Adaptation of re-better-fixempties.patch from Tom Lane @ postgres.

1 files changed, 134 insertions, 131 deletions

diff --git a/generic/regc_nfa.c b/generic/regc_nfa.c
index 0f572b8..5582e4e 100644
--- a/generic/regc_nfa.c
+++ b/generic/regc_nfa.c
@@ -603,44 +603,6 @@ hasnonemptyout(
 }
 
 /*
- - nonemptyouts - count non-EMPTY out arcs of a state
- ^ static int nonemptyouts(struct state *);
- */
-static int
-nonemptyouts(
-    struct state *s)
-{
-    int n = 0;
-    struct arc *a;
-
-    for (a = s->outs; a != NULL; a = a->outchain) {
-	if (a->type != EMPTY) {
-	    n++;
-	}
-    }
-    return n;
-}
-
-/*
- - nonemptyins - count non-EMPTY in arcs of a state
- ^ static int nonemptyins(struct state *);
- */
-static int
-nonemptyins(
-    struct state *s)
-{
-    int n = 0;
-    struct arc *a;
-
-    for (a = s->ins; a != NULL; a = a->inchain) {
-	if (a->type != EMPTY) {
-	    n++;
-	}
-    }
-    return n;
-}
-
-/*
  - findarc - find arc, if any, from given source with given type and color
  * If there is more than one such arc, the result is random.
  ^ static struct arc *findarc(struct state *, int, pcolor);
@@ -1897,6 +1859,12 @@ fixempties(
     struct state *nexts;
     struct arc *a;
     struct arc *nexta;
+    int totalinarcs;
+    struct arc **inarcsorig;
+    struct arc **arcarray;
+    int arccount;
+    int prevnins;
+    int nskip;
 
     /*
      * First, get rid of any states whose sole out-arc is an EMPTY,
@@ -1942,42 +1910,129 @@ fixempties(
 	dropstate(nfa, s);
     }
 
