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

1 files changed, 223 insertions, 167 deletions

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);
     }
 }