lbrycrd/src/wallet/test/coinselector_tests.cpp

// Copyright (c) 2017 The Bitcoin Core developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.

#include "wallet/wallet.h"
#include "wallet/coinselection.h"
#include "amount.h"
#include "primitives/transaction.h"
#include "random.h"
#include "test/test_bitcoin.h"
#include "wallet/test/wallet_test_fixture.h"

#include <boost/test/unit_test.hpp>
#include <random>

BOOST_FIXTURE_TEST_SUITE(coin_selection_tests, WalletTestingSetup)

// how many times to run all the tests to have a chance to catch errors that only show up with particular random shuffles
#define RUN_TESTS 100

// some tests fail 1% of the time due to bad luck.
// we repeat those tests this many times and only complain if all iterations of the test fail
#define RANDOM_REPEATS 5

std::vector<std::unique_ptr<CWalletTx>> wtxn;

typedef std::set<CInputCoin> CoinSet;

static std::vector<COutput> vCoins;
static const CWallet testWallet("dummy", CWalletDBWrapper::CreateDummy());

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;
    set.emplace_back(MakeTransactionRef(tx), nInput);
}

static void add_coin(const CAmount& nValue, int nInput, CoinSet& set)
{
    CMutableTransaction tx;
    tx.vout.resize(nInput + 1);
    tx.vout[nInput].nValue = nValue;
    set.emplace(MakeTransactionRef(tx), nInput);
}

static void add_coin(const CAmount& nValue, int nAge = 6*24, bool fIsFromMe = false, int nInput=0)
{
    static int nextLockTime = 0;
    CMutableTransaction tx;
    tx.nLockTime = nextLockTime++;        // so all transactions get different hashes
    tx.vout.resize(nInput + 1);
    tx.vout[nInput].nValue = nValue;
    if (fIsFromMe) {
        // IsFromMe() returns (GetDebit() > 0), and GetDebit() is 0 if vin.empty(),
        // so stop vin being empty, and cache a non-zero Debit to fake out IsFromMe()
        tx.vin.resize(1);
    }
    std::unique_ptr<CWalletTx> wtx(new CWalletTx(&testWallet, MakeTransactionRef(std::move(tx))));
    if (fIsFromMe)
    {
        wtx->fDebitCached = true;
        wtx->nDebitCached = 1;
    }
    COutput output(wtx.get(), nInput, nAge, true /* spendable */, true /* solvable */, true /* safe */);
    vCoins.push_back(output);
    wtxn.emplace_back(std::move(wtx));
}

static void empty_wallet(void)
{
    vCoins.clear();
    wtxn.clear();
}

static bool equal_sets(CoinSet a, CoinSet b)
{
    std::pair<CoinSet::iterator, CoinSet::iterator> ret = mismatch(a.begin(), a.end(), b.begin());
    return ret.first == a.end() && ret.second == b.end();
}

static CAmount make_hard_case(int utxos, std::vector<CInputCoin>& utxo_pool)
{
    utxo_pool.clear();
    CAmount target = 0;
    for (int i = 0; i < utxos; ++i) {
        target += (CAmount)1 << (utxos+i);
        add_coin((CAmount)1 << (utxos+i), 2*i, utxo_pool);
        add_coin(((CAmount)1 << (utxos+i)) + ((CAmount)1 << (utxos-1-i)), 2*i + 1, utxo_pool);
    }
    return target;
}

// Branch and bound coin selection tests
BOOST_AUTO_TEST_CASE(bnb_search_test)
{

    LOCK(testWallet.cs_wallet);

    // Setup
    std::vector<CInputCoin> utxo_pool;
    CoinSet selection;
    CoinSet actual_selection;
    CAmount value_ret = 0;
    CAmount not_input_fees = 0;

    /////////////////////////
    // Known Outcome tests //
    /////////////////////////
    BOOST_TEST_MESSAGE("Testing known outcomes");

