From 5e0f91a9839fa3f95f0d6930d63d9499cb231296 Mon Sep 17 00:00:00 2001
From: Reza Rastak <rastak@stanford.edu>
Date: Sat, 27 Jun 2020 15:48:31 -0700
Subject: [PATCH] DiscreteTime::set_next_step_size() added

---
 doc/news/changes/minor/20200627RezaRastak |  3 +++
 include/deal.II/base/discrete_time.h      | 16 ++++++++++++++++
 source/base/discrete_time.cc              | 14 ++++++++++++++
 tests/base/discrete_time_1.cc             |  2 +-
 4 files changed, 34 insertions(+), 1 deletion(-)
 create mode 100644 doc/news/changes/minor/20200627RezaRastak

diff --git a/doc/news/changes/minor/20200627RezaRastak b/doc/news/changes/minor/20200627RezaRastak
new file mode 100644
index 0000000000..c951c0b639
--- /dev/null
+++ b/doc/news/changes/minor/20200627RezaRastak
@@ -0,0 +1,3 @@
+New: The member function DiscreteTime::set_next_step_size() is added.
+<br>
+(Reza Rastak, 2020/06/27)
diff --git a/include/deal.II/base/discrete_time.h b/include/deal.II/base/discrete_time.h
index a656f77138..51c81f85f2 100644
--- a/include/deal.II/base/discrete_time.h
+++ b/include/deal.II/base/discrete_time.h
@@ -349,6 +349,22 @@ public:
   void
   set_desired_next_step_size(const double time_step_size);
 
+  /**
+   * Set the *actual* value of the next time step size. By calling this
+   * method, we are indicating the next time advance_time() is called,
+   * @p time_step_size is to be used to advance the simulation time.
+   *
+   * @note The difference between set_next_step_size() and
+   * set_desired_next_step_size() is that the former uses the provided $dt$
+   * exactly without any adjustment, but produces an
+   * error (in debug mode) if $dt$ is not in the acceptable range.
+   * Generally, set_desired_next_step_size() is the preferred method because
+   * it can adjust the $dt$ intelligently, based on $T_{\text{end}}$.
+   * @pre $0 < dt \le T_{\text{end}} - t$.
+   */
+  void
+  set_next_step_size(const double time_step_size);
+
   /**
    * Advance the current time based on the value of the current step.
    * If you want to adjust the next time step size, call the method
diff --git a/source/base/discrete_time.cc b/source/base/discrete_time.cc
index 1ce499e1a5..28636a7d54 100644
--- a/source/base/discrete_time.cc
+++ b/source/base/discrete_time.cc
@@ -66,6 +66,20 @@ DiscreteTime::set_desired_next_step_size(const double next_step_size)
 
 
 
+void
+DiscreteTime::set_next_step_size(const double next_step_size)
+{
+  Assert(next_step_size > 0,
+         ExcMessage("Only positive time step size is allowed."));
+  next_time = current_time + next_step_size;
+  Assert(
+    next_time <= end_time,
+    ExcMessage(
+      "Time step size is too large. The next time cannot exceed the end time."));
+}
+
+
+
 void
 DiscreteTime::advance_time()
 {
diff --git a/tests/base/discrete_time_1.cc b/tests/base/discrete_time_1.cc
index e5461cf198..b37ee917d4 100644
--- a/tests/base/discrete_time_1.cc
+++ b/tests/base/discrete_time_1.cc
@@ -69,7 +69,7 @@ test_adjust_time_step_size()
   print_time(time);
   time.advance_time();
   print_time(time);
-  time.set_desired_next_step_size(0.36);
+  time.set_next_step_size(0.36);
   time.advance_time();
   print_time(time);
   time.advance_time();
-- 
2.39.5