Merge bitcoin/bitcoin#22019: wallet: Introduce SelectionResult for encapsulating a coin selection solution

05300c14392facf38330eb4fcd8e695a838b76f3 Use SelectionResult in SelectCoins (Andrew Chow)
9d9b101d2019d8237546eedd022e74519feb07bb Use SelectionResult in AttemptSelection (Andrew Chow)
bb50850a447bdf461ffb76d47d4a4db904fce324 Use SelectionResult for waste calculation (Andrew Chow)
e8f7ae5eb3c682d1a80b503f71e06ce76af1b65c Make an OutputGroup for preset inputs (Andrew Chow)
51a9c00b4de707e0a6a1a68ca6f8e38d86c72d94 Return SelectionResult from SelectCoinsSRD (Andrew Chow)
0ef6184575e77b17f5ec6d7ca086900aca79f6d7 Return SelectionResult from KnapsackSolver (Andrew Chow)
60d2ca72e3f4c56433c63b929a88e7a2def06399 Return SelectionResult from SelectCoinsBnB (Andrew Chow)
a339add471717623915cd1a846ade4dab2c89deb Make member variables of SelectionResult private (Andrew Chow)
cbf0b9f4ff438865a71c7ceb0a543c18a34f41f0 scripted-diff: Use SelectionResult in coin selector tests (Andrew Chow)
9d1d86da04d5d4768975338841285e90b01130b8 Introduce SelectionResult struct (Andrew Chow)
94d851d28cb909a8f1f8ab795f1d9fc74bebfc7f Fix bnb_search_test to use set equivalence for (Andrew Chow)

Pull request description:

  Instead of returning a set of selected coins and their total value as separate items, encapsulate both of these, and other variables, into a new `SelectionResult` struct. This allows us to have all of the things relevant to a coin selection solution be in a single object. `SelectionResult` enables us to implement the waste calculation in a cleaner way.

  All of the coin selection functions (`SelectCoinsBnB`, `KnapsackSolver`, `AttemptSelection`, and `SelectCoins`) are changed to use a `SelectionResult` as the output parameter.

  Based on #22009

ACKs for top commit:
  laanwj:
    Code review ACK 05300c14392facf38330eb4fcd8e695a838b76f3

Tree-SHA512: e4dbb4d78a6cda9c237d230b19e7265591efac5a101a64e6970f0654e2c4f93d13bb5d07b98e8c7b8d37321753dbfc94c28c3a7810cb1c59b5bc29b08a8493ef

6 files changed, 433 insertions, 310 deletions

diff --git a/src/bench/coin_selection.cpp b/src/bench/coin_selection.cpp
index 3c24fee60f..f97bf8028a 100644
--- a/src/bench/coin_selection.cpp
+++ b/src/bench/coin_selection.cpp
@@ -54,12 +54,10 @@ static void CoinSelection(benchmark::Bench& bench)
                                                     /* long_term_feerate= */ CFeeRate(0), /* discard_feerate= */ CFeeRate(0),
                                                     /* tx_noinputs_size= */ 0, /* avoid_partial= */ false);
     bench.run([&] {
-        std::set<CInputCoin> setCoinsRet;
-        CAmount nValueRet;
-        bool success = AttemptSelection(wallet, 1003 * COIN, filter_standard, coins, setCoinsRet, nValueRet, coin_selection_params);
-        assert(success);
-        assert(nValueRet == 1003 * COIN);
-        assert(setCoinsRet.size() == 2);
+        auto result = AttemptSelection(wallet, 1003 * COIN, filter_standard, coins, coin_selection_params);
+        assert(result);
+        assert(result->GetSelectedValue() == 1003 * COIN);
+        assert(result->GetInputSet().size() == 2);
     });
 }
 
@@ -92,17 +90,14 @@ static void BnBExhaustion(benchmark::Bench& bench)
 {
     // Setup
     std::vector<OutputGroup> utxo_pool;
-    CoinSet selection;
-    CAmount value_ret = 0;
 
     bench.run([&] {
         // Benchmark
         CAmount target = make_hard_case(17, utxo_pool);
-        SelectCoinsBnB(utxo_pool, target, 0, selection, value_ret); // Should exhaust
+        SelectCoinsBnB(utxo_pool, target, 0); // Should exhaust
 
         // Cleanup
         utxo_pool.clear();
-        selection.clear();
     });
 }
 
diff --git a/src/wallet/coinselection.cpp b/src/wallet/coinselection.cpp
index c0ca931364..ddf6a8b829 100644
--- a/src/wallet/coinselection.cpp
+++ b/src/wallet/coinselection.cpp
@@ -56,17 +56,14 @@ struct {
  *        bound of the range.
  * @param const CAmount& cost_of_change This is the cost of creating and spending a change output.
  *        This plus selection_target is the upper bound of the range.
- * @param std::set<CInputCoin>& out_set -> This is an output parameter for the set of CInputCoins
- *        that have been selected.
- * @param CAmount& value_ret -> This is an output parameter for the total value of the CInputCoins
- *        that were selected.
+ * @returns The result of this coin selection algorithm, or std::nullopt
  */
 
 static const size_t TOTAL_TRIES = 100000;
 
-bool SelectCoinsBnB(std::vector<OutputGroup>& utxo_pool, const CAmount& selection_target, const CAmount& cost_of_change, std::set<CInputCoin>& out_set, CAmount& value_ret)
+std::optional<SelectionResult> SelectCoinsBnB(std::vector<OutputGroup>& utxo_pool, const CAmount& selection_target, const CAmount& cost_of_change)
 {
-    out_set.clear();
+    SelectionResult result(selection_target);
     CAmount curr_value = 0;
 
     std::vector<bool> curr_selection; // select the utxo at this index
@@ -80,7 +77,7 @@ bool SelectCoinsBnB(std::vector<OutputGroup>& utxo_pool, const CAmount& selectio
         curr_available_value += utxo.GetSelectionAmount();
     }
     if (curr_available_value < selection_target) {
-        return false;
+        return std::nullopt;
     }
 
     // Sort the utxo_pool
@@ -156,25 +153,22 @@ bool SelectCoinsBnB(std::vector<OutputGroup>& utxo_pool, const CAmount& selectio
 
     // Check for solution
     if (best_selection.empty()) {
-        return false;
+        return std::nullopt;
     }
 
     // Set output set
-    value_ret = 0;
     for (size_t i = 0; i < best_selection.size(); ++i) {
         if (best_selection.at(i)) {
-            util::insert(out_set, utxo_pool.at(i).m_outputs);
-            value_ret += utxo_pool.at(i).m_value;
+            result.AddInput(utxo_pool.at(i));
         }
     }
 
-    return true;
+    return result;
 }
 
-std::optional<std::pair<std::set<CInputCoin>, CAmount>> SelectCoinsSRD(const std::vector<OutputGroup>& utxo_pool, CAmount target_value)
+std::optional<SelectionResult> SelectCoinsSRD(const std::vector<OutputGroup>& utxo_pool, CAmount target_value)
 {
-    std::set<CInputCoin> out_set;
-    CAmount value_ret = 0;
+    SelectionResult result(target_value);
 
     std::vector<size_t> indexes;
     indexes.resize(utxo_pool.size());
@@ -186,10 +180,9 @@ std::optional<std::pair<std::set<CInputCoin>, CAmount>> SelectCoinsSRD(const std
         const OutputGroup& group = utxo_pool.at(i);
         Assume(group.GetSelectionAmount() > 0);
         selected_eff_value += group.GetSelectionAmount();
-        value_ret += group.m_value;
-        util::insert(out_set, group.m_outputs);
+        result.AddInput(group);
         if (selected_eff_value >= target_value) {
-            return std::make_pair(out_set, value_ret);
+            return result;
         }
     }
     return std::nullopt;
@@ -241,10 +234,9 @@ static void ApproximateBestSubset(const std::vector<OutputGroup>& groups, const
     }
 }
 
-bool KnapsackSolver(const CAmount& nTargetValue, std::vector<OutputGroup>& groups, std::set<CInputCoin>& setCoinsRet, CAmount& nValueRet)
+std::optional<SelectionResult> KnapsackSolver(std::vector<OutputGroup>& groups, const CAmount& nTargetValue)
 {
-    setCoinsRet.clear();
-    nValueRet = 0;
+    SelectionResult result(nTargetValue);
 
     // List of values less than target
     std::optional<OutputGroup> lowest_larger;
@@ -255,9 +247,8 @@ bool KnapsackSolver(const CAmount& nTargetValue, std::vector<OutputGroup>& group
 
     for (const OutputGroup& group : groups) {
         if (group.GetSelectionAmount() == nTargetValue) {
-            util::insert(setCoinsRet, group.m_outputs);
-            nValueRet += group.m_value;
-            return true;
+            result.AddInput(group);
+            return result;
         } else if (group.GetSelectionAmount() < nTargetValue + MIN_CHANGE) {
             applicable_groups.push_back(group);
             nTotalLower += group.GetSelectionAmount();
@@ -268,17 +259,15 @@ bool KnapsackSolver(const CAmount& nTargetValue, std::vector<OutputGroup>& group
 
     if (nTotalLower == nTargetValue) {
         for (const auto& group : applicable_groups) {
-            util::insert(setCoinsRet, group.m_outputs);
-            nValueRet += group.m_value;
+            result.AddInput(group);
         }
-        return true;
+        return result;
     }
 
     if (nTotalLower < nTargetValue) {
-        if (!lowest_larger) return false;
-        util::insert(setCoinsRet, lowest_larger->m_outputs);
-        nValueRet += lowest_larger->m_value;
-        return true;
+        if (!lowest_larger) return std::nullopt;
+        result.AddInput(*lowest_larger);
+        return result;
     }
 
     // Solve subset sum by stochastic approximation
@@ -295,13 +284,11 @@ bool KnapsackSolver(const CAmount& nTargetValue, std::vector<OutputGroup>& group
     //                                   or the next bigger coin is closer), return the bigger coin
     if (lowest_larger &&
         ((nBest != nTargetValue && nBest < nTargetValue + MIN_CHANGE) || lowest_larger->GetSelectionAmount() <= nBest)) {
-        util::insert(setCoinsRet, lowest_larger->m_outputs);
-        nValueRet += lowest_larger->m_value;
+        result.AddInput(*lowest_larger);
     } else {
         for (unsigned int i = 0; i < applicable_groups.size(); i++) {
             if (vfBest[i]) {
-                util::insert(setCoinsRet, applicable_groups[i].m_outputs);
-                nValueRet += applicable_groups[i].m_value;
+                result.AddInput(applicable_groups[i]);
             }
         }
 