    // Empty utxo pool
    BOOST_CHECK(!SelectCoinsBnB(utxo_pool, 1 * CENT, 0.5 * CENT, selection, value_ret, not_input_fees));
    selection.clear();

    // Add utxos
    add_coin(1 * CENT, 1, utxo_pool);
    add_coin(2 * CENT, 2, utxo_pool);
    add_coin(3 * CENT, 3, utxo_pool);
    add_coin(4 * CENT, 4, utxo_pool);

    // Select 1 Cent
    add_coin(1 * CENT, 1, actual_selection);
    BOOST_CHECK(SelectCoinsBnB(utxo_pool, 1 * CENT, 0.5 * CENT, selection, value_ret, not_input_fees));
    BOOST_CHECK(equal_sets(selection, actual_selection));
    actual_selection.clear();
    selection.clear();

    // Select 2 Cent
    add_coin(2 * CENT, 2, actual_selection);
    BOOST_CHECK(SelectCoinsBnB(utxo_pool, 2 * CENT, 0.5 * CENT, selection, value_ret, not_input_fees));
    BOOST_CHECK(equal_sets(selection, actual_selection));
    actual_selection.clear();
    selection.clear();

    // Select 5 Cent
    add_coin(3 * CENT, 3, actual_selection);
    add_coin(2 * CENT, 2, actual_selection);
    BOOST_CHECK(SelectCoinsBnB(utxo_pool, 5 * CENT, 0.5 * CENT, selection, value_ret, not_input_fees));
    BOOST_CHECK(equal_sets(selection, actual_selection));
    actual_selection.clear();
    selection.clear();

    // Select 11 Cent, not possible
    BOOST_CHECK(!SelectCoinsBnB(utxo_pool, 11 * CENT, 0.5 * CENT, selection, value_ret, not_input_fees));
    actual_selection.clear();
    selection.clear();

    // Select 10 Cent
    add_coin(5 * CENT, 5, utxo_pool);
    add_coin(4 * CENT, 4, actual_selection);
    add_coin(3 * CENT, 3, actual_selection);
    add_coin(2 * CENT, 2, actual_selection);
    add_coin(1 * CENT, 1, actual_selection);
    BOOST_CHECK(SelectCoinsBnB(utxo_pool, 10 * CENT, 0.5 * CENT, selection, value_ret, not_input_fees));
    BOOST_CHECK(equal_sets(selection, actual_selection));
    actual_selection.clear();
    selection.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(utxo_pool, 10 * CENT, 5000, selection, value_ret, not_input_fees));

    // Select 0.25 Cent, not possible
    BOOST_CHECK(!SelectCoinsBnB(utxo_pool, 0.25 * CENT, 0.5 * CENT, selection, value_ret, not_input_fees));
    actual_selection.clear();
    selection.clear();

    // Iteration exhaustion test
    CAmount target = make_hard_case(17, utxo_pool);
    BOOST_CHECK(!SelectCoinsBnB(utxo_pool, target, 0, selection, value_ret, not_input_fees)); // Should exhaust
    target = make_hard_case(14, utxo_pool);
    BOOST_CHECK(SelectCoinsBnB(utxo_pool, target, 0, selection, value_ret, not_input_fees)); // Should not exhaust

    // Test same value early bailout optimization
    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, utxo_pool);
    add_coin(7 * CENT, 7, utxo_pool);
    add_coin(7 * CENT, 7, utxo_pool);
    add_coin(7 * CENT, 7, utxo_pool);
    add_coin(2 * CENT, 7, utxo_pool);
    for (int i = 0; i < 50000; ++i) {
        add_coin(5 * CENT, 7, utxo_pool);
    }
    BOOST_CHECK(SelectCoinsBnB(utxo_pool, 30 * CENT, 5000, selection, value_ret, not_input_fees));

