tree-diff: use the name "tail" to refer to list tail

The ll_diff_tree_paths() function and its helpers all append to a
running list by taking in a pointer to the old tail and returning the
new tail. But they just call this argument "p", which is not very
descriptive.

It gets particularly confusing in emit_path(), where we actually add to
the list, because "p" does double-duty: it is the tail of the list, but
it is also the entry which we add. Except that in some cases we _don't_
add a new entry (or we might even add it and roll it back) if the path
isn't interesting. At first glance, this makes it look like a bug that
we pass "p" on to ll_diff_tree_paths() to recurse; sometimes it is
getting the new entry we made and sometimes not!

But it's not a bug, because ll_diff_tree_paths() does not care about the
entry itself at all. It is only using its "next" pointer as the tail of
the list.

Let's swap out "p" for "tail" to make this obvious. And then in
emit_path() we'll continue to use "p" for our newly allocated entry.

Signed-off-by: Jeff King <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: peff

tree-diff.c

@@ -49,7 +49,7 @@
 } while(0)
 
 static struct combine_diff_path *ll_diff_tree_paths(
-	struct combine_diff_path *p, const struct object_id *oid,
+	struct combine_diff_path *tail, const struct object_id *oid,
 	const struct object_id **parents_oid, int nparent,
 	struct strbuf *base, struct diff_options *opt,
 	int depth);
@@ -134,7 +134,7 @@ static int emit_diff_first_parent_only(struct diff_options *opt, struct combine_
  *	 t,  tp		-> path modified/added
  *			   (M for tp[i]=tp[imin], A otherwise)
  */
-static struct combine_diff_path *emit_path(struct combine_diff_path *p,
+static struct combine_diff_path *emit_path(struct combine_diff_path *tail,
 	struct strbuf *base, struct diff_options *opt, int nparent,
 	struct tree_desc *t, struct tree_desc *tp,
 	int imin, int depth)
@@ -177,13 +177,14 @@ static struct combine_diff_path *emit_path(struct combine_diff_path *p,
 
 	if (emitthis) {
 		int keep;
-		struct combine_diff_path *pprev = p;
+		struct combine_diff_path *pprev = tail, *p;
 
 		strbuf_add(base, path, pathlen);
 		p = combine_diff_path_new(base->buf, base->len, mode,
 					  oid ? oid : null_oid(),
 					  nparent);
-		pprev->next = p;
+		tail->next = p;
+		tail = p;
 		strbuf_setlen(base, old_baselen);
 
 		for (i = 0; i < nparent; ++i) {
@@ -222,7 +223,7 @@ static struct combine_diff_path *emit_path(struct combine_diff_path *p,
 		if (!keep) {
 			free(p);
 			pprev->next = NULL;
-			p = pprev;
+			tail = pprev;
 		}
 	}
 
@@ -239,13 +240,13 @@ static struct combine_diff_path *emit_path(struct combine_diff_path *p,
 
 		strbuf_add(base, path, pathlen);
 		strbuf_addch(base, '/');
-		p = ll_diff_tree_paths(p, oid, parents_oid, nparent, base, opt,
-				       depth + 1);
+		tail = ll_diff_tree_paths(tail, oid, parents_oid, nparent, base, opt,
+					  depth + 1);
 		FAST_ARRAY_FREE(parents_oid, nparent);
 	}
 
 	strbuf_setlen(base, old_baselen);
-	return p;
+	return tail;
 }
 
 static void skip_uninteresting(struct tree_desc *t, struct strbuf *base,
@@ -359,7 +360,7 @@ static inline void update_tp_entries(struct tree_desc *tp, int nparent)
 }
 
 static struct combine_diff_path *ll_diff_tree_paths(
-	struct combine_diff_path *p, const struct object_id *oid,
+	struct combine_diff_path *tail, const struct object_id *oid,
 	const struct object_id **parents_oid, int nparent,
 	struct strbuf *base, struct diff_options *opt,
 	int depth)
@@ -463,8 +464,8 @@ static struct combine_diff_path *ll_diff_tree_paths(
 			}
 
 			/* D += {δ(t,pi) if pi=p[imin];  "+a" if pi > p[imin]} */
-			p = emit_path(p, base, opt, nparent,
-					&t, tp, imin, depth);
+			tail = emit_path(tail, base, opt, nparent,
+					 &t, tp, imin, depth);
 
 		skip_emit_t_tp:
 			/* t↓,  ∀ pi=p[imin]  pi↓ */
@@ -475,8 +476,8 @@ static struct combine_diff_path *ll_diff_tree_paths(
 		/* t < p[imin] */
 		else if (cmp < 0) {
 			/* D += "+t" */
-			p = emit_path(p, base, opt, nparent,
-					&t, /*tp=*/NULL, -1, depth);
+			tail = emit_path(tail, base, opt, nparent,
+					 &t, /*tp=*/NULL, -1, depth);
 
 			/* t↓ */
 			update_tree_entry(&t);
@@ -491,8 +492,8 @@ static struct combine_diff_path *ll_diff_tree_paths(
 						goto skip_emit_tp;
 			}
 
-			p = emit_path(p, base, opt, nparent,
-					/*t=*/NULL, tp, imin, depth);
+			tail = emit_path(tail, base, opt, nparent,
+					 /*t=*/NULL, tp, imin, depth);
 
 		skip_emit_tp:
 			/* ∀ pi=p[imin]  pi↓ */
@@ -506,7 +507,7 @@ static struct combine_diff_path *ll_diff_tree_paths(
 	FAST_ARRAY_FREE(tptree, nparent);
 	FAST_ARRAY_FREE(tp, nparent);
 
-	return p;
+	return tail;
 }
 
 struct combine_diff_path *diff_tree_paths(