@@ -316,7 +303,7 @@ bool KnapsackSolver(const CAmount& nTargetValue, std::vector<OutputGroup>& group
         }
     }
 
-    return true;
+    return result;
 }
 
 /******************************************************************************
@@ -395,3 +382,51 @@ CAmount GetSelectionWaste(const std::set<CInputCoin>& inputs, CAmount change_cos
 
     return waste;
 }
+
+void SelectionResult::ComputeAndSetWaste(CAmount change_cost)
+{
+    m_waste = GetSelectionWaste(m_selected_inputs, change_cost, m_target, m_use_effective);
+}
+
+CAmount SelectionResult::GetWaste() const
+{
+    Assume(m_waste != std::nullopt);
+    return *m_waste;
+}
+
+CAmount SelectionResult::GetSelectedValue() const
+{
+    return std::accumulate(m_selected_inputs.cbegin(), m_selected_inputs.cend(), CAmount{0}, [](CAmount sum, const auto& coin) { return sum + coin.txout.nValue; });
+}
+
+void SelectionResult::Clear()
+{
+    m_selected_inputs.clear();
+    m_waste.reset();
+}
+
+void SelectionResult::AddInput(const OutputGroup& group)
+{
+    util::insert(m_selected_inputs, group.m_outputs);
+    m_use_effective = !group.m_subtract_fee_outputs;
+}
+
+const std::set<CInputCoin>& SelectionResult::GetInputSet() const
+{
+    return m_selected_inputs;
+}
+
+std::vector<CInputCoin> SelectionResult::GetShuffledInputVector() const
+{
+    std::vector<CInputCoin> coins(m_selected_inputs.begin(), m_selected_inputs.end());
+    Shuffle(coins.begin(), coins.end(), FastRandomContext());
+    return coins;
+}
+
+bool SelectionResult::operator<(SelectionResult other) const
+{
+    Assume(m_waste != std::nullopt);
+    Assume(other.m_waste != std::nullopt);
+    // As this operator is only used in std::min_element, we want the result that has more inputs when waste are equal.
+    return *m_waste < *other.m_waste || (*m_waste == *other.m_waste && m_selected_inputs.size() > other.m_selected_inputs.size());
+}
diff --git a/src/wallet/coinselection.h b/src/wallet/coinselection.h
index e7d467660f..637afcdb2b 100644
--- a/src/wallet/coinselection.h
+++ b/src/wallet/coinselection.h
@@ -187,6 +187,8 @@ struct OutputGroup
  * where excess = selected_effective_value - target
  * change_cost = effective_feerate * change_output_size + long_term_feerate * change_spend_size
  *
+ * Note this function is separate from SelectionResult for the tests.
+ *
  * @param[in] inputs The selected inputs
  * @param[in] change_cost The cost of creating change and spending it in the future.
  *                        Only used if there is change, in which case it must be positive.
@@ -197,18 +199,55 @@ struct OutputGroup
  */
 [[nodiscard]] CAmount GetSelectionWaste(const std::set<CInputCoin>& inputs, CAmount change_cost, CAmount target, bool use_effective_value = true);
 
-bool SelectCoinsBnB(std::vector<OutputGroup>& utxo_pool, const CAmount& selection_target, const CAmount& cost_of_change, std::set<CInputCoin>& out_set, CAmount& value_ret);
+struct SelectionResult
+{
+private:
+    /** Set of inputs selected by the algorithm to use in the transaction */
+    std::set<CInputCoin> m_selected_inputs;
+    /** The target the algorithm selected for. Note that this may not be equal to the recipient amount as it can include non-input fees */
+    const CAmount m_target;
+    /** Whether the input values for calculations should be the effective value (true) or normal value (false) */
+    bool m_use_effective{false};
+    /** The computed waste */
+    std::optional<CAmount> m_waste;
+
+public:
+    explicit SelectionResult(const CAmount target)
+        : m_target(target) {}
+
+    SelectionResult() = delete;
+
+    /** Get the sum of the input values */
+    [[nodiscard]] CAmount GetSelectedValue() const;
+
+    void Clear();
+
+    void AddInput(const OutputGroup& group);
+
+    /** Calculates and stores the waste for this selection via GetSelectionWaste */
+    void ComputeAndSetWaste(CAmount change_cost);
+    [[nodiscard]] CAmount GetWaste() const;
+
+    /** Get m_selected_inputs */
+    const std::set<CInputCoin>& GetInputSet() const;
+    /** Get the vector of CInputCoins that will be used to fill in a CTransaction's vin */
+    std::vector<CInputCoin> GetShuffledInputVector() const;
+
+    bool operator<(SelectionResult other) const;
+};
+
+std::optional<SelectionResult> SelectCoinsBnB(std::vector<OutputGroup>& utxo_pool, const CAmount& selection_target, const CAmount& cost_of_change);
 
 /** Select coins by Single Random Draw. OutputGroups are selected randomly from the eligible
  * outputs until the target is satisfied
  *
  * @param[in]  utxo_pool    The positive effective value OutputGroups eligible for selection
  * @param[in]  target_value The target value to select for
- * @returns If successful, a pair of set of outputs and total selected value, otherwise, std::nullopt
+ * @returns If successful, a SelectionResult, otherwise, std::nullopt
  */
-std::optional<std::pair<std::set<CInputCoin>, CAmount>> SelectCoinsSRD(const std::vector<OutputGroup>& utxo_pool, CAmount target_value);
+std::optional<SelectionResult> SelectCoinsSRD(const std::vector<OutputGroup>& utxo_pool, CAmount target_value);
 
 // Original coin selection algorithm as a fallback
-bool KnapsackSolver(const CAmount& nTargetValue, std::vector<OutputGroup>& groups, std::set<CInputCoin>& setCoinsRet, CAmount& nValueRet);
+std::optional<SelectionResult> KnapsackSolver(std::vector<OutputGroup>& groups, const CAmount& nTargetValue);
 
 #endif // BITCOIN_WALLET_COINSELECTION_H
diff --git a/src/wallet/spend.cpp b/src/wallet/spend.cpp
index 5470177440..5d9cc7bf6b 100644
--- a/src/wallet/spend.cpp
+++ b/src/wallet/spend.cpp
@@ -373,81 +373,72 @@ std::vector<OutputGroup> GroupOutputs(const CWallet& wallet, const std::vector<C
     return groups_out;
 }
 
-bool AttemptSelection(const CWallet& wallet, const CAmount& nTargetValue, const CoinEligibilityFilter& eligibility_filter, std::vector<COutput> coins,
-                                 std::set<CInputCoin>& setCoinsRet, CAmount& nValueRet, const CoinSelectionParams& coin_selection_params)
+std::optional<SelectionResult> AttemptSelection(const CWallet& wallet, const CAmount& nTargetValue, const CoinEligibilityFilter& eligibility_filter, std::vector<COutput> coins,
+                               const CoinSelectionParams& coin_selection_params)
 {
-    setCoinsRet.clear();
-    nValueRet = 0;
-    // Vector of results for use with waste calculation
-    // In order: calculated waste, selected inputs, selected input value (sum of input values)
-    // TODO: Use a struct representing the selection result
-    std::vector<std::tuple<CAmount, std::set<CInputCoin>, CAmount>> results;
+    // Vector of results. We will choose the best one based on waste.
+    std::vector<SelectionResult> results;
 
     // Note that unlike KnapsackSolver, we do not include the fee for creating a change output as BnB will not create a change output.
     std::vector<OutputGroup> positive_groups = GroupOutputs(wallet, coins, coin_selection_params, eligibility_filter, true /* positive_only */);
-    std::set<CInputCoin> bnb_coins;
-    CAmount bnb_value;
-    if (SelectCoinsBnB(positive_groups, nTargetValue, coin_selection_params.m_cost_of_change, bnb_coins, bnb_value)) {
-        const auto waste = GetSelectionWaste(bnb_coins, /* cost of change */ CAmount(0), nTargetValue, !coin_selection_params.m_subtract_fee_outputs);
-        results.emplace_back(std::make_tuple(waste, std::move(bnb_coins), bnb_value));
+    if (auto bnb_result{SelectCoinsBnB(positive_groups, nTargetValue, coin_selection_params.m_cost_of_change)}) {
+        bnb_result->ComputeAndSetWaste(CAmount(0));
+        results.push_back(*bnb_result);
     }
 
     // The knapsack solver has some legacy behavior where it will spend dust outputs. We retain this behavior, so don't filter for positive only here.
     std::vector<OutputGroup> all_groups = GroupOutputs(wallet, coins, coin_selection_params, eligibility_filter, false /* positive_only */);
     // While nTargetValue includes the transaction fees for non-input things, it does not include the fee for creating a change output.
     // So we need to include that for KnapsackSolver as well, as we are expecting to create a change output.
-    std::set<CInputCoin> knapsack_coins;
-    CAmount knapsack_value;
-    if (KnapsackSolver(nTargetValue + coin_selection_params.m_change_fee, all_groups, knapsack_coins, knapsack_value)) {
-        const auto waste = GetSelectionWaste(knapsack_coins, coin_selection_params.m_cost_of_change, nTargetValue + coin_selection_params.m_change_fee, !coin_selection_params.m_subtract_fee_outputs);
-        results.emplace_back(std::make_tuple(waste, std::move(knapsack_coins), knapsack_value));
+    if (auto knapsack_result{KnapsackSolver(all_groups, nTargetValue + coin_selection_params.m_change_fee)}) {
+        knapsack_result->ComputeAndSetWaste(coin_selection_params.m_cost_of_change);
+        results.push_back(*knapsack_result);
     }
 
     // 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 (auto srd_result{SelectCoinsSRD(positive_groups, srd_target)}) {
+        srd_result->ComputeAndSetWaste(coin_selection_params.m_cost_of_change);
+        results.push_back(*srd_result);
     }
 
     if (results.size() == 0) {
         // No solution found
-        return false;
+        return std::nullopt;
     }
 