    ////////////////////
    // Behavior tests //
    ////////////////////
    // Select 1 Cent with pool of only greater than 5 Cent
    utxo_pool.clear();
    for (int i = 5; i <= 20; ++i) {
        add_coin(i * CENT, i, utxo_pool);
    }
    // Run 100 times, to make sure it is never finding a solution
    for (int i = 0; i < 100; ++i) {
        BOOST_CHECK(!SelectCoinsBnB(utxo_pool, 1 * CENT, 2 * CENT, selection, value_ret, not_input_fees));
    }
}

CoinEligibilityFilter filter_standard(1, 6, 0);
CoinEligibilityFilter filter_confirmed(1, 1, 0);
CoinEligibilityFilter filter_standard_extra(6, 6, 0);

BOOST_AUTO_TEST_CASE(knapsack_solver_test)
{
    CoinSet setCoinsRet, setCoinsRet2;
    CAmount nValueRet;

    LOCK(testWallet.cs_wallet);

    // test multiple times to allow for differences in the shuffle order
    for (int i = 0; i < RUN_TESTS; i++)
    {
        empty_wallet();

        // with an empty wallet we can't even pay one cent
        BOOST_CHECK(!testWallet.SelectCoinsMinConf( 1 * CENT, filter_standard, vCoins, setCoinsRet, nValueRet));

        add_coin(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(!testWallet.SelectCoinsMinConf( 1 * CENT, filter_standard, vCoins, setCoinsRet, nValueRet));

        // but we can find a new 1 cent
        BOOST_CHECK( testWallet.SelectCoinsMinConf( 1 * CENT, filter_confirmed, vCoins, setCoinsRet, nValueRet));
        BOOST_CHECK_EQUAL(nValueRet, 1 * CENT);

        add_coin(2*CENT);           // add a mature 2 cent coin

        // we can't make 3 cents of mature coins
        BOOST_CHECK(!testWallet.SelectCoinsMinConf( 3 * CENT, filter_standard, vCoins, setCoinsRet, nValueRet));

        // we can make 3 cents of new coins
        BOOST_CHECK( testWallet.SelectCoinsMinConf( 3 * CENT, filter_confirmed, vCoins, setCoinsRet, nValueRet));
        BOOST_CHECK_EQUAL(nValueRet, 3 * CENT);

        add_coin(5*CENT);           // add a mature 5 cent coin,
        add_coin(10*CENT, 3, true); // a new 10 cent coin sent from one of our own addresses
        add_coin(20*CENT);          // and a mature 20 cent coin

        // 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(!testWallet.SelectCoinsMinConf(38 * CENT, filter_standard, vCoins, setCoinsRet, nValueRet));
        // we can't even make 37 cents if we don't allow new coins even if they're from us
        BOOST_CHECK(!testWallet.SelectCoinsMinConf(38 * CENT, filter_standard_extra, vCoins, setCoinsRet, nValueRet));
        // but we can make 37 cents if we accept new coins from ourself
        BOOST_CHECK( testWallet.SelectCoinsMinConf(37 * CENT, filter_standard, vCoins, setCoinsRet, nValueRet));
        BOOST_CHECK_EQUAL(nValueRet, 37 * CENT);
        // and we can make 38 cents if we accept all new coins
        BOOST_CHECK( testWallet.SelectCoinsMinConf(38 * CENT, filter_confirmed, vCoins, setCoinsRet, nValueRet));
        BOOST_CHECK_EQUAL(nValueRet, 38 * CENT);

        // try making 34 cents from 1,2,5,10,20 - we can't do it exactly
        BOOST_CHECK( testWallet.SelectCoinsMinConf(34 * CENT, filter_confirmed, vCoins, 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)

        // when we try making 7 cents, the smaller coins (1,2,5) are enough.  We should see just 2+5
        BOOST_CHECK( testWallet.SelectCoinsMinConf( 7 * CENT, filter_confirmed, vCoins, setCoinsRet, nValueRet));
        BOOST_CHECK_EQUAL(nValueRet, 7 * CENT);
        BOOST_CHECK_EQUAL(setCoinsRet.size(), 2U);

