const std::vector<bool>& allow_reversals) {

std::vector<std::pair<size_t, segment_type_p>> intersected_segment_pairs;

boost::polygon::intersect_segments(intersected_segment_pairs, all_segments.cbegin(), all_segments.cend());

std::vector<std::pair<segment_type_p, bool>> intersected_segments;

for (const auto& p : intersected_segment_pairs) {

const auto index_in_input = p.first;

const auto& allow_reversal = allow_reversals[index_in_input];

intersected_segments.push_back(std::make_pair(p.second, allow_reversal));

if (!allow_reversal) {

// Reverse segments that are now pointing in the wrong direction.

const auto& input_segment = all_segments[index_in_input];

auto& new_segment = intersected_segments.back().first;

auto input_delta_x = input_segment.high().x() - input_segment.low().x();

auto input_delta_y = input_segment.high().y() - input_segment.low().y();

auto new_delta_x = new_segment.high().x() - new_segment.low().x();

auto new_delta_y = new_segment.high().y() - new_segment.low().y();

if (sgn(input_delta_x) != sgn(new_delta_x) ||

sgn(input_delta_y) != sgn(new_delta_y)) {

// Swap low and high.

auto low = new_segment.low();

new_segment.low(new_segment.high());

new_segment.high(low);

}

}

}

std::sort(intersected_segments.begin(), intersected_segments.end());

auto last = std::unique(intersected_segments.begin(), intersected_segments.end());

intersected_segments.erase(last, intersected_segments.end());

return intersected_segments;