From: Wolfgang Bangerth <bangerth@colostate.edu>
Date: Thu, 24 Aug 2023 23:33:47 +0000 (-0600)
Subject: Introduce IndexSet::is_subset_of().
X-Git-Tag: relicensing~549^2~2
X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=bd35b086bac8565387f540c3d90ed27165848ced;p=dealii.git

Introduce IndexSet::is_subset_of().
---

diff --git a/include/deal.II/base/index_set.h b/include/deal.II/base/index_set.h
index 76776af802..fe0b4de11d 100644
--- a/include/deal.II/base/index_set.h
+++ b/include/deal.II/base/index_set.h
@@ -444,6 +444,16 @@ public:
   void
   fill_binary_vector(VectorType &vector) const;
 
+  /**
+   * Determine whether the current object represents a set of indices
+   * that is a subset of the set represented by the argument. This
+   * function returns `true` if the two sets are the same, that is, it
+   * considers the "subset" comparison typically used in set theory,
+   * rather than the "strict subset" comparison.
+   */
+  bool
+  is_subset_of(const IndexSet &other) const;
+
   /**
    * Output a text representation of this IndexSet to the given stream. Used
    * for testing.
diff --git a/source/base/index_set.cc b/source/base/index_set.cc
index 8582fd7f9c..f3c7b66b66 100644
--- a/source/base/index_set.cc
+++ b/source/base/index_set.cc
@@ -497,6 +497,23 @@ IndexSet::add_indices(const IndexSet &other, const size_type offset)
 
 
 
+bool
+IndexSet::is_subset_of(const IndexSet &other) const
+{
+  // See whether there are indices in the current set that are not in 'other'.
+  // If so, then this is clearly not a subset of 'other'.
+  IndexSet A_minus_B = *this;
+  A_minus_B.subtract_set(other);
+  if (A_minus_B.n_elements() > 0)
+    return false;
+  else
+    // Else, every index in 'this' is also in 'other', since we ended up
+    // with an empty set upon subtraction. This means that we have a subset:
+    return true;
+}
+
+
+
 void
 IndexSet::write(std::ostream &out) const
 {