     // Choose the result with the least waste
     // If the waste is the same, choose the one which spends more inputs.
-    const auto& best_result = std::min_element(results.begin(), results.end(), [](const auto& a, const auto& b) {
-        return std::get<0>(a) < std::get<0>(b) || (std::get<0>(a) == std::get<0>(b) && std::get<1>(a).size() > std::get<1>(b).size());
-    });
-    setCoinsRet = std::get<1>(*best_result);
-    nValueRet = std::get<2>(*best_result);
-    return true;
+    auto& best_result = *std::min_element(results.begin(), results.end());
+    return best_result;
 }
 
-bool SelectCoins(const CWallet& wallet, const std::vector<COutput>& vAvailableCoins, const CAmount& nTargetValue, std::set<CInputCoin>& setCoinsRet, CAmount& nValueRet, const CCoinControl& coin_control, CoinSelectionParams& coin_selection_params)
+std::optional<SelectionResult> SelectCoins(const CWallet& wallet, const std::vector<COutput>& vAvailableCoins, const CAmount& nTargetValue, const CCoinControl& coin_control, const CoinSelectionParams& coin_selection_params)
 {
     std::vector<COutput> vCoins(vAvailableCoins);
     CAmount value_to_select = nTargetValue;
 
+    OutputGroup preset_inputs(coin_selection_params);
+
     // coin control -> return all selected outputs (we want all selected to go into the transaction for sure)
     if (coin_control.HasSelected() && !coin_control.fAllowOtherInputs)
     {
-        for (const COutput& out : vCoins)
-        {
-            if (!out.fSpendable)
-                 continue;
-            nValueRet += out.tx->tx->vout[out.i].nValue;
-            setCoinsRet.insert(out.GetInputCoin());
+        for (const COutput& out : vCoins) {
+            if (!out.fSpendable) continue;
+            /* Set depth, from_me, ancestors, and descendants to 0 or false as these don't matter for preset inputs as no actual selection is being done.
+             * positive_only is set to false because we want to include all preset inputs, even if they are dust.
+             */
+            preset_inputs.Insert(out.GetInputCoin(), 0, false, 0, 0, false);
         }
-        return (nValueRet >= nTargetValue);
+        SelectionResult result(nTargetValue);
+        result.AddInput(preset_inputs);
+        if (result.GetSelectedValue() < nTargetValue) return std::nullopt;
+        return result;
     }
 
     // calculate value from preset inputs and store them
     std::set<CInputCoin> setPresetCoins;
-    CAmount nValueFromPresetInputs = 0;
 
     std::vector<COutPoint> vPresetInputs;
     coin_control.ListSelected(vPresetInputs);
@@ -459,7 +450,7 @@ bool SelectCoins(const CWallet& wallet, const std::vector<COutput>& vAvailableCo
             const CWalletTx& wtx = it->second;
             // Clearly invalid input, fail
             if (wtx.tx->vout.size() <= outpoint.n) {
-                return false;
+                return std::nullopt;
             }
             input_bytes = GetTxSpendSize(wallet, wtx, outpoint.n, false);
             txout = wtx.tx->vout.at(outpoint.n);
@@ -468,15 +459,14 @@ bool SelectCoins(const CWallet& wallet, const std::vector<COutput>& vAvailableCo
             // The input is external. We either did not find the tx in mapWallet, or we did but couldn't compute the input size with wallet data
             if (!coin_control.GetExternalOutput(outpoint, txout)) {
                 // Not ours, and we don't have solving data.
-                return false;
+                return std::nullopt;
             }
             input_bytes = CalculateMaximumSignedInputSize(txout, &coin_control.m_external_provider, /* use_max_sig */ true);
         }
 
         CInputCoin coin(outpoint, txout, input_bytes);
-        nValueFromPresetInputs += coin.txout.nValue;
         if (coin.m_input_bytes == -1) {
-            return false; // Not solvable, can't estimate size for fee
+            return std::nullopt; // Not solvable, can't estimate size for fee
         }
         coin.effective_value = coin.txout.nValue - coin_selection_params.m_effective_feerate.GetFee(coin.m_input_bytes);
         if (coin_selection_params.m_subtract_fee_outputs) {
@@ -485,6 +475,10 @@ bool SelectCoins(const CWallet& wallet, const std::vector<COutput>& vAvailableCo
             value_to_select -= coin.effective_value;
         }
         setPresetCoins.insert(coin);
+        /* Set depth, from_me, ancestors, and descendants to 0 or false as don't matter for preset inputs as no actual selection is being done.
+         * positive_only is set to false because we want to include all preset inputs, even if they are dust.
+         */
+        preset_inputs.Insert(coin, 0, false, 0, 0, false);
     }
 
     // remove preset inputs from vCoins so that Coin Selection doesn't pick them.
@@ -515,60 +509,62 @@ bool SelectCoins(const CWallet& wallet, const std::vector<COutput>& vAvailableCo
     // Coin Selection attempts to select inputs from a pool of eligible UTXOs to fund the
     // transaction at a target feerate. If an attempt fails, more attempts may be made using a more
     // permissive CoinEligibilityFilter.
-    const bool res = [&] {
+    std::optional<SelectionResult> res = [&] {
         // Pre-selected inputs already cover the target amount.
-        if (value_to_select <= 0) return true;
+        if (value_to_select <= 0) return std::make_optional(SelectionResult(nTargetValue));
 
         // If possible, fund the transaction with confirmed UTXOs only. Prefer at least six
         // confirmations on outputs received from other wallets and only spend confirmed change.
-        if (AttemptSelection(wallet, value_to_select, CoinEligibilityFilter(1, 6, 0), vCoins, setCoinsRet, nValueRet, coin_selection_params)) return true;
-        if (AttemptSelection(wallet, value_to_select, CoinEligibilityFilter(1, 1, 0), vCoins, setCoinsRet, nValueRet, coin_selection_params)) return true;
+        if (auto r1{AttemptSelection(wallet, value_to_select, CoinEligibilityFilter(1, 6, 0), vCoins, coin_selection_params)}) return r1;
+        if (auto r2{AttemptSelection(wallet, value_to_select, CoinEligibilityFilter(1, 1, 0), vCoins, coin_selection_params)}) return r2;
 
         // Fall back to using zero confirmation change (but with as few ancestors in the mempool as
         // possible) if we cannot fund the transaction otherwise.
         if (wallet.m_spend_zero_conf_change) {
-            if (AttemptSelection(wallet, value_to_select, CoinEligibilityFilter(0, 1, 2), vCoins, setCoinsRet, nValueRet, coin_selection_params)) return true;
-            if (AttemptSelection(wallet, value_to_select, CoinEligibilityFilter(0, 1, std::min((size_t)4, max_ancestors/3), std::min((size_t)4, max_descendants/3)),
-                                   vCoins, setCoinsRet, nValueRet, coin_selection_params)) {
-                return true;
+            if (auto r3{AttemptSelection(wallet, value_to_select, CoinEligibilityFilter(0, 1, 2), vCoins, coin_selection_params)}) return r3;
+            if (auto r4{AttemptSelection(wallet, value_to_select, CoinEligibilityFilter(0, 1, std::min((size_t)4, max_ancestors/3), std::min((size_t)4, max_descendants/3)),
+                                   vCoins, coin_selection_params)}) {
+                return r4;
             }
-            if (AttemptSelection(wallet, value_to_select, CoinEligibilityFilter(0, 1, max_ancestors/2, max_descendants/2),
-                                   vCoins, setCoinsRet, nValueRet, coin_selection_params)) {
-                return true;
+            if (auto r5{AttemptSelection(wallet, value_to_select, CoinEligibilityFilter(0, 1, max_ancestors/2, max_descendants/2),
+                                   vCoins, coin_selection_params)}) {
+                return r5;
             }
             // If partial groups are allowed, relax the requirement of spending OutputGroups (groups
             // of UTXOs sent to the same address, which are obviously controlled by a single wallet)
             // in their entirety.
-            if (AttemptSelection(wallet, value_to_select, CoinEligibilityFilter(0, 1, max_ancestors-1, max_descendants-1, true /* include_partial_groups */),
-                                   vCoins, setCoinsRet, nValueRet, coin_selection_params)) {
-                return true;
+            if (auto r6{AttemptSelection(wallet, value_to_select, CoinEligibilityFilter(0, 1, max_ancestors-1, max_descendants-1, true /* include_partial_groups */),
+                                   vCoins, coin_selection_params)}) {
+                return r6;
             }
             // Try with unsafe inputs if they are allowed. This may spend unconfirmed outputs
             // received from other wallets.
-            if (coin_control.m_include_unsafe_inputs
-                && AttemptSelection(wallet, value_to_select,
+            if (coin_control.m_include_unsafe_inputs) {
+                if (auto r7{AttemptSelection(wallet, value_to_select,
                     CoinEligibilityFilter(0 /* conf_mine */, 0 /* conf_theirs */, max_ancestors-1, max_descendants-1, true /* include_partial_groups */),
-                    vCoins, setCoinsRet, nValueRet, coin_selection_params)) {
-                return true;
+                    vCoins, coin_selection_params)}) {
+                    return r7;
+                }
             }
             // Try with unlimited ancestors/descendants. The transaction will still need to meet
             // mempool ancestor/descendant policy to be accepted to mempool and broadcasted, but
             // OutputGroups use heuristics that may overestimate ancestor/descendant counts.
-            if (!fRejectLongChains && AttemptSelection(wallet, value_to_select,
+            if (!fRejectLongChains) {
+                if (auto r8{AttemptSelection(wallet, value_to_select,
                                       CoinEligibilityFilter(0, 1, std::numeric_limits<uint64_t>::max(), std::numeric_limits<uint64_t>::max(), true /* include_partial_groups */),
-                                      vCoins, setCoinsRet, nValueRet, coin_selection_params)) {
-                return true;
+                                      vCoins, coin_selection_params)}) {
+                    return r8;
+                }
             }
         }
         // Coin Selection failed.
-        return false;
+        return std::optional<SelectionResult>();
     }();
 
-    // AttemptSelection clears setCoinsRet, so add the preset inputs from coin_control to the coinset
-    util::insert(setCoinsRet, setPresetCoins);
+    if (!res) return std::nullopt;
 
-    // add preset inputs to the total value selected
-    nValueRet += nValueFromPresetInputs;
+    // Add preset inputs to result
+    res->AddInput(preset_inputs);
 
     return res;
 }
@@ -766,17 +762,15 @@ static bool CreateTransactionInternal(
     AvailableCoins(wallet, vAvailableCoins, &coin_control, 1, MAX_MONEY, MAX_MONEY, 0);
 
