* Garbage collector: added an option `--use-atime' to delete paths in

order of ascending last access time.  This is useful in conjunction
  with --max-freed or --max-links to prefer deleting non-recently used
  garbage, which is good (especially in the build farm) since garbage
  may become live again.

  The code could easily be modified to accept other criteria for
  ordering garbage by changing the comparison operator used by the
  priority queue in collectGarbage().

src/libutil/util.hh

@@ -303,6 +303,29 @@ bool hasSuffix(const string & s, const string & suffix);
 void ignoreException();
 
 
+/* STL functions such as sort() pass a binary function object around
+   by value, so it gets cloned a lot.  This is bad if the function
+   object has state or is simply large.  This adapter wraps the
+   function object to simulate passing by reference. */
+template<class F>
+struct binary_function_ref_adapter
+{
+    F * p;
+
+    binary_function_ref_adapter(F * _p)
+    {
+        p = _p;
+    }
+    
+    typename F::result_type operator () (
+        const typename F::first_argument_type & x,
+        const typename F::second_argument_type & y)
+    {
+        return (*p)(x, y);
+    }
+};
+
+
 }