From: Martin Kronbichler <martin.kronbichler@uni-a.de>
Date: Fri, 30 Jun 2023 09:34:12 +0000 (+0200)
Subject: Avoid quadratic complexity in IndexSet::add_ranges
X-Git-Tag: relicensing~843^2~2
X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=18b0c16b6348fcb4bc52455e217cb5d4722f2831;p=dealii.git

Avoid quadratic complexity in IndexSet::add_ranges
---

diff --git a/source/base/index_set.cc b/source/base/index_set.cc
index f51d30ddfb..cf73cf524b 100644
--- a/source/base/index_set.cc
+++ b/source/base/index_set.cc
@@ -388,8 +388,15 @@ IndexSet::pop_front()
 void
 IndexSet::add_range_lower_bound(const Range &new_range)
 {
-  ranges.insert(Utilities::lower_bound(ranges.begin(), ranges.end(), new_range),
-                new_range);
+  // if the inserted range is already within the range we find by lower_bound,
+  // there is no need to do anything; we do not try to be clever here and
+  // leave all other work to compress().
+  const auto insert_position =
+    Utilities::lower_bound(ranges.begin(), ranges.end(), new_range);
+  if (insert_position == ranges.end() ||
+      insert_position->begin > new_range.begin ||
+      insert_position->end < new_range.end)
+    ranges.insert(insert_position, new_range);
 }