git/query: fix spurious merge requests

Due to the way LCA is defined, a using a strict LCA
on a graph like this:

 <--a--b--c--d--e--f--g
     \               /
       +-----h-------

can lead to spurious requests to merge. This happens
because 'lca(b, g)' would return 'a', since it can be
reached in one step from 'b', and 2 steps from 'g', while
reaching 'b' from 'a' would be a longer path.

As a result, we need to implement an lca variant that
returns the starting node if one is reachable from the
other, even if it's already found the technically correct
least common ancestor.

This replaces our LCA algorithm with one based on the
painting we do while finding a twixt, making it give
the resutls we want.
git/query: fix spurious merge requests

Due to the way LCA is defined, a using a strict LCA
on a graph like this:

 <--a--b--c--d--e--f--g
     \               /
       +-----h-------

can lead to spurious requests to merge. This happens
because 'lca(b, g)' would return 'a', since it can be
reached in one step from 'b', and 2 steps from 'g', while
reaching 'b' from 'a' would be a longer path.

As a result, we need to implement an lca variant that
returns the starting node if one is reachable from the
other, even if it's already found the technically correct
least common ancestor.

This replaces our LCA algorithm with one based on the
painting we do while finding a twixt.

1 files changed, 169 insertions, 182 deletions

diff --git a/sys/src/cmd/git/ref.c b/sys/src/cmd/git/ref.c
index 5f67dacb0..ba6f6f830 100644
--- a/sys/src/cmd/git/ref.c
+++ b/sys/src/cmd/git/ref.c
@@ -5,8 +5,20 @@
 #include "git.h"
 
 typedef struct Eval	Eval;
-typedef struct XObject	XObject;
-typedef struct Objq	Objq;
+typedef struct Lcaq	Lcaq;
+
+struct Lcaq {
+	Objq;
+
+	Hash	*head;
+	Hash	*tail;
+	int	nhead;
+	int	ntail;
+
+	Object	*best;
+	int	dist;
+};
+
 
 enum {
 	Blank,
@@ -22,17 +34,10 @@ struct Eval {
 	int	stksz;
 };
 
-struct XObject {
-	Object	*obj;
-	Object	*mark;
-	XObject	*queue;
-	XObject	*next;
-};
-
-struct Objq {
-	Objq	*next;
-	Object	*o;
-	int	color;
+static char *colors[] = {
+[Keep] "keep",
+[Drop] "drop",
+[Blank] "blank",
 };
 
 static Object zcommit = {
@@ -128,150 +133,98 @@ take(Eval *ev, char *m)
 	return 1;
 }
 
-XObject*
-hnode(XObject *ht[], Object *o)
+static int
+pickbest(Lcaq *q, Qelt *e, int color)
 {
-	XObject *h;
-	int	hh;
-
-	hh = o->hash.h[0] & 0xff;
-	for(h = ht[hh]; h; h = h->next)
-		if(hasheq(&o->hash, &h->obj->hash))
-			return h;
-
-	h = emalloc(sizeof(*h));
-	h->obj = o;
-	h->mark = nil;
-	h->queue = nil;
-	h->next = ht[hh];
-	ht[hh] = h;
-	return h;
-}
+	int i, best, exact;
 
-Object*
-ancestor(Object *a, Object *b)
-{
-	Object *o, *p, *r;
-	XObject *ht[256];
-	XObject *h, *q, *q1, *q2;
-	int i;
-
-	if(a == b)
-		return a;
-	if(a == nil || b == nil)
-		return nil;
-	r = nil;
-	memset(ht, 0, sizeof(ht));
-	q1 = nil;
-
-	h = hnode(ht, a);
-	h->mark = a;
-	h->queue = q1;
-	q1 = h;
-
-	h = hnode(ht, b);
-	h->mark = b;
-	h->queue = q1;
-	q1 = h;
-
-	while(1){
-		q2 = nil;
-		while(q = q1){
-			q1 = q->queue;
-			q->queue = nil;
-			o = q->obj;
-			for(i = 0; i < o->commit->nparent; i++){
-				p = readobject(o->commit->parent[i]);
-				if(p == nil)
-					goto err;
-				h = hnode(ht, p);
-				if(h->mark != nil){
-					if(h->mark != q->mark){
-						r = h->obj;
-						goto done;
-					}
-				} else {
-					h->mark = q->mark;
-					h->queue = q2;
-					q2 = h;
-				}
-			}
-		}
-		if(q2 == nil){
-err:
-			werrstr("no common ancestor");
-			break;
-		}
-		q1 = q2;
+	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;
 	}
-done:
-	for(i=0; i<nelem(ht); i++){
-		while(h = ht[i]){
-			ht[i] = h->next;
-			free(h);
+	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 r;
-}
-
-int
-lca(Eval *ev)
-{
-	Object *a, *b, *o;
-
-	if(ev->nstk < 2){
-		werrstr("ancestor needs 2 objects");
-		return -1;
-	}
-	a = pop(ev);
-	b = pop(ev);
-	o = ancestor(a, b);
-	if(o == nil)
-		return -1;
-	push(ev, o);
-	return 0;
+	return exact;
 }
 
