🐛 Fix DD node leak (#385)

include/dd/Package.hpp

@@ -602,9 +602,7 @@ template <class Config> class Package {
 
       // all equal to zero
       if (argmax == -1) {
-        if (!cached && !e.isTerminal()) {
-          // If it is not a cached computation, the node has to be put back into
-          // the chain
+        if (!e.isTerminal()) {
           getMemoryManager<Node>().returnEntry(e.p);
         }
         return Edge<Node>::zero;

test/dd/test_package.cpp

@@ -805,6 +805,7 @@ TEST(DDPackageTest, NearZeroNormalize) {
   auto meNormalizedCached = dd->normalize(me, true);
   EXPECT_EQ(meNormalizedCached, dd::mEdge::zero);
 
+  me.p = dd->mMemoryManager.get();
   for (auto& edge : me.p->e) {
     edge.p = dd->mMemoryManager.get();
     edge.p->v = 0;
@@ -1824,3 +1825,25 @@ TEST(DDPackageTest, InnerProductTopNodeConjugation) {
   // it will result in +0.416.
   EXPECT_NEAR(dd->expectationValue(op, evolvedState), -0.416, 0.001);
 }
+
+/**
+ * @brief This is a regression test for a long lasting memory leak in the DD
+ * package.
+ * @details The memory leak was caused by a bug in the normalization routine
+ * which was not properly returning a node to the memory manager. This occurred
+ * whenever the multiplication of two DDs resulted in a zero terminal.
+ */
+TEST(DDPackageTest, DDNodeLeakRegressionTest) {
+  const auto nqubits = 1U;
+  auto matrix = dd::GateMatrix{dd::complex_one, dd::complex_zero,
+                               dd::complex_zero, dd::complex_zero};
+  auto dd = std::make_unique<dd::Package<>>(nqubits);
+
+  auto dd1 = dd->makeGateDD(matrix, nqubits, 0U);
+  matrix[0] = dd::complex_zero;
+  matrix[3] = dd::complex_one;
+  auto dd2 = dd->makeGateDD(matrix, nqubits, 0U);
+  dd->multiply(dd1, dd2);
+  dd->garbageCollect(true);
+  EXPECT_EQ(dd->mMemoryManager.getUsedCount(), 0U);
+}