     // Choose coins to use
-    CAmount inputs_sum = 0;
-    std::set<CInputCoin> setCoins;
-    if (!SelectCoins(wallet, vAvailableCoins, /* nTargetValue */ selection_target, setCoins, inputs_sum, coin_control, coin_selection_params))
-    {
+    std::optional<SelectionResult> result = SelectCoins(wallet, vAvailableCoins, /* nTargetValue */ selection_target, coin_control, coin_selection_params);
+    if (!result) {
         error = _("Insufficient funds");
         return false;
     }
 
     // Always make a change output
     // We will reduce the fee from this change output later, and remove the output if it is too small.
-    const CAmount change_and_fee = inputs_sum - recipients_sum;
+    const CAmount change_and_fee = result->GetSelectedValue() - recipients_sum;
     assert(change_and_fee >= 0);
     CTxOut newTxOut(change_and_fee, scriptChange);
 
@@ -795,8 +789,7 @@ static bool CreateTransactionInternal(
     auto change_position = txNew.vout.insert(txNew.vout.begin() + nChangePosInOut, newTxOut);
 
     // Shuffle selected coins and fill in final vin
-    std::vector<CInputCoin> selected_coins(setCoins.begin(), setCoins.end());
-    Shuffle(selected_coins.begin(), selected_coins.end(), FastRandomContext());
+    std::vector<CInputCoin> selected_coins = result->GetShuffledInputVector();
 
     // Note how the sequence number is set to non-maxint so that
     // the nLockTime set above actually works.
diff --git a/src/wallet/spend.h b/src/wallet/spend.h
index 7467dd9fa3..268bcfc033 100644
--- a/src/wallet/spend.h
+++ b/src/wallet/spend.h
@@ -101,29 +101,34 @@ std::map<CTxDestination, std::vector<COutput>> ListCoins(const CWallet& wallet)
 std::vector<OutputGroup> GroupOutputs(const CWallet& wallet, const std::vector<COutput>& outputs, const CoinSelectionParams& coin_sel_params, const CoinEligibilityFilter& filter, bool positive_only);
 
 /**
- * Shuffle and select coins until nTargetValue is reached while avoiding
- * small change; This method is stochastic for some inputs and upon
- * completion the coin set and corresponding actual target value is
- * assembled
- * param@[in]   coins           Set of UTXOs to consider. These will be categorized into
- *                              OutputGroups and filtered using eligibility_filter before
- *                              selecting coins.
- * param@[out]  setCoinsRet     Populated with the coins selected if successful.
- * param@[out]  nValueRet       Used to return the total value of selected coins.
+ * Attempt to find a valid input set that meets the provided eligibility filter and target.
+ * Multiple coin selection algorithms will be run and the input set that produces the least waste
+ * (according to the waste metric) will be chosen.
+ *
+ * param@[in]  wallet                 The wallet which provides solving data for the coins
+ * param@[in]  nTargetValue           The target value
+ * param@[in]  eligilibity_filter     A filter containing rules for which coins are allowed to be included in this selection
+ * param@[in]  coins                  The vector of coins available for selection prior to filtering
+ * param@[in]  coin_selection_params  Parameters for the coin selection
+ * returns                            If successful, a SelectionResult containing the input set
+ *                                    If failed, a nullopt
  */
-bool AttemptSelection(const CWallet& wallet, const CAmount& nTargetValue, const CoinEligibilityFilter& eligibility_filter, std::vector<COutput> coins,
-                        std::set<CInputCoin>& setCoinsRet, CAmount& nValueRet, const CoinSelectionParams& coin_selection_params);
+std::optional<SelectionResult> AttemptSelection(const CWallet& wallet, const CAmount& nTargetValue, const CoinEligibilityFilter& eligibility_filter, std::vector<COutput> coins,
+                        const CoinSelectionParams& coin_selection_params);
 
 /**
- * Select a set of coins such that nValueRet >= nTargetValue and at least
+ * Select a set of coins such that nTargetValue is met and at least
  * all coins from coin_control are selected; never select unconfirmed coins if they are not ours
- * param@[out]  setCoinsRet         Populated with inputs including pre-selected inputs from
- *                                  coin_control and Coin Selection if successful.
- * param@[out]  nValueRet           Total value of selected coins including pre-selected ones
- *                                  from coin_control and Coin Selection if successful.
+ * param@[in]   wallet                 The wallet which provides data necessary to spend the selected coins
+ * param@[in]   vAvailableCoins        The vector of coins available to be spent
+ * param@[in]   nTargetValue           The target value
+ * param@[in]   coin_selection_params  Parameters for this coin selection such as feerates, whether to avoid partial spends,
+ *                                     and whether to subtract the fee from the outputs.
+ * returns                             If successful, a SelectionResult containing the selected coins
+ *                                     If failed, a nullopt.
  */
-bool SelectCoins(const CWallet& wallet, const std::vector<COutput>& vAvailableCoins, const CAmount& nTargetValue, std::set<CInputCoin>& setCoinsRet, CAmount& nValueRet,
-                 const CCoinControl& coin_control, CoinSelectionParams& coin_selection_params) EXCLUSIVE_LOCKS_REQUIRED(wallet.cs_wallet);
+std::optional<SelectionResult> SelectCoins(const CWallet& wallet, const std::vector<COutput>& vAvailableCoins, const CAmount& nTargetValue, const CCoinControl& coin_control,
+                 const CoinSelectionParams& coin_selection_params) EXCLUSIVE_LOCKS_REQUIRED(wallet.cs_wallet);
 
 /**
  * Create a new transaction paying the recipients with a set of coins
diff --git a/src/wallet/test/coinselector_tests.cpp b/src/wallet/test/coinselector_tests.cpp
index 0acd8a811f..a1fa871ccb 100644
--- a/src/wallet/test/coinselector_tests.cpp
+++ b/src/wallet/test/coinselector_tests.cpp
@@ -14,6 +14,7 @@
 #include <wallet/test/wallet_test_fixture.h>
 #include <wallet/wallet.h>
 
+#include <algorithm>
 #include <boost/test/unit_test.hpp>
 #include <random>
 
@@ -31,20 +32,35 @@ typedef std::set<CInputCoin> CoinSet;
 static const CoinEligibilityFilter filter_standard(1, 6, 0);
 static const CoinEligibilityFilter filter_confirmed(1, 1, 0);
 static const CoinEligibilityFilter filter_standard_extra(6, 6, 0);
+static int nextLockTime = 0;
 
 static void add_coin(const CAmount& nValue, int nInput, std::vector<CInputCoin>& set)
 {
     CMutableTransaction tx;
     tx.vout.resize(nInput + 1);
     tx.vout[nInput].nValue = nValue;
+    tx.nLockTime = nextLockTime++;        // so all transactions get different hashes
     set.emplace_back(MakeTransactionRef(tx), nInput);
 }
 
+static void add_coin(const CAmount& nValue, int nInput, SelectionResult& result)
+{
+    CMutableTransaction tx;
+    tx.vout.resize(nInput + 1);
+    tx.vout[nInput].nValue = nValue;
+    tx.nLockTime = nextLockTime++;        // so all transactions get different hashes
+    CInputCoin coin(MakeTransactionRef(tx), nInput);
+    OutputGroup group;
+    group.Insert(coin, 1, false, 0, 0, true);
+    result.AddInput(group);
+}
+
 static void add_coin(const CAmount& nValue, int nInput, CoinSet& set, CAmount fee = 0, CAmount long_term_fee = 0)
 {
     CMutableTransaction tx;
     tx.vout.resize(nInput + 1);
     tx.vout[nInput].nValue = nValue;
+    tx.nLockTime = nextLockTime++;        // so all transactions get different hashes
     CInputCoin coin(MakeTransactionRef(tx), nInput);
     coin.effective_value = nValue - fee;
     coin.m_fee = fee;
@@ -54,7 +70,6 @@ static void add_coin(const CAmount& nValue, int nInput, CoinSet& set, CAmount fe
 
 static void add_coin(std::vector<COutput>& coins, CWallet& wallet, const CAmount& nValue, int nAge = 6*24, bool fIsFromMe = false, int nInput=0, bool spendable = false)
 {
-    static int nextLockTime = 0;
     CMutableTransaction tx;
     tx.nLockTime = nextLockTime++;        // so all transactions get different hashes
     tx.vout.resize(nInput + 1);
@@ -86,10 +101,30 @@ static void add_coin(std::vector<COutput>& coins, CWallet& wallet, const CAmount
     coins.push_back(output);
 }
 
-static bool equal_sets(CoinSet a, CoinSet b)
+/** Check if SelectionResult a is equivalent to SelectionResult b.
+ * Equivalent means same input values, but maybe different inputs (i.e. same value, different prevout) */
+static bool EquivalentResult(const SelectionResult& a, const SelectionResult& b)
+{
+    std::vector<CAmount> a_amts;
+    std::vector<CAmount> b_amts;
+    for (const auto& coin : a.GetInputSet()) {
+        a_amts.push_back(coin.txout.nValue);
+    }
+    for (const auto& coin : b.GetInputSet()) {
+        b_amts.push_back(coin.txout.nValue);
+    }
+    std::sort(a_amts.begin(), a_amts.end());
+    std::sort(b_amts.begin(), b_amts.end());
+
+    std::pair<std::vector<CAmount>::iterator, std::vector<CAmount>::iterator> ret = std::mismatch(a_amts.begin(), a_amts.end(), b_amts.begin());
+    return ret.first == a_amts.end() && ret.second == b_amts.end();
+}
+
+/** Check if this selection is equal to another one. Equal means same inputs (i.e same value and prevout) */
+static bool EqualResult(const SelectionResult& a, const SelectionResult& b)
 {
-    std::pair<CoinSet::iterator, CoinSet::iterator> ret = mismatch(a.begin(), a.end(), b.begin());
-    return ret.first == a.end() && ret.second == b.end();
+    std::pair<CoinSet::iterator, CoinSet::iterator> ret = std::mismatch(a.GetInputSet().begin(), a.GetInputSet().end(), b.GetInputSet().begin());
+    return ret.first == a.GetInputSet().end() && ret.second == b.GetInputSet().end();
 }
 
 static CAmount make_hard_case(int utxos, std::vector<CInputCoin>& utxo_pool)
@@ -142,17 +177,14 @@ BOOST_AUTO_TEST_CASE(bnb_search_test)
 {
     // Setup
     std::vector<CInputCoin> utxo_pool;
-    CoinSet selection;
-    CoinSet actual_selection;
-    CAmount value_ret = 0;
+    SelectionResult expected_result(CAmount(0));
 
