Now I need to fix the BacktrackState struct which references `coord_key` — since we removed the lambda, the struct's `key_fn` member needs updating. Let me read that section:


   430→    // scratch if greedy left unused sub-ways. The second pass gets clean
   431→    // used flags so backtracking makes optimal branch choices.
   432→
   433→    // Backtracking with depth limit (for fallback only)
   434→    struct BacktrackState {
   435→        const std::vector<SubWay>& sways;
   436→        const std::unordered_map<int64_t, std::vector<std::pair<size_t, bool>>>& adj;
   437→        std::vector<bool>& used;
   438→        decltype(coord_key)& key_fn;
   439→        int calls;
   440→
   441→        bool try_close(int64_t first_key, int64_t last_key,
   442→                       std::vector<std::pair<size_t, bool>>& path, int depth) {
   443→            if (++calls > 100000) return false; // limit total work, not just depth
   444→            if (!path.empty() && first_key == last_key) return true;
   445→            if (depth > 200) return false;
   446→
   447→            auto it = adj.find(last_key);
   448→            if (it == adj.end()) return false;
   449→
   450→            for (const auto& [wi, is_last] : it->second) {
   451→                if (used[wi]) continue;
   452→                auto& endpoint = is_last ? sways[wi].coords.front() : sways[wi].coords.back();
   453→                int64_t new_key = key_fn(endpoint.first, endpoint.second);
   454→
   455→                used[wi] = true;
   456→                path.push_back({wi, is_last});
   457→
   458→                if (try_close(first_key, new_key, path, depth + 1)) return true;
   459→
   460→                path.pop_back();
   461→                used[wi] = false;
   462→            }
   463→            return false;
   464→        }
   465→    };
   466→
   467→    BacktrackState bt{sub_ways, coord_adj, used, coord_key, 0};
   468→
   469→    // Pass 1: Greedy (fast, O(n) per ring)
   470→    for (size_t start_idx = 0; start_idx < sub_ways.size(); start_idx++) {
   471→        if (used[start_idx]) continue;
   472→
   473→        const auto& sg = sub_ways[start_idx];
   474→        int64_t first_key = coord_key(sg.coords.front().first, sg.coords.front().second);
   475→        int64_t last_key = coord_key(sg.coords.back().first, sg.coords.back().second);
   476→
   477→        if (sg.coords.size() >= 4 && first_key == last_key) {
   478→            used[start_idx] = true;
   479→            rings.push_back(sg.coords);

    // scratch if greedy left unused sub-ways. The second pass gets clean
    // used flags so backtracking makes optimal branch choices.

    // Backtracking with depth limit (for fallback only)
    struct BacktrackState {
        const std::vector<SubWay>& sways;
        const std::unordered_map<int64_t, std::vector<std::pair<size_t, bool>>>& adj;
        std::vector<bool>& used;
        decltype(coord_key)& key_fn;
        int calls;

        bool try_close(int64_t first_key, int64_t last_key,
                       std::vector<std::pair<size_t, bool>>& path, int depth) {
            if (++calls > 100000) return false; // limit total work, not just depth
            if (!path.empty() && first_key == last_key) return true;
            if (depth > 200) return false;

            auto it = adj.find(last_key);
            if (it == adj.end()) return false;

            for (const auto& [wi, is_last] : it->second) {
                if (used[wi]) continue;
                auto& endpoint = is_last ? sways[wi].coords.front() : sways[wi].coords.back();
                int64_t new_key = key_fn(endpoint.first, endpoint.second);

                used[wi] = true;
                path.push_back({wi, is_last});

                if (try_close(first_key, new_key, path, depth + 1)) return true;

                path.pop_back();
                used[wi] = false;
            }
            return false;
        }
    };

    BacktrackState bt{sub_ways, coord_adj, used, coord_key, 0};

    // Pass 1: Greedy (fast, O(n) per ring)
    for (size_t start_idx = 0; start_idx < sub_ways.size(); start_idx++) {
        if (used[start_idx]) continue;

        const auto& sg = sub_ways[start_idx];
        int64_t first_key = coord_key(sg.coords.front().first, sg.coords.front().second);
        int64_t last_key = coord_key(sg.coords.back().first, sg.coords.back().second);

        if (sg.coords.size() >= 4 && first_key == last_key) {
            used[start_idx] = true;
            rings.push_back(sg.coords);


I need to replace the `key_fn` member with a direct call to `coord_key`:


Edit

    struct BacktrackState {
        const std::vector<SubWay>& sways;
        const std::unordered_map<int64_t, std::vector<std::pair<size_t, bool>>>& adj;
        std::vector<bool>& used;
        decltype(coord_key)& key_fn;
        int calls;

        bool try_close(int64_t first_key, int64_t last_key,
                       std::vector<std::pair<size_t, bool>>& path, int depth) {
            if (++calls > 100000) return false; // limit total work, not just depth
            if (!path.empty() && first_key == last_key) return true;
            if (depth > 200) return false;

            auto it = adj.find(last_key);
            if (it == adj.end()) return false;

            for (const auto& [wi, is_last] : it->second) {
                if (used[wi]) continue;
                auto& endpoint = is_last ? sways[wi].coords.front() : sways[wi].coords.back();
                int64_t new_key = key_fn(endpoint.first, endpoint.second);

                used[wi] = true;
                path.push_back({wi, is_last});

                if (try_close(first_key, new_key, path, depth + 1)) return true;

                path.pop_back();
                used[wi] = false;
            }
            return false;
        }
    };

    BacktrackState bt{sub_ways, coord_adj, used, coord_key, 0};

    struct BacktrackState {
        const std::vector<SubWay>& sways;
        const std::unordered_map<int64_t, std::vector<std::pair<size_t, bool>>>& adj;
        std::vector<bool>& used;
        int calls;

        bool try_close(int64_t first_key, int64_t last_key,
                       std::vector<std::pair<size_t, bool>>& path, int depth) {
            if (++calls > 100000) return false; // limit total work, not just depth
            if (!path.empty() && first_key == last_key) return true;
            if (depth > 200) return false;

            auto it = adj.find(last_key);
            if (it == adj.end()) return false;

            for (const auto& [wi, is_last] : it->second) {
                if (used[wi]) continue;
                auto& endpoint = is_last ? sways[wi].coords.front() : sways[wi].coords.back();
                int64_t new_key = coord_key(endpoint.first, endpoint.second);

                used[wi] = true;
                path.push_back({wi, is_last});

                if (try_close(first_key, new_key, path, depth + 1)) return true;

                path.pop_back();
                used[wi] = false;
            }
            return false;
        }
    };

    BacktrackState bt{sub_ways, coord_adj, used, 0};