git/query: refactor graph painting algorithm (findtwixt, lca)

We now keep track of 3 sets during traversal:
- keep: commits we've reached from head commits
- drop: commits we've reached from tail commits
- skip: ancestors of commits in both 'keep' and 'drop'

Commits in 'keep' and/or 'drop' may be added later to the 'skip' set
if we discover later that they are part of a common subgraph of the
head and tail commits.

From these sets we can calculate the commits we are interested in:
lca commits are those in 'keep' and 'drop', but not in 'skip'.
findtwixt commits are those in 'keep', but not in 'drop' or 'skip'.

The "LCA" commit returned is a common ancestor such that there are no
other common ancestors that can reach that commit.  Although there can
be multiple commits that meet this criteria, where one is technically
lower on the commit-graph than the other, these cases only happen in
complex merge arrangements and any choice is likely a decent merge
base.

Repainting is now done in paint() directly.  When we find a boundary
commit, we switch our paint color to 'skip'.  'skip' painting does
not stop when it hits another color; we continue until we are left
with only 'skip' commits on the queue.

This fixes several mishandled cases in the current algorithm:
1. If we hit the common subgraph from tail commits first (if the tail
   commit was newer than the head commit), we ended up traversing the
   entire commit graph.  This is because we couldn't distinguish
   between 'drop' commits that were part of the common subgraph, and
   those that were still looking for it.
2. If we traversed through an initial part of the common subgraph from
   head commits before reaching it from tail commits, these commits
   were returned from findtwixt even though they were also reachable
   from tail commits.
3. In the same case as 2, we might end up choosing an incorrect
   commit as the LCA, which is an ancestor of the real LCA.

sys/src/cmd/git/git.h

@@ -160,14 +160,12 @@ struct Qelt {
 	Object	*o;
 	vlong	mtime;
 	int	color;
-	int	dist;
 };
 
 struct Objq {
 	Qelt	*heap;
 	int	nheap;
 	int	heapsz;
-	int	nkeep;
 };
 
 struct Dtab {
@@ -324,5 +322,5 @@ void	closeconn(Conn *);
 /* queues */
 void	qinit(Objq*);
 void	qclear(Objq*);
-void	qput(Objq*, Object*, int, int);
+void	qput(Objq*, Object*, int);
 int	qpop(Objq*, Qelt*);

sys/src/cmd/git/log.c

@@ -205,7 +205,7 @@ showcommits(char *c)
 		sysfatal("load %H: %r", h);
 	qinit(&objq);
 	osinit(&done);
-	qput(&objq, o, 0, 0);
+	qput(&objq, o, 0);
 	while(qpop(&objq, &e)){
 		show(e.o);
 		for(i = 0; i < e.o->commit->nparent; i++){
@@ -214,7 +214,7 @@ showcommits(char *c)
 			if((p = readobject(e.o->commit->parent[i])) == nil)
 				sysfatal("load %H: %r", o->commit->parent[i]);
 			osadd(&done, p);
-			qput(&objq, p, 0, 0);
+			qput(&objq, p, 0);
 		}
 		unref(e.o);
 	}

sys/src/cmd/git/ref.c

@@ -5,25 +5,12 @@
 #include "git.h"
 
 typedef struct Eval	Eval;
-typedef struct Lcaq	Lcaq;
-
-struct Lcaq {
-	Objq;
-
-	Hash	*head;
-	Hash	*tail;
-	int	nhead;
-	int	ntail;
-
-	Object	*best;
-	int	dist;
-};
-
 
 enum {
 	Blank,
 	Keep,
 	Drop,
+	Skip,
 };
 
 struct Eval {
@@ -38,6 +25,7 @@ static char *colors[] = {
 [Keep] "keep",
 [Drop] "drop",
 [Blank] "blank",
+[Skip] "skip",
 };
 
 static Object zcommit = {
@@ -113,97 +101,20 @@ take(Eval *ev, char *m)
 	return 1;
 }
 