     /////////////////////////
     // Known Outcome tests //
     /////////////////////////
 
     // Empty utxo pool
-    BOOST_CHECK(!SelectCoinsBnB(GroupCoins(utxo_pool), 1 * CENT, 0.5 * CENT, selection, value_ret));
-    selection.clear();
+    BOOST_CHECK(!SelectCoinsBnB(GroupCoins(utxo_pool), 1 * CENT, 0.5 * CENT));
 
     // Add utxos
     add_coin(1 * CENT, 1, utxo_pool);
@@ -161,87 +193,86 @@ BOOST_AUTO_TEST_CASE(bnb_search_test)
     add_coin(4 * CENT, 4, utxo_pool);
 
     // Select 1 Cent
-    add_coin(1 * CENT, 1, actual_selection);
-    BOOST_CHECK(SelectCoinsBnB(GroupCoins(utxo_pool), 1 * CENT, 0.5 * CENT, selection, value_ret));
-    BOOST_CHECK(equal_sets(selection, actual_selection));
-    BOOST_CHECK_EQUAL(value_ret, 1 * CENT);
-    actual_selection.clear();
-    selection.clear();
+    add_coin(1 * CENT, 1, expected_result);
+    const auto result1 = SelectCoinsBnB(GroupCoins(utxo_pool), 1 * CENT, 0.5 * CENT);
+    BOOST_CHECK(result1);
+    BOOST_CHECK(EquivalentResult(expected_result, *result1));
+    BOOST_CHECK_EQUAL(result1->GetSelectedValue(), 1 * CENT);
+    expected_result.Clear();
 
     // Select 2 Cent
-    add_coin(2 * CENT, 2, actual_selection);
-    BOOST_CHECK(SelectCoinsBnB(GroupCoins(utxo_pool), 2 * CENT, 0.5 * CENT, selection, value_ret));
-    BOOST_CHECK(equal_sets(selection, actual_selection));
-    BOOST_CHECK_EQUAL(value_ret, 2 * CENT);
-    actual_selection.clear();
-    selection.clear();
+    add_coin(2 * CENT, 2, expected_result);
+    const auto result2 = SelectCoinsBnB(GroupCoins(utxo_pool), 2 * CENT, 0.5 * CENT);
+    BOOST_CHECK(result2);
+    BOOST_CHECK(EquivalentResult(expected_result, *result2));
+    BOOST_CHECK_EQUAL(result2->GetSelectedValue(), 2 * CENT);
+    expected_result.Clear();
 
     // Select 5 Cent
-    add_coin(4 * CENT, 4, actual_selection);
-    add_coin(1 * CENT, 1, actual_selection);
-    BOOST_CHECK(SelectCoinsBnB(GroupCoins(utxo_pool), 5 * CENT, 0.5 * CENT, selection, value_ret));
-    BOOST_CHECK(equal_sets(selection, actual_selection));
-    BOOST_CHECK_EQUAL(value_ret, 5 * CENT);
-    actual_selection.clear();
-    selection.clear();
+    add_coin(4 * CENT, 4, expected_result);
+    add_coin(1 * CENT, 1, expected_result);
+    const auto result3 = SelectCoinsBnB(GroupCoins(utxo_pool), 5 * CENT, 0.5 * CENT);
+    BOOST_CHECK(result3);
+    BOOST_CHECK(EquivalentResult(expected_result, *result3));
+    BOOST_CHECK_EQUAL(result3->GetSelectedValue(), 5 * CENT);
+    expected_result.Clear();
 
     // Select 11 Cent, not possible
-    BOOST_CHECK(!SelectCoinsBnB(GroupCoins(utxo_pool), 11 * CENT, 0.5 * CENT, selection, value_ret));
-    actual_selection.clear();
-    selection.clear();
+    BOOST_CHECK(!SelectCoinsBnB(GroupCoins(utxo_pool), 11 * CENT, 0.5 * CENT));
+    expected_result.Clear();
 
     // Cost of change is greater than the difference between target value and utxo sum
-    add_coin(1 * CENT, 1, actual_selection);
-    BOOST_CHECK(SelectCoinsBnB(GroupCoins(utxo_pool), 0.9 * CENT, 0.5 * CENT, selection, value_ret));
-    BOOST_CHECK_EQUAL(value_ret, 1 * CENT);
-    BOOST_CHECK(equal_sets(selection, actual_selection));
-    actual_selection.clear();
-    selection.clear();
+    add_coin(1 * CENT, 1, expected_result);
+    const auto result4 = SelectCoinsBnB(GroupCoins(utxo_pool), 0.9 * CENT, 0.5 * CENT);
+    BOOST_CHECK(result4);
+    BOOST_CHECK_EQUAL(result4->GetSelectedValue(), 1 * CENT);
+    BOOST_CHECK(EquivalentResult(expected_result, *result4));
+    expected_result.Clear();
 
     // Cost of change is less than the difference between target value and utxo sum
-    BOOST_CHECK(!SelectCoinsBnB(GroupCoins(utxo_pool), 0.9 * CENT, 0, selection, value_ret));
-    actual_selection.clear();
-    selection.clear();
+    BOOST_CHECK(!SelectCoinsBnB(GroupCoins(utxo_pool), 0.9 * CENT, 0));
+    expected_result.Clear();
 
     // Select 10 Cent
     add_coin(5 * CENT, 5, utxo_pool);
-    add_coin(5 * CENT, 5, actual_selection);
-    add_coin(4 * CENT, 4, actual_selection);
-    add_coin(1 * CENT, 1, actual_selection);
-    BOOST_CHECK(SelectCoinsBnB(GroupCoins(utxo_pool), 10 * CENT, 0.5 * CENT, selection, value_ret));
-    BOOST_CHECK(equal_sets(selection, actual_selection));
-    BOOST_CHECK_EQUAL(value_ret, 10 * CENT);
-    actual_selection.clear();
-    selection.clear();
+    add_coin(5 * CENT, 5, expected_result);
+    add_coin(4 * CENT, 4, expected_result);
+    add_coin(1 * CENT, 1, expected_result);
+    const auto result5 = SelectCoinsBnB(GroupCoins(utxo_pool), 10 * CENT, 0.5 * CENT);
+    BOOST_CHECK(result5);
+    BOOST_CHECK(EquivalentResult(expected_result, *result5));
+    BOOST_CHECK_EQUAL(result5->GetSelectedValue(), 10 * CENT);
+    expected_result.Clear();
 
     // Negative effective value
     // Select 10 Cent but have 1 Cent not be possible because too small
-    add_coin(5 * CENT, 5, actual_selection);
-    add_coin(3 * CENT, 3, actual_selection);
-    add_coin(2 * CENT, 2, actual_selection);
-    BOOST_CHECK(SelectCoinsBnB(GroupCoins(utxo_pool), 10 * CENT, 5000, selection, value_ret));
-    BOOST_CHECK_EQUAL(value_ret, 10 * CENT);
+    add_coin(5 * CENT, 5, expected_result);
+    add_coin(3 * CENT, 3, expected_result);
+    add_coin(2 * CENT, 2, expected_result);
+    const auto result6 = SelectCoinsBnB(GroupCoins(utxo_pool), 10 * CENT, 5000);
+    BOOST_CHECK(result6);
+    BOOST_CHECK_EQUAL(result6->GetSelectedValue(), 10 * CENT);
     // FIXME: this test is redundant with the above, because 1 Cent is selected, not "too small"
-    // BOOST_CHECK(equal_sets(selection, actual_selection));
+    // BOOST_CHECK(EquivalentResult(expected_result, *result));
 
     // Select 0.25 Cent, not possible
-    BOOST_CHECK(!SelectCoinsBnB(GroupCoins(utxo_pool), 0.25 * CENT, 0.5 * CENT, selection, value_ret));
-    actual_selection.clear();
-    selection.clear();
+    BOOST_CHECK(!SelectCoinsBnB(GroupCoins(utxo_pool), 0.25 * CENT, 0.5 * CENT));
+    expected_result.Clear();
 
     // Iteration exhaustion test
     CAmount target = make_hard_case(17, utxo_pool);
-    BOOST_CHECK(!SelectCoinsBnB(GroupCoins(utxo_pool), target, 0, selection, value_ret)); // Should exhaust
+    BOOST_CHECK(!SelectCoinsBnB(GroupCoins(utxo_pool), target, 0)); // Should exhaust
     target = make_hard_case(14, utxo_pool);
-    BOOST_CHECK(SelectCoinsBnB(GroupCoins(utxo_pool), target, 0, selection, value_ret)); // Should not exhaust
+    const auto result7 = SelectCoinsBnB(GroupCoins(utxo_pool), target, 0); // Should not exhaust
+    BOOST_CHECK(result7);
 
     // Test same value early bailout optimization
     utxo_pool.clear();
-    add_coin(7 * CENT, 7, actual_selection);
-    add_coin(7 * CENT, 7, actual_selection);
-    add_coin(7 * CENT, 7, actual_selection);
-    add_coin(7 * CENT, 7, actual_selection);
-    add_coin(2 * CENT, 7, actual_selection);
+    add_coin(7 * CENT, 7, expected_result);
+    add_coin(7 * CENT, 7, expected_result);
+    add_coin(7 * CENT, 7, expected_result);
+    add_coin(7 * CENT, 7, expected_result);
+    add_coin(2 * CENT, 7, expected_result);
     add_coin(7 * CENT, 7, utxo_pool);
     add_coin(7 * CENT, 7, utxo_pool);
     add_coin(7 * CENT, 7, utxo_pool);
@@ -250,9 +281,10 @@ BOOST_AUTO_TEST_CASE(bnb_search_test)
     for (int i = 0; i < 50000; ++i) {
         add_coin(5 * CENT, 7, utxo_pool);
     }
-    BOOST_CHECK(SelectCoinsBnB(GroupCoins(utxo_pool), 30 * CENT, 5000, selection, value_ret));
-    BOOST_CHECK_EQUAL(value_ret, 30 * CENT);
-    BOOST_CHECK(equal_sets(selection, actual_selection));
+    const auto result8 = SelectCoinsBnB(GroupCoins(utxo_pool), 30 * CENT, 5000);
+    BOOST_CHECK(result8);
+    BOOST_CHECK_EQUAL(result8->GetSelectedValue(), 30 * CENT);
+    BOOST_CHECK(EquivalentResult(expected_result, *result8));
 
