From a1a56e529f9f194f14339b8528732aa7487c8962 Mon Sep 17 00:00:00 2001
From: bangerth <bangerth@0785d39b-7218-0410-832d-ea1e28bc413d>
Date: Wed, 22 Jan 2014 19:13:57 +0000
Subject: [PATCH] Patch by Martin Steigemann: Handle the case of zero error
 indicators better when finding the threshold for mesh refinement.

git-svn-id: https://svn.dealii.org/trunk@32286 0785d39b-7218-0410-832d-ea1e28bc413d
---
 deal.II/doc/news/changes.h                    | 11 +++++++++++
 deal.II/source/distributed/grid_refinement.cc | 11 ++++++++---
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/deal.II/doc/news/changes.h b/deal.II/doc/news/changes.h
index 69ee2024b2..40dee84da4 100644
--- a/deal.II/doc/news/changes.h
+++ b/deal.II/doc/news/changes.h
@@ -84,6 +84,17 @@ inconvenience this causes.
 <h3>Specific improvements</h3>
 
 <ol>
+  <li> Improved: In rare cases when the vector of error indicators
+  has entries equal to zero the adjust_interesting_range method
+  produces a negative lower bound. As a result the
+  parallel::distributed::GridRefinement::refine_and_coarsen_fixed_*
+  methods flagged the wrong number of cells for coarsening and refinement.
+  This is now changed by adjusting the lower bound in adjust_interesting_range
+  only, if not equal to zero.
+  <br>
+  (Martin Steigemann, 2014/01/22)
+  </li>
+
   <li> Changed: It was previously possible to set the
   <code>active_fe_index</code> on non-active cells of an hp::DoFHandler.
   However, this was prone to mistakes because it may lead to the assumption
diff --git a/deal.II/source/distributed/grid_refinement.cc b/deal.II/source/distributed/grid_refinement.cc
index f7f185ff4e..e4a5e8f414 100644
--- a/deal.II/source/distributed/grid_refinement.cc
+++ b/deal.II/source/distributed/grid_refinement.cc
@@ -199,11 +199,16 @@ namespace
     Assert (interesting_range[0] <= interesting_range[1],
             ExcInternalError());
 
+    Assert (interesting_range[0] >= 0,
+            ExcInternalError());
+
+    // adjust the lower bound only
+    // if the end point is not equal
+    // to zero, otherwise it could
+    // happen, that the result
+    // becomes negative
     if (interesting_range[0] > 0)
       interesting_range[0] *= 0.99;
-    else
-      interesting_range[0]
-      -= 0.01 * (interesting_range[1] - interesting_range[0]);
 
     if (interesting_range[1] > 0)
       interesting_range[1] *= 1.01;
-- 
2.39.5