 static int
-repaint(Objset *keep, Objset *drop, Object *o)
+repaint(Lcaq *lcaq, Objset *keep, Objset *drop, Object *o, int dist, int ancestor)
 {
+	Lcaq objq;
+	Qelt e;
 	Object *p;
 	int i;
 
-	if(!oshas(keep, o->hash) && !oshas(drop, o->hash)){
-		dprint(2, "repaint: blank => drop %H\n", o->hash);
-		osadd(drop, o);
-		return 0;
-	}
-	if(oshas(keep, o->hash))
-		dprint(2, "repaint: keep => drop %H\n", o->hash);
-	osadd(drop, o);
-	for(i = 0; i < o->commit->nparent; i++){
-		if((p = readobject(o->commit->parent[i])) == nil)
-			return -1;
-		if(repaint(keep, drop, p) == -1)
-			return -1;
-		unref(p);
+	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;
 }
 
-int
-findtwixt(Hash *head, int nhead, Hash *tail, int ntail, Object ***res, int *nres)
+static int
+paint(Hash *head, int nhead, Hash *tail, int ntail, Object ***res, int *nres, int ancestor)
 {
-	Objq *q, *e, *n, **p;
+	Qelt e;
+	Lcaq objq;
 	Objset keep, drop;
 	Object *o, *c;
 	int i, ncolor;
 
-	e = nil;
-	q = nil;
-	p = &q;
 	osinit(&keep);
 	osinit(&drop);
+	qinit(&objq);
+	objq.head = head;
+	objq.nhead = nhead;
+	objq.tail = tail;
+	objq.ntail = ntail;
+	objq.dist = 1<<30;
+
 	for(i = 0; i < nhead; i++){
-		if(hasheq(&head[i], &Zhash))
-			continue;
 		if((o = readobject(head[i])) == nil){
 			fprint(2, "warning: %H does not point at commit\n", o->hash);
 			werrstr("read head %H: %r", head[i]);
@@ -282,17 +235,11 @@ findtwixt(Hash *head, int nhead, Hash *tail, int ntail, Object ***res, int *nres
 			unref(o);
 			continue;
 		}
-		dprint(1, "twixt init: keep %H\n", o->hash);
-		e = emalloc(sizeof(Objq));
-		e->o = o;
-		e->color = Keep;
-		*p = e;
-		p = &e->next;
+		dprint(1, "init: keep %H\n", o->hash);
+		qput(&objq, o, Keep, 0);
 		unref(o);
 	}		
 	for(i = 0; i < ntail; i++){
-		if(hasheq(&tail[i], &Zhash))
-			continue;
 		if((o = readobject(tail[i])) == nil){
 			werrstr("read tail %H: %r", tail[i]);
 			return -1;
@@ -303,77 +250,117 @@ findtwixt(Hash *head, int nhead, Hash *tail, int ntail, Object ***res, int *nres
 			continue;
 		}
 		dprint(1, "init: drop %H\n", o->hash);
-		e = emalloc(sizeof(Objq));
-		e->o = o;
-		e->color = Drop;
-		*p = e;
-		p = &e->next;
+		qput(&objq, o, Drop, 0);
 		unref(o);
 	}
 
