Bitcoin QT - What order are coins sent?

[BitcoinBoss]

If I send a payment using Bitcoin QT and am not sending the entire wallet balance, what order are coins taken in regards to my received transaction history? For example do the oldest coins (the one with the most confirmations) take priority and will be sent first in any outgoing transaction? Hope this question makes sense.

[DannyHamilton]

Any chance you can read C++ code?  Here is the selection routine:

Code:

bool CWallet::SelectCoins(int64 nTargetValue, set<pair<const CWalletTx*,unsigned int> >& setCoinsRet, int64& nValueRet) const
{
    vector<COutput> vCoins;
    AvailableCoins(vCoins);

    return (SelectCoinsMinConf(nTargetValue, 1, 6, vCoins, setCoinsRet, nValueRet) ||
            SelectCoinsMinConf(nTargetValue, 1, 1, vCoins, setCoinsRet, nValueRet) ||
            SelectCoinsMinConf(nTargetValue, 0, 1, vCoins, setCoinsRet, nValueRet));
}


bool CWallet::SelectCoinsMinConf(int64 nTargetValue, int nConfMine, int nConfTheirs, vector<COutput> vCoins,
                                 set<pair<const CWalletTx*,unsigned int> >& setCoinsRet, int64& nValueRet) const
{
    setCoinsRet.clear();
    nValueRet = 0;

    // List of values less than target
    pair<int64, pair<const CWalletTx*,unsigned int> > coinLowestLarger;
    coinLowestLarger.first = std::numeric_limits<int64>::max();
    coinLowestLarger.second.first = NULL;
    vector<pair<int64, pair<const CWalletTx*,unsigned int> > > vValue;
    int64 nTotalLower = 0;

    random_shuffle(vCoins.begin(), vCoins.end(), GetRandInt);

    BOOST_FOREACH(COutput output, vCoins)
    {
        const CWalletTx *pcoin = output.tx;

        if (output.nDepth < (pcoin->IsFromMe() ? nConfMine : nConfTheirs))
            continue;

        int i = output.i;
        int64 n = pcoin->vout[i].nValue;

        pair<int64,pair<const CWalletTx*,unsigned int> > coin = make_pair(n,make_pair(pcoin, i));

        if (n == nTargetValue)
        {
            setCoinsRet.insert(coin.second);
            nValueRet += coin.first;
            return true;
        }
        else if (n < nTargetValue + CENT)
        {
            vValue.push_back(coin);
            nTotalLower += n;
        }
        else if (n < coinLowestLarger.first)
        {
            coinLowestLarger = coin;
        }
    }

    if (nTotalLower == nTargetValue)
    {
        for (unsigned int i = 0; i < vValue.size(); ++i)
        {
            setCoinsRet.insert(vValue[i].second);
            nValueRet += vValue[i].first;
        }
        return true;
    }

    if (nTotalLower < nTargetValue)
    {
        if (coinLowestLarger.second.first == NULL)
            return false;
        setCoinsRet.insert(coinLowestLarger.second);
        nValueRet += coinLowestLarger.first;
        return true;
    }

    // Solve subset sum by stochastic approximation
    sort(vValue.rbegin(), vValue.rend(), CompareValueOnly());
    vector<char> vfBest;
    int64 nBest;

    ApproximateBestSubset(vValue, nTotalLower, nTargetValue, vfBest, nBest, 1000);
    if (nBest != nTargetValue && nTotalLower >= nTargetValue + CENT)
        ApproximateBestSubset(vValue, nTotalLower, nTargetValue + CENT, vfBest, nBest, 1000);

    // If we have a bigger coin and (either the stochastic approximation didn't find a good solution,
    //                                   or the next bigger coin is closer), return the bigger coin
    if (coinLowestLarger.second.first &&
        ((nBest != nTargetValue && nBest < nTargetValue + CENT) || coinLowestLarger.first <= nBest))
    {
        setCoinsRet.insert(coinLowestLarger.second);
        nValueRet += coinLowestLarger.first;
    }
    else {
        for (unsigned int i = 0; i < vValue.size(); i++)
            if (vfBest[i])
            {
                setCoinsRet.insert(vValue[i].second);
                nValueRet += vValue[i].first;
            }

        //// debug print
        printf("SelectCoins() best subset: ");
        for (unsigned int i = 0; i < vValue.size(); i++)
            if (vfBest[i])
                printf("%s ", FormatMoney(vValue[i].first).c_str());
        printf("total %s\n", FormatMoney(nBest).c_str());
    }

    return true;
}