clusterlin: add reordering support for DepGraph

Add a DepGraph(depgraph, reordering) function that constructs a new DepGraph
corresponding to an old one, but with its transactions is a modified order
(given as a vector from old to new positions).

Also use this reordering feature inside DepGraphFormatter::Unser, which needs
a small modification so that its reordering mapping is old-to-new (rather than
the new-to-old it used before).

src/cluster_linearize.h

@@ -118,6 +118,28 @@ public:
         }
     }
 
+    /** Construct a DepGraph object given another DepGraph and a mapping from old to new.
+     *
+     * Complexity: O(N^2) where N=depgraph.TxCount().
+     */
+    DepGraph(const DepGraph<SetType>& depgraph, Span<const ClusterIndex> mapping) noexcept : entries(depgraph.TxCount())
+    {
+        Assert(mapping.size() == depgraph.TxCount());
+        // Fill in fee, size, ancestors.
+        for (ClusterIndex i = 0; i < depgraph.TxCount(); ++i) {
+            const auto& input = depgraph.entries[i];
+            auto& output = entries[mapping[i]];
+            output.feerate = input.feerate;
+            for (auto j : input.ancestors) output.ancestors.Set(mapping[j]);
+        }
+        // Fill in descendant information.
+        for (ClusterIndex i = 0; i < entries.size(); ++i) {
+            for (auto j : entries[i].ancestors) {
+                entries[j].descendants.Set(i);
+            }
+        }
+    }
+
     /** Get the number of transactions in the graph. Complexity: O(1). */
     auto TxCount() const noexcept { return entries.size(); }
     /** Get the feerate of a given transaction i. Complexity: O(1). */

src/test/util/cluster_linearize.h

@@ -186,7 +186,7 @@ struct DepGraphFormatter
         /** The dependency graph which we deserialize into first, with transactions in
          *  topological serialization order, not original cluster order. */
         DepGraph<SetType> topo_depgraph;
-        /** Mapping from cluster order to serialization order, used later to reconstruct the
+        /** Mapping from serialization order to cluster order, used later to reconstruct the
          *  cluster order. */
         std::vector<ClusterIndex> reordering;
 
@@ -205,9 +205,9 @@ struct DepGraphFormatter
                 coded_fee &= 0xFFFFFFFFFFFFF; // Enough for fee between -21M...21M BTC.
                 static_assert(0xFFFFFFFFFFFFF > uint64_t{2} * 21000000 * 100000000);
                 auto fee = UnsignedToSigned(coded_fee);
-                // Extend topo_depgraph with the new transaction (at the end).
+                // Extend topo_depgraph with the new transaction (preliminarily at the end).
                 auto topo_idx = topo_depgraph.AddTransaction({fee, size});
-                reordering.push_back(topo_idx);
+                reordering.push_back(reordering.size());
                 // Read dependency information.
                 uint64_t diff = 0; //!< How many potential parents we have to skip.
                 s >> VARINT(diff);
@@ -226,31 +226,23 @@ struct DepGraphFormatter
                         --diff;
                     }
                 }
-                // If we reach this point, we can interpret the remaining skip value as how far from the
-                // end of reordering topo_idx should be placed (wrapping around), so move it to its
-                // correct location. The preliminary reordering.push_back(topo_idx) above was to make
-                // sure that if a deserialization exception occurs, topo_idx still appears somewhere.
+                // If we reach this point, we can interpret the remaining skip value as how far
+                // from the end of reordering the new transaction should be placed (wrapping
+                // around), so remove the preliminary position it was put in above (which was to
+                // make sure that if a deserialization exception occurs, the new transaction still
+                // has some entry in reordering).
                 reordering.pop_back();
-                reordering.insert(reordering.end() - (diff % (reordering.size() + 1)), topo_idx);
+                ClusterIndex insert_distance = diff % (reordering.size() + 1);
+                // And then update reordering to reflect this new transaction's insertion.
+                for (auto& pos : reordering) {
+                    pos += (pos >= reordering.size() - insert_distance);
+                }
+                reordering.push_back(reordering.size() - insert_distance);
             }
         } catch (const std::ios_base::failure&) {}
 
         // Construct the original cluster order depgraph.
-        depgraph = {};
-        // Add transactions to depgraph in the original cluster order.
-        for (auto topo_idx : reordering) {
-            depgraph.AddTransaction(topo_depgraph.FeeRate(topo_idx));
-        }
-        // Translate dependencies from topological to cluster order.
-        for (ClusterIndex idx = 0; idx < reordering.size(); ++idx) {
-            ClusterIndex topo_idx = reordering[idx];
-            for (ClusterIndex dep_idx = 0; dep_idx < reordering.size(); ++dep_idx) {
-                ClusterIndex dep_topo_idx = reordering[dep_idx];
-                if (topo_depgraph.Ancestors(topo_idx)[dep_topo_idx]) {
-                    depgraph.AddDependency(dep_idx, idx);
-                }
-            }
-        }
+        depgraph = DepGraph(topo_depgraph, reordering);
     }
 };