-static int
-pickbest(Lcaq *q, Qelt *e, int color)
-{
-	int i, best, exact;
-
-	best = 0;
-	exact = 0;
-	if(color == Blank || e->color == color)
-		return 0;
-	if(e->dist < q->dist){
-		dprint(1, "found best (dist %d < %d): %H\n", e->dist, q->dist, e->o->hash);
-		best = 1;
-	}
-	for(i = 0; i < q->nhead; i++)
-		if(hasheq(&q->head[i], &e->o->hash)){
-			dprint(1, "found best (exact head): %H\n", e->o->hash);
-			best = 1;
-			exact = 1;
-		}
-	for(i = 0; i < q->ntail; i++)
-		if(hasheq(&q->tail[i], &e->o->hash)){
-			dprint(1, "found best (exact tail): %H\n", e->o->hash);
-			best = 1;
-			exact = 1;
-		}
-	if(best){
-		q->best = e->o;
-		q->dist = e->dist;
-	}
-	return exact;
-}
-
-static int
-repaint(Lcaq *lcaq, Objset *keep, Objset *drop, Object *o, int dist, int ancestor)
-{
-	Lcaq objq;
-	Qelt e;
-	Object *p;
-	int i;
-
-	qinit(&objq);
-	if((o = readobject(o->hash)) == nil)
-		return -1;
-	qput(&objq, o, Drop, dist);
-	while(qpop(&objq, &e)){
-		o = e.o;
-		if(oshas(drop, o->hash))
-			continue;
-		if(ancestor && pickbest(lcaq, &e, Keep))
-			goto out;
-		if(!oshas(keep, o->hash)){
-			dprint(2, "repaint: blank => drop %H\n", o->hash);
-			osadd(drop, o);
-			continue;
-		}
-		for(i = 0; i < o->commit->nparent; i++){
-			if(oshas(drop, o->commit->parent[i]))
-				continue;
-			if((p = readobject(o->commit->parent[i])) == nil)
-				goto out;
-			if(p->type != GCommit){
-				fprint(2, "hash %H not commit\n", p->hash);
-				unref(p);
-			}
-			qput(&objq, p, Drop, e.dist+1);
-		}
-		unref(e.o);
-	}
-out:
-	qclear(&objq);
-	return 0;
-}
-
 static int
 paint(Hash *head, int nhead, Hash *tail, int ntail, Object ***res, int *nres, int ancestor)
 {
 	Qelt e;
-	Lcaq objq;
+	Objq objq;
-	Objset keep, drop;
+	Objset keep, drop, skip;
 	Object *o, *c;
-	int i, ncolor;
+	int i, nskip;
 
 	osinit(&keep);
 	osinit(&drop);
-	memset(&objq, 0, sizeof(objq));
+	osinit(&skip);
 	qinit(&objq);
-	objq.head = head;
+	nskip = 0;
-	objq.nhead = nhead;
-	objq.tail = tail;
-	objq.ntail = ntail;
-	objq.dist = 1<<30;
 
 	for(i = 0; i < nhead; i++){
 		if((o = readobject(head[i])) == nil){
@@ -217,7 +128,7 @@ paint(Hash *head, int nhead, Hash *tail, int ntail, Object ***res, int *nres, in
 			continue;
 		}
 		dprint(1, "init: keep %H\n", o->hash);
-		qput(&objq, o, Keep, 0);
+		qput(&objq, o, Keep);
 		unref(o);
 	}		
 	for(i = 0; i < ntail; i++){
@@ -231,30 +142,35 @@ paint(Hash *head, int nhead, Hash *tail, int ntail, Object ***res, int *nres, in
 			continue;
 		}
 		dprint(1, "init: drop %H\n", o->hash);
-		qput(&objq, o, Drop, 0);
+		qput(&objq, o, Drop);
 		unref(o);
 	}
 
