qpkg: optimize duplicate lookup when inserting a new name

- jrb.h, jrb.c (jrb_find_or_insert): new function to look for a node and
  to insert a new one if not found
- id.c (make_id): use jrb_find_or_insert to avoid looking up new nodes
  twice
- TODO: report the improvement

qpkg/TODO

@@ -10,9 +10,12 @@
 
   Update: after switching to red-black trees, we get only 1497604 calls
   to comp_id. This is 130% of the "good case" estimate above. Insertion
-  of a new node is currently done with two lookup, so we'll get rid of
+  of a new node is currently done with two lookups, so we'll get rid of
   some more lookups after further optimization.
 
+  Update: after merging the two lookups per new node into one, we're at
+  1172642 calls to comp_id, or 102% of the predicted "good case".
+
 - check whether introducing a new package would cause a conflict
 
 - compile the list of conflicts of installed packages

qpkg/id.c

@@ -38,17 +38,15 @@ struct tree *make_tree(int (*comp)(const void *a, const void *b))
 struct id *make_id(struct tree *tree, const char *s, size_t len)
 {
 	struct id *id;
-	struct jrb *node;
 
 	if (!free_id)
 		free_id = alloc_type(struct id);
 	id = free_id;
 	id->s = s;
 	id->len = len;
-	node = jrb_find(tree->root, id, tree->comp);
-	if (node)
-		return node->key;
-	id->jrb = jrb_insert(tree->root, id, NULL, tree->comp);
+	id->jrb = jrb_find_or_insert(tree->root, id, NULL, tree->comp);
+	if (id->jrb->key != id)
+		return id->jrb->key;
 	free_id = NULL;
 	return id;
 }

qpkg/jrb.c

@@ -552,3 +552,14 @@ struct jrb *jrb_insert(struct jrb *tree, void *key, void *val,
 
 	return jrb_insert_b(jrb_find_gte(tree, key, func, &fnd), key, val);
 }
+
+
+struct jrb *jrb_find_or_insert(struct jrb *tree, void *key, void *val,
+    int (*func)(const void *a, const void *b))
+{
+	struct jrb *n;
+	int fnd;
+
+	n = jrb_find_gte(tree, key, func, &fnd);
+	return fnd ? n : jrb_insert_b(n, key, val);
+}

qpkg/jrb.h

@@ -72,6 +72,12 @@ struct jrb *make_jrb(void);   /* Creates a new rb-tree */
 struct jrb *jrb_insert(struct jrb *tree, void *key, void *val,
     int (*func)(const void *a, const void *b));
 
+/* If the key already exists, return the node (without altering it).
+   Otherwise, insert a new node. */
+
+struct jrb *jrb_find_or_insert(struct jrb *tree, void *key, void *val,
+    int (*func)(const void *a, const void *b));
+
 /* returns an external node in t whose value is equal k. Returns NULL if
    there is no such node in the tree */