🐛 Fix cost contribution of direction reverse in exact mapper (#252)

## Description

Fixes #251.

For some reason the cost contribution of direction reverses in the exact
mapper was inverted, i.e., the mapper effectively considered direction
reverses to be "free". This PR fixes the underlying condition and adds a
corresponding regression test.

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Author: burgholzer

src/exact/ExactMapper.cpp

@@ -724,22 +724,23 @@ number of variables: (|L|-1) * m!
 
   // cost for reversed directions
   if (!architecture.bidirectional()) {
-    cost = LogicTerm(0);
-    for (std::size_t k = 0; k < reducedLayerIndices.size(); ++k) {
+    cost                 = LogicTerm(0);
+    const auto numLayers = reducedLayerIndices.size();
+    for (std::size_t k = 0; k < numLayers; ++k) {
       for (const auto& gate : layers.at(reducedLayerIndices.at(k))) {
         if (gate.singleQubit()) {
           continue;
         }
 
-        auto reverse = LogicTerm(true);
-        for (const auto& edge : rcm) {
-          auto indexFT = x[k][physicalQubitIndex[edge.first]][gate.target];
-          auto indexSC = x[k][physicalQubitIndex[edge.second]]
-                          [static_cast<std::size_t>(gate.control)];
-          reverse = reverse && (!indexFT || !indexSC);
+        auto reverse = LogicTerm(false);
+        for (const auto& [q0, q1] : rcm) {
+          const auto indexFT = x[k][physicalQubitIndex[q0]][gate.target];
+          const auto indexSC = x[k][physicalQubitIndex[q1]]
+                                [static_cast<std::size_t>(gate.control)];
+          reverse = reverse || (indexFT && indexSC);
         }
         cost = cost + LogicTerm::ite(reverse,
-                                     LogicTerm(GATES_OF_DIRECTION_REVERSE),
+                                     LogicTerm(::GATES_OF_DIRECTION_REVERSE),
                                      LogicTerm(0));
       }
     }

test/test_exact.cpp

@@ -5,7 +5,6 @@
 
 #include "exact/ExactMapper.hpp"
 
-#include "gmock/gmock.h"
 #include "gtest/gtest.h"
 
 class ExactTest : public testing::TestWithParam<std::string> {
@@ -534,3 +533,26 @@ TEST_F(ExactTest, Test4QCircuitThatUsesAll5Q) {
   const auto& results = mapper.getResults();
   EXPECT_EQ(results.output.swaps, 1);
 }
+
+TEST_F(ExactTest, Test) {
+  // Regression test for https://github.com/cda-tum/qmap/issues/251
+  using namespace qc::literals;
+
+  Architecture      arch;
+  const CouplingMap cm = {{1, 0}, {2, 0}, {2, 1}, {4, 2}, {3, 2}, {3, 4}};
+  arch.loadCouplingMap(5, cm);
+
+  Architecture::printCouplingMap(cm, std::cout);
+
+  qc = qc::QuantumComputation(4);
+  qc.x(0, 1_pc);
+  qc.x(1, 0_pc);
+  qc.x(1, 2_pc);
+  qc.x(2, 1_pc);
+  qc.x(2, 3_pc);
+
+  auto mapper = ExactMapper(qc, arch);
+  mapper.map(settings);
+  EXPECT_EQ(mapper.getResults().output.swaps, 0);
+  EXPECT_EQ(mapper.getResults().output.directionReverse, 2);
+}