        // when we try making 8 cents, the smaller coins (1,2,5) are exactly enough.
        BOOST_CHECK( testWallet.SelectCoinsMinConf( 8 * CENT, filter_confirmed, vCoins, setCoinsRet, nValueRet));
        BOOST_CHECK(nValueRet == 8 * CENT);
        BOOST_CHECK_EQUAL(setCoinsRet.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( testWallet.SelectCoinsMinConf( 9 * CENT, filter_confirmed, vCoins, setCoinsRet, nValueRet));
        BOOST_CHECK_EQUAL(nValueRet, 10 * CENT);
        BOOST_CHECK_EQUAL(setCoinsRet.size(), 1U);

        // now clear out the wallet and start again to test choosing between subsets of smaller coins and the next biggest coin
        empty_wallet();

        add_coin( 6*CENT);
        add_coin( 7*CENT);
        add_coin( 8*CENT);
        add_coin(20*CENT);
        add_coin(30*CENT); // now we have 6+7+8+20+30 = 71 cents total

        // check that we have 71 and not 72
        BOOST_CHECK( testWallet.SelectCoinsMinConf(71 * CENT, filter_confirmed, vCoins, setCoinsRet, nValueRet));
        BOOST_CHECK(!testWallet.SelectCoinsMinConf(72 * CENT, filter_confirmed, vCoins, setCoinsRet, nValueRet));

        // 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( testWallet.SelectCoinsMinConf(16 * CENT, filter_confirmed, vCoins, setCoinsRet, nValueRet));
        BOOST_CHECK_EQUAL(nValueRet, 20 * CENT); // we should get 20 in one coin
        BOOST_CHECK_EQUAL(setCoinsRet.size(), 1U);

        add_coin( 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( testWallet.SelectCoinsMinConf(16 * CENT, filter_confirmed, vCoins, setCoinsRet, nValueRet));
        BOOST_CHECK_EQUAL(nValueRet, 18 * CENT); // we should get 18 in 3 coins
        BOOST_CHECK_EQUAL(setCoinsRet.size(), 3U);

        add_coin( 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( testWallet.SelectCoinsMinConf(16 * CENT, filter_confirmed, vCoins, 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

        // now try making 11 cents.  we should get 5+6
        BOOST_CHECK( testWallet.SelectCoinsMinConf(11 * CENT, filter_confirmed, vCoins, setCoinsRet, nValueRet));
        BOOST_CHECK_EQUAL(nValueRet, 11 * CENT);
        BOOST_CHECK_EQUAL(setCoinsRet.size(), 2U);

        // check that the smallest bigger coin is used
        add_coin( 1*COIN);
        add_coin( 2*COIN);
        add_coin( 3*COIN);
        add_coin( 4*COIN); // now we have 5+6+7+8+18+20+30+100+200+300+400 = 1094 cents
        BOOST_CHECK( testWallet.SelectCoinsMinConf(95 * CENT, filter_confirmed, vCoins, setCoinsRet, nValueRet));
        BOOST_CHECK_EQUAL(nValueRet, 1 * COIN);  // we should get 1 BTC in 1 coin
        BOOST_CHECK_EQUAL(setCoinsRet.size(), 1U);

        BOOST_CHECK( testWallet.SelectCoinsMinConf(195 * CENT, filter_confirmed, vCoins, setCoinsRet, nValueRet));
        BOOST_CHECK_EQUAL(nValueRet, 2 * COIN);  // we should get 2 BTC in 1 coin
        BOOST_CHECK_EQUAL(setCoinsRet.size(), 1U);

        // empty the wallet and start again, now with fractions of a cent, to test small change avoidance

        empty_wallet();
        add_coin(MIN_CHANGE * 1 / 10);
        add_coin(MIN_CHANGE * 2 / 10);
        add_coin(MIN_CHANGE * 3 / 10);
        add_coin(MIN_CHANGE * 4 / 10);
        add_coin(MIN_CHANGE * 5 / 10);

        // 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( testWallet.SelectCoinsMinConf(MIN_CHANGE, filter_confirmed, vCoins, setCoinsRet, nValueRet));
        BOOST_CHECK_EQUAL(nValueRet, MIN_CHANGE);