 	dprint(1, "finding twixt commits\n");
-	while(qpop(&objq, &e)){
+	while(nskip != objq.nheap && qpop(&objq, &e)){
-		if(oshas(&drop, e.o->hash))
+		if(e.color == Skip)
-			ncolor = Drop;
+			nskip--;
-		else if(oshas(&keep, e.o->hash))
+		if(oshas(&skip, e.o->hash))
-			ncolor = Keep;
-		else
-			ncolor = Blank;
-		if(ancestor && pickbest(&objq, &e, ncolor))
-			goto exactlca;
-		if(ncolor == Keep && e.color == Keep || ncolor == Drop)
 			continue;
-		if(ncolor == Keep && e.color == Drop){
+		switch(e.color){
-			if(repaint(&objq, &keep, &drop, e.o, e.dist, ancestor) == -1)
+		case Keep:
-				goto error;
+			if(oshas(&keep, e.o->hash))
-		}else if (ncolor == Blank) {
+				continue;
-			if(e.color == Keep)
+			if(oshas(&drop, e.o->hash))
+				e.color = Skip;
 			osadd(&keep, e.o);
-			else
+			break;
+		case Drop:
+			if(oshas(&drop, e.o->hash))
+				continue;
+			if(oshas(&keep, e.o->hash))
+				e.color = Skip;
 			osadd(&drop, e.o);
+			break;
+		case Skip:
+			osadd(&skip, e.o);
+			break;
+		}
 		o = readobject(e.o->hash);
 		for(i = 0; i < o->commit->nparent; i++){
 			if((c = readobject(e.o->commit->parent[i])) == nil)
@@ -265,36 +181,44 @@ paint(Hash *head, int nhead, Hash *tail, int ntail, Object ***res, int *nres, in
 				continue;
 			}
 			dprint(2, "\tenqueue: %s %H\n", colors[e.color], c->hash);
-				qput(&objq, c, e.color, e.dist+1);
+			qput(&objq, c, e.color);
 			unref(c);
+			if(e.color == Skip)
+				nskip++;
 		}
 		unref(o);
 	}
-	}
-exactlca:
 	if(ancestor){
 		dprint(1, "found ancestor\n");
-		if(objq.best == nil){
+		o = nil;
+		for(i = 0; i < keep.sz; i++){
+			o = keep.obj[i];
+			if(o != nil && oshas(&drop, o->hash) && !oshas(&skip, o->hash))
+				break;
+		}
+		if(i == keep.sz){
 			*nres = 0;
 			*res = nil;
 		}else{
 			*nres = 1;
 			*res = eamalloc(1, sizeof(Object*));
-			(*res)[0] = objq.best;
+			(*res)[0] = o;
 		}
 	}else{
 		dprint(1, "found twixt\n");
 		*res = eamalloc(keep.nobj, sizeof(Object*));
 		*nres = 0;
 		for(i = 0; i < keep.sz; i++){
-			if(keep.obj[i] != nil && !oshas(&drop, keep.obj[i]->hash)){
+			o = keep.obj[i];
-				(*res)[*nres] = keep.obj[i];
+			if(o != nil && !oshas(&drop, o->hash) && !oshas(&skip, o->hash)){
+				(*res)[*nres] = o;
 				(*nres)++;
 			}
 		}
 	}
 	osclear(&keep);
 	osclear(&drop);
+	osclear(&skip);
 	return 0;
 error:
 	dprint(1, "twixt error: %r\n");

sys/src/cmd/git/util.c

@@ -338,7 +338,7 @@ qclear(Objq *q)
 }
 
 void
-qput(Objq *q, Object *o, int color, int dist)
+qput(Objq *q, Object *o, int color)
 {
 	Qelt t;
 	int i;
@@ -349,7 +349,6 @@ qput(Objq *q, Object *o, int color, int dist)
 	}
 	q->heap[q->nheap].o = o;
 	q->heap[q->nheap].color = color;
-	q->heap[q->nheap].dist = dist;
 	q->heap[q->nheap].mtime = o->commit->mtime;
 	for(i = q->nheap; i > 0; i = (i-1)/2){
 		if(q->heap[i].mtime < q->heap[(i-1)/2].mtime)