From 1be4c06866137120eb2c884b886384dec2590a93 Mon Sep 17 00:00:00 2001
From: heister <heister@0785d39b-7218-0410-832d-ea1e28bc413d>
Date: Mon, 27 Dec 2010 19:28:39 +0000
Subject: [PATCH] make IndexSet::n_elements() an O(1) operation.

git-svn-id: https://svn.dealii.org/trunk@23073 0785d39b-7218-0410-832d-ea1e28bc413d
---
 deal.II/include/deal.II/base/index_set.h | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/deal.II/include/deal.II/base/index_set.h b/deal.II/include/deal.II/base/index_set.h
index 1a17164e81..0c0b59213c 100644
--- a/deal.II/include/deal.II/base/index_set.h
+++ b/deal.II/include/deal.II/base/index_set.h
@@ -745,13 +745,23 @@ IndexSet::n_elements () const
 				   // non-overlapping ranges
   compress ();
 
+  unsigned int v = 0;
+  if (!ranges.empty())
+    {
+      Range & r = ranges.back();
+      v = r.nth_index_in_set + r.end - r.begin;
+    }
+  
+#ifdef DEBUG  
   unsigned int s = 0;
   for (std::vector<Range>::iterator range = ranges.begin();
        range != ranges.end();
        ++range)
     s += (range->end - range->begin);
-
-  return s;
+  Assert(s==v, ExcInternalError());
+#endif  
+  
+  return v;
 }
 
 
-- 
2.39.5