Use SelectCoinsSRD if it has less waste

Try to find a solution with SelectCoinsSRD. If we do have one, add it to
the list of solutions from which we choose the one with the least waste
as the solution to use.

Author: achow101

src/wallet/spend.cpp

@@ -387,6 +387,15 @@ bool AttemptSelection(const CWallet& wallet, const CAmount& nTargetValue, const
         results.emplace_back(std::make_tuple(waste, std::move(knapsack_coins), knapsack_value));
     }
 
+    // We include the minimum final change for SRD as we do want to avoid making really small change.
+    // KnapsackSolver does not need this because it includes MIN_CHANGE internally.
+    const CAmount srd_target = nTargetValue + coin_selection_params.m_change_fee + MIN_FINAL_CHANGE;
+    auto srd_result = SelectCoinsSRD(positive_groups, srd_target);
+    if (srd_result != std::nullopt) {
+        const auto waste = GetSelectionWaste(srd_result->first, coin_selection_params.m_cost_of_change, srd_target, !coin_selection_params.m_subtract_fee_outputs);
+        results.emplace_back(std::make_tuple(waste, std::move(srd_result->first), srd_result->second));
+    }
+
     if (results.size() == 0) {
         // No solution found
         return false;