     ////////////////////
     // Behavior tests //
@@ -264,7 +296,7 @@ BOOST_AUTO_TEST_CASE(bnb_search_test)
     }
     // Run 100 times, to make sure it is never finding a solution
     for (int i = 0; i < 100; ++i) {
-        BOOST_CHECK(!SelectCoinsBnB(GroupCoins(utxo_pool), 1 * CENT, 2 * CENT, selection, value_ret));
+        BOOST_CHECK(!SelectCoinsBnB(GroupCoins(utxo_pool), 1 * CENT, 2 * CENT));
     }
 
     // Make sure that effective value is working in AttemptSelection when BnB is used
@@ -280,20 +312,19 @@ BOOST_AUTO_TEST_CASE(bnb_search_test)
         wallet->SetupDescriptorScriptPubKeyMans();
 
         std::vector<COutput> coins;
-        CoinSet setCoinsRet;
-        CAmount nValueRet;
 
         add_coin(coins, *wallet, 1);
         coins.at(0).nInputBytes = 40; // Make sure that it has a negative effective value. The next check should assert if this somehow got through. Otherwise it will fail
-        BOOST_CHECK(!SelectCoinsBnB(GroupCoins(coins), 1 * CENT, coin_selection_params_bnb.m_cost_of_change, setCoinsRet, nValueRet));
+        BOOST_CHECK(!SelectCoinsBnB(GroupCoins(coins), 1 * CENT, coin_selection_params_bnb.m_cost_of_change));
 
         // Test fees subtracted from output:
         coins.clear();
         add_coin(coins, *wallet, 1 * CENT);
         coins.at(0).nInputBytes = 40;
         coin_selection_params_bnb.m_subtract_fee_outputs = true;
-        BOOST_CHECK(SelectCoinsBnB(GroupCoins(coins), 1 * CENT, coin_selection_params_bnb.m_cost_of_change, setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 1 * CENT);
+        const auto result9 = SelectCoinsBnB(GroupCoins(coins), 1 * CENT, coin_selection_params_bnb.m_cost_of_change);
+        BOOST_CHECK(result9);
+        BOOST_CHECK_EQUAL(result9->GetSelectedValue(), 1 * CENT);
     }
 
     {
@@ -304,8 +335,6 @@ BOOST_AUTO_TEST_CASE(bnb_search_test)
         wallet->SetupDescriptorScriptPubKeyMans();
 
         std::vector<COutput> coins;
-        CoinSet setCoinsRet;
-        CAmount nValueRet;
 
         add_coin(coins, *wallet, 5 * CENT, 6 * 24, false, 0, true);
         add_coin(coins, *wallet, 3 * CENT, 6 * 24, false, 0, true);
@@ -314,7 +343,8 @@ BOOST_AUTO_TEST_CASE(bnb_search_test)
         coin_control.fAllowOtherInputs = true;
         coin_control.Select(COutPoint(coins.at(0).tx->GetHash(), coins.at(0).i));
         coin_selection_params_bnb.m_effective_feerate = CFeeRate(0);
-        BOOST_CHECK(SelectCoins(*wallet, coins, 10 * CENT, setCoinsRet, nValueRet, coin_control, coin_selection_params_bnb));
+        const auto result10 = SelectCoins(*wallet, coins, 10 * CENT, coin_control, coin_selection_params_bnb);
+        BOOST_CHECK(result10);
     }
 }
 
@@ -326,8 +356,6 @@ BOOST_AUTO_TEST_CASE(knapsack_solver_test)
     wallet->SetWalletFlag(WALLET_FLAG_DESCRIPTORS);
     wallet->SetupDescriptorScriptPubKeyMans();
 
-    CoinSet setCoinsRet, setCoinsRet2;
-    CAmount nValueRet;
     std::vector<COutput> coins;
 
     // test multiple times to allow for differences in the shuffle order
@@ -336,25 +364,27 @@ BOOST_AUTO_TEST_CASE(knapsack_solver_test)
         coins.clear();
 
         // with an empty wallet we can't even pay one cent
-        BOOST_CHECK(!KnapsackSolver(1 * CENT, KnapsackGroupOutputs(coins, *wallet, filter_standard), setCoinsRet, nValueRet));
+        BOOST_CHECK(!KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_standard), 1 * CENT));
 
         add_coin(coins, *wallet, 1*CENT, 4);        // add a new 1 cent coin
 
         // with a new 1 cent coin, we still can't find a mature 1 cent
-        BOOST_CHECK(!KnapsackSolver(1 * CENT, KnapsackGroupOutputs(coins, *wallet, filter_standard), setCoinsRet, nValueRet));
+        BOOST_CHECK(!KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_standard), 1 * CENT));
 
         // but we can find a new 1 cent
-        BOOST_CHECK(KnapsackSolver(1 * CENT, KnapsackGroupOutputs(coins, *wallet, filter_confirmed), setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 1 * CENT);
+        const auto result1 = KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_confirmed), 1 * CENT);
+        BOOST_CHECK(result1);
+        BOOST_CHECK_EQUAL(result1->GetSelectedValue(), 1 * CENT);
 
         add_coin(coins, *wallet, 2*CENT);           // add a mature 2 cent coin
 
         // we can't make 3 cents of mature coins
-        BOOST_CHECK(!KnapsackSolver(3 * CENT, KnapsackGroupOutputs(coins, *wallet, filter_standard), setCoinsRet, nValueRet));
+        BOOST_CHECK(!KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_standard), 3 * CENT));
 
         // we can make 3 cents of new coins
-        BOOST_CHECK(KnapsackSolver(3 * CENT, KnapsackGroupOutputs(coins, *wallet, filter_confirmed), setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 3 * CENT);
+        const auto result2 = KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_confirmed), 3 * CENT);
+        BOOST_CHECK(result2);
+        BOOST_CHECK_EQUAL(result2->GetSelectedValue(), 3 * CENT);
 
         add_coin(coins, *wallet, 5*CENT);           // add a mature 5 cent coin,
         add_coin(coins, *wallet, 10*CENT, 3, true); // a new 10 cent coin sent from one of our own addresses
@@ -363,35 +393,41 @@ BOOST_AUTO_TEST_CASE(knapsack_solver_test)
         // now we have new: 1+10=11 (of which 10 was self-sent), and mature: 2+5+20=27.  total = 38
 
         // we can't make 38 cents only if we disallow new coins:
-        BOOST_CHECK(!KnapsackSolver(38 * CENT, KnapsackGroupOutputs(coins, *wallet, filter_standard), setCoinsRet, nValueRet));
+        BOOST_CHECK(!KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_standard), 38 * CENT));
         // we can't even make 37 cents if we don't allow new coins even if they're from us
-        BOOST_CHECK(!KnapsackSolver(38 * CENT, KnapsackGroupOutputs(coins, *wallet, filter_standard_extra), setCoinsRet, nValueRet));
+        BOOST_CHECK(!KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_standard_extra), 38 * CENT));
         // but we can make 37 cents if we accept new coins from ourself
-        BOOST_CHECK(KnapsackSolver(37 * CENT, KnapsackGroupOutputs(coins, *wallet, filter_standard), setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 37 * CENT);
+        const auto result3 = KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_standard), 37 * CENT);
+        BOOST_CHECK(result3);
+        BOOST_CHECK_EQUAL(result3->GetSelectedValue(), 37 * CENT);
         // and we can make 38 cents if we accept all new coins
-        BOOST_CHECK(KnapsackSolver(38 * CENT, KnapsackGroupOutputs(coins, *wallet, filter_confirmed), setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 38 * CENT);
+        const auto result4 = KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_confirmed), 38 * CENT);
+        BOOST_CHECK(result4);
+        BOOST_CHECK_EQUAL(result4->GetSelectedValue(), 38 * CENT);
 
         // try making 34 cents from 1,2,5,10,20 - we can't do it exactly
-        BOOST_CHECK(KnapsackSolver(34 * CENT, KnapsackGroupOutputs(coins, *wallet, filter_confirmed), setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 35 * CENT);       // but 35 cents is closest
-        BOOST_CHECK_EQUAL(setCoinsRet.size(), 3U);     // the best should be 20+10+5.  it's incredibly unlikely the 1 or 2 got included (but possible)
+        const auto result5 = KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_confirmed), 34 * CENT);
+        BOOST_CHECK(result5);
+        BOOST_CHECK_EQUAL(result5->GetSelectedValue(), 35 * CENT);       // but 35 cents is closest
+        BOOST_CHECK_EQUAL(result5->GetInputSet().size(), 3U);     // the best should be 20+10+5.  it's incredibly unlikely the 1 or 2 got included (but possible)
 
         // when we try making 7 cents, the smaller coins (1,2,5) are enough.  We should see just 2+5
-        BOOST_CHECK(KnapsackSolver(7 * CENT, KnapsackGroupOutputs(coins, *wallet, filter_confirmed), setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 7 * CENT);
-        BOOST_CHECK_EQUAL(setCoinsRet.size(), 2U);
+        const auto result6 = KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_confirmed), 7 * CENT);
+        BOOST_CHECK(result6);
+        BOOST_CHECK_EQUAL(result6->GetSelectedValue(), 7 * CENT);
+        BOOST_CHECK_EQUAL(result6->GetInputSet().size(), 2U);
 
         // when we try making 8 cents, the smaller coins (1,2,5) are exactly enough.
-        BOOST_CHECK(KnapsackSolver(8 * CENT, KnapsackGroupOutputs(coins, *wallet, filter_confirmed), setCoinsRet, nValueRet));
-        BOOST_CHECK(nValueRet == 8 * CENT);
-        BOOST_CHECK_EQUAL(setCoinsRet.size(), 3U);
+        const auto result7 = KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_confirmed), 8 * CENT);
+        BOOST_CHECK(result7);
+        BOOST_CHECK(result7->GetSelectedValue() == 8 * CENT);
+        BOOST_CHECK_EQUAL(result7->GetInputSet().size(), 3U);
 
