        * the next match length on the left or to below the minimum match
+        * length.  */
+       for (;;) {
+               lz_sarray_pos_t offset;
+               lz_sarray_cost_t cost;
+               lz_sarray_pos_t old_R;
+               lz_sarray_pos_t old_lenR;
+
+               /* Check for hard cutoff on amount of work done.  */
+               if (count_remaining-- == 0) {
+                       if (pending_offset != 0) {
+                               /* Save pending match.  */
+                               matches[nmatches++] = (struct raw_match){
+                                       .len = lenR,
+                                       .offset = pending_offset,
+                               };
+                       }
+                       return nmatches;
+               }
 
-               /* Extend right.  */
-               if (lenR >= min_match_len && lenR > lenL) {
-                       for (;;) {
+               if (SA[R] < i) {
+                       /* Suffix is in LZ77 dictionary.  (Check was needed
+                        * because the salink array caps distances to save
+                        * memory.)  */
 
-                               if (--count_remaining == 0)
-                                       goto out_save_pending;
+                       offset = i - SA[R];
 
-                               lz_sarray_pos_t offset = i - SA[R];
+                       /* Save match offset if it results in lower cost.  */
+                       cost = (*eval_match_cost)(lenR,
+                                                 offset,
+                                                 eval_match_cost_ctx);
+                       if (cost < best_cost) {
+                               best_cost = cost;
+                               pending_offset = offset;
+                       }
+               }
 
-                               /* Save match if it has smaller cost.  */
-                               cost = (*eval_match_cost)(lenR,
-                                                         offset,
-                                                         eval_match_cost_ctx);
-                               if (cost < pending_cost) {
-                                       pending.offset = offset;
-                                       pending_cost = cost;
-                               }
+               /* Advance right to next suffix.  */
 
-                               if (link[R].lcpnext < lenR) {
-                                       /* Match length decreased.  */
+               old_R = R;
+               old_lenR = lenR;
 
-                                       lenR = link[R].lcpnext;
+               R += link[R].dist_to_next;
 
-                                       /* Save the pending match unless a
-                                        * previous (longer) match had lower
-                                        * cost.  */
-                                       if (pending_cost < best_cost) {
-                                               matches[nmatches++] = pending;
-                                               best_cost = pending_cost;
-                                               if (nmatches == max_matches_to_return)
-                                                       return nmatches;
-                                       }
+               if (link[old_R].lcpnext < lenR) {
+                       /* Match length decreased.  */
 
-                                       if (lenR < min_match_len || lenR <= lenL)
-                                               break;
+                       lenR = link[old_R].lcpnext;
 
-                                       pending.len = lenR;
-                                       pending_cost = LZ_SARRAY_INFINITE_COST;
-                               }
-                               R = link[R].next;
+                       /* Neither the right side nor the left size has any more
+                        * matches of length @old_lenR.  If a pending match
+                        * exists, save it.  */
+                       if (pending_offset != 0) {
+                               matches[nmatches++] = (struct raw_match){
+                                       .len = old_lenR,
+                                       .offset = pending_offset,
+                               };
+                               if (nmatches == max_matches_to_return)
+                                       return nmatches;
+
+                               pending_offset = 0;
+                       }
+
+                       if (lenR > lenL) {
+                               /* lenR > lenL:  Keep extending right, unless
+                                * the minimum match length would be underrun.
+                                */
+                               if (lenR == 0)
+                                       return nmatches;
+                       } else {
+                               /* lenL >= lenR:  Extend left, unless the
+                                * minimum match length would be underrun, in
+                                * which case we are done.  */
+                               if (lenL == 0)
+                                       return nmatches;
+
+                               goto extend_left;
                        }
                }
        }
-       goto out;
-
-out_save_pending:
-       if (pending_cost != LZ_SARRAY_INFINITE_COST)
-               matches[nmatches++] = pending;
-
-out:
-       return nmatches;
 }
 
 #endif /* _WIMLIB_LZ_SARRAY_H */