clusterlin: abstract out DepGraph::GetReduced{Parents,Children}

A fuzz test already relies on these operations, and a future commit will need
the same logic too. Therefore, abstract them out into proper member functions,
with proper testing.

src/cluster_linearize.h

@@ -184,6 +184,48 @@ public:
         }
     }
 
+    /** Compute the (reduced) set of parents of node i in this graph.
+     *
+     * This returns the minimal subset of the parents of i whose ancestors together equal all of
+     * i's ancestors (unless i is part of a cycle of dependencies). Note that DepGraph does not
+     * store the set of parents; this information is inferred from the ancestor sets.
+     *
+     * Complexity: O(N) where N=Ancestors(i).Count() (which is bounded by TxCount()).
+     */
+    SetType GetReducedParents(ClusterIndex i) const noexcept
+    {
+        SetType parents = Ancestors(i);
+        parents.Reset(i);
+        for (auto parent : parents) {
+            if (parents[parent]) {
+                parents -= Ancestors(parent);
+                parents.Set(parent);
+            }
+        }
+        return parents;
+    }
+
+    /** Compute the (reduced) set of children of node i in this graph.
+     *
+     * This returns the minimal subset of the children of i whose descendants together equal all of
+     * i's descendants (unless i is part of a cycle of dependencies). Note that DepGraph does not
+     * store the set of children; this information is inferred from the descendant sets.
+     *
+     * Complexity: O(N) where N=Descendants(i).Count() (which is bounded by TxCount()).
+     */
+    SetType GetReducedChildren(ClusterIndex i) const noexcept
+    {
+        SetType children = Descendants(i);
+        children.Reset(i);
+        for (auto child : children) {
+            if (children[child]) {
+                children -= Descendants(child);
+                children.Set(child);
+            }
+        }
+        return children;
+    }
+
     /** Compute the aggregate feerate of a set of nodes in this graph.
      *
      * Complexity: O(N) where N=elems.Count().

src/test/fuzz/cluster_linearize.cpp

@@ -896,24 +896,12 @@ FUZZ_TARGET(clusterlin_postlinearize_tree)
     }
     if (direction & 1) {
         for (ClusterIndex i = 0; i < depgraph_gen.TxCount(); ++i) {
-            auto children = depgraph_gen.Descendants(i) - TestBitSet::Singleton(i);
-            // Remove descendants that are children of other descendants.
-            for (auto j : children) {
-                if (!children[j]) continue;
-                children -= depgraph_gen.Descendants(j);
-                children.Set(j);
-            }
+            auto children = depgraph_gen.GetReducedChildren(i);
             if (children.Any()) depgraph_tree.AddDependency(i, children.First());
          }
     } else {
         for (ClusterIndex i = 0; i < depgraph_gen.TxCount(); ++i) {
-            auto parents = depgraph_gen.Ancestors(i) - TestBitSet::Singleton(i);
-            // Remove ancestors that are parents of other ancestors.
-            for (auto j : parents) {
-                if (!parents[j]) continue;
-                parents -= depgraph_gen.Ancestors(j);
-                parents.Set(j);
-            }
+            auto parents = depgraph_gen.GetReducedParents(i);
             if (parents.Any()) depgraph_tree.AddDependency(parents.First(), i);
         }
     }

src/test/util/cluster_linearize.h

@@ -264,9 +264,23 @@ void SanityCheck(const DepGraph<SetType>& depgraph)
         for (ClusterIndex j = 0; j < depgraph.TxCount(); ++j) {
             assert(depgraph.Ancestors(i)[j] == depgraph.Descendants(j)[i]);
         }
+        // No transaction is a parent or child of itself.
+        auto parents = depgraph.GetReducedParents(i);
+        auto children = depgraph.GetReducedChildren(i);
+        assert(!parents[i]);
+        assert(!children[i]);
+        // Parents of a transaction do not have ancestors inside those parents (except itself).
+        // Note that even the transaction itself may be missing (if it is part of a cycle).
+        for (auto parent : parents) {
+            assert((depgraph.Ancestors(parent) & parents).IsSubsetOf(SetType::Singleton(parent)));
+        }
+        // Similar for children and descendants.
+        for (auto child : children) {
+            assert((depgraph.Descendants(child) & children).IsSubsetOf(SetType::Singleton(child)));
+        }
     }
-    // If DepGraph is acyclic, serialize + deserialize must roundtrip.
     if (IsAcyclic(depgraph)) {
+        // If DepGraph is acyclic, serialize + deserialize must roundtrip.
         std::vector<unsigned char> ser;
         VectorWriter writer(ser, 0);
         writer << Using<DepGraphFormatter>(depgraph);
@@ -284,6 +298,37 @@ void SanityCheck(const DepGraph<SetType>& depgraph)
         reader >> Using<DepGraphFormatter>(decoded_depgraph);
         assert(depgraph == decoded_depgraph);
         assert(reader.empty());
+
+        // In acyclic graphs, the union of parents with parents of parents etc. yields the
+        // full ancestor set (and similar for children and descendants).
+        std::vector<SetType> parents, children;
+        for (ClusterIndex i = 0; i < depgraph.TxCount(); ++i) {
+            parents.push_back(depgraph.GetReducedParents(i));
+            children.push_back(depgraph.GetReducedChildren(i));
+        }
+        for (ClusterIndex i = 0; i < depgraph.TxCount(); ++i) {
+            // Initialize the set of ancestors with just the current transaction itself.
+            SetType ancestors = SetType::Singleton(i);
+            // Iteratively add parents of all transactions in the ancestor set to itself.
+            while (true) {
+                const auto old_ancestors = ancestors;
+                for (auto j : ancestors) ancestors |= parents[j];
+                // Stop when no more changes are being made.
+                if (old_ancestors == ancestors) break;
+            }
+            assert(ancestors == depgraph.Ancestors(i));
+
+            // Initialize the set of descendants with just the current transaction itself.
+            SetType descendants = SetType::Singleton(i);
+            // Iteratively add children of all transactions in the descendant set to itself.
+            while (true) {
+                const auto old_descendants = descendants;
+                for (auto j : descendants) descendants |= children[j];
+                // Stop when no more changes are being made.
+                if (old_descendants == descendants) break;
+            }
+            assert(descendants == depgraph.Descendants(i));
+        }
     }
 }