+    if (NISERR()) {
+	return;
+    }
+
     /*
-     * For each remaining NFA state, find all other states that are
-     * reachable from it by a chain of one or more EMPTY arcs.  Then
-     * generate new arcs that eliminate the need for each such chain.
+     * For each remaining NFA state, find all other states from which it is
+     * reachable by a chain of one or more EMPTY arcs.  Then generate new arcs
+     * that eliminate the need for each such chain.
+     *
+     * We could replace a chain of EMPTY arcs that leads from a "from" state
+     * to a "to" state either by pushing non-EMPTY arcs forward (linking
+     * directly from "from"'s predecessors to "to") or by pulling them back
+     * (linking directly from "from" to "to"'s successors).  We choose to
+     * always do the former; this choice is somewhat arbitrary, but the
+     * approach below requires that we uniformly do one or the other.
+     *
+     * Suppose we have a chain of N successive EMPTY arcs (where N can easily
+     * approach the size of the NFA).  All of the intermediate states must
+     * have additional inarcs and outarcs, else they'd have been removed by
+     * the steps above.  Assuming their inarcs are mostly not empties, we will
+     * add O(N^2) arcs to the NFA, since a non-EMPTY inarc leading to any one
+     * state in the chain must be duplicated to lead to all its successor
+     * states as well.  So there is no hope of doing less than O(N^2) work;
+     * however, we should endeavor to keep the big-O cost from being even
+     * worse than that, which it can easily become without care.  In
+     * particular, suppose we were to copy all S1's inarcs forward to S2, and
+     * then also to S3, and then later we consider pushing S2's inarcs forward
+     * to S3.  If we include the arcs already copied from S1 in that, we'd be
+     * doing O(N^3) work.  (The duplicate-arc elimination built into newarc()
+     * and its cohorts would get rid of the extra arcs, but not without cost.)
+     *
+     * We can avoid this cost by treating only arcs that existed at the start
+     * of this phase as candidates to be pushed forward.  To identify those,
+     * we remember the first inarc each state had to start with.  We rely on
+     * the fact that newarc() and friends put new arcs on the front of their
+     * to-states' inchains, and that this phase never deletes arcs, so that
+     * the original arcs must be the last arcs in their to-states' inchains.
+     *
+     * So the process here is that, for each state in the NFA, we gather up
+     * all non-EMPTY inarcs of states that can reach the target state via
+     * EMPTY arcs.  We then sort, de-duplicate, and merge these arcs into the
+     * target state's inchain.  (We can safely use sort-merge for this as long
+     * as we update each state's original-arcs pointer after we add arcs to
+     * it; the sort step of mergeins probably changed the order of the old
+     * arcs.)
      *
-     * If we just do this straightforwardly, the algorithm gets slow in
-     * complex graphs, because the same arcs get copied to all
-     * intermediate states of an EMPTY chain, and then uselessly pushed
-     * repeatedly to the chain's final state; we waste a lot of time in
-     * newarc's duplicate checking.  To improve matters, we decree that
-     * any state with only EMPTY out-arcs is "doomed" and will not be
-     * part of the final NFA. That can be ensured by not adding any new
-     * out-arcs to such a state. Having ensured that, we need not update
-     * the state's in-arcs list either; all arcs that might have gotten
-     * pushed forward to it will just get pushed directly to successor
-     * states.  This eliminates most of the useless duplicate arcs.
+     * Another refinement worth making is that, because we only add non-EMPTY
+     * arcs during this phase, and all added arcs have the same from-state as
+     * the non-EMPTY arc they were cloned from, we know ahead of time that any
+     * states having only EMPTY outarcs will be useless for lack of outarcs
+     * after we drop the EMPTY arcs.  (They cannot gain non-EMPTY outarcs if
+     * they had none to start with.)  So we need not bother to update the
+     * inchains of such states at all.
      */
+
+    /* Remember the states' first original inarcs */
+    /* ... and while at it, count how many old inarcs there are altogether */
+    inarcsorig = (struct arc **) MALLOC(nfa->nstates * sizeof(struct arc *));
+    if (inarcsorig == NULL) {
+	NERR(REG_ESPACE);
+	return;
+    }
+    totalinarcs = 0;
+    for (s = nfa->states; s != NULL; s = s->next) {
+	inarcsorig[s->no] = s->ins;
+	totalinarcs += s->nins;
+    }
+
+    /*
+     * Create a workspace for accumulating the inarcs to be added to the
+     * current target state.  totalinarcs is probably a considerable
+     * overestimate of the space needed, but the NFA is unlikely to be large
+     * enough at this point to make it worth being smarter.
+     */
+    arcarray = (struct arc **) MALLOC(totalinarcs * sizeof(struct arc *));
+    if (arcarray == NULL) {
+	NERR(REG_ESPACE);
+	FREE(inarcsorig);
+	return;
+    }
+
+    /* And iterate over the target states */
     for (s = nfa->states; s != NULL && !NISERR(); s = s->next) {
-	for (s2 = emptyreachable(s, s); s2 != s && !NISERR();
-		s2 = nexts) {
-	    /*
-	     * If s2 is doomed, we decide that (1) we will always push
-	     * arcs forward to it, not pull them back to s; and (2) we
-	     * can optimize away the push-forward, per comment above.
-	     * So do nothing.
-	     */
-	    if (s2->flag || hasnonemptyout(s2)) {
-		replaceempty(nfa, s, s2);
+	/* Ignore target states without non-EMPTY outarcs, per note above */
+	if (!s->flag && !hasnonemptyout(s)) {
+	    continue;
+	}
+
+	/* Find predecessor states and accumulate their original inarcs */
+	arccount = 0;
+	for (s2 = emptyreachable(nfa, s, s, inarcsorig); s2 != s; s2 = nexts) {
+	    /* Add s2's original inarcs to arcarray[], but ignore empties */
+	    for (a = inarcsorig[s2->no]; a != NULL; a = a->inchain) {
+		if (a->type != EMPTY) {
+		    arcarray[arccount++] = a;
+		}
 	    }
+  
+  	    /* Reset the tmp fields as we walk back */
+  	    nexts = s2->tmp;
+  	    s2->tmp = NULL;
+  	}
+  	s->tmp = NULL;
+	assert(arccount <= totalinarcs);
 