         // when we try making 9 cents, no subset of smaller coins is enough, and we get the next bigger coin (10)
-        BOOST_CHECK(KnapsackSolver(9 * CENT, KnapsackGroupOutputs(coins, *wallet, filter_confirmed), setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 10 * CENT);
-        BOOST_CHECK_EQUAL(setCoinsRet.size(), 1U);
+        const auto result8 = KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_confirmed), 9 * CENT);
+        BOOST_CHECK(result8);
+        BOOST_CHECK_EQUAL(result8->GetSelectedValue(), 10 * CENT);
+        BOOST_CHECK_EQUAL(result8->GetInputSet().size(), 1U);
 
         // now clear out the wallet and start again to test choosing between subsets of smaller coins and the next biggest coin
         coins.clear();
@@ -403,45 +439,52 @@ BOOST_AUTO_TEST_CASE(knapsack_solver_test)
         add_coin(coins, *wallet, 30*CENT); // now we have 6+7+8+20+30 = 71 cents total
 
         // check that we have 71 and not 72
-        BOOST_CHECK(KnapsackSolver(71 * CENT, KnapsackGroupOutputs(coins, *wallet, filter_confirmed), setCoinsRet, nValueRet));
-        BOOST_CHECK(!KnapsackSolver(72 * CENT, KnapsackGroupOutputs(coins, *wallet, filter_confirmed), setCoinsRet, nValueRet));
+        const auto result9 = KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_confirmed), 71 * CENT);
+        BOOST_CHECK(result9);
+        BOOST_CHECK(!KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_confirmed), 72 * CENT));
 
         // now try making 16 cents.  the best smaller coins can do is 6+7+8 = 21; not as good at the next biggest coin, 20
-        BOOST_CHECK(KnapsackSolver(16 * CENT, KnapsackGroupOutputs(coins, *wallet, filter_confirmed), setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 20 * CENT); // we should get 20 in one coin
-        BOOST_CHECK_EQUAL(setCoinsRet.size(), 1U);
+        const auto result10 = KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_confirmed), 16 * CENT);
+        BOOST_CHECK(result10);
+        BOOST_CHECK_EQUAL(result10->GetSelectedValue(), 20 * CENT); // we should get 20 in one coin
+        BOOST_CHECK_EQUAL(result10->GetInputSet().size(), 1U);
 
         add_coin(coins, *wallet,  5*CENT); // now we have 5+6+7+8+20+30 = 75 cents total
 
         // now if we try making 16 cents again, the smaller coins can make 5+6+7 = 18 cents, better than the next biggest coin, 20
-        BOOST_CHECK(KnapsackSolver(16 * CENT, KnapsackGroupOutputs(coins, *wallet, filter_confirmed), setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 18 * CENT); // we should get 18 in 3 coins
-        BOOST_CHECK_EQUAL(setCoinsRet.size(), 3U);
+        const auto result11 = KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_confirmed), 16 * CENT);
+        BOOST_CHECK(result11);
+        BOOST_CHECK_EQUAL(result11->GetSelectedValue(), 18 * CENT); // we should get 18 in 3 coins
+        BOOST_CHECK_EQUAL(result11->GetInputSet().size(), 3U);
 
         add_coin(coins, *wallet,  18*CENT); // now we have 5+6+7+8+18+20+30
 
         // and now if we try making 16 cents again, the smaller coins can make 5+6+7 = 18 cents, the same as the next biggest coin, 18
-        BOOST_CHECK(KnapsackSolver(16 * CENT, KnapsackGroupOutputs(coins, *wallet, filter_confirmed), setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 18 * CENT);  // we should get 18 in 1 coin
-        BOOST_CHECK_EQUAL(setCoinsRet.size(), 1U); // because in the event of a tie, the biggest coin wins
+        const auto result12 = KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_confirmed), 16 * CENT);
+        BOOST_CHECK(result12);
+        BOOST_CHECK_EQUAL(result12->GetSelectedValue(), 18 * CENT);  // we should get 18 in 1 coin
+        BOOST_CHECK_EQUAL(result12->GetInputSet().size(), 1U); // because in the event of a tie, the biggest coin wins
 
         // now try making 11 cents.  we should get 5+6
-        BOOST_CHECK(KnapsackSolver(11 * CENT, KnapsackGroupOutputs(coins, *wallet, filter_confirmed), setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 11 * CENT);
-        BOOST_CHECK_EQUAL(setCoinsRet.size(), 2U);
+        const auto result13 = KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_confirmed), 11 * CENT);
+        BOOST_CHECK(result13);
+        BOOST_CHECK_EQUAL(result13->GetSelectedValue(), 11 * CENT);
+        BOOST_CHECK_EQUAL(result13->GetInputSet().size(), 2U);
 
         // check that the smallest bigger coin is used
         add_coin(coins, *wallet,  1*COIN);
         add_coin(coins, *wallet,  2*COIN);
         add_coin(coins, *wallet,  3*COIN);
         add_coin(coins, *wallet,  4*COIN); // now we have 5+6+7+8+18+20+30+100+200+300+400 = 1094 cents
-        BOOST_CHECK(KnapsackSolver(95 * CENT, KnapsackGroupOutputs(coins, *wallet, filter_confirmed), setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 1 * COIN);  // we should get 1 BTC in 1 coin
-        BOOST_CHECK_EQUAL(setCoinsRet.size(), 1U);
+        const auto result14 = KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_confirmed), 95 * CENT);
+        BOOST_CHECK(result14);
+        BOOST_CHECK_EQUAL(result14->GetSelectedValue(), 1 * COIN);  // we should get 1 BTC in 1 coin
+        BOOST_CHECK_EQUAL(result14->GetInputSet().size(), 1U);
 
-        BOOST_CHECK(KnapsackSolver(195 * CENT, KnapsackGroupOutputs(coins, *wallet, filter_confirmed), setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 2 * COIN);  // we should get 2 BTC in 1 coin
-        BOOST_CHECK_EQUAL(setCoinsRet.size(), 1U);
+        const auto result15 = KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_confirmed), 195 * CENT);
+        BOOST_CHECK(result15);
+        BOOST_CHECK_EQUAL(result15->GetSelectedValue(), 2 * COIN);  // we should get 2 BTC in 1 coin
+        BOOST_CHECK_EQUAL(result15->GetInputSet().size(), 1U);
 
         // empty the wallet and start again, now with fractions of a cent, to test small change avoidance
 
@@ -454,23 +497,26 @@ BOOST_AUTO_TEST_CASE(knapsack_solver_test)
 
         // try making 1 * MIN_CHANGE from the 1.5 * MIN_CHANGE
         // we'll get change smaller than MIN_CHANGE whatever happens, so can expect MIN_CHANGE exactly
-        BOOST_CHECK(KnapsackSolver(MIN_CHANGE, KnapsackGroupOutputs(coins, *wallet, filter_confirmed), setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, MIN_CHANGE);
+        const auto result16 = KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_confirmed), MIN_CHANGE);
+        BOOST_CHECK(result16);
+        BOOST_CHECK_EQUAL(result16->GetSelectedValue(), MIN_CHANGE);
 
         // but if we add a bigger coin, small change is avoided
         add_coin(coins, *wallet, 1111*MIN_CHANGE);
 
         // try making 1 from 0.1 + 0.2 + 0.3 + 0.4 + 0.5 + 1111 = 1112.5
-        BOOST_CHECK(KnapsackSolver(1 * MIN_CHANGE, KnapsackGroupOutputs(coins, *wallet, filter_confirmed), setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 1 * MIN_CHANGE); // we should get the exact amount
+        const auto result17 = KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_confirmed), 1 * MIN_CHANGE);
+        BOOST_CHECK(result17);
+        BOOST_CHECK_EQUAL(result17->GetSelectedValue(), 1 * MIN_CHANGE); // we should get the exact amount
 
         // if we add more small coins:
         add_coin(coins, *wallet, MIN_CHANGE * 6 / 10);
         add_coin(coins, *wallet, MIN_CHANGE * 7 / 10);
 
         // and try again to make 1.0 * MIN_CHANGE
-        BOOST_CHECK(KnapsackSolver(1 * MIN_CHANGE, KnapsackGroupOutputs(coins, *wallet, filter_confirmed), setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 1 * MIN_CHANGE); // we should get the exact amount
+        const auto result18 = KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_confirmed), 1 * MIN_CHANGE);
+        BOOST_CHECK(result18);
+        BOOST_CHECK_EQUAL(result18->GetSelectedValue(), 1 * MIN_CHANGE); // we should get the exact amount
 
         // run the 'mtgox' test (see https://blockexplorer.com/tx/29a3efd3ef04f9153d47a990bd7b048a4b2d213daaa5fb8ed670fb85f13bdbcf)
         // they tried to consolidate 10 50k coins into one 500k coin, and ended up with 50k in change
@@ -478,9 +524,10 @@ BOOST_AUTO_TEST_CASE(knapsack_solver_test)
         for (int j = 0; j < 20; j++)
             add_coin(coins, *wallet, 50000 * COIN);
 
-        BOOST_CHECK(KnapsackSolver(500000 * COIN, KnapsackGroupOutputs(coins, *wallet, filter_confirmed), setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 500000 * COIN); // we should get the exact amount
-        BOOST_CHECK_EQUAL(setCoinsRet.size(), 10U); // in ten coins
+        const auto result19 = KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_confirmed), 500000 * COIN);
+        BOOST_CHECK(result19);
+        BOOST_CHECK_EQUAL(result19->GetSelectedValue(), 500000 * COIN); // we should get the exact amount
+        BOOST_CHECK_EQUAL(result19->GetInputSet().size(), 10U); // in ten coins
 