        // but if we add a bigger coin, small change is avoided
        add_coin(1111*MIN_CHANGE);

        // try making 1 from 0.1 + 0.2 + 0.3 + 0.4 + 0.5 + 1111 = 1112.5
        BOOST_CHECK( testWallet.SelectCoinsMinConf(1 * MIN_CHANGE, filter_confirmed, vCoins, setCoinsRet, nValueRet));
        BOOST_CHECK_EQUAL(nValueRet, 1 * MIN_CHANGE); // we should get the exact amount

        // if we add more small coins:
        add_coin(MIN_CHANGE * 6 / 10);
        add_coin(MIN_CHANGE * 7 / 10);

        // and try again to make 1.0 * MIN_CHANGE
        BOOST_CHECK( testWallet.SelectCoinsMinConf(1 * MIN_CHANGE, filter_confirmed, vCoins, setCoinsRet, nValueRet));
        BOOST_CHECK_EQUAL(nValueRet, 1 * MIN_CHANGE); // we should get the exact amount

        // run the 'mtgox' test (see http://blockexplorer.com/tx/29a3efd3ef04f9153d47a990bd7b048a4b2d213daaa5fb8ed670fb85f13bdbcf)
        // they tried to consolidate 10 50k coins into one 500k coin, and ended up with 50k in change
        empty_wallet();
        for (int j = 0; j < 20; j++)
            add_coin(50000 * COIN);

        BOOST_CHECK( testWallet.SelectCoinsMinConf(500000 * COIN, filter_confirmed, vCoins, setCoinsRet, nValueRet));
        BOOST_CHECK_EQUAL(nValueRet, 500000 * COIN); // we should get the exact amount
        BOOST_CHECK_EQUAL(setCoinsRet.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

        // sometimes it will fail, and so we use the next biggest coin:
        empty_wallet();
        add_coin(MIN_CHANGE * 5 / 10);
        add_coin(MIN_CHANGE * 6 / 10);
        add_coin(MIN_CHANGE * 7 / 10);
        add_coin(1111 * MIN_CHANGE);
        BOOST_CHECK( testWallet.SelectCoinsMinConf(1 * MIN_CHANGE, filter_confirmed, vCoins, setCoinsRet, nValueRet));
        BOOST_CHECK_EQUAL(nValueRet, 1111 * MIN_CHANGE); // we get the bigger coin
        BOOST_CHECK_EQUAL(setCoinsRet.size(), 1U);

        // but sometimes it's possible, and we use an exact subset (0.4 + 0.6 = 1.0)
        empty_wallet();
        add_coin(MIN_CHANGE * 4 / 10);
        add_coin(MIN_CHANGE * 6 / 10);
        add_coin(MIN_CHANGE * 8 / 10);
        add_coin(1111 * MIN_CHANGE);
        BOOST_CHECK( testWallet.SelectCoinsMinConf(MIN_CHANGE, filter_confirmed, vCoins, 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

        // test avoiding small change
        empty_wallet();
        add_coin(MIN_CHANGE * 5 / 100);
        add_coin(MIN_CHANGE * 1);
        add_coin(MIN_CHANGE * 100);

        // trying to make 100.01 from these three coins
        BOOST_CHECK(testWallet.SelectCoinsMinConf(MIN_CHANGE * 10001 / 100, filter_confirmed, vCoins, setCoinsRet, nValueRet));
        BOOST_CHECK_EQUAL(nValueRet, MIN_CHANGE * 10105 / 100); // we should get all coins
        BOOST_CHECK_EQUAL(setCoinsRet.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(testWallet.SelectCoinsMinConf(MIN_CHANGE * 9990 / 100, filter_confirmed, vCoins, setCoinsRet, nValueRet));
        BOOST_CHECK_EQUAL(nValueRet, 101 * MIN_CHANGE);
        BOOST_CHECK_EQUAL(setCoinsRet.size(), 2U);

