Function AttemptSelection

src/wallet/spend.cpp:698–723

static bool HasErrorMsg(const util::Result<SelectionResult>& res) { return !util::ErrorString(res).empty(); }

util::Result<SelectionResult> AttemptSelection(interfaces::Chain& chain, const CAmount& nTargetValue, OutputGroupTypeMap& groups,
                               const CoinSelectionParams& coin_selection_params, bool allow_mixed_output_types)
{
    // Run coin selection on each OutputType and compute the Waste Metric
    std::vector<SelectionResult> results;
    for (auto& [type, group] : groups.groups_by_type) {
        auto result{ChooseSelectionResult(chain, nTargetValue, group, coin_selection_params)};
        // If any specific error message appears here, then something particularly wrong happened.
        if (HasErrorMsg(result)) return result; // So let's return the specific error.
        // Append the favorable result.
        if (result) results.push_back(*result);
    }
    // If we have at least one solution for funding the transaction without mixing, choose the minimum one according to waste metric
    // and return the result
    if (results.size() > 0) return *std::min_element(results.begin(), results.end());

    // If we can't fund the transaction from any individual OutputType, run coin selection one last time
    // over all available coins, which would allow mixing.
    // If TypesCount() <= 1, there is nothing to mix.
    if (allow_mixed_output_types && groups.TypesCount() > 1) {
        return ChooseSelectionResult(chain, nTargetValue, groups.all_groups, coin_selection_params);
    }
    // Either mixing is not allowed and we couldn't find a solution from any single OutputType, or mixing was allowed and we still couldn't
    // find a solution using all available coins
    return util::Error();
};

util::Result<SelectionResult> ChooseSelectionResult(interfaces::Chain& chain, const CAmount& nTargetValue, Groups& groups, const CoinSelectionParams& coin_selection_params)
{