🐛 Fix Coupling Limit Computation (#216)

## Description

For whatever reason, the computation of the coupling limit of an
architecture is broken since its introduction.
It remains unclear, why this was not detected earlier, but definitely
shows the lack of tests for that feature.

## Checklist:

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- [x] The pull request only contains commits that are related to it.
- [x] I have added appropriate tests and documentation.
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project's style guidelines.

Author: burgholzer

src/Architecture.cpp

@@ -552,8 +552,9 @@ void Architecture::findCouplingLimit(
   }
   visited[node] = true;
 
-  if (d.at(node) < curSum) {
-    d[node] = curSum;
+  auto& elem = d[node];
+  if (elem == 0 || elem > curSum) {
+    elem = curSum;
   }
   if (connections.at(node).empty()) {
     visited[node] = false;

test/test_architecture.cpp

@@ -152,3 +152,11 @@ TEST(TestArchitecture, MinimumNumberOfSwapsError) {
   EXPECT_THROW(architecture.minimumNumberOfSwaps(permutation, swaps),
                std::runtime_error);
 }
+
+TEST(TestArchitecture, TestCouplingLimitRing) {
+  Architecture      architecture{};
+  const CouplingMap cm = {{0, 1}, {1, 0}, {1, 2}, {2, 1}, {2, 3},
+                          {3, 2}, {3, 4}, {4, 3}, {4, 0}, {0, 4}};
+  architecture.loadCouplingMap(5, cm);
+  EXPECT_EQ(architecture.getCouplingLimit(), 2);
+}