 	dprint(1, "finding twixt commits\n");
-	while(q != nil){
-		if(oshas(&drop, q->o->hash))
+	while(qpop(&objq, &e)){
+		if(oshas(&drop, e.o->hash))
 			ncolor = Drop;
-		else if(oshas(&keep, q->o->hash))
+		else if(oshas(&keep, e.o->hash))
 			ncolor = Keep;
 		else
 			ncolor = Blank;
-		if(ncolor == Drop || ncolor == Keep && q->color == Keep)
-			goto next;
-		if(ncolor == Keep && q->color == Drop){
-			if(repaint(&keep, &drop, q->o) == -1)
+		if(ancestor && pickbest(&objq, &e, ncolor))
+			goto exactlca;
+		if(ncolor == Keep && e.color == Keep || ncolor == Drop)
+			continue;
+		if(ncolor == Keep && e.color == Drop){
+			if(repaint(&objq, &keep, &drop, e.o, e.dist, ancestor) == -1)
 				goto error;
 		}else if (ncolor == Blank) {
-			dprint(2, "visit: %s %H\n", q->color == Keep ? "keep" : "drop", q->o->hash);
-			if(q->color == Keep)
-				osadd(&keep, q->o);
+			if(e.color == Keep)
+				osadd(&keep, e.o);
 			else
-				osadd(&drop, q->o);
-			for(i = 0; i < q->o->commit->nparent; i++){
-				if((c = readobject(q->o->commit->parent[i])) == nil)
+				osadd(&drop, e.o);
+			o = readobject(e.o->hash);
+			for(i = 0; i < o->commit->nparent; i++){
+				if((c = readobject(e.o->commit->parent[i])) == nil)
 					goto error;
 				if(c->type != GCommit){
 					fprint(2, "warning: %H does not point at commit\n", c->hash);
 					unref(c);
 					continue;
 				}
-				dprint(2, "enqueue: %s %H\n", q->color == Keep ? "keep" : "drop", c->hash);
-				n = emalloc(sizeof(Objq));
-				n->color = q->color;
-				n->next = nil;
-				n->o = c;
-				e->next = n;
-				e = n;
+				dprint(2, "\tenqueue: %s %H\n", colors[e.color], c->hash);
+				qput(&objq, c, e.color, e.dist+1);
 				unref(c);
 			}
-		}else{
-			sysfatal("oops");
+			unref(o);
 		}
-next:
-		n = q->next;
-		free(q);
-		q = 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)){
-			(*res)[*nres] = keep.obj[i];
-			(*nres)++;
+exactlca:
+	if(ancestor){
+		dprint(1, "found ancestor\n");
+		if(objq.best == nil){
+			*nres = 0;
+			*res = nil;
+		}else{
+			*nres = 1;
+			*res = eamalloc(1, sizeof(Object*));
+			(*res)[0] = objq.best;
+		}
+	}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)){
+				(*res)[*nres] = keep.obj[i];
+				(*nres)++;
+			}
 		}
 	}
 	osclear(&keep);
 	osclear(&drop);
 	return 0;
 error:
-	for(; q != nil; q = n) {
-		n = q->next;
-		free(q);
-	}
+	dprint(1, "twixt error: %r\n");
+	free(objq.heap);
 	return -1;
 }
 
+int
+findtwixt(Hash *head, int nhead, Hash *tail, int ntail, Object ***res, int *nres)
+{
+	return paint(head, nhead, tail, ntail, res, nres, 0);
+}
+
+Object*
+ancestor(Object *a, Object *b)
+{
+	Object **o, *r;
+	int n;
+
+	if(paint(&a->hash, 1, &b->hash, 1, &o, &n, 1) == -1 || n == 0)
+		return nil;
+	r = o[0];
+	free(o);
+	return ref(r);
+}
+
+int
+lca(Eval *ev)
+{
+	Object *a, *b, **o;
+	int n;
+
+	if(ev->nstk < 2){
+		werrstr("ancestor needs 2 objects");
+		return -1;
+	}
+	n = 0;
+	b = pop(ev);
+	a = pop(ev);
+	paint(&a->hash, 1, &b->hash, 1, &o, &n, 1);
+	if(n == 0)
+		return -1;
+	push(ev, *o);
+	free(o);
+	return 0;
+}
+
 static int
 parent(Eval *ev)
 {