        // test with many inputs
        for (CAmount amt=1500; amt < COIN; amt*=10) {
             empty_wallet();
             // Create 676 inputs (=  (old MAX_STANDARD_TX_SIZE == 100000)  / 148 bytes per input)
             for (uint16_t j = 0; j < 676; j++)
                 add_coin(amt);
             BOOST_CHECK(testWallet.SelectCoinsMinConf(2000, filter_confirmed, vCoins, setCoinsRet, nValueRet));
             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);
             } else {
                 // one input is sufficient:
                 BOOST_CHECK_EQUAL(nValueRet, amt);
                 BOOST_CHECK_EQUAL(setCoinsRet.size(), 1U);
             }
        }

        // test randomness
        {
            empty_wallet();
            for (int i2 = 0; i2 < 100; i2++)
                add_coin(COIN);

            // picking 50 from 100 coins doesn't depend on the shuffle,
            // but does depend on randomness in the stochastic approximation code
            BOOST_CHECK(testWallet.SelectCoinsMinConf(50 * COIN, filter_standard, vCoins, setCoinsRet , nValueRet));
            BOOST_CHECK(testWallet.SelectCoinsMinConf(50 * COIN, filter_standard, vCoins, setCoinsRet2, nValueRet));
            BOOST_CHECK(!equal_sets(setCoinsRet, setCoinsRet2));

            int fails = 0;
            for (int j = 0; j < RANDOM_REPEATS; j++)
            {
                // selecting 1 from 100 identical coins depends on the shuffle; this test will fail 1% of the time
                // run the test RANDOM_REPEATS times and only complain if all of them fail
                BOOST_CHECK(testWallet.SelectCoinsMinConf(COIN, filter_standard, vCoins, setCoinsRet , nValueRet));
                BOOST_CHECK(testWallet.SelectCoinsMinConf(COIN, filter_standard, vCoins, setCoinsRet2, nValueRet));
                if (equal_sets(setCoinsRet, setCoinsRet2))
                    fails++;
            }
            BOOST_CHECK_NE(fails, RANDOM_REPEATS);

            // add 75 cents in small change.  not enough to make 90 cents,
            // then try making 90 cents.  there are multiple competing "smallest bigger" coins,
            // one of which should be picked at random
            add_coin(5 * CENT);
            add_coin(10 * CENT);
            add_coin(15 * CENT);
            add_coin(20 * CENT);
            add_coin(25 * CENT);

            fails = 0;
            for (int j = 0; j < RANDOM_REPEATS; j++)
            {
                // selecting 1 from 100 identical coins depends on the shuffle; this test will fail 1% of the time
                // run the test RANDOM_REPEATS times and only complain if all of them fail
                BOOST_CHECK(testWallet.SelectCoinsMinConf(90*CENT, filter_standard, vCoins, setCoinsRet , nValueRet));
                BOOST_CHECK(testWallet.SelectCoinsMinConf(90*CENT, filter_standard, vCoins, setCoinsRet2, nValueRet));
                if (equal_sets(setCoinsRet, setCoinsRet2))
                    fails++;
            }
            BOOST_CHECK_NE(fails, RANDOM_REPEATS);
        }
    }
    empty_wallet();
}

BOOST_AUTO_TEST_CASE(ApproximateBestSubset)
{
    CoinSet setCoinsRet;
    CAmount nValueRet;

    LOCK(testWallet.cs_wallet);

    empty_wallet();

    // Test vValue sort order
    for (int i = 0; i < 1000; i++)
        add_coin(1000 * COIN);
    add_coin(3 * COIN);

    BOOST_CHECK(testWallet.SelectCoinsMinConf(1003 * COIN, filter_standard, vCoins, setCoinsRet, nValueRet));
    BOOST_CHECK_EQUAL(nValueRet, 1003 * COIN);
    BOOST_CHECK_EQUAL(setCoinsRet.size(), 2U);

    empty_wallet();
}

BOOST_AUTO_TEST_SUITE_END()