         // if there's not enough in the smaller coins to make at least 1 * MIN_CHANGE change (0.5+0.6+0.7 < 1.0+1.0),
         // we need to try finding an exact subset anyway
@@ -491,9 +538,10 @@ BOOST_AUTO_TEST_CASE(knapsack_solver_test)
         add_coin(coins, *wallet, MIN_CHANGE * 6 / 10);
         add_coin(coins, *wallet, MIN_CHANGE * 7 / 10);
         add_coin(coins, *wallet, 1111 * MIN_CHANGE);
-        BOOST_CHECK(KnapsackSolver(1 * MIN_CHANGE, KnapsackGroupOutputs(coins, *wallet, filter_confirmed), setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 1111 * MIN_CHANGE); // we get the bigger coin
-        BOOST_CHECK_EQUAL(setCoinsRet.size(), 1U);
+        const auto result20 = KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_confirmed), 1 * MIN_CHANGE);
+        BOOST_CHECK(result20);
+        BOOST_CHECK_EQUAL(result20->GetSelectedValue(), 1111 * MIN_CHANGE); // we get the bigger coin
+        BOOST_CHECK_EQUAL(result20->GetInputSet().size(), 1U);
 
         // but sometimes it's possible, and we use an exact subset (0.4 + 0.6 = 1.0)
         coins.clear();
@@ -501,9 +549,10 @@ BOOST_AUTO_TEST_CASE(knapsack_solver_test)
         add_coin(coins, *wallet, MIN_CHANGE * 6 / 10);
         add_coin(coins, *wallet, MIN_CHANGE * 8 / 10);
         add_coin(coins, *wallet, 1111 * MIN_CHANGE);
-        BOOST_CHECK(KnapsackSolver(MIN_CHANGE, KnapsackGroupOutputs(coins, *wallet, filter_confirmed), setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, MIN_CHANGE);   // we should get the exact amount
-        BOOST_CHECK_EQUAL(setCoinsRet.size(), 2U); // in two coins 0.4+0.6
+        const auto result21 = KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_confirmed), MIN_CHANGE);
+        BOOST_CHECK(result21);
+        BOOST_CHECK_EQUAL(result21->GetSelectedValue(), MIN_CHANGE);   // we should get the exact amount
+        BOOST_CHECK_EQUAL(result21->GetInputSet().size(), 2U); // in two coins 0.4+0.6
 
         // test avoiding small change
         coins.clear();
@@ -512,14 +561,16 @@ BOOST_AUTO_TEST_CASE(knapsack_solver_test)
         add_coin(coins, *wallet, MIN_CHANGE * 100);
 
         // trying to make 100.01 from these three coins
-        BOOST_CHECK(KnapsackSolver(MIN_CHANGE * 10001 / 100, KnapsackGroupOutputs(coins, *wallet, filter_confirmed), setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, MIN_CHANGE * 10105 / 100); // we should get all coins
-        BOOST_CHECK_EQUAL(setCoinsRet.size(), 3U);
+        const auto result22 = KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_confirmed), MIN_CHANGE * 10001 / 100);
+        BOOST_CHECK(result22);
+        BOOST_CHECK_EQUAL(result22->GetSelectedValue(), MIN_CHANGE * 10105 / 100); // we should get all coins
+        BOOST_CHECK_EQUAL(result22->GetInputSet().size(), 3U);
 
         // but if we try to make 99.9, we should take the bigger of the two small coins to avoid small change
-        BOOST_CHECK(KnapsackSolver(MIN_CHANGE * 9990 / 100, KnapsackGroupOutputs(coins, *wallet, filter_confirmed), setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 101 * MIN_CHANGE);
-        BOOST_CHECK_EQUAL(setCoinsRet.size(), 2U);
+        const auto result23 = KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_confirmed), MIN_CHANGE * 9990 / 100);
+        BOOST_CHECK(result23);
+        BOOST_CHECK_EQUAL(result23->GetSelectedValue(), 101 * MIN_CHANGE);
+        BOOST_CHECK_EQUAL(result23->GetInputSet().size(), 2U);
     }
 
     // test with many inputs
@@ -531,18 +582,19 @@ BOOST_AUTO_TEST_CASE(knapsack_solver_test)
 
         // We only create the wallet once to save time, but we still run the coin selection RUN_TESTS times.
         for (int i = 0; i < RUN_TESTS; i++) {
-            BOOST_CHECK(KnapsackSolver(2000, KnapsackGroupOutputs(coins, *wallet, filter_confirmed), setCoinsRet, nValueRet));
+            const auto result24 = KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_confirmed), 2000);
+            BOOST_CHECK(result24);
 
             if (amt - 2000 < MIN_CHANGE) {
                 // needs more than one input:
                 uint16_t returnSize = std::ceil((2000.0 + MIN_CHANGE)/amt);
                 CAmount returnValue = amt * returnSize;
-                BOOST_CHECK_EQUAL(nValueRet, returnValue);
-                BOOST_CHECK_EQUAL(setCoinsRet.size(), returnSize);
+                BOOST_CHECK_EQUAL(result24->GetSelectedValue(), returnValue);
+                BOOST_CHECK_EQUAL(result24->GetInputSet().size(), returnSize);
             } else {
                 // one input is sufficient:
-                BOOST_CHECK_EQUAL(nValueRet, amt);
-                BOOST_CHECK_EQUAL(setCoinsRet.size(), 1U);
+                BOOST_CHECK_EQUAL(result24->GetSelectedValue(), amt);
+                BOOST_CHECK_EQUAL(result24->GetInputSet().size(), 1U);
             }
         }
     }
@@ -557,9 +609,11 @@ BOOST_AUTO_TEST_CASE(knapsack_solver_test)
         for (int i = 0; i < RUN_TESTS; i++) {
             // picking 50 from 100 coins doesn't depend on the shuffle,
             // but does depend on randomness in the stochastic approximation code
-            BOOST_CHECK(KnapsackSolver(50 * COIN, GroupCoins(coins), setCoinsRet, nValueRet));
-            BOOST_CHECK(KnapsackSolver(50 * COIN, GroupCoins(coins), setCoinsRet2, nValueRet));
-            BOOST_CHECK(!equal_sets(setCoinsRet, setCoinsRet2));
+            const auto result25 = KnapsackSolver(GroupCoins(coins), 50 * COIN);
+            BOOST_CHECK(result25);
+            const auto result26 = KnapsackSolver(GroupCoins(coins), 50 * COIN);
+            BOOST_CHECK(result26);
+            BOOST_CHECK(!EqualResult(*result25, *result26));
 
             int fails = 0;
             for (int j = 0; j < RANDOM_REPEATS; j++)
@@ -568,9 +622,11 @@ BOOST_AUTO_TEST_CASE(knapsack_solver_test)
                 // When choosing 1 from 100 identical coins, 1% of the time, this test will choose the same coin twice
                 // which will cause it to fail.
                 // To avoid that issue, run the test RANDOM_REPEATS times and only complain if all of them fail
-                BOOST_CHECK(KnapsackSolver(COIN, GroupCoins(coins), setCoinsRet, nValueRet));
-                BOOST_CHECK(KnapsackSolver(COIN, GroupCoins(coins), setCoinsRet2, nValueRet));
-                if (equal_sets(setCoinsRet, setCoinsRet2))
+                const auto result27 = KnapsackSolver(GroupCoins(coins), COIN);
+                BOOST_CHECK(result27);
+                const auto result28 = KnapsackSolver(GroupCoins(coins), COIN);
+                BOOST_CHECK(result28);
+                if (EqualResult(*result27, *result28))
                     fails++;
             }
             BOOST_CHECK_NE(fails, RANDOM_REPEATS);
@@ -589,9 +645,11 @@ BOOST_AUTO_TEST_CASE(knapsack_solver_test)
             int fails = 0;
             for (int j = 0; j < RANDOM_REPEATS; j++)
             {
-                BOOST_CHECK(KnapsackSolver(90*CENT, GroupCoins(coins), setCoinsRet, nValueRet));
-                BOOST_CHECK(KnapsackSolver(90*CENT, GroupCoins(coins), setCoinsRet2, nValueRet));
-                if (equal_sets(setCoinsRet, setCoinsRet2))
+                const auto result29 = KnapsackSolver(GroupCoins(coins), 90 * CENT);
+                BOOST_CHECK(result29);
+                const auto result30 = KnapsackSolver(GroupCoins(coins), 90 * CENT);
+                BOOST_CHECK(result30);
+                if (EqualResult(*result29, *result30))
                     fails++;
             }
             BOOST_CHECK_NE(fails, RANDOM_REPEATS);
@@ -607,8 +665,6 @@ BOOST_AUTO_TEST_CASE(ApproximateBestSubset)
     wallet->SetWalletFlag(WALLET_FLAG_DESCRIPTORS);
     wallet->SetupDescriptorScriptPubKeyMans();
 
-    CoinSet setCoinsRet;
-    CAmount nValueRet;
     std::vector<COutput> coins;
 
     // Test vValue sort order
@@ -616,9 +672,10 @@ BOOST_AUTO_TEST_CASE(ApproximateBestSubset)
         add_coin(coins, *wallet, 1000 * COIN);
     add_coin(coins, *wallet, 3 * COIN);
 
-    BOOST_CHECK(KnapsackSolver(1003 * COIN, KnapsackGroupOutputs(coins, *wallet, filter_standard), setCoinsRet, nValueRet));
-    BOOST_CHECK_EQUAL(nValueRet, 1003 * COIN);
-    BOOST_CHECK_EQUAL(setCoinsRet.size(), 2U);
+    const auto result = KnapsackSolver(KnapsackGroupOutputs(coins, *wallet, filter_standard), 1003 * COIN);
+    BOOST_CHECK(result);
+    BOOST_CHECK_EQUAL(result->GetSelectedValue(), 1003 * COIN);
+    BOOST_CHECK_EQUAL(result->GetInputSet().size(), 2U);
 }
 
 // Tests that with the ideal conditions, the coin selector will always be able to find a solution that can pay the target value
@@ -660,11 +717,10 @@ BOOST_AUTO_TEST_CASE(SelectCoins_test)
                                       /* change_spend_size= */ 148, /* effective_feerate= */ CFeeRate(0),
                                       /* long_term_feerate= */ CFeeRate(0), /* discard_feerate= */ CFeeRate(0),
                                       /* tx_noinputs_size= */ 0, /* avoid_partial= */ false);
-        CoinSet out_set;
-        CAmount out_value = 0;
         CCoinControl cc;
-        BOOST_CHECK(SelectCoins(*wallet, coins, target, out_set, out_value, cc, cs_params));
-        BOOST_CHECK_GE(out_value, target);
+        const auto result = SelectCoins(*wallet, coins, target, cc, cs_params);
+        BOOST_CHECK(result);
+        BOOST_CHECK_GE(result->GetSelectedValue(), target);
     }
 }