-	    /* Reset the tmp fields as we walk back */
-	    nexts = s2->tmp;
-	    s2->tmp = NULL;
+	/* Remember how many original inarcs this state has */
+	prevnins = s->nins;
+
+	/* Add non-duplicate inarcs to target state */
+	mergeins(nfa, s, arcarray, arccount);
+
+	/* Now we must update the state's inarcsorig pointer */
+	nskip = s->nins - prevnins;
+	a = s->ins;
+	while (nskip-- > 0) {
+	    a = a->inchain;
 	}
-	s->tmp = NULL;
+	inarcsorig[s->no] = a;
     }
+  
+    FREE(arcarray);
+    FREE(inarcsorig);
+
     if (NISERR()) {
 	return;
     }
@@ -2013,90 +2068,38 @@ fixempties(
 }
 
 /*
- - emptyreachable - recursively find all states reachable from s by EMPTY arcs
+ - emptyreachable - recursively find all states that can reach s by EMPTY arcs
  * The return value is the last such state found.  Its tmp field links back
  * to the next-to-last such state, and so on back to s, so that all these
  * states can be located without searching the whole NFA.
+ *
+ * Since this is only used in fixempties(), we pass in the inarcsorig[] array
+ * maintained by that function.  This lets us skip over all new inarcs, which
+ * are certainly not EMPTY arcs.
+ *
  * The maximum recursion depth here is equal to the length of the longest
  * loop-free chain of EMPTY arcs, which is surely no more than the size of
- * the NFA, and in practice will be a lot less than that.
+ * the NFA, and in practice will be less than that.
  ^ static struct state *emptyreachable(struct state *, struct state *);
  */
 static struct state *
 emptyreachable(
+    struct nfa *nfa,
     struct state *s,
-    struct state *lastfound)
+    struct state *lastfound,
+    struct arc **inarcsorig)
 {
     struct arc *a;
 
     s->tmp = lastfound;
     lastfound = s;
-    for (a = s->outs; a != NULL; a = a->outchain) {
-	if (a->type == EMPTY && a->to->tmp == NULL) {
-	    lastfound = emptyreachable(a->to, lastfound);
+    for (a = inarcsorig[s->no]; a != NULL; a = a->inchain) {
+	if (a->type == EMPTY && a->from->tmp == NULL) {
+	    lastfound = emptyreachable(nfa, a->from, lastfound, inarcsorig);
 	}
     }
     return lastfound;
 }
-
-/*
- - replaceempty - replace an EMPTY arc chain with some non-empty arcs
- * The EMPTY arc(s) should be deleted later, but we can't do it here because
- * they may still be needed to identify other arc chains during fixempties().
- ^ static void replaceempty(struct nfa *, struct state *, struct state *);
- */
-static void
-replaceempty(
-    struct nfa *nfa,
-    struct state *from,
-    struct state *to)
-{
-    int fromouts;
-    int toins;
-
-    assert(from != to);
-
-    /*
-     * Create replacement arcs that bypass the need for the EMPTY chain.  We
-     * can do this either by pushing arcs forward (linking directly from
-     * "from"'s predecessors to "to") or by pulling them back (linking
-     * directly from "from" to "to"'s successors).  In general, we choose
-     * whichever way creates greater fan-out or fan-in, so as to improve the
-     * odds of reducing the other state to zero in-arcs or out-arcs and
-     * thereby being able to delete it.  However, if "from" is doomed (has no
-     * non-EMPTY out-arcs), we must keep it so, so always push forward in that
-     * case.
-     *
-     * The fan-out/fan-in comparison should count only non-EMPTY arcs.  If
-     * "from" is doomed, we can skip counting "to"'s arcs, since we want to
-     * force taking the copynonemptyins path in that case.
-     */
-    fromouts = nonemptyouts(from);
-    toins = (fromouts == 0) ? 1 : nonemptyins(to);
-
-    if (fromouts > toins) {
-	copyouts(nfa, to, from, 0);
-	return;
-    }
-    if (fromouts < toins) {
-	copyins(nfa, from, to, 0);
-	return;
-    }
-
-    /*
-     * fromouts == toins.  Decide on secondary issue: copy fewest arcs.
-     *
-     * Doesn't seem to be worth the trouble to exclude empties from these
-     * comparisons; that takes extra time and doesn't seem to improve the
-     * resulting graph much.
-     */
-    if (from->nins > to->nouts) {
-	copyouts(nfa, to, from, 0);
-	return;
-    }
-
-    copyins(nfa, from, to, 0);
-}
 
 /*
  * isconstraintarc - detect whether an arc is of a constraint type