Return SelectionResult from KnapsackSolver

Returns a std::optional<SelectionResult> from KnapsackSolver instead of
using out parameters for the inputs set and selected value.

4 files changed, 134 insertions, 114 deletions

diff --git a/src/wallet/coinselection.cpp b/src/wallet/coinselection.cpp
index 4d90eefbfe..97918429e0 100644
--- a/src/wallet/coinselection.cpp
+++ b/src/wallet/coinselection.cpp
@@ -236,10 +236,9 @@ 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;
@@ -250,9 +249,8 @@ 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();
@@ -263,17 +261,15 @@ 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
@@ -290,13 +286,11 @@ 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]);
             }
         }
 
@@ -311,7 +305,7 @@ bool KnapsackSolver(const CAmount& nTargetValue, std::vector<OutputGroup>& group
         }
     }
 
-    return true;
+    return result;
 }
 
 /******************************************************************************
diff --git a/src/wallet/coinselection.h b/src/wallet/coinselection.h
index 3e42ed2aa2..23bdff73c2 100644
--- a/src/wallet/coinselection.h
+++ b/src/wallet/coinselection.h
@@ -246,6 +246,6 @@ std::optional<SelectionResult> SelectCoinsBnB(std::vector<OutputGroup>& utxo_poo
 std::optional<std::pair<std::set<CInputCoin>, CAmount>> 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 022420c57d..e953c83c10 100644
--- a/src/wallet/spend.cpp
+++ b/src/wallet/spend.cpp
@@ -394,11 +394,9 @@ bool AttemptSelection(const CWallet& wallet, const CAmount& nTargetValue, const
     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.emplace_back(std::make_tuple(knapsack_result->GetWaste(), knapsack_result->GetInputSet(), knapsack_result->GetSelectedValue()));
     }
 
     // We include the minimum final change for SRD as we do want to avoid making really small change.
diff --git a/src/wallet/test/coinselector_tests.cpp b/src/wallet/test/coinselector_tests.cpp
index 8bb2105242..7d3a022285 100644
--- a/src/wallet/test/coinselector_tests.cpp
+++ b/src/wallet/test/coinselector_tests.cpp
@@ -115,10 +115,11 @@ static bool EquivalentResult(const SelectionResult& a, const SelectionResult& b)
     return ret.first == a_amts.end() && ret.second == b_amts.end();
 }
 
-static bool equal_sets(CoinSet a, CoinSet b)
+/** 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)
@@ -351,8 +352,6 @@ 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
@@ -361,25 +360,27 @@ 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
@@ -388,35 +389,41 @@ 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();
@@ -428,45 +435,52 @@ 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
 
@@ -479,23 +493,26 @@ 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
@@ -503,9 +520,10 @@ 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
@@ -516,9 +534,10 @@ 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();
@@ -526,9 +545,10 @@ 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();
@@ -537,14 +557,16 @@ 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
@@ -556,18 +578,19 @@ 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);
             }
         }
     }
@@ -582,9 +605,11 @@ 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++)
@@ -593,9 +618,11 @@ 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);
@@ -614,9 +641,11 @@ 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);
@@ -632,8 +661,6 @@ BOOST_AUTO_TEST_CASE(ApproximateBestSubset)
     wallet->SetWalletFlag(WALLET_FLAG_DESCRIPTORS);
     wallet->SetupDescriptorScriptPubKeyMans();
 
-    CoinSet setCoinsRet;
-    CAmount nValueRet;
     std::vector<COutput> coins;
 
     // Test vValue sort order
@@ -641,